https://nanoscalereslett.springeropen.com/articles/10.1186/s11671-018-2760-6

Skip to main content

Advertisement

Advertisement
SpringerOpen
Search

  * Get published
  * Explore Journals
  * Books
  * About
  * My account

Search all SpringerOpen articles
[                    ]
Search
Nanoscale Research Letters

  * About
  * Articles
  * Submission Guidelines

Dye-Sensitized Solar Cells: Fundamentals and Current Status
Download PDF
Download PDF

  * Nano Review
  * Open Access
  * Published: 28 November 2018

Dye-Sensitized Solar Cells: Fundamentals and Current Status

  * Khushboo Sharma^1,
  * Vinay Sharma^2 &
  * S. S. Sharma^3 

Nanoscale Research Letters volume 13, Article number: 381 (2018) Cite
this article

  * 111k Accesses

  * 364 Citations

  * 51 Altmetric

  * Metrics details

Abstract

Dye-sensitized solar cells (DSSCs) belong to the group of thin-film
solar cells which have been under extensive research for more than
two decades due to their low cost, simple preparation methodology,
low toxicity and ease of production. Still, there is lot of scope for
the replacement of current DSSC materials due to their high cost,
less abundance, and long-term stability. The efficiency of existing
DSSCs reaches up to 12%, using Ru(II) dyes by optimizing material and
structural properties which is still less than the efficiency offered
by first- and second-generation solar cells, i.e., other thin-film
solar cells and Si-based solar cells which offer ~ 20-30% efficiency.
This article provides an in-depth review on DSSC construction,
operating principle, key problems (low efficiency, low scalability,
and low stability), prospective efficient materials, and finally a
brief insight to commercialization.

[11671_2018_2760_Figa_HTML]

Introduction

Dye-sensitized solar cells (DSSCs) have arisen as a technically and
economically credible alternative to the p-n junction photovoltaic
devices. In the late 1960s, it was discovered that electricity can be
generated through illuminated organic dyes in electrochemical cells.
At the University of California at Berkeley, chlorophyll was
extracted from spinach (photosynthesis). First chlorophyll-sensitized
zinc oxide (ZnO) electrode was synthesized in 1972. For the first
time, through electron injection of excited dye molecules into a wide
band gap of semiconductor, photons were converted into electricity [1
]. A lot of research has been done on ZnO-single crystals [2], but
the efficiency of these dye-sensitized solar cells was very poor, as
the monolayer of dye molecules was able to absorb incident light only
up to 1%. Thus, the efficiency was improved by optimizing the
porosity of the electrode made up of fine oxide powder, so that the
absorption of dye over electrode could be enhanced and as a result
light harvesting efficiency (LHE) could also be enhanced. As a
result, nanoporous titanium dioxide (TiO[2]) electrodes with a
roughness factor of ca.1000 were discovered, and in 1991, DSSCs with
7% efficiency were invented [3]. These cells, also known as Gratzel
cells, were originally co-invented in 1988 by Brian O'Regan and
Michael Gratzel at UC Berkeley [3] and were further developed by the
aforementioned scientists at Ecole Polytechnique Federale de Lausanne
(EPFL) till 1991.

Brian O'Regan and Michael Gratzel fabricated a device based on a
10-mm-thick, high surface area and optically transparent film of TiO
[2] nanoparticles, coated with a monolayer of a charge transfer dye
with ideal spectral characteristics to sensitize the film for light
harvesting. The device harvested a high proportion of the incident
solar energy flux of 46% and showed exceptionally high efficiencies,
even more than 80% efficiencies for the conversion of incident
photons to electrical current. The overall incident photon to current
conversion efficiency (IPCE) yield was 7.1-7.9% in simulated solar
light and 12% in diffuse daylight. A large short circuit current
density J[SC] (greater than 12 mAcm^- 2) and exceptional stability
(sustaining at least five million turnovers without decomposition)
and low cost made the practical application feasible [3]. In 1993,
Gratzel et al. reported 9.6% efficiency of cells, and then in 1997,
they achieved 10% at the National Renewable Energy Laboratory (NREL).
The sensitizers are usually designed to have functional groups such
as -COOH, -PO[3]H[2], and -B(OH)[2] for stable adsorption onto the
semiconductor substrate [4, 5]. Recently in 2018, an efficiency of
8.75% was reported for hybrid dye-titania nanoparticle-based DSSC for
superior low temperature by Costa et al. [6]. In a traditional solar
cell, Si provides two functions: acts as source of photoelectrons and
provides electric field to separate the charges and create a current.
But, in DSSCs, the bulk of semiconductor is only used as a charge
transporter and the photoelectrons are provided by photosensitive
dyes. The theoretically predicted power conversion efficiency (PCE)
of DSSCs was approximately 20% [7, 8]; thus, an extensive research
has been made over the years on DSSCs to improve the efficiency and
to augment its commercialization. However, in the last few decades, a
lot of experiments were carried out to improve the performance of
DSSCs. For instance, if one goes through the review articles or
papers published around 1920 and 1921, a remarkable difference may be
observed in the performance as well as fabrication of these cells.
Few review papers are discussed below with the objective and main
results shown in a respective article to get an idea how the
performance of these cells has been improved and, thus, how the DSSCs
became a hot topic for researchers.

Anandan reviewed the improvements and arising challenges in
dye-sensitized solar cells till 2007 [9]. The main components of his
review study were light harvesting inorganic dye molecules, p-CuO
nanorod counter electrodes, and self-organization of electroactive
polymers, and he showed how these materials perform in a rationally
designed solar cell. However, the maximum IPCE of 7% was discussed in
the review paper for naphthyridine coordinated Ru complex [10] which
was good till 2007 but is almost half to the efficiencies shown in
later work.

The main emphasis of the review paper published by Bose et al. [11]
was the current state and developments in the field of
photoelectrode, photosensitizer, and electrolyte for DSSCs till 2015.
They have included an interesting study of comparing the performance
of the DSSC module with that of the Si-based module by the graph
shown in Fig. 1 [12] and concluded that the performance of the DSSC
module is far better than that of the Si module. Also, the highest
efficiency discussed in this review paper was 11.2% for N719
dye-based DSSC.

Fig. 1
figure 1

The performance of dye PV modules increases with temperature,
contrary to Si-based modules [(Web reference [available online at
http://www.sta.com.au/downloads/DSC%20Booklet.pdf] [11, 12]

Full size image

Shalini et al. [13] emphasized on sensitizers, including ruthenium
complexes, metal-free organic dyes, quantum-dot sensitizer,
perovskite-based sensitizer, mordant dyes, and natural dyes. However,
this article provides a great knowledge about the different types of
sensitizers, but lacks the information regarding other important
components of the DSSCs. Again, apart from discussing all different
components of DSSCs, the review article by Jihuai Wu et al. [14] was
concentrated over the counter electrode part. They have discussed the
study of different types of counter electrodes based on transparency
and flexibility, metals and alloys, carbon materials, conductive
polymers, transition metal compounds, and hybrids. A highest
efficiency of 14.3% was discussed for the DSSC fabricated with Au/GNP
as a counter electrode, Co^3+/2+ as a redox couple, and
LEG4 + ADEKA-1 as a sensitizer [15] and was shown in the review
article. Similarly, Yeoh et al. and Fan et al. [16, 17] have given a
brief review over the photoanode of DSSC. They have classified
modification of photoanode into three categories, namely interfacial
modification through the introduction of blocking and scattering
layer, compositing, doping with non-metallic anions and metallic
cations, interfacial engineering, and replacing the conventional
mesoporous semiconducting metal oxide films like with 1-D or 2-D
nanostructures.

Thus, by comparing different review articles published earlier, it
can be easily seen that the present review article "Dye Sensitized
solar Cells: Fundamentals and Current Status" gives the in-depth
study of different components and their application in DSSCs as well
as construction and working of these cells.

Construction and Working of DSSCs

The working electrode, sensitizer (dye), redox-mediator
(electrolyte), and counter electrode are four key parameters for a
DSSC. DSSC is an assembly of working electrode soaked with a
sensitizer or a dye and sealed to the counter electrode soaked with a
thin layer of electrolyte with the help of a hot melt tape to prevent
the leakage of the electrolyte (as shown in Fig. 2). The components
as well as the construction and working of DSSCs are shown below:

Fig. 2
figure 2

Construction and working principle of the dye-sensitized
nanocrystalline solar cells

Full size image

Transparent and Conductive Substrate

DSSCs are typically constructed with two sheets of conductive
transparent materials, which help a substrate for the deposition of
the semiconductor and catalyst, acting also as current collectors [18
, 19] There are two main characteristics of a substrate being used in
a DSSC: Firstly, more than 80% of transparency is required by the
substrate to permit the passage of optimum sunlight to the effective
area of the cell. Secondly, for the efficient charge transfer and
reduced energy loss in DSSCs, it should have a high electrical
conductivity. The fluorine-doped tin oxide (FTO, SnO[2]: F) and
indium-doped tin oxide (ITO, In[2]O[3]: Sn) are usually applied as a
conductive substrate in DSSCs. These substrates consist of soda lime
glass coated with the layers of indium-doped tin oxide and
fluorine-doped tin oxide. The ITO films have a transmittance > 80%
and 18 O/cm^2 of sheet resistance, while FTO films show a lower
transmittance of ~ 75% in the visible region and sheet resistance of
8.5 O/cm^2 [18].

Working Electrode (WE)

The working electrodes (WE) are prepared by depositing a thin layer
of oxide semiconducting materials such as TiO[2], Nb[2]O[5], ZnO, SnO
[2] (n-type), and NiO (p-type) on a transparent conducting glass
plate made of FTO or ITO. These oxides have a wide energy band gap of
3-3.2 eV. The application of an anatase allotropic form of TiO[2] is
more commendable in DSSCs as compared to a rutile form due to its
higher energy band gap of 3.2 eV whereas the rutile form has a band
gap of about 3 eV [20, 21], although alternative wide band gap oxides
such as ZnO and Nb[2]O[5] have also given promising results [22, 23].
Due to being non-toxic and less expensive and its easy availability,
TiO[2] is mostly used as a semiconducting layer. However, these
semiconducting layers absorb only a small fraction of light in the UV
region; hence, these working electrodes are then immersed in a
mixture of a photosensitive molecular sensitizer and a solvent. After
soaking the film within the dye solution, the dye gets covalently
bonded to the TiO[2] surface. Due to the highly porous structure and
the large surface area of the electrode, a high number of dye
molecules get attached on the nanocrystalline TiO[2] surface, and
thus, light absorption at the semiconductor surface increases.

Photosensitizer or Dye

Dye is the component of DSSC responsible for the maximum absorption
of the incident light. Any material being dye should have the
following photophysical and electrochemical properties:

 1. 1.

    Firstly, the dye should be luminescent.

 2. 2.

    Secondly, the absorption spectra of the dye should cover
    ultraviolet-visible (UV-vis) and near-infrared region (NIR)
    regions.

 3. 3.

    The highest occupied molecular orbital (HOMO) should be located
    far from the surface of the conduction band of TiO[2] and the
    lowest unoccupied molecular orbital (LUMO) should be placed as
    close to the surface of the TiO[2], and subsequently should be
    higher with respect to the TiO[2] conduction band potential.

 4. 4.

    HOMO should lie lower than that of redox electrolytes.

 5. 5.

    The periphery of the dye should be hydrophobic to enhance the
    long-term stability of cells, as it results in minimized direct
    contact between electrolyte and anode; otherwise, water-induced
    distortion of the dye from the TiO[2] surface can appear which
    may reduce the stability of cells.

 6. 6.

    To avoid the aggregation of the dye over the TiO[2] surface,
    co-absorbents like chenodeoxycholic acid (CDCA) and anchoring
    groups like alkoxy-silyl [24], phosphoric acid [25], and
    carboxylic acid group [26, 27] were inserted between the dye and
    TiO[2]. This results in the prevention of dye aggregation and
    thus limits the recombination reaction [28] between redox
    electrolyte and electrons in the TiO[2] nanolayer as well as
    results in the formation of stable linkage.

Electrolyte

An electrolyte (such as I^-/I^- [3], Br^-/Br^- [2] [29], SCN^-/SCN[2]
[30], and Co(II)/Co(III) [31]) has five main components, i.e., redox
couple, solvent, additives, ionic liquids, and cations. The following
properties should be present in an electrolyte:

 1. 1.

    Redox couple should be able to regenerate the oxidized dye
    efficiently.

 2. 2.

    Should have long-term chemical, thermal, and electrochemical
    stability.

 3. 3.

    Should be non-corrosive with DSSC components.

 4. 4.

    Should be able to permit fast diffusion of charge carriers,
    enhance conductivity, and create effective contact between the
    working and counter electrodes.

 5. 5.

    Absorption spectra of an electrolyte should not overlap with the
    absorption spectra of a dye.

I^-/I^- [3] has been demonstrated as a highly efficient electrolyte [
32], but there are certain limitations associated with its
application in DSSCs. I^-/I^- [3] electrolyte corrodes glass/TiO[2]/
Pt; it is highly volatile and responsible for photodegradation and
dye desorption and has poor long-term stability [33, 34].
Acetonitrile (ACN), N-methylpyrrolidine (NMP), and solvent mixtures,
such as ACN/valeronitrile, have been used as a solvent having high
dielectric constants. 4-Tert-butylpyridine (TBP) is mostly used as an
additive to shift the conduction band of TiO[2] upwards, which
results in an increase in the value of open circuit voltage (V[OC]),
reduced cell photocurrent (J[SC]), and less injection driving force.
It is believed that TBP on a TiO[2] surface reduces recombination
through back transfer to an electrolyte [35]. However, the biggest
drawback allied with the ionic liquid is their leakage factor. Thus,
solid-state electrolytes are developed to avoid the drawbacks
associated with ionic liquid (IL) electrolytes [36]. Also, to test
the failure of the redox electrolyte or the sealing under long-term
illumination, long-term light soaking tests on sealed cells have also
progressed significantly over the years [37].

Counter Electrode (CE)

CE in DSSCs are mostly prepared by using platinum (Pt) or carbon (C).
Both working and counter electrodes are sealed together, and
subsequently, an electrolyte is filled with a help of a syringe.
Counter electrode catalyzes the reduction of I^-/I^- [3] liquid
electrolyte and collects holes from the hole transport materials
(HTMs). Pt is used mostly as a counter electrode as it demonstrates
higher efficiencies [38], but the replacement of Pt was much needed
due to its higher cost and less abundance. Thus, several alternatives
have developed to replace Pt in DSSCs, such as carbon [39],
carbonylsulfide (CoS) [40], Au/GNP [15], alloy CEs like FeSe [41],
and CoNi[0.25] [42], although the different types of the CEs are also
discussed by Jihuai Wu et al. [14].

Working Principle

The working principle of DSSC involves four basic steps: light
absorption, electron injection, transportation of carrier, and
collection of current. The following steps are involved in the
conversion of photons into current (as shown in Fig. 2):

 1. 1.

    Firstly, the incident light (photon) is absorbed by a
    photosensitizer, and thus, due to the photon absorption,
    electrons get promoted from the ground state (S^+/S) to the
    excited state (S^+/S*) of the dye, where the absorption for most
    of the dye is in the range of 700 nm which corresponds to the
    photon energy almost about 1.72 eV.

 2. 2.

    Now, the excited electrons with a lifetime of nanosecond range
    are injected into the conduction band of nanoporous TiO[2]
    electrode which lies below the excited state of the dye, where
    the TiO[2] absorbs a small fraction of the solar photons from the
    UV region [43]. As a result, the dye gets oxidized.

    $$ {\mathrm{S}}^{+}/\mathrm{S}+\mathrm{h}\upnu \rightarrow {\
    mathrm{S}}^{+}/{\mathrm{S}}^{\ast } $$
    $$ {\mathrm{S}}^{+}/{\mathrm{S}}^{\ast}\rightarrow {\mathrm{S}}^
    {+}/\mathrm{S}+{\mathrm{e}}^{-}\ \left({\mathrm{TiO}}_2\right) $$
 3. 3.

    These injected electrons are transported between TiO[2]
    nanoparticles and diffuse towards the back contact (transparent
    conducting oxide [TCO]). Through the external circuit, electrons
    reach at the counter electrode.

 4. 4.

    The electrons at the counter electrode reduce I^- [3] to I^-;
    thus, dye regeneration or the regeneration of the ground state of
    the dye takes place due to the acceptance of electrons from I^-
    ion redox mediator, and I^- gets oxidized to I^- [3] (oxidized
    state).

    $$ {\mathrm{S}}^{+}/{\mathrm{S}}^{\ast }+{\mathrm{e}}^{-}\
    rightarrow {\mathrm{S}}^{+}/\mathrm{S} $$
 5. 5.

    Again, the oxidized mediator (I^- [3]) diffuses towards the
    counter electrode and reduces to I ion.

    $$ {{\mathrm{I}}^{-}}_3+{2\mathrm{e}}^{-}\rightarrow {3\mathrm
    {I}}^{-} $$

Evaluation of Dye-Sensitized Solar Cell Performance

The performance of a dye-sensitized solar cell can be evaluated by
using incident photon to current conversion efficiency (IPCE, %),
short circuit current (J[SC], mAcm^- 2), open circuit voltage (V[OC],
V), maximum power output [P[max]], overall efficiency [e, %], and
fill factor [FF] (as shown in Fig. 3) at a constant light level
exposure as shown in Eq. 1 [44].

Fig. 3
figure 3

I-V curve to evaluate the cells performance

Full size image

The current produces when negative and positive electrodes of the
cell are short circuited at a zero mV voltage. V[OC] (V) is the
voltage across negative and positive electrodes under open circuit
condition at zero milliampere (mA) current or simply, the potential
difference between the conduction band energy of semiconducting
material and the redox potential of electrolyte. P[max] is the
maximum efficiency of the DSSC to convert sunlight into electricity.
The ratio of maximum power output (J[mp] x V[mp]) to the product (V
[OC] x J[SC]) gives FF.

$$ \mathrm{FF}=\frac{\mathrm{Area}\ A}{\mathrm{Area}\ B}=\frac{J_{\
mathrm{mp}}\times {V}_{\mathrm{mp}}}{J_{SC}\times {V}_{OC}} $$

Also, the overall efficiency (%) is the percentage of the solar
energy (shining on a photovoltaic [PV] device) converting into
electrical energy, where n increases with the decrease in the value
of J[SC] and increase in the values of V[OC], FF, and molar
coefficient of dye, respectively.

$$ \eta\ \left(\%\right)=\frac{J_{\mathrm{SC}}\times {V}_{\mathrm
{OC}}\times \mathrm{FF}}{P_{in}} $$
(1)

External quantum efficiency (also known as IPCE) is the ratio of
number of electrons flowing through the external circuit to the
number of photons incident on the cells surface at any wavelength l.
It is given as follows:

$$ \mathrm{IPCE}\%\left(\lambda \right)=1240\times \frac{J_{\mathrm
{SC}}}{P_{in}\lambda } $$
(2)

IPCE values are also related to LHE, phE1, and eEC. As shown in Eq. 3
[45],

$$ \mathrm{IPCE}\ \left(\lambda, \mathrm{nm}\right)=\mathrm{LHE}\
upvarphi \mathrm{E}1\upeta \mathrm{E}\mathrm{C} $$
(3)

where LHE is the light harvesting efficiency, phE1 is electron
injection quantum efficiency, and eEC is the efficiency of collecting
electrons in the external circuit.

Limitations of the Devices

In the recent years, comparable efficiencies have been demonstrated
for the DSSCs, but still they need a further modification due to some
of the limitations associated with these cells. In terms of
limitations, stability failure can be characterized in two different
classes: (i) limitation towards extrinsic stability and (ii)
limitation towards intrinsic stability. Also, a huge amount of loss
in energy of oxidized dye takes place during the process of
regeneration, due to the energy mismatch between the oxidized dye and
an electrolyte. Thus, in the queue to enhance the efficacy of these
cells, different electrolytes have been developed. Gratzel et al.
showed over 900-mV open circuit voltages and short circuit currents I
[SC] up to 5.1 mA by blending the hole conductor matrix with a
combination of TBP and Li[CF[3]SO[2]][2]N, yielding an overall
efficiency of 2.56% at air mass 1.5 (AM 1.5) illuminations [46].
Also, the sheet resistance of FTO glass sheet is about 10 O/sq.;
thus, this makes scaling of the device difficult and acts as a
limiting factor for an active cell area > 1 cm^2. Therefore, to
increase the sheet resistance as well as to maintain spacing between
working and counter electrode, the short circuiting of the solar cell
is required or either the spacing should be increased by 25 to 50 mm
[47] in small modules (where these modules consist of small stripes
of an active cell area (1 cm^2) with adjacent silver lines [47, 48]).
As a consequence, a drop in the IPCE value from 10.4 to 6.3% for a
1-cm^2 cell was observed for a submodule of 26.5 cm^2 [49].

To upscale the cell performance, silver fingers can be used to
collect the current and using a sealant material like hotmelt tape,
for the protection from the leakage of the electrolytes. Although due
to the chemically aggressive nature of the electrolyte, the use of
silver fingers is less feasible. And due to the small modules, the
chances of leakage increase which results in reduced active cell area
by 32% [47]. Another factor is the conductivity of the glass sheet
that affects the performance of the DSSCs. Therefore, the
conductivity of the transparent conducting oxide (TCO) can be
improved by combining the indium-doped tin oxide (ITO, highly
conductive but less chemically stable) and fluorine-doped tin oxide
(FTO, highly chemically stable but less conductive) together. This
results in the reduction of the sheet resistance of TCO glass to
1.3 O/sq. [50].

Limitation Towards the Stability of the Devices

The DSSCs need to be stable extrinsically as well as intrinsically as
to be comparable to that of Si-solar cells, so that they can fulfill
market needs, and thus, their commercialization can be increased. The
limitations towards the stability are discussed below:

Limitation Towards Extrinsic Stability (Stability of Sealant
Material)

Sealant materials like Surlyn(r) and Bynel(r) hotmelt foils are used in
DSSCs to seal the cells [48]. Their sealing capability decreases when
the pressure builds up inside the cell [51] and also if exposed
within a cyclic or regular temperature variation [52]. But due to
their low cost and easy processing, their utilization cannot be
neglected. Thus, it is required to increase their adhesion with glass
by pretreatment of the glass with metal oxide particles. As an
alternative, sealants based on low melting glass frits [53] were also
developed which offer more stability than the hotmelt foils, but
these sealants are not suitable for the large area module production.

Limitation Towards Intrinsic Stability

To examine the intrinsic stability of the cell, accelerated aging
experiments were performed. These accelerated aging experiments lasts
for 1000 h to show the thermal stability of the dye, electrolyte, and
Pt-counter electrode at 80 degC of temperature. Through these
experiments, it was found that small test cells can maintain 90% of
the initial efficiency under elevated temperatures and the observed
initial efficiencies were 7.65% [54] and 8% [55], respectively. Also,
under AM 1.5 and 55-60 degC moderate temperatures, the device was
stable for 1000 h. But when both the stress factors, i.e.,
temperature about 80 degC and light soaking, were combined, a rapid
degradation in the performance of the cell was observed [52].
Therefore, improvement in the intrinsic stability of the cell is
required as 80 degC temperature can be easily attained during sunny
days.

Different Ways to Augment the Efficiency of DSSCs

To enhance the efficiency as well as the stability of the DSSCs,
researchers have to focus on fundamental fabrication methods and
materials, as well working of these cells. Different ways to improve
the efficiency of these solar cells (SCs) are discussed below:

 1. 1.

    To increase the efficiency of DSSCs, the oxidized dye must be
    firmly reduced to its original ground state after electron
    injection. In other words, the regeneration process (which occurs
    in the nanosecond range [56]) should be fast as compared to the
    process of oxidation of dye [the process of recombination (0.1 to
    30 ms)]. As the redox mediator potential (I^- ion) strongly
    effects the maximum photovoltage, thus the potential of the redox
    couple should be close to the ground state of the dye. To carry
    out this viable repeated process, about 210 mV driving force is
    required (or ca. 0.6 V [56]).

 2. 2.

    By increasing the porosity of the TiO[2] nanoparticles, the
    maximum dye absorption takes place at WE.

 3. 3.

    Reducing or prohibiting the formation of the dark current by
    depositing a uniform thin layer or under layer of the TiO[2]
    nanoparticles over the conduction glass plate. Thus, the
    electrolyte does not have a direct contact with the FTO or back
    contact and hence not reduced by the collector electrons, which
    restricts the formation of the dark current.

 4. 4.

    Preventing the trapping of nanoporous TiO[2] nanoparticles by TBP
    molecules or by an electrolyte solvent. Thus, uniform
    sensitization of the WE by a sensitizer is required. If the
    entire surface of the nanoporous TiO[2] electrode is not
    uniformly covered by the sensitizer, then the naked spots of
    nanoporous TiO[2] can be captured by TBP molecules or by an
    electrolyte solvent.

 5. 5.

    Co-sensitization is another way to optimize the performance of
    DSSC. In co-sensitization, two or more sensitizing dyes with
    different absorption spectrum ranges are mixed together. to
    broaden the spectrum response range [57].

 6. 6.

    By promoting the use of different materials in the manufacture of
    electrodes like nanotubes, nanowires of carbon, graphene; using
    varied electrolytes instead of a liquid one like gel electrolyte
    and quasi-solid electrolytes; providing different pre-post
    treatments to the working electrode like anodization
    pre-treatment and TiCl[4] treatment; using different types of CEs
    [14] and by developing hydrophobic sensitizers, the performance
    as well as the efficiency of these cells can be tremendously
    improved.

 7. 7.

    By inserting phosphorescence or luminescent chromophores, such as
    applying rare-earth doped oxides into the DSSC [58,59,60],
    coating a luminescent layer on the glass of the photoanode [60,61
    ,62], i.e., using plasmonic phenomenon [63] and adding energy
    relay dyes (ERDs) to the electrolyte [57, 64, 65].

Previous and Further Improvements in DSSCs

To fabricate low cost, more flexible, and stable DSSCs with higher
efficiencies, new materials that are light weight, thin, low cost,
and easy to synthesize are required. Thus, previous as well as
further improvement in the field of DSSCs is included in this
section. This section gives a brief account on the work done by the
different researchers in the last 10-12 years and the results they
observed for respective cells.

Working and Counter Electrodes

Gratzel and co-workers showed drastic improvements in the performance
of DSSCs. They demonstrated efficiency of 7-10% under AM 1.5
irradiation using nanocrystalline (nc) TiO[2] thin-film electrode
with nanoporous structure and large surface area, and used a novel Ru
bipyridyl complex as a sensitizer and an ionic redox electrolyte at
EPFL [3, 26]. The conduction band level of TiO[2] electrode and the
redox potential of I^-/I^- [3] as - 0.7 V versus saturated calomel
electrode (SCE) and 0.2 V versus SCE has been evaluated [66, 67]. A
binary oxide photoelectrode with coffee as a natural dye was
demonstrated, in 2014 [68]. SnO[2] (x)-ZnO (1 - x) binary system with
two different SnO[2] composition (x = 3, 5 mol%) were prepared by
solid-state reaction at high temperature and employed as a
photoanode. An improved efficiency was demonstrated for the larger
SnO[2] composition and an overall power conversion efficiency (PCE)
observed for SnO[2]: ZnO device was increased from 0.18% (3:97 mol%)
to 0.26% for a device with SnO[2]:ZnO (5:95 mol%) photoanode. Hu et
al. observed that the performance of the DSSCs with graphite-P25
composites as photoanodes has been significantly enhanced by 30%
improvement of conversion efficiency compared with P25 alone. They
found an enhancement in the value of J[SC] from 9.03 to 12.59 mA/cm^2
under the condition of 0.01 wt% graphite amount and attained the
conversion efficiency of 5.76% [69]. Figure 4 shows the SEM images of
the photoanodes. Apart from TiO[2], carbon and its different
allotropes are also widely applied in DSSCs to fulfill future demand
and arisen as a perfect surrogate material for DSSCs. Some reports
have shown that incorporating carbon nanotube (CNT) in TiO[2] by
hydrothermal or sol-gel methods greatly improved the cell's
performance [70,71,72]. Also, by improving the interconnectivity
between the TiO[2] and CNT, an increase in the IPCE can be found [70
]. Sun et al. reported that the DSSCs incorporating graphene in TiO
[2] photoanode showed a PCE of 4.28%, which was 59% higher than that
without graphene [73]. Sharma et al. has shown the improvement in the
PCE value from 7.35 to 8.15% of the co-sensitized solar cell using
modified TiO[2] (G-TiO[2]) photoanode, instead of pure TiO[2]
photoanode [74]. In 2014, it was shown that the electronically and
catalytically functional carbon cloth works as a permeable and
flexible counter electrode for DSSC [75]. The researchers have found
that the TiN nanotube arrays and TiN nanoparticles supported on
carbon nanotubes showed high electrocatalytic activity for the
reduction of triiodide ions in DSSCs [76, 77]. Single-crystal CoSe[2]
nanorods were applied as an efficient electrocatalyst for DSSCs by
Sun et al. in 2014 [78]. They prepared single-crystal CoSe[2]
nanorods with a facile one step hydrothermal method. By drop-casting
the CoSe[2] nanorod suspension onto conductive substrates followed by
simple drying without sintering, they fabricated the thin CoSe[2]
films and used as a highly efficient electrocatalyst for the
reduction of I^- [3]. They showed a power conversion efficiency of
10.20% under AM1.5G one-sun illumination for DSSCs with the standard
N719 dye. Park et al. prepared a mesoporous TiO[2] Bragg stack
templated by graft copolymer for dye-sensitized solar cells [79]. To
enhance dye loading, electron transport, light harvesting and
electrolyte pore-infiltration in DSSCs, they prepared organized
mesoporous TiO[2] Bragg stacks (om-TiO[2] BS) consisting of
alternating high and low refractive index organized mesoporous TiO[2]
(om-TiO[2]) films. They synthesized om-TiO[2] films through sol-gel
reaction using amphiphilic graft copolymers consisting of poly(vinyl
chloride) backbones and poly(oxyethylene methacrylate) side chains,
i.e., PVC-g-POEM as templates. They showed that a polymerized ionic
liquid (PIL)-based DSSC fabricated with a 1.2-mm-thick om-TiO[2]
BS-based photoanode exhibited an efficiency of 4.3%, which was much
higher than that of conventional DSSCs with a nanocrystalline TiO[2]
layer (nc-TiO[2] layer) with an efficiency of 1.7%. An excellent
efficiency of 7.5% was demonstrated for a polymerized ionic liquid
(PIL)-based DSSC with a heterostructured photoanode consisting of
400-nm-thick organized mesoporous TiO[2] interfacial (om-TiO[2] IF)
layer, 7-mm-thick nc-TiO[2], and 1.2-mm-thick om-TiO[2] BS as the
bottom, middle, and top layers, respectively, which was again much
higher than that of nanocrystalline TiO[2] photoanode with an
efficiency of 3.5%. Lee et al. reported platinum-free, low-cost, and
flexible DSSCs using graphene film coated with a conducting polymer
as a counter electrode [80]. In 2014, Banerjee et al. demonstrated
nickel cobalt sulfide nanoneedle-array as an effective alternative to
Pt as a counter electrode in dye-sensitized solar cells [81].

Fig. 4
figure 4

SEM images of a P25 film, b 1 wt% graphite-P25 composite film, and (c
) the cross section of P25 film on FTO [69]

Full size image

Calogero et al. invented a transparent and low-cost counter electrode
based on platinum nanoparticles prepared by a bottom-up synthetic
approach. They demonstrated that with such a type of cathode, the
observed solar energy conversion efficiency was the same as that
obtained for a platinum-sputtered counter electrode and even was more
than 50% obtained with a standard electrode, i.e., one prepared by
chlorine platinum acid thermal decomposition, in similar working
condition [82]. By using a special back-reflecting layer of silver,
they improved upon the performance of a counter electrode based on
platinum sputtering and achieved an overall e of 4.75% under 100 mWcm
^- 2 (AM 1.5) of simulated sunlight. They showed that, for the
optical transmittance at different wavelengths of platinum-based
films, i.e., Pt nanoparticles, Pt thermal decomposition, and Pt
sputtered deposited onto FTO glass, the platinum nanoparticle-based
cathode electrode (CE) prepared by Pt sputtering deposition method
appeared more transparent than the platinum CE prepared using the Pt
acid thermal decomposition method. Meanwhile, when Pt nanoparticle
deposition method was employed, the transmittance was very poor (as
shown in Fig. 5). Anothumakkool et al. showed a highly conducting 1-D
aligned polyethylenedioxythiophene (PEDOT) along the inner and outer
surfaces of a hollow carbon nanofiber (CNF), as a counter electrode
in a DSSC to enhance the electrocatalytic activity of the cell [83].
They showed that the hybrid material (CP-25) displayed a conversion
efficiency of 7.16% compared to 7.30% for the standard Pt counter
electrode, 4.48% for bulk PEDOT and 5.56% for CNF, respectively. The
enhanced conversion efficiency of CP-25 was accredited to the
accomplishment of high conductivity and surface area of PEDOT through
the 1-D alignment compared to its bulk counterpart. Further, through
a long-term stability test involving efficiency profiling for
20 days, it was observed that CP-25 exhibited extraordinary
durability compared to the bulk PEDOT. Recently, Huang et al.
improved the performance of the device by inserting a H[3]PW[12]O[40]
layer between the transparent conductive oxide layer and the compact
TiO[2] layer [84]. They observed the reduction in the recombination
of the electrons upon the addition of H[3]PW[12]O[40] layer,
resulting in longer electron lifetime and obtained a e = 9.3%,
respectively.

Fig. 5
figure 5

Optical transmittance of platinum-based films (Pt nanoparticles, Pt
thermal decomposition, Pt sputtered) deposited onto FTO glass [82]

Full size image

Li et al. reported that the transition metal nitrides MoN, WN, and Fe
[2]N show Pt-like electrocatalytic activity for dye-sensitized solar
cells, where MoN showed superior electrocatalytic activity and a
higher PV performance [85]. Characteristic J-V curves of DSSCs using
different metal nitrides and Pt counter electrodes showed that the
cell fabricated with the MoN counter electrode achieved a FF = 0.66,
which was higher than that of the Pt electrode (as shown in Fig. 6).
However, J[SC] = 11.55 mAcm^- 2 was relatively high and the V[OC] of
0.735 V was almost same to the V[OC] = 0.740 V offered by Pt
electrode. In the case of WN, V[OC] and J[SC] were relatively low,
indicating a low efficiency of 3.67%. DSSC with the Fe[2]N electrode
attained lower values of V[OC] and FF, i.e., 0.535 V of V[OC] and
0.41 of FF, resulting in a poor e = 2.65%. Thus, above data shows
superior performance of MoN-based DSSC among all other metal nitrides
as CE material. Gokhale et al. showed a laser-synthesized
super-hydrophobic conducting carbon with broccoli-type morphology as
a CE for dye-sensitized solar cells in 2012 [86]. In 2014, plasmonic
light harvesting of dye-sensitized solar cells by Au
nanoparticle-loaded TiO[2] nanofibers was demonstrated by Naphade et
al. [87] because the surface morphology of a WE and a CE play a key
role in the performance of DSSC. Usually, mesoporous TiO[2]
nanoparticle films are used in WE fabrication because they provide
large surface area for efficient dye adsorption. However, there are
certain limitations associated with them as short electron diffusion
length (10-35 mm) and random electrical pathway induced by the
substantial trapping and detrapping phenomena that take place within
excessive surface states, defects, and grain boundaries of
nanoparticles [88] and disorganized stacking of TiO[2] films which
limits the electron transport [89]. Thus, doping of metallic cations
and non-metallic anions in TiO[2], treating FTOs [90], applying 1-D
nanostructures like nanowires, nanorods, nanosheets, nanoplates [16],
and hollow spheres are approaches to modify the WE. However due to
the low surface area, these 1-D nanostructures show poor dye loading.
In 2015, Zhao et al. studied the influence of the incorporation of
CNT-G-TiO[2] NPs into TiO[2] NT arrays and attained an efficiency of
6.17% for the DSSC based on CNT-G-TiO[2] nanoparticles/TiO[2]
nanotube double-layer structure photoanode [91]. An efficiency of
8.30% was demonstrated by Qiu et al. for the DSSC based on
double-layered anatase TiO[2] nanospindle photoanodes [92].

Fig. 6
figure 6

Characteristic J-V curves of DSSCs using different metal nitrides and
Pt counter electrodes, measured under simulated sunlight at 100 mWcm^
- 2 (AM 1.5) [85]

Full size image

Apart from NTs, bilayer TiO[2] hollow spheres/TiO[2] nanotube
array-based DSSC also showed an effective efficiency of 6.90% [93].
Efficiency can also be improved by incorporating SnO[2] as a shell
material on a photoanode [94]. The integration of SnO[2] as a shell
material on ZnO nanoneedle arrays results in a larger surface area
and reduced recombination rate [94], thus increasing the dye
adsorption which plays a crucial role in the performance of a cell.
Huang and co-workers synthesized mesoporous TiO[2] spheres of high
crystallinity and large surface area and applied it as a WE in the
device. An excellent efficiency of 10.3% was achieved for the
DSSC-employed TiO[2] spheres with long-term stability due to the
terrific dye-loading and light-scattering abilities as well as
attenuated charge recombination. Further, the efficiency was improved
by performing the TiCl[4] treatment [95].

Maheswari et al. reported various DSSCs employing zirconia-doped TiO
[2] nanoparticle and nanowire composite photoanode film. They
demonstrated highest e = 9.93% for Zirconia/TNPW photoanode with a
hafnium oxide (HfO[2]) blocking layer and observed that the
combination of zirconia-doped photoanode with blocking layer possibly
restrains the recombination process and increases the PCE of the
DSSCs effectively [96]. However, many ideas do not achieve a great
efficiency initially but at least embed different ideas and aspects
for the synthesis of new materials. For instance, by using
carbon-coated stainless steel as a CE for DSSC, Shejale et al.
demonstrated a e = 1.98%, respectively [97]. Recently in 2018, a
study was carried out to determine the effect of microwave exposure
on photoanode and found an enhancement in the efficiency of the cell
upon exposure. For the preparation of the DSSC, a LiI electrolyte, Pt
cathode, TiO[2] photoanode, and Alizarin red as a natural sensitizer
were used. An efficiency of 0.144% was found for the cell, where
10 min of microwave exposure was carried upon the photoanode [98].

Similarly, varied materials as mentioned earlier are synthesized as
CE for efficient DSSCs. Last year, Guo et al. synthesized an In[2.77]
S[4]@conductive carbon (In[2.77S4]@CC) hybrid CE via a two-step
method and achieved e = 8.71% for the DSSC with superior
electrocatalytic activity for the reduction of triiodide and, also,
comparable to the commercial Pt-based DSSC that showed PCE of 8.75%,
respectively [99]. The doping of an organic acid, 1S-(+)
-camphorsulfonic acid, with the conductive polymer poly
(o-methoxyaniline) to form a hybrid (CSA/POMA) and its application in
DSSCs as CE has been examined by Tsai et al. This CE showed increased
surface roughness, decreased impedance, and increased crystallinity [
100]. In 2017, Liu et al. fabricated DSSCs employing Co(bpy)[3] ^3+/
2+ as the redox couple and carbon black (CB) as the CE [101]. The
observation revealed superior electrocatalytic activity of a
well-prepared CB film compared to that of conventional sputtered Pt.
Due to the flexible nature of Cu foil substrates, Cu[2]O has also
been employed as a CE in DSSC [102]. The fabrication of different
samples by varying the sintering temperature of the CEs and obtaining
the maximum efficiency of 3.62% at 600 degC of temperature has been
reported [102]. Figure 7 shows the I-V characteristics and IPCE
curves of DSSCs employing different Cu[2]O CEs. In 2013, by replacing
the FTO with Mo as the conductor for the counter electrode, an
increase in the value of FF as well as e was found [103]. The EIS
Nyquist plots (as shown in Fig. 8) showed the difference in R[s]
between the devices employed FTO (15.11 Ocm^2) and Mo (7.25 Ocm^2)
due to the dissimilarity of the sheet resistance between FTO (8.2 O/
sq) and Mo (0.16 O/sq). Also, by replacing FTO with Mo, a decrease in
the R[ct1] value from 6.87 to 3.14 Ocm^2 was induced by the higher
redox reactivity of Pt on Mo than that on FTO. In the queue of
developing new materials, Maiaugree et al. fabricated DSSCs employing
carbonized mangosteen peel (MPC) as a natural counter electrode with
a mangosteen peel dye as a sensitizer [104]. They observed a typical
mesoporous honeycomb-like carbon structure with a rough nanoscale
surface in carbonized mangosteen peels and achieved the highest value
of e = 2.63%. By analyzing the Raman spectra (shown in Fig. 9), they
found a broad D-peak (130.6 cm^- 1 of FWHM) located at 1350 cm^- 1
indicating the high disorder of sp3 carbon and a narrower G peak
(68.8 cm^- 1 of FWHM) at 1595 cm^- 1 which correlated with a graphite
oxide phase observed in 2008 [105]. Thus, it was concluded the
graphite oxide from MPC was a highly ordered sp2 hexagonal carbon
oxide network. Furthermore, I-V characteristics of DSSCs employing
different WE and CE are summarized in Table 1.

Fig. 7
figure 7

The a current density-voltage (J-V) and b incident monochromatic
photon-to-current conversion efficiency (IPCE) curves of DSSCs using
various Cu[2]O CE [102]

Full size image
Fig. 8
figure 8

Nyquist plots of the Device_FTO and Device_Mo. The inset indicates an
equivalent circuit model used for the devices [103]

Full size image
Fig. 9
figure 9

Raman spectra of mangosteen peel carbon [104]

Full size image
Table 1 Photovoltaic parameters of DSSCs employing different types of
WEs and CEs
Full size table

Electrolyte

To improve and study the performance of DSSCs, different electrolytes
like gel electrolytes, quasi-solid-state electrolytes, ionic liquid
electrolytes etc. have been applied as mediators so far. However, a
different trend to optimize the performance of the DSSCs has been
initiated by adding the energy relay dyes to the electrolyte.

Liquid Electrolyte

The cells efficiency through liquid electrolyte can be augmented by
introducing iodide/triiodide redox couple and high dielectric
constant organic solvents like ACN, 3-methoxypropionitrile (MePN),
propylene carbonate (PC), g-butyrolactone (GBL), N
-methyl-2-pyrrolidone (NMP), ethylene carbonate (EC), and counter
ions of iodides, where solvents are the key component of a liquid
electrolyte. On the basis of their stability, organic solvents can be
sequenced as imidazolium < picolinium < alkylpyridinium. Among
various characteristics of solvents like donor number, dielectric
constants, and viscosity, the donor number shows manifest influence
on the V[OC] and J[SC] of DSSCs. Adding the small amount of electric
additives like N-methylbenzimidazole (NMBI), guanidinium thiocyanate
(GuSCN), and TBP hugely improves the cell performance. Just like
solvents, a coabsorbent also plays a key role in the functioning and
performance of an electrolyte. The addition of coabsorbents in an
electrolyte trims down the charge recombination of photoelectrons in
the semiconductor with the redox shuttle of the electrolyte.
Secondly, a coabsorbent may alter the band edge position of the TiO
[2]-conduction band, thus resulting in an augmentation in the value
of V[OC] of the cell. This suppresses the dye aggregation over the
TiO[2] surface and results in long-term stability of the cell as well
as increase in V[OC]. Although the best regeneration of the oxidized
dye is observed for iodide/triiodide as a redox couple for a liquid
electrolyte, its characteristic of severe corrosion for many sealing
materials results in a poor long-term stability of the DSSC. Thus SCN
^-/SCN[2], Br^-/Br[2], and SeCN^-/SeCN[2] bipyridine cobalt (III/II)
complexes are some of the other redox couples applied in DSSCs. The
ionic liquids (IL) or room temperature ionic liquids (RTIL) are
stand-in material for organic solvents in a liquid electrolyte.
Despite the many advantages, i.e., negligible vapor pressure, low
flammability, high electrical conductivity at room temperature (RT),
and wide electrochemical window, they are less applicable in DSSCs.
Because of their higher viscosity, restoration of oxidized dye
restricts due to the lower transport speed of iodide/triioide in
solvent-free IL electrolytes. Thus, the performance of the
dye-sensitized solar cells can also be enhanced by modifying the TiO
[2] dye interface, i.e., by reducing vapor pressure of the
electrolyte's solvent. In 2017, Puspitasari et al. investigated the
effect of mixing dyes and solvent in electrolyte and thus fabricated
various devices. They have used two types of gel electrolyte based on
PEG that mixed with liquid electrolyte for analyzing the lifetime of
DSSC. They also changed solvents as distilled water (type I) and ACN
(type II) with the addition of concentration of KI and iodine, and
achieved better efficiency for the electrolyte type II [106].

As low-viscous solution can cause leakage in the cell, thus,
application of solidified electrolytes obtained by in situ
polymerization of precursor solution containing monomer or oligomer
and the iodide/iodine redox couple results in a completely filled
quasi-solid-state electrolyte within the TiO[2] network with
negligible vapor pressure [107]. Komiya et al. obtained initial
efficiency of 8.1% by applying the aforementioned approach [107]. But
still a question arises whether the polymer matrix will degrade under
prolonged UV radiations or not. The effect on the addition of SiO[2]
nanoparticles to solidify the solvent was also studied as to increase
the cell efficiency [108], where only inorganic materials were
applied in this technique. However, there are certain limitations
associated with the addition of organic solvents within a liquid
electrolyte, i.e., this leads hermetic sealing of the cell and the
evaporation of solvents at higher temperature, and thus the cells do
not uphold long-term stability. Therefore, more research was carried
over the developments and implementation of gel, polymer, and
solid-state electrolytes in the DSSCs with various approaches, such
as the usages of the electrolytes containing p-type inorganic
semiconductors [109], organic hole transporting materials (HTMs) [110
], and polymer gelator (PG) [111]. Chen et al. fabricated a
solid-state DSSC using PVB-SPE (polyvinyl butyral-quasi-solid
polymeric electrolyte) as an electrolyte. They measured the
efficiency approximately 5.46%, which was approximately 94% compared
to that of corresponding liquid-state devices, and the lifetime
observed for the devices was over 3000 h [112]. Recently, a study
explained the stability of the current characteristics of DSSCs in
the second quadrant of the I-V characteristics [113]. The study
explains the continuous flow of the forward current and the operating
voltage point that gradually shift towards more negative voltages in
the second quadrant of the I-V characteristics. The increase in the
ratio of iodide to tri-iodide in the electrolyte rather than to the
decomposition or the coupling reactions of the constituent materials
was considered to be the reason behind it. According to the studies,
these changes were also considered as reversible reactions that can
be detected based on the changes in the color of the electrolyte or
the I-V measurements.

However, ILs with lower viscosity and higher iodine concentration are
needed as to increase J[SC] by increasing iodine mass transport.
Laser transient measurements have been attempted and revealed that
the high iodide concentration present in the pure ILs leads to a
reductive quenching of the excited dye molecule [114]. Due to the low
cost, thermal stability, and good conductivity of the conductive
polymers based on polytiophenes and polypyrroles, they can be widely
applied in DSSCs despite using ILs [115]. For the application point
of view, the IL should have a high number of delocalized negative
charge and counterions with a high chemical stability. Also, the
derivatives of imidazolium salts are one of the best applicable in
DSSCs. When 1-ethyl-3-methylimidazolium dicyanamide [EMIM] [DCA] with
a viscosity of only 21 mPa s [116] was combined with
1-propyl-3-methylimidayolium iodide (PMII, volume ratio 1:1), an
efficiency of 7.4% was observed and, after prolonged illumination,
some degradation was also found. A cell with a binary IL of
1-ethyl-3-methylimidazolium tetracyanoborate in combination with PMII
showed a stable efficiency of 7% that retained at least 90% of its
initial efficiency after 1000 h at 80 degC in darkness and 1000 h at
60 degC, at AM 1.5 [117]. Moudam et al. studied the effect of
water-based electrolytes in DSSC and demonstrated a highly efficient
glass and printable flexible dye-sensitized solar cells upon
application [118]. They used high concentrations of alkylamidazoliums
to overcome the deleterious effect of water. The DSSCs employed pure
water-based electrolyte and were tested under a simulated air mass
1.5 solar spectrum illumination at 100 mWcm^- 2 and found the highest
recorded efficiency of 3.45% and 6% for flexible and glass cells,
respectively. An increase in the value of V[OC] from 0.38 to 0.72 V
on the addition of TBP to the electrolyte has been observed [26].
Thus, to improve the efficiencies of DSSC, new materials were
synthesized and applied in DSSCs. L-cysteine/L-cystine redox couple
was employed in DSSC by Chen et al. which showed a comparable
efficiency of 7.70%, as compared to the cell using I^-/I[3]^- redox
couple (8.10%) [119]. In 2016, Huang et al. studied the effect of
liquid crystals (LCs) on the PCE of dye-sensitized solar cells. They
observed that the addition of minute amounts of LC decreases the J
[SC] because it reduces the electrochemical reaction rate between the
counter electrode and an electrolyte. Also, it delays the degradation
rates of the cell because of the interaction between cyano groups of
the doped LCs and organic solvent in the liquid electrolyte [120].
Main components of different kinds of electrolytes are discussed
below:

Pyridine Derivatives (Like 4-Tert-Butylpyridine [TBP],
2-Propylpyridine, N-Methylbenzimidazole [NMBI])

The improved efficiency for a DSSC can be achieved by adding about
0.5 M of pyridine derivative within the electrolyte, due to which an
increase in the value of V[OC] occurs. This improved V[OC] can be
attributed to the positive band edge movement and slightly affected
charge recombination rate on the basis of intensity-modulated
photovoltage spectroscopy (IMVS) [121]. The study showed that after
the adsorption of pyridine ring on TiO[2] surface, the pyridine ring
induced electron density into the TiO[2] creating a surface dipole.
But, the band edge movement results in the slight decrease in J[SC]
as compared to the untreated cell, due to the diminution in the
driving force for electron injection [122]. Further, application of
NMBI over TBP was studied in 2003, due to its long-term stability
under elevated temperature [54].

Alkyl Phosphonic/Carboxylic Acids (Like Decylphosphonic Acid [DPA],
Hexadecylmalonic Acid [HDMA])

An improved V[OC] with slight decrease in the J[SC] have been
observed when DPA [54] and HDMA [123] were combined. This was due to
the presence of self-assembled long alkyl chain on the surface of TiO
[2], which is responsible for the formation of densely packed
hydrophobic monolayer and reduction in recombination rate too, as
these long alkyl chains repel iodide from TiO[2] surface.

Guanidinium Derivatives (GuSCN)

The addition of guanidium thiocyanate as a co-absorbent in an
electrolyte results in enhanced V[OC] by ca. 120 mV with a downward
shift in the conduction band by ca. 100 mV [124] at the same time due
to the suppression in the recombination rate by a factor of 20 and a
difference of 20 mV gained for V[OC]. By limiting the downward shift
in the conduction band, an improvement in the overall efficacy can be
attained.

Solid-State Electrolyte (SSE)

The SSE falls in two subcategories: (1) where hole transport
materials are used as a transport medium and (2) SSE containing
iodide/triiodide redox couple as a transport medium. Both kinds of
SSEs are discussed below:

Hole Transport Materials (HTMs)

HTMs fall in the category of solid-state electrolytes, where HTMs are
used a medium. These materials have set a great milestone in DSSCs
and effectively applicable in cells because iodine/iodide
electrolytes are highly chemically aggressive by nature and corrodes
other materials easily, mostly metals. Most of the HTMs are
chemically less-aggressive inorganic solids, organic polymers, or
p-conducting molecules, although the results are still unmatched with
the one obtained for iodine/iodide redox electrolytes because of the
following reasons:

 1. 1.

    Due to their solid form, an incomplete penetration of solid HTMs
    within nanoporous TiO[2]-layer leads to poor electronic contact
    between HTMs and the dye. Thus, incomplete dye regeneration takes
    place.

 2. 2.

    The high frequencies of charge recombination from TiO[2] to HTMs.

 3. 3.

    Due to the presence of organic hole conducting molecules, the
    series resistance of the cell increases due to the low hole
    mobility in the organic HTMs as compared to IL electrolytes.

 4. 4.

    HTM results in a drop in V[OC], as the recombination rate of
    electrons of CB with HTM becomes higher as compared to iodine/
    iodide redox electrolytes.

 5. 5.

    Low intrinsic conductivities of HTMs.

Thus, researchers need to synthesize and focus on HTMs whose VB
energy should be slightly above the energy of the oxidized dye,
should not absorb light, and must be photochemically stable, so that
they can keep a healthy contact with the dye. Among a number of HTMs,
some of the HTMs are discussed below:

Inorganic CuI Salt

CuI halogens and pseudohalogens are two classes of inorganic CuI
salts that can be applied as HTMs in a DSSC. Copper bromide (CuBr),
copper iodide (CuI), and copper thiocyanate (CuSCN) are some
copper-based compounds which work as a hole conductor and are more
effective due to their good conductivity. Although CuSCN is one of
the best pseudohalogen HTMs and despite its high hole mobility, its
application results in high series resistance and does not support
high current and also shows poor electronic contact between CuSCN and
the dye, and poor pore filling due to their fast crystallization
rates, which resulted in low e of < 4% for the corresponding
solid-state DSSCs [125]. Thus, to reduce the high recombination rate
of electrons, additional blocking layers of insulating materials like
SiO[2] or Al[2]O[3] can be applied or coated around the TiO[2]
particles which enhance the V[OC] due to the suppressed recombination
rate. With respect to halogens, CuBr showed an efficiency of 1.53%
with thioether as an additive [126] and demonstrated high stability
under prolonged irradiation of about 200 h at RT and the application
of nickel oxide (NiO) showed moderate PCEs of 3% [127]. But, due to
the easy poor solubility as well as crystallization of these
materials, their application became a challenge and, thus,
pseudohalogens have proven to be more stable and efficient in DSSCs.
But the devices were found to be highly unstable and the
reproducibility became dubious.

Hole-Conducting Molecules

spiro-OMeTAD {2,2',7,7'-tetrakis(N,N'-di-p methoxyphenylamine)
-9,9'-spirobifluorene} is one of the most suitable candidate in the
prospect of hole conducting molecules and thus also widely applicable
in integrated devices [128]. It was first introduced in 1998 [110]
with a high glass transition temperature of ca. 120 degC. The
researchers observed the formation of amorphous layers that are
necessary for the complete pore filling and showed an IPCE of 33%,
yielding overall efficiency to about 0.74% [110], and finally 4% of
efficiency with an ambiphillic dye Z907 was demonstrated [129]. Some
other triphenylamine detrivatives also demonstrated sufficient
efficiencies in DSSCs [130]. Again, spiro-OMeTAD has certain
limitations as it has low charge carrier mobility, ca. 104 cm^2/Vs [
130], that limits the thickness of the TiO[2] layer up to 2 mm and
thus leads to incomplete light harvesting efficiency (LHE) of dye.
Also, a high recombination rate between TiO[2] and FTO leads to low
efficiencies in DSSCs.

Triphenylamine (TPA)

Phenylamines demonstrate a remarkable charge transporting property
which makes them great hole transporting materials in organic
electroluminescent devices [131]. However, despite a huge range of
non-conjugated polymers of di- and triphenylamine which are
synthesized and used efficiently as HTMs in organic
electroluminescent devices, their conjugated polymers are still rare.
Polyaniline (PANI) is the only well-recognized conjugated
diphenylamine polymer [132] due to its highly electrical conductive
property and is environmentally stable in the doped state. In 1991,
triphenylamine (TPA)-conjugated polymers were synthesized by
Ni-catalyzed coupling polymerization [133]. Okada et al. reported
dimer (TPD 9), trimer (TPTR 10), tetramer (TPTE 11), and pentamer
(TPPE 12) of TPA with the aid of Ullmann coupling reaction between
the corresponding primary or secondary arylamines and aryl iodides [
134].

SSE Containing Iodide/Triiodide Redox Couple

These SSEs have larger applications than those of HTMs, because
interfacial contact properties of these solid-state electrolytes are
better than those of HTMs. Fabrication of a DSSC based on solid-state
electrolyte was reported by adding TiO[2] nanoparticle into poly
(ethylene oxide) (PEO) and the overall light-to-electricity
conversion efficiency of 4.2% for the cell was obtained under
irradiation of AM 1.5100 mWcm^- 2 [135].

Quasi-Solid-State Electrolyte (QSSE)

QSSE has a hybrid network structure, because it consists of a polymer
host network swollen with liquid electrolytes, thus showing the
property of both solid (cohesive property) and liquid (diffusive
transport property), simultaneously. Thus, to overcome the
volatilization and leakage problems of liquid electrolytes, ILs like
1-propargyl-3- methylimidazolium iodide, bis(imidazolium) iodides and
1-ethyl-1-methylpyrrolidinium and polymer gel-like PEO, and poly
(vinylidinefluoride) and polyvinyl acetate containing redox couples
are commonly used as QSSEs [136, 137]. In 2015, Sun et al. fabricated
a DSSC employing wet-laid polyethylene terephthalate (PET) membrane
electrolyte, where PET is a commonly used textile fiber used in the
form of a wet-laid non-woven fabric as a matrix for an electrolyte.
According to their observation, this membrane can better absorb
electrolyte turning into a quasi-solid, providing excellent
interfacial contact between both electrodes of the DSSC and
preventing a short circuit. The quasi-solid-state DSSC assembled with
an optimized membrane exhibited a PCE = 10.248% at 100 mWcm^-2. To
improve the absorbance, they plasma-treated the membrane separately
with argon and oxygen, which resulted in the retention of the
electrolyte, avoiding its evaporation, and a 15% longer lifetime of
the DSSC compared to liquid electrolyte [138]. Figure 10 shows the
polarization curves of DSSCs with various electrolytes under
simulated AM 1.5 global sunlight (1 Sun, 100 mWcm^-2).

Fig. 10
figure 10

Polarization curves of DSSCs with various electrolytes under
simulated AM 1.5 global sunlight (1 sun, 100 mWcm^-2) [138]

Full size image

Hole-Conducting Polymers

IPCE of 3.5% by the application of C60/polythiphene derivative in
DSSCs has been achieved for pure organic solar cells [139]. However,
this field is developing slowly, as its deposition by standard
methods (like CBD) is difficult, because solid polymer does not
penetrate the TiO[2]-nanoporous layer. Hence, there are only few
groups applied as conducting polymers in DSSCs. Ravirajan et al.
demonstrated a monochromatic efficiency of 1.4% at 440 nm by applying
fluorene-thiophene copolymer [140]. Researchers are working hard so
long to develop new efficient materials for electrolytes. Jeon et al.
reported that the addition of alkylpyridinium iodide salts in
electrolytes enhanced the performance of the dye-sensitized solar
cells. They observed better J-V characteristics, 7.92% efficiency
with V[OC] = 0.696 V, J[SC] = 17.74 mA/cm^2, and FF = 0.641 for the
cell applying EC6PI (pyridinium salts) as compared to EC3ImI
(imidazolium salts), whose e = 7.46% with V[OC] = 0.686 V, J[SC] =
 16.99 mA/cm^2, and FF = 0.64 [141]. For a comparison, they added UV
spectra for C6ImI and observed that the higher quantum efficiencies
from the cell with EC6PI were obtained within the wide range from 460
to 800 nm. The quantum efficiencies were almost the same in the range
of shorter wavelengths, may be due to the ability of C6PI to absorb
more incident light than C3ImI at shorter wavelengths. Even so, the
absorption coefficients for C6PI were higher than those for C6ImI
over all the range, but the cell efficiencies are quite comparable
(as shown in Fig. 11) [141]. Lee et al. developed and utilized the
conjugated polymer electrolytes (CPEs) like MPF-E, MPCZ-E, MPCF-E,
and MPCT-E containing quaternized ammonium iodide groups in polymer
solution and gel electrolytes for DSSCs. They observed, as the
polymer content in the electrolyte solution increased, the
electrochemical impedance also increased for the cells based on CPE
containing polymer solution electrolytes, whereas the PV performances
showed the reverse trend [142]. Table 2 shows the FF and efficiencies
for the DSSCs employing various dyes and mediators.

Fig. 11
figure 11

UV-vis spectroscopy selected pyridinium and imidazolium salts. The
inset is the IPCE data for the cells with EC3ImI and EC6PI, which are
the best cells among each series [141]

Full size image
Table 2 Efficiencies for different dyes and electrolytes
Full size table

Developments in Dye Synthesis

As dyes play a key role in DSSCs, numerous inorganic and organic/
metal-free dyes/natural dyes, like N3 [26], N719 [143], N749 (black
dye) [144], K19 [145], CYC-B11 [146], C101 [32], K8 [147], D102 [148
], SQ [149], Y123 [101], Z907 [150], Mangosteen [106], and many more
have been utilized as sensitizers in DSSCs. Few of them will be
discussed below briefly:

Metal (Ru) Complexes

Metal complex dyes produced from the heavy transition metals such as
the complexes of ruthenium (Ru), Osmium (Os), and Iridium (Ir) have
widely been used as inorganic dyes in DSSCs because of their long
excited lifetime, highly efficient metal-to-ligand charge transfer
spectra, and high redox properties. ML2(X)2 is the general structure
of the sensitizer preferred as a dye, where M represents a metal, L
is a ligand like 2,2'-bipyridyl-4,4'-dicarboxylic acid and X presents
a halide, cyanide, thiocyanate, acetyl acetonate, and thiocarbamate
or water substituent group [151]. Due to the thermal and chemical
stability and wide absorption range from visible to NIR, the
ruthenium polypyridyl complexes show best efficiencies and, thus,
have been under extensive use so far.

Ru Complexes

In 1991, O'Regan and Gratzel reported the efficiency of 7.12% for the
very first DSSC based on the ruthenium dye (black dye) [3]. Later, an
efficiency of about 10% was reported by them using Ru-based dye
(N749) which has given this topic a new sight. Most of the Ru
complexes consist of Ru(II) atoms coordinated by polypyridyl ligands
and thiocyanate moieties in octahedral geometery, and because of the
metal to ligand charge transfer (MLCT) transitions, they exhibit
moderate absorption coefficient, i.e., < 18,000 M^- 1 cm^- 1. Ru (II)
complexes lead the inter crossing of excited electron to the long
lived triplet state and augmentation in the electron injection.
Further, to improve the absorption and emission as well as
electrochemical properties of Ru complexes, bipyridyl moieties can be
replaced by the carboxylate polypyridine Ru dyes, phosphate Ru dyes,
and poly nuclear bipyridyl Ru dyes. Table 3 and Fig. 12 show the
molecular structure, the absorption spectra, and photoelectric
performance for DSSCs based on different metal complex [polypyridyl
(RuII)] dyes.

Table 3 Absorption spectra and photoelectric performance for DSSCs
based on different metal complex [polypyridyl (RuII)] dyes
Full size table

N3/N719/N712 Dyes

In 1993, Nazeeruddin et al. reported DSSC based on Ru-complex dye
known as N3 dye {cis-di(thiocyanato)bis
(2,2-bipyridine-4,4-dicarboxylate)ruthenium}, which contained one Ru
center and two thiocyanate ligands (LL') with additional carboxylate
groups as anchoring sites and absorbed up to 800 nm radiations [26].
They obtained 10.3% efficiency for a system containing N3 dye and
treated the dye covered film with TBP. At 518 and 380 nm wavelength,
this dye attained maximum absorption spectra with respective
extinction coefficients as 1.3 x 10^4 M^- 1 cm^- 1 and 1.33 x 10^4 M^
- 1 cm^- 1, respectively. The dye has showed the 60 ns of excited
state lifetime and sustained for more than 10^7 turnovers without the
significant decomposition since the beginning of the illumination [26
]. Further, the absorption of the dye can be extended into the red
and NIR by substituting the ligands such as thiocyanate ligands and
halogen ligands. For example, a device containing acetylacetonate
showed e = 6.0% [152], followed by a pteridinedione complex with 3.8%
efficiency [153] and a diimine dithiolate complex with 3.7%
efficiency [154].

It has been investigated that during esterification, the dye gets
bounded to the TiO[2] chemically which results in the partial
transformation of protons of the anchoring group to the surface of
the TiO[2]. Thus, it was concluded that the photovoltaic (PV)
performance of the cell gets influenced by the presence of the number
of protons on the N3 photosensitizer or, in other words, the
modification in protonation level of N3 (N712, N719) affects the
performance of the device [155, 156], in two major aspects. Firstly,
the increase in the concentration of the protons results in the
positively charged TiO[2] surface and the downward shift in the Fermi
level of TiO[2]. Hence, a drop in the V[OC] takes place due to the
positive shift of the conduction band edge induced by the surface
protonation. Secondly, the electric field associated with the surface
dipole enhances the absorption of the anionic Ru(II) complexes and,
thus, insists the electron injection from the excited state of the
dye to the conduction band of the TiO[2]. In 2001, Nazeeruddin et al.
reported a 10.4% of efficiency for the DSSCs using a ruthenium dye,
i.e., "black dye" [157], where its wide absorption band covers the
entire visible range of wavelengths. Gratzel and group demonstrated
the PCE of 9.3% for the monoprotonated sensitizer N3 [TBA][3] closely
followed by a diprotonated sensitizer N3[TBA][2] or N719 with a
conversion efficiency of 8.4% [156]. Later, Wang et al. and Chiba et
al. reported a e = 10.5% [158] and e = 11.1% [159], for the devices
that used black dye as a sensitizer in DSSCs.

A new dye "N719" was reported by Nazeeruddin et al. by replacing four
H^+ counterions of N3 dye by three TBA^+ and one H^+ counterions and
achieved e = 11.2% for the respective device [155]. Despite having
almost the same structure to the N3 dye, the higher value of e for
N719 was accredited to the change in the counterions, as they altered
the speed of adsorption onto the porous TiO[2] electrode, i.e., N3 is
fast (3 h) whereas N719 is slow (24 h). The dye-sensitized solar cell
database (DSSCDB) yields around 329 results assembled from over 250
articles when queried as "N719," where the reported efficiencies
range between 2 and 11% [160].

Recently, Shazly et al. fabricated the solid-state dye-sensitized
solar cells based on Zn[1-x]Sn[x]O nanocomposite photoanodes
sensitized with N719 and insinuated with spiro-OMeTAD as a solid hole
transport layer [161] and achieved highest efficiency of 4.3% with J
[SC] = 12.45 mAcm^- 2, V[OC] = 0.740 V, and FF = 46.70. Similarly, by
applying different techniques, like post treatment of photoanode,
optimizing the thickness of the nc-TiO[2] layer, and the
antireflective filming, Gratzel group reported e = 11.3% [32] for the
device containing the dyes C101 and e = 12.3% [162] for Z991
dye-based DSSCs (molecular structure shown in Fig. 13). Again, if a
sensitizer does not carry even a single proton, the value expected
for V[OC] will be high but the value for J[SC] becomes low. Thus,
there should be an optimal amount of protonation of the sensitizer
required, so that the product of both J[SC] and V[OC] can determine
the conversion efficiency of the cell as a maximum. And thus,
deprotonation levels of N3, N719, and N712 in solar cells were
investigated, where the doubly protonated salt form of N3 or N719
showed higher PCE as compared to the other two sensitizers [143].
Figure 14 shows the effect of dye protonation on the I-V
characteristics of TiO[2] photoanode sensitized with different Ru
dyes as N3 (4 protons), N719 (2 protons), N3[TBA][3] (1 proton), and
N712 (0 protons) dyes, measured under AM 1.5 [156]. However, the main
limitations of N3 sensitizers are their relatively low molar
extinction coefficient and less of absorption in the red region of
the visible spectrum.

Fig. 12
figure 12

Molecular structure of Ruthenium complex based dye sensitizers

Full size image
Fig. 13
figure 13

Molecular structure of C101 and Z991 sensitizers

Full size image

p-System Extension (N945, Z910, K19, K73, K8, K9)

As compared to the other organic dyes, standard Ru complexes have
significantly lower absorption coefficient and thus a thick layer of
TiO[2] was required, which results in the higher electron
recombination probability. Thus, two carboxylic acid groups of N3 can
be replaced by the ligands containing conjugated p-systems to enhance
the absorption and the cell efficiency, simultaneously. Thus, the
reason behind the p-system extension in dyes is to create sensitizers
with higher molar extinction coefficients (e), so that the LUMO of
the dye can be tuned to get directionality in the excited state and
to introduce hydrophobic side chains that repel water and triiodide
from the TiO[2] surface. Recently, Rawashdeh et al. have demonstrated
an efficiency of 0.45% by modifying the photoanode as graphene-based
transparent electrode sensitized with 0.2 mM N749 dye in ethanolic
solution [163].

Styryl-ligands attached to the bipyridil ring showed the utmost
results. The e = 1.69 x 10^4 M^- 1 cm^- 1 for the Z910 dye [164], e =
 1.82 x 10^4 M^- 1 cm^- 1 for the K19 dye [145], and e = 1.89 x 10^
4 M^- 1 cm^- 1 for the N945 dye [165] have been found, which were at
least 16% more as compared to the standard N3 dye. An efficiency of
10.2% was demonstrated by Wang et al. for Z910 dye [166]. 10.8% of
the efficiency was observed for the N945 dye [167] in 2007, on thick
electrodes and with volatile electrolytes which was about the same as
for the N3 as reference, but, when applied on thin electrodes and
with non-volatile electrolytes, the observed PCE was significantly
higher. At the same time, a remarkable stability at 80 degC (in
darkness) and 60 degC temperature (under AM 1.5) was observed [168],
and between - 0.71 V and - 0.79 V vs. normal hydrogen electrode (NHE)
[145, 165, 166, 168], the excited state of these dyes has been
reported and was observed sufficiently more negative than the
conduction band of TiO[2] (ca. - 0.1 V vs. NHE) to ensure the
complete charge injection. In terms of higher molar extinction
coefficient, Nazeeruddin et al. synthesized K8 and K9 dyes that
showed even better results as compared to the previous ones. K8 and
K9 complexes showed broad and intense absorption bands between 370
and 570 nm. In DMF solution, the K9 complex showed the maxima at
534 nm (l[max]) with a e = 14,500 M^- 1 cm^- 1 which was blue shifted
by 22 nm compared to K8 complex which showed maxima at 556 nm with a
e = 17,400 M^- 1 cm^- 1, respectively. Thus, due to the substitution
of 4, 4'-bis (carboxylvinyl)-2, 2'-bipyridine by 4,4'-
dinonyl-2,2'-bipyridine, e observed for K9 complex was ~ 20% less
than that of the K8 complex. The overall PCE observed for K8 and K9
complexes were 8.46% with J[SC] = 18 mA/cm^2 and V[OC] = 640 mV and
7.81% with J[SC] = 16 mA/cm^2 and V[OC] = 666 mV [169], respectively.
Gratzel group synthesized K19 as a second amphiphilic dye and
demonstrated that K19 shows 18,200 M^- 1 cm^- 1 M extinction
coefficient, 7.0% overall conversion efficiency and a low energy
metal-to-ligand transition (MLCT) absorption band at 543 nm, which
was higher than the corresponding values for the first amphiphilic
dye Z907 with a molar extinction coefficient of 12,200 M^- 1 cm^- 1,
6.0% overall conversion efficiency, and standard N719 dye with a
molar extinction coefficient of 14,000 M^- 1 cm^- 1 with 6.7% overall
PCE under the same fabrication and evaluation conditions. They
appraised the performance of the device using N719, Z907, and K19 as
sensitizers during thermal aging at 80 degC and observed a lower
stability for N719 dye may be due to the desorption of the sensitizer
at higher temperature; however, K19 and Z907, both retained over 92%
of their initial performances under the thermal stress at 80 degC for
1000 h [145, 169].

Thiophene ligands containing Ru sensitizers also showed good
efficiencies. In 2006, Yanagida et al. reported a Ru complex, by
replacing a phenylvinyl group of K19 by thienylvinyl group in HRS-1 [
170] and an improved stability along with respectable LHE in vis-NIR
and a reversible one electron oxidation process was reported. They
found a e up to 9.5% for HRS-1 (substituted thiophene derivatives).
Several thiophene containing sensitizers have been developed without
conjugation, such as C101 [32] and CYC-B1 [171]. After the
development of C101 dye, Ru (II) thiophene compounds gained special
attention as having set a new DSSC efficiency record of 11.3-11.5%
and became the first sensitizer to triumph over the well-known N3 dye
[32].

Amphiphilic Dyes with Alkyl Chains

Two of the four carboxylic groups of N3 dye are replaced by long
alkyl chains because the ester linkage of the dye to the TiO[2] was
prone to hydrolyze, if water gets adsorbed on the TiO[2] surface [172
], thus resulting in usually lower absorption spectrum in these
sensitizers due to the smaller conjugated p-system of the
bipyridil-ligand. Even though the PCE offered by these sensitizers
were appreciable, ranging from 7.3% for Z907 (with 9 carbon atoms) [
54] to 9.6% for N621 (with 13 carbon atoms) [155] and were highly
stable, Z907 sensitized DSSCs passed 1000 h at 80 degC in darkness and
at 55 degC under illumination without any degradation [173]. It has
been found that by coadsorption of decylphosphonic acid on the TiO[2]
NPs, the hydrophobicity of the surface could be even enhanced and,
thus, stable cells have been demonstrated [54].

Different Anchoring Groups

Most of the sensitizers in DSSCs have carboxylic acid groups as an
anchor on the surface of TiO[2]. But, the dye molecules get desorbed
at the semiconductor surface at a pH > 9, due to the shifting of the
equilibrium towards the reactant side. Thus, dyes with different
anchoring groups are much needed. Again, most of the research focuses
on phosphonic acid and the credit goes to its binding strength to a
metal oxide surface, as the binding strength to a metal oxide surface
decreases in the order, phosphonic acid > carboxylic acid > ester >
acid chloride > carboxylate salts > amides [174]. Z955 is a
Ru-complex containing phosphonic acid as an anchoring group and
demonstrated a e = 8% accompanied by good stability under prolonged
light soaking for about 1000 h at AM 1.5 and 55 degC [175].
Triethoxysilane [176] and boronic acid are some other anchoring
groups. p-Extended ferrocene with varied anchoring groups (-COOH,
-OH, and -CHO) has been applied as photosensitizers in DSSCs [177].
Chauhan and co-workers has synthesized and characterized two new
compounds as FcCH=NC[6]H[4]COOH (1) and FcCH=NCH[2]CH[2]OH (2), where
Fc = C[5]H[4]FeC[5]H[5] and FcCHO are used as the starting material [
177]. By cyclic voltammetry (CV) in dichloromethane solution and
using density functional theory (DFT) calculations, they have
explained the quasi-reversible redox behavior of the dyes. The
redox-active ferrocenyl group exhibited a single quasi-reversible
oxidation wave with E' = 0.34, 0.44, and 0.44 V for 1, 2, and 3,
respectively. In 2017, a study was carried out to inspect the
influence of a cyano group in the anchoring part of the dye on its
adsorption stability and the overall PV properties like electron
injection ability to the surface and V[OC] [178]. The results
indicated that the addition of the cyano group increased the
stability of adsorption only when it adsorbs via CN with the surface
and it decreased the photovoltaic properties when it was not involved
in binding.

However, in the race of improved efficiency and efficient DSSCs, Ru
(II) dyes are still an ace. The most vital reason following usage of
Ru dyes in DSSCs is their extraordinary stability when being absorbed
on the TiO[2] surface. N749 and Z907 are the two important Ru dyes,
although N749 which shows broad absorption and high efficiency, in
contrast, has low absorption coefficient about ~ 7000 M^- 1 cm^- 1
and the stability of this dye was not so good as compared to other Ru
sensitizers. PCE of 10.4% has been observed for black dye, under AM
1.5 and full sunlight [48]. It achieved sensitization over the whole
visible range extending into the NIR up to 920 nm with 80% IPCE and
10.4% overall efficiency, when anchored on TiO[2] nanocrystalline
film. At NREL, black dye (N749)-sensitized DSSC showed efficiency of
10.4% with J[SC] = 20.53 mA/cm^2, V[OC] = 0.721 V, and FF = 0.704,
where the active area of the cell was 0.186 cm^2 [157, 179].
Nazeeruddin et al. reported a comparative study between the spectral
response of the photocurrent of the two dyes, N3 and N749 [26], as
shown in Fig. 15. In the vis-range, both chromophores showed very
high IPCE values. The response of N749 dye was observed to be
extended 100 nm further into the IR compared to that of N3. The
recorded photocurrent onset was close to 920 nm and there on the IPCE
rose gradually until at 700 nm it reached to a plateau of ca. 80%.
From overlap integral of the curves in Fig. 8 with the AM 1.5 solar
emission, it could be predicted that J[SC] of the N3 and black
dye-sensitized cells to be 16 and 20.5 mA/cm^2, respectively [37]. In
Z907 sensitizer, one of the dicarboxy bipyridine ligands in N3
molecule was replaced by a nonyl bipyridine, which resulted in the
formation of a hydrophobic environment on the device. However, the
dye has set a precedent for hundreds of tris-heteroleptic Ru
complexes with isothiocyanate ligands that were developed in the last
15 years, but provides efficiencies rarely comparable to N719 [180].

Fig. 14
figure 14

Effect of dye protonation on photocurrent-voltage characteristics of
nanocrystalline TiO[2] cell sensitized with N3 (4 protons), N719 (2
protons), N3[TBA][3] (1 proton), and N712 (0 protons) dyes, measured
under AM 1.5 sun using 1 cm^2 TiO[2] electrodes with an I^-/I[3]^-
redox couple in methoxyacetonitrile [156]

Full size image

Metal-Free, Organic Dyes

Despite the capability to provide highly efficient DSSCs, the range
of application of Ru dyes are limited to DSSCs, as Ru is a rare and
expensive metal and, thus, not suitable for cost-effective,
environmentally friendly PV systems. Therefore, development and
application of new metal-free/organic dyes and natural dyes is much
needed. The efficiency of DSSCs with organic dyes has been increased
significantly in the last few years and an efficiency of 9% [181] was
shown by Ito and co-workers. The molecular structure and the
efficiency for DSSCs based on different metal-free organic dyes are
shown in Table 4 and Fig. 16.

Table 4 The efficiency for DSSCs based on different metal-free
organic dyes
Full size table

Thus, metal-free organic dyes are developing at a fast pace to
overcome the limitations discussed above and especially promising is
the fast learning curve, which raises hope of the further synthesis
of new materials with higher stability and, thus, designing highly
efficient DSSCs at much lower prices. Though the efficiencies offered
by these dyes are less comparable to those by Ru dyes, their
application is vast as they are potentially very cheap because of the
incorporation of rare noble metals in organic dyes; thus, their cost
mainly depends on the number of synthesis steps involved. Other
advantages associated with organic dyes are their structure
variations, low cost, simple preparation process, and environmental
friendliness as compared to Ru complexes; also, the absorption
coefficient of these organic dyes is typically one order of magnitude
higher than Ru complexes which makes the thin TiO[2] layer feasible.
Thus, there is a huge demand to develop new pure organic dyes, so
that the commercialization of DSSCs becomes easier.

However, there are certain limitations associated with these dyes
too, as under high elevated temperatures the observed stability of
the organic dyes were not as good as expected. Therefore, to get a
larger photocurrent response for newly designed and developed organic
dyes, it is essential to attain predominant light-harvesting
abilities in the whole visible region and NIR of dyes, with a
sufficiently positive HOMO than I^-/I^- [3] redox potential and
sufficiently negative LUMO than the conduction band edge level of the
TiO[2] semiconductor, respectively [182, 183]. In 2008, Tian et al.
fabricated DSSCs based on a novel dye (2TPA-R), containing two
triphenylamine (TPA) units connected by a vinyl group and
rhodanine-3-acetic acid as the electron acceptor to study the
intramolecular energy transfer (E[n]T) and charge transfer (ICT) [184
]. They found that the intramolecular E[n]T and ICT processes showed
a positive effect on the performance of DSSCs, but the less amount of
dye was adsorbed on TiO[2] which may make it difficult to improve the
efficiency of DSSCs [184]. An efficiency of 2.3% was attained for the
DSSC used 2TPA-R dye and an imidazolium iodide electrolyte, whereas e
 = 2% was achieved for TPA-R dye. This improved efficiency for 2TPA-R
device was possibly due to the contribution of the E[n]T and ICT.
They studied the effect of 2TPA-R via absorption spectrum (as shown
in Fig. 17) and found that the two absorption bands, i.e., l[abs] at
383 nm and 485 nm obtained for 2TPA-R, are almost the same as those
observed for 2TPA (l[abs] at 388 nm) and TPA-R (l[abs] at 476 nm) in
CH[2]Cl[2] solution (2 x 10^- 5 M). Thus, the study on intramolecular
E[n]T and ICT could help in the design and synthesis of efficient
organic dyes. Hence, a suitable anchoring group which can chemically
bind over the TiO[2] surface with a suitable structure and effective
intramolecular E[n]T and ICT processes, is also required for
synthesis.

Fig. 15
figure 15

Photocurrent action spectra obtained with the N3 (ligand L) and the
black dye (ligand L_) as sensitizer. The photocurrent response of
bare TiO^2 films is also shown for comparison [26]

Full size image

The construction of most of the organic dyes is based on the donor-
acceptor (D-A)-like structure linked through a p-conjugated bridge
(D-p-A) and usually has a rod-like configuration. Moieties like
indoline, triarylamine, coumarin, and fluorine are employed as an
electron donor unit, whereas carboxylic acid, cyanoacrylic acid, and
rhodamine units are best applicable as electron acceptors to fulfill
the requirement. The linking of donor and acceptor is brought about
by adding p spacer such as polyene and oligothiophene [185, 186].
This type of the structure results in a higher photoinduced electron
transfer from the donor to acceptor through linker (spacer) to the
conduction band of the TiO[2] layer, where the p-conjugation can be
extended either by increasing the methine unit or by introducing
aromatic rings such as benzene, thiophene, and furan or in other
words by adding either electron donating or withdrawing groups, which
results in the enhanced light harvesting ability of the dye, and by
using different donor, linker, and acceptor groups, the photophysical
properties of the organic dyes can also be tuned [187, 188]. Whereas
the photophysical properties change with the expansion of
p-conjugation due to the shift of the both HOMO and LUMO energy
levels, thus, D-p-A structure was considered to be the most promising
class of organic dyes in DSSCs as they can be easily tuned [189].
Moreover, in 2010, the encouraging efficiency up to 10.3% was
reported using organic dyes [190]. Fuse et al. demonstrated a one-pot
procedure to clarify the structure-property relationships of
donor-p-acceptor dyes for DSSCs through rapid library synthesis [191
]. Four novel organic dyes IDB-1, ISB-1, IDB-2, and ISB-2, based on
5-phenyl-iminodibenzyl (IDB) and 5-phenyliminostlbene (ISB) as
electron donors and cyanoacrylic acid moiety as an electron acceptor
connected with a thiophene as a p-conjugated system, were designed by
Wang et al. in 2012 [192]. The highest efficiencies for the devices
based on ISB-2 were observed due to the larger red shifts of 48 nm
for ISB-2, indicating the more powerful electron-donating ability due
to the increased linker conjugation. The absorption peaks for IDB-1,
ISB-1, IDB-2, and ISB-2 were obtained at 422, 470, 467, and 498 nm in
dichloromethane-diluted solution, respectively.

Tetrahydroquinolines [193, 194], pyrolidine [195], diphenylamine [196
], triphenylamine (TPA) [27, 197], coumarin [198, 199], indoline [200
, 201], fluorine [202], carbazole (CBZ) [203], phenothiazine (PTZ) [
204, 205], phenoxazine (POZ) [206], hemicyanine dyes [207],
merocyanine dyes [208], squaraine dyes [209], perylene dyes [210],
anthraquinone dyes [211], boradiazaindacene (BODIPY) dyes [212],
oligothiophene dyes [213], and polymeric dyes [214] are widely used
in DSSCs and are still under development. Jia et al. designed
quasi-solid-state DSSCs employing two efficient sensitizers FNE55 and
FNE56, based on fluorinated quinoxaline moiety, i.e., 6,
7-difluoroquinoxaline moiety, and an organic dye FNE54 without
fluorine was designed for comparison [215]. From the studies, it was
concluded that the absorption properties of the dye enhanced
bathochromically from 504 nm for FNE54 to 511 nm and 525 nm for FNE55
and FNE56 sensitizers upon the addition of fluorine into the dye. The
addition of fluorine resulted in the improved electron-withdrawing
ability of the quinoxaline and, thus, enhanced the push-pull
interactions and narrowed the energy band gap. Due to the high
polarizability, spectroscopically and electrochemically tunable
properties and high chemical stability, p-conjugated oligothiophenes
were well applied as spacers in DSSCs [194, 204]. To induce a
bathochromic shift and augment the absorption, a number of thiophene
units could be increased in the spacers, and by controlling the
length of these thiophene units or chain, higher efficiencies up to
two to three units can be achieved [216, 217], as the p-conjugated
spacers used previously were thiophenes linked directly or through
double bonds to the donor moiety [218].

As good electron injection is one of the parameters for higher
efficiency in the DSSCs, cyanoacetic acid and cyanoacrylic acid are
well employed as acceptor units due to their strong electron
withdrawing capability. Yu et al. concluded cyanoacrylic acid as a
strong electron acceptor for D-p-A-based dyes because the dye
incorporating cyanoacrylic acid as an electron acceptor showed the
best results and, due to the maximum absorption spectrum and the
highest molar excitation coefficient, the DSSC achieved e = 4.93% [
219]. Wang and co-workers designed organic dyes based on
thienothiophene as p conjugation unit, where they used triphenylamine
as donor and cyanoacetic acid as an acceptor. They substituted
different alkyl chains on the triphenylamine unit and found the best
efficiency of about 7.05% for the sensitizers with longer alkoxy
chains due to the longer electron lifetime [220]. Acceptors based on
rhodanine-3-acetic acid were also used as an alternative, but due to
the low lying molecular LUMO, the results obtained were not pleasing
[221, 222].

Coumarin Dyes

Coumarin is a synthetic organic dye and is a natural compound found
in many plants like tonka bean, woodruff, and bison grass (molecular
structure shown in Fig. 18a). In 1996, Gratzel et al. found the
efficient electron injection rates of 200 fs from C343 into the
conduction band of the TiO[2], where for the first time the transient
studies on a coumarin dye in DSSCs were performed [223]. But the
narrow absorption spectrum of C343, i.e., lack of absorption in the
visible region, resulted in the lower conversion efficiency of the
device. This can be altered by adding more methane groups that result
in expanding the p-conjugation linkers and an increased efficiency of
the DSSC [224]. Giribabu and co-workers synthesized RD-Cou
sensitizers and obtained the conversion efficiency of 4.24% using
liquid electrolyte, where coumarin moiety was bridged to the pyridyl
groups by thiophene, which resulted in the extended p-conjugation and
broadening of the metal-to-ligand charge transfer spectra [225]. They
found that the absorption spectrum of RD-Cou dye was centered at
498 nm with a e = 16,046 M^- 1 cm^- 1. Despite the lower efficiency
offered by these cells, the thermal stability of the sensitizer make
its rooftop applications possible because the dye showed stability of
up to 220 degC during thermal analysis.

Fig. 16
figure 16

Molecular Structure of metal-free organic dyes

Full size image

Indole Dyes

Indole occurs naturally as a building block in amino acid tryptophan,
and in many alkaloids and dyes too (molecular structure shown in
Fig. 18b). It is substituted with an electron withdrawing anchoring
group on the benzene ring and an electron donating group on the
nitrogen atom, and these dyes have demonstrated good potential as a
sensitizer. Generally, the D-A structure of an indole dye is such
that the indole moiety acts as an electron donor and is connected to
a rhodanine group that acts as an electron acceptor. Also by
introducing the aromatic units into the core of the indoline
structure, the absorbance in the infrared (IR) region of the visible
spectra as well as the absorption coefficient of the dye can be
enhanced significantly [226]. An efficiency of 6.1% was demonstrated
for DSSCs with D102 dye, and by optimizing the substituents, 8% of
the efficiency was attained with D149 dye [200]. Another dye "D205"
was synthesized by controlling the aggregation between the dye
molecules, as an indoline dye with an n-octyl substituent on the
rhodanine ring of D149 [227]. They investigated that n-octyl
substitution increased the V[OC] without acknowledging the presence
of CDCA too much. However, the increase in the V[OC] of D205 due to
the CDCA was approximately 0.054 V but showed little effect on D149
with an increase of 0.006 V only. But the CDCA and n-octyl chain
(D205) together improved the V[OC] by up to 0.710 V significantly,
which was 0.066 V higher (by 10.2%) than that of D149 with CDCA.

Further in 2012, e = 9.4% was shown by Wu et al. with the observed J
[SC] = 18 mAcm^- 2, V[OC] = 0.69 V, and FF = 0.78, by employing
indoline as an organic dye in the respective DSSC [228]. Suzuka et
al. fabricated a DSSC sensitized with indoline dyes in conjunction
with the highly reactive but robust nitroxide radical molecules as
redox mediator in a quasi-solid gel form of the electrolyte. They
obtained an appreciable efficacy of 10.1% at 1 sun. To suppress a
charge-recombination process at the dye interface, they introduced
long alkyl chains, which specifically interact with the radical
mediator [229]. Recently in 2017, Irgashev et al. synthesized a novel
push-pull thieno[2,3-b]indole-based metal-free dyes and investigated
their application in DSSCs [230]. They designed IK 3-6 dyes based on
the thieno[2,3-b]indole ring system, bearing various aliphatic
substituents such as the nitrogen atom as an electron-donating part,
several thiophene units as a p-bridge linker, and 2-cyanoacrylic acid
as the electron-accepting and anchoring group. An efficiency of 6.3%
was achieved for the DSSCs employing 2-cyano-3-{5-[8-(2-ethylhexyl)
-8H-thieno[2,3-b]indol-2-yl]thiophen-2-yl}acrylic acid (IK 3), under
simulated AM 1.5 G irradiation (100 mWcm^- 2), whereas the lower
values of e = 1.3% and 1.4%, respectively, were shown by the dyes IK
5 and IK 6. The LUMO energy levels are more negative than the
conduction edge of the TiO[2] (- 3.9 eV), and their HOMO energy
levels of all four dyes were found to be more positive than the I^-/I
[3]^- redox couple (- 4.9 eV), making possible regeneration of
oxidized dye molecules after injection of excited electrons into TiO
[2] electrode (as shown in Fig. 19) [230]. The less efficiency of
other dyes was contributed by the intermolecular p-stacking and
aggregation processes in these dyes, proceeding on the photoanode
surface.

Fig. 17
figure 17

Absorption spectra of 2TPA, TPA-R, and 2TPA-R in CH[2]Cl[2] solutions
[184]

Full size image

Porphyrins

Porphyrin shows strong absorption and emission in the visible region
and has a long lifetime in its excited singlet state (> 1 ns), very
fast electron injection rate (femtosecond range) [231], millisecond
time scale electron recombination rate [232], and tunable redox
potentials [13]. In 1987, the first paper was published on DSSCs
based on efficient sensitization of TiO[2] with porphyrins [233].
This led researchers in the direction to make efforts for the
synthesis of novel porphyrin derivatives with the underlying idea to
mimic nature's photosystems I and II, so that the large molar
extinction coefficient of the Soret bands and Q bands can be
exploited. In 2007, a Zn-porphyrin dye-based DSSC was fabricated by
Campbell et al. and has given the exceptional PCE of 7.1% [195].
Krishna and co-workers investigated the application of bulky nature
phenanthroimidazole-based porphyrin sensitizers in DSSCs [234]. The
group designed a novel D-p-A-based porphyrin sensitizer having strong
electron-donating methyl phenanthroimidazole ring and
ethynylcarboxyphenyl group at meso-position of porphyrin framework
(LG11). They have attached the hexyl phenyl chains to the
phenanthroimidazole moiety to reduce the unwanted loss of V[OC]
caused by dye aggregation and charge recombination effect, thus
achieving an increase in V[OC] to 460 and 650 mV. The energy level
diagram and the absorption-emission spectra for the sensitizers
(LG11-14) are shown in Fig. 20 [234].

Fig. 18
figure 18

Molecular structure of a Coumarin and b indole

Full size image

Wang et al. have synthesized zinc porphyrins in a series bearing a
phenylethynyl, naphthalenylethynyl, anthracenylethynyl,
phenanthrenylethynyl, or pyrenylethynyl substituent, namely LD1, LD2,
LD3a, LD3p, and LD4, as photosensitizers for DSSCs (as shown in Fig. 
21). The overall efficiencies of the corresponding devices resulted
as LD4 (with e = 10.06%) > LD3p > LD2 > LD3a > LD1. The higher value
of e and V[OC] = 0.711 V was achieved for LD4 due to the broader and
more red-shifted spectral feature; thus, the IPCE spectrum was
covered broadly over the entire visible region [235]. Later for a
push-pull zinc porphyrin DSSCs, changes in the structural design were
carried out and structures with long alkoxyl chains enveloping the
porphyrin core were built. By following the process, a e = 12.3% was
achieved by Yella et al. for DSSC with cobalt as the mediator [236].

Fig. 19
figure 19

HOMO and LUMO energy level diagram of dyes IK 3-6 [230]

Full size image

Giovannetti et al. investigated the free base, Cu(II) and Zn(II)
complexes of the 2,7,12,17-tetrapropionic acid of
3,8,13,18-tetramethyl-21H,23H porphyrin (CPI) in solution and bounded
to transparent monolayer TiO[2] nanoparticle films to determine their
adsorption on the TiO[2] surface, to measure the adsorption kinetics
and isotherms, and to use the obtained results to optimize the
preparation of DSSC PVCs (photovoltaic cells) [237]. The absorption
spectra study of CPI, CPIZn, and CPICu molecules onto the TiO[2]
surface (as shown in Fig. 22) revealed the presence of typical strong
Soret and weak Q bands of porphyrin molecules in the region
400-450 nm and 500-650 nm, which were not changed with respect to the
solution spectra. They observed no modification in the structural
properties of the adsorbed molecules.

Fig. 20
figure 20

Energy level diagram of LG-11 to LG-14 porphyrins, electrolyte and
TiO[2] (a) and absorption (left, solid line) and emission (right,
dashed line) spectra of porphyrin sensitizers LG-13 and LG-14 in the
THF solvent (b)

Full size image

Triarylamine Dyes

Due to the good electron as well as transporting capability and its
special propeller starburst molecular structure with a nonplanar
configuration, the triarylamine group is widely applied as a HTM in
various electronic devices. Triarylamine derivative distributes the
p-p stacking and, thus, improves the cells performance by reducing
the charge recombination, minimizing the dye aggregation and
enhancing the molar extinction coefficient of the organic dye [202,
217, 238]. By the addition of alkyl chains or donating groups, the
structural modification of the triarylamine derivatives could be
performed [218, 220, 239]. The performance of a basic D-p-A organic
dye can be improved by simply binding donor substitutions on the
p-linker of the dye [240]. Thus, Prachumrak and co-workers have
synthesized three new molecularly engineered D-p-A dyes, namely T2-4,
comprising TPA as a donor, terthiophene containing different numbers
of TPA substitutions as a p-conjugated linker and cyanoacrylic acid
as an acceptor [240]. To minimize the electron recombination between
redox electrolyte and the TiO[2] surface as well as an increase the
electron correction efficiency, the introduction of electron donating
TPA substitutes on the p-linker of the D-p-A dye can play a favorable
game, leading to improved V[OC] and J[SC], respectively [240]. In
2006, Hagberg et al. published a paper on TPA-based D5 dye [241],
where the overall PCE demonstrated for D5 dye was 5.1% in comparison
with the standard N719 dye with an efficiency of 6.40% under the
similar fabrication conditions. Thus, D5 appeared as an underpinning
structure to design the next series of TPA derivatives.

In 2007, a series L0-L4 of TPA-based organic dyes were published by
extending the conjugation in a systematic way [218]. By increasing
the p conjugation, the absorption spectra and molar extinction
coefficients of L0-L4 were increased. The observed IPCE spectra for
L0 and L1 dyes were high, but the spectra of these dyes were not
broad; as a result, lower conversion efficiencies were obtained for
L0 and L1, whereas the broad absorption spectrum as well as the broad
IPCE was obtained for L3 and L4 by the augmentation of linker
conjugation, but the efficiencies observed were less than the L0 and
L1 due to the amount of dye loading, i.e., with the increase in the
size of dye there appears a decrease in the dye amount. Thus, the
lower IPCE obtained for longer L3 and L4 may be accredited to
unfavorable binding with the TiO[2] surface. Higher efficiencies were
obtained for solar cells based on L1 and L2, 2.75% and 3.08%,
respectively. Baheti et al. synthesized DSSCs based on
nanocrystalline anatase TiO[2] and simple triarylamine-based dyes
containing fluorene and biphenyl linkers [242]. They reported that
the fluorene-based dyes showed better solar cell parameters than
those of the biphenyl analogues. In 2011, Lu et al. reported the
synthesis and photophysical/electrochemical properties of three
functional triarylamine organic dyes (MXD5-7) as well as their
application in dye-sensitized solar cells. They used the nonplanar
structures of bishexapropyltruxeneamino as an electron donor [243]
and investigated the impact of addition of chenodeoxycholic acid
(CDCA) in the respective dyes, as MXD5-7 without CDCA showed lower
photocurrent and efficiency as compared to the dyes MXD5-7 with 3 mM
CDCA. However, the highest efficiency of 6.18% was observed for MXD7
(with 3 mM CDCA) with electron lifetime (t) = 63 ms, under standard
global AM 1.5 solar conditions (molecular structure is given in
Table 4, where R = propyl).

Using furan as a linker, different TPA-based chromophores were
studied by Lin and co-workers [244]. When D5 and its furan
counterpart were compared, the results were exciting, still the light
harvesting abilities observed for D5 were higher (l[abs] = 476 nm
with e = 45,900 M^- 1 cm^- 1 in ACN) than those for the furan
counterpart (l[abs] = 439 nm with e = 33,000 M^- 1 cm^- 1 in ACN).
However, the performance of the solar cells based on the furan
counterpart (n[max] = 7.36%) was better as compared to the one based
on D5 (n[max] = 6.09%) because of the faster recombination lifetimes
in D5. Again, the tendency of trapping of charge from the TPA moeity
was higher in thiophene than the furan. In 2016, Simon et al.
reported an enhancement in the photovoltage for DSSCs that employed
triarylamine-based dyes, where halogen-bonding interactions existed
between a nucleophilic electrolyte species (I^-) and a photo-oxidized
dye immobilized on a TiO[2] surface. They found larger rate constants
for dye regeneration (k[reg]) by the nucleophilic electrolyte species
when heavier halogen substituents were positioned on the dye. Through
the observations, they concluded that the halogen-bonding
interactions between the dye and the electrolyte can boost the
performance of DSSC [245]. However, the most efficient metal-free
organic dye-based DSSC has shown PCE of 10.3% in combination with a
cobalt redox shuttle, by using the phenyl dihexyloxy-substituted
triphenylamine (TPA) (DHO-TPA) Y123 dye [246]. In 2018, Manfredi and
group have designed di-branched dyes based on a triphenylamino (TPA)
donor core with different aromatic and heteroaromatic peripheral
groups bonded to TPA as auxiliary donors [247]. Thus, due to the
improved strategic interface interactions between the dye sensitized
titania and the liquid electrolyte, better optical properties were
achieved.

Phenothiazine (PTZ) Dyes

Phenothiazine is a heterocyclic compound containing electron-rich
sulfur and nitrogen heteroatoms, with a non-planar and butterfly
conformation in the ground state, which can obstruct the molecular
aggregation and the intermolecular excimer formation. Thus, PTZ
results as a promising hole transport semiconductor in the organic
devices, presenting unique electronic and optical properties [248].

Tian and co-workers investigated the effect of PTZ as an
electron-donating unit in DSSCs, and because of the stronger electron
donating tendency of PTZ unit than the TPA unit (0.848 and 1.04 V vs.
the normal hydrogen electrode (NHE), respectively) [249], they found
efficient results for the sensitizers based on PTZ rather than those
based on the TPA [250]. In 2007, a new series of PTZ-based dyes as
T2-1 to T2-4 was demonstrated [251]. In these dyes, PTZ unit acted as
an electron donor, cyanoacrylic acid or rhodanine-3-acetic acid was
used as an electron acceptor, and alkyl chains were used to increase
the solubility. They found a red shift in the absorption spectra of
T2-3 (e = 1.9%) and T2-4 (e = 2.4%) dyes with low IPCE values for
rhodanine-3-acetic acid as an anchoring group, as compared to T2-1 (e
 = 5.5%) and T2-2 (e = 4.8%) dyes with cyanoacrylic acid as an
anchoring group. This proved the use of the cyanoacrylic acid is more
viable than a rhodanine-3-acetic acid. In 2010, Tian et al. reported
modified phenothiazine (P1-P3) dyes [252] with the molecular
structure containing the same acceptor and conjugation chain but
different donors. Due to the presence of two methoxy groups attached
to TPA, a red shift was observed in the absorption spectra of P1 as
compared to P2 and P3. This resulted in an increment in the extent of
electron delocalization over the whole molecule and, thus, a little
red shift in the maximum absorption peak was observed. Xie et al.
synthesized two novel organic dyes (PTZ-1 and PTZ-2) using
electron-rich phenothiazine as electron donors and oligothiophene
vinylene as conjugation spacers. They employed 13 mm transparent and
1.5 mm scattering TiO[2] electrode and used an electrolyte composed
of 0.6 M butylmethylimidazolium iodide (BMII), 0.03 M I[2], 0.1 M
GuSCN, 0.5 M 4-tert-butylpyridine in acetonitrile (TBP in ACN), and
valeronitrile. They demonstrated that the (2E)-2-cyano-3-(5-(5-((E)
-2-(10-(2-ethylhexyl)-10H-phenothiazin-7-yl)vinyl)thiophen-2-yl)
thiophen-2-yl)acrylic acid (PTZ-1) and (2E)-3-(5-(5-(4,5-bis((E)-2-
(10-(2-ethylhexyl)-10Hphenothiazin-3-yl)vinyl)thiophen-2-yl)
thiophen-2-yl)thiophen-2-yl)-2cyanoacrylic acid (PTZ-2)-based DSSC
showed V[OC] = 0.70 V, J[SC] = 11.69 mAcm^- 2, FF = 65.3, and e =
 5.4% and V[OC] = 0.706 V, J[SC] = 7.14 mAcm^- 2, FF = 55.6, and e =
 2.80% [150] under AM 1.5100 mWcm^-2 illumination, respectively. The
effect of hydrophilic sensitizer PTZ-TEG together with an aqueous
choline chloride-based deep eutectic solvent (used as an electrolyte)
has been reported [253]. In the study, glucuronic acid (GA) was used
as a co-absorbent because it as has a simple structure and polar
nature and is also able to better interact with hydrophilic media and
components and possibly participates to the hydrogen bind interaction
operated in the DES medium. PCE of 0.50% was achieved for the 1:1 dye
/coabsorbent ratio.

Carbazole Dyes

It is a non-planar compound and can improve the hole transporting
ability of the materials as well as avert the dye aggregate formation
[235]. Due to its unique optical, electrical, and chemical
properties, this compound has been applied as an active component in
solar cells [254, 255]. Even with the addition of carbazole unit into
the structure, the thermal stability and glassy state durability of
the organic molecules were observed to be improved significantly [256
, 257]. Tian et al. reported an efficiency of 6.02% for the DSSCs
using S4 dye as a sensitizer, with an additional carbazole moiety to
the outside of the donor group and found that the additional moiety
facilitated the charge separation thereby decreasing the
recombination rate between conduction band electrons and the oxidized
sensitizer [185].

A series of MK-1, MK-2, and MK-3 dyes based on carbazole were
reported by Koumura et al., where MK-1 and MK-2 have alkyl groups but
MK-3 had no alkyl group. They showed that the presence of alkyl
groups increased the electron lifetime and consequently V[OC] in MK-1
and MK-2 [203, 258, 259], and due to the absence of alkyl groups,
lower electron lifetime values could be responsible for the
recombination process between the conduction band electrons and dye
cations in MK-3. New structured dyes, i.e., D-A-p-A-type and
D-D-p-A-type organic dyes, have been developed by inserting the
subordinate donor-acceptor such as
3,6-ditert-butylcarbazole-2,3-diphenylquinoxaline to facilitate
electron migration, restrain dye aggregation, and improve
photostability [260]. Thus, by further extending the p conjugation of
the linkers, mounting the electron-donating and electron-accepting
capability of donors and acceptors, and substituting long alkyl
chains, more stable DSSCs with lower dye aggregation and higher
efficiency can be achieved.

Phenoxazine (POZ) Dyes

Phenoxazine is a tricyclic isoster of PTZ. The PTZ and POZ units
display a stronger electron donating ability than the TPA unit
(0.848, 0.880, and 1.04 V vs. normal hydrogen electrode (NHE),
respectively) [261]. However, DSSCs based on POZ dyes show better
cell performance as compared to PTZ dye-based DSSCs [261]. In 2009,
two POZ-based dyes were demonstrated by Tian et al., i.e., a simple
POZ dye TH301 and triphenylamine attached to TH301, named as TH305.
Due to the insertion of TPA unit in TH305, a red shift in the
absorption band was seen because of the higher electron donating
capability of POZ. The efficiencies obtained for TH301 and TH305 were
6.2% and 7.7%, respectively, where standard N719 sensitizer showed an
efficiency of 8.0% under similar conditions [206]. Thus, in 2011,
Karslson reported a series of dyes MP03, MK05, MK08, MK12, and MK13,
based on POZ unit, to increase the absorption properties of the
sensitizers [261]. Further, two novel metal-free dyes (DPP-I and
DPP-II) with a diketopyrrolopyrrole (DPP) core were synthesized for
dye-sensitized solar cells (DSSCs) by Qu et al. [262]. They
demonstrated the better photovoltaic performance with a maximum
monochromatic IPCE of 80% and e = 4.14% with J[SC] = 9.78 mAcm^- 2, V
[OC] = 605 mV, and FF = 0.69, for the DSSC based on dye DPP-I.

Singh et al. have demonstrated nanocrystalline TiO[2] dye-sensitized
solar cells with PCE of 4.47% successfully designed two metal-free
dyes (TPA-CN1-R2 and TPA-CN2-R1), containing triphenylamine and
cyanovinylene 4-nitrophenyls as donors and carboxylic acid as an
acceptor [263].

Semiconductor quantum dots (QDs) are another attractive approach to
being sensitizers. These are II-VI and III-V type semiconductor
particles whose size is small enough to produce quantum confinement
effects. QD is a fluorescent semiconductor nanocrystal or
nanoparticle typically between 10 and 100 atoms in diameter and
confines the motion of electrons in conduction band, holes in valence
band, or simply excitons in all three spatial directions. Thus, by
changing the size of the particle, the absorption spectrum of such
QDs can be easily varied. An efficiency of 7.0% has been recorded by
collaborating groups from the University of Toronto and EPFL [264].
This recorded efficiency was higher than the solid-state DSSCs and
lower than the DSSCs based on liquid electrolytes. A high performance
QDSSC with 4.2% of PCE was demonstrated by Li et al. This cell
consisted of TiO[2]/CuInS[2]-QDs/CdS/ZnS photoanode, a polysulfide
electrolyte, and a CuS counter electrode [265]. In 2014, a conversion
efficiency of 8.55% has been reported by Chuang et al. [266].
Recently, Saad and co-workers investigated the influence on the
absorbance peak on N719 dye due to the combination between cadmium
selenide (CdSe) QDs and zinc sulfide (ZnS) QDs [267]. The cyclic
voltammetry (CV) of varying wt% of ZnS found that the 40 wt% of ZnS
is an apposite combination for a DSSC's photoanode and has produced
the higher current. However, 50 wt% of ZnS was found to be the best
concerto to increase the absorbance peak of the photoanode.

Natural dyes

New dye materials are also under extensive research, due to the
intrinsic properties of Ru(II)-based dyes, and as a result to replace
these rare and expensive Ru(II) complexes, the cheaper and
environmentally friendly natural dyes overcome as an alternative [268
].

Natural dyes provide low-cost and environmentally friendly DSSCs.
There are various natural dyes containing anthocyanin [268],
chlorophyll [269], flavonoid [270], carotenoid [271], etc. which have
been used as sensitizers in DSSCs. Table 5 provides the general
characteristics of these dyes, i.e., their availability and color
range.

Table 5 Availability and color range for the natural dyes
(anthocyanin, carotenoid, chlorophyll, and flavonoid)
Full size table

Molecular Structure

Anthocyanin: The molecular structure of anthocyanin is shown in Fig. 
23a. In anthocyanin molecule, the carbonyl and hydroxyl groups are
bound to the semiconductor (TiO[2]) surface, which stimulates the
electron transfer from the sensitizer (anthocyanin molecules) to the
conduction band of porous semiconducting (TiO[2]) film. Anthocyanin
can absorb light and transfer that light energy by resonance energy
transfers to the anthocyanin pair in the reaction center of the
photosystems [272].

Fig. 21
figure 21

Molecular structures of LD porphyrins

Full size image

Flavonoid: Flavonoid is an enormous compilation of natural dyes which
shows a carbon framework (C[6]-C[3]-C[6]) or more particularly the
phenylbenzopyran functionality, as shown in Fig. 23b [273]. It
contains 15 carbons with two phenyl rings connected by three carbon
bridges, forming a third ring, where the different colors of
flavonoids depend on the degree of phenyl ring oxidation (C-ring).
Its adsorption onto mesoporous TiO[2] surface is quite fast by
displacing an OH counter ion from the Ti sites that combines with a
proton donated by the flavonoid [274].

Carotenoid: Andanthocyanin, flavonoids, and carotenoids are often
found in the same organs [275]. Carotenoids are the compounds having
eight isoprenoid units that are widespread in nature (as shown in
Fig. 23c). Beta-carotene dye has an absorbance in wavelength zones
from 415 to 508 nm, has the largest photoconductivity of 8.2 x 10^- 4
and 28.3 x 10^- 4 (O.m)^- 1 in dark and bright conditions [276], and
has great potential as energy harvesters and sensitizers for DSSCs [
277].

Cholorophyll: Among six different types of chlorophyll pigments that
actually exist, Chl a is the most occurring type. Its molecular
structure comprises a chlorine ring with a Mg center, along with
different side chains and a hydrocarbon trail, depending on the Chl
type (as shown in Fig. 23d).

In 1997, antocyanins extracted from blackberries gave a conversion
efficiency of 0.56% [268]. The roselle (Hibiscus sabdariffa
containing anthocyanin) flowers and papaw (Carica papaya containing
chlorophyll) leaves were also investigated as natural sensitizers for
DSSCs. Eli et al. sensitized TiO[2] photoelectrode with roselle
extract (e = 0.046%) and papaw leaves (e = 0.022%), respectively and
found better efficiency for roselle extract-sensitized cell because
of the broader absorption of the roselle extract onto TiO[2] [278].
Tannins have also been attracted as a sensitizer in DSSCs due to
their photochemical stability. DSSCs using natural dyes tannins and
other polyphenols (extracted from Ceylon black tea) have given
photocurrents of up to 8 mAcm^- 2 [168]. Haryanto et al. fabricated a
DSSC using annato seeds (Bixa orellana Linn) as a sensitizer [279].
They demonstrated V[OC] and J[SC] for 30 g, 40 g, and 50 g as
0.4000 V, 0.4251 V, and 0.4502 V and 0.000074 A, 0.000458 A, and
0.000857 A, respectively. The efficiencies of the fabricated solar
cells using annato seeds as a sensitizer for each varying mass were
0.00799%, 0.01237%, and 0.05696%. They observed 328-515 nm wavelength
range for annato seeds with the help of a UV-vis spectrometer.
Hemalatha et al. reported a PCE of 0.22% for the Kerria japonica
carotenoid dye-sensitized solar cells in 2012 [280].

In 2017, a paper was published on DSSCs sensitized with four natural
dyes (viz. Indian jamun, plum, black currant, and berries). The cell
achieved highest PCE of 0.55% and 0.53%, respectively, for
anthocyanin extracts of blackcurrant and mixed berry juice [281].
Flavonoid dye extracted from Botuje (Jathopha curcas Linn) has been
used a sensitizer in DSSCs. Boyo et al. achieved e = 0.12% with the J
[SC] = 0.69 mAcm^- 2, V[OC] = 0.054 V, and FF = 0.87 for the
flavonoid dye-sensitized solar cell [282]. Bougainvillea and
bottlebrush flower can also be used as a sensitizer in DSSCs because
both of them show a good absorption level in the range of 400 to
600 nm as a sensitizer, with peak absorption at 520 nm for
bougainvillea and 510 nm for bottlebrush flower [283]. A study of
color stability of anthocyanin (mangosteen pericarp) with
co-pigmentation method has been conducted by Munawaroh et al. They
have found higher color retention for anthocyanin-malic acid and
anthocyanin-ascorbic acid than that of pure anthocyanin [284]. Thus,
the addition of ascorbic acid and malic acid as a co-pigment can be
performed to protect the color retention of anthocyanin (mangosteen
pericarp) from the degradation process. The I-V characteristics of
DSSCs employing different natural dyes are shown in Table 6.

Table 6 PV characteristics for different natural dye-sensitized solar
cells
Full size table

Organic Complexes of Other Metals

  Os, Fe and Pt complexes [285, 286, 287] are considered to be some
other promising materials in DSSCs. Besides the fact that Os
complexes are highly toxic, they are applied as a sensitizer in DSSCs
due to its intense absorption (a811nm = 1.5 x 103 M- 1 cm- 1) and for
the utilization of spin forbidden singlet-triplet MLCT transition in
the NIR. Higher IPCE values were obtained in this spectral region;
however, the overall conversion efficiency was only 50% of a standard
Ru dye. Pt complexes have given modest efficiencies of ca. 0.64% [286
] a and iron complexes, which are very interesting due to the vast
abundance of the metal and its non-toxicity; the solvatochromism of
complexes like [FeIIL2(CN)2] can be used to adjust their ground and
excited state potentials and increase the driving force for electron
injection into the semiconductor conduction band or for regeneration
of the oxidized dye by the electrolyte couple [287].

Thus, a number of metal dyes, metal-free organic dyes, and natural
dyes have been synthesized till today. Many other dyes like K51 [288
], K60 [289], K68 [290]; D5, D6 (containing oligophenylenevinylene
p-conjugated backbones, each with one N,N-dibutylamino moiety) [291];
K77 [292]; SJW-E1 [293]; S8 [294]; JK91 and JK92 [295]; CBTR, CfBTR,
CiPoR, CifPoR, and CifPR [296, 297]; Complexes A1, A2, and A3 [298];
T18 [299]; A597 [300]; YS-1-YS-5 [301]; YE05 [302]; and TFRS-1-3 [303
] were developed and applied as sensitizers in DSSCs.

Latest Approaches and Trends

However, a different trend to optimize the performance of the DSSCs
has been started by adding the energy relay dyes (ERDs) to the
electrolyte [57, 304]; inserting phosphorescence or luminescent
chromophores, such as applying rare-earth doped oxides [58,59,60]
into the DSSC; and coating a luminescent layer on the glass of the
photoanode [61, 62]. In the process of adding the ERDs to the
electrolyte or to the HTM, some highly luminescent fluorophores have
to be chosen. The main role of ERD molecules in DSSCs is to absorb
the light that is not in the primary absorption spectrum range of the
sensitizing dye and then transfer the energy non-radiatively to the
sensitizing dyes by the fluorescence (Forster) resonance energy
transfer (FRET) effect [305]. An improvement in the external quantum
efficiency of 5 to 10% in the spectrum range from 400 to 500 nm has
been demonstrated by Siegers and colleagues [306]. Recently, Lin et
al. reported the doping of 1,8-naphthalimide (N-Bu) derivative
fluorophore directly into a TiO[2] mesoporous film with N719 for
application in DSSCs [307], in which the N-Bu functioned as the FRET
donor and transferred the energy via spectral down-conversion to the
N719 molecules (FRET acceptor). An improvement of the PCE from 7.63
to 8.13% under 1 sun (AM 1.5) illumination was attained by the cell.
Similarly, Prathiwi et al. fabricated a DSSC by adding a synthetic
dye into the natural dye containing anthocyanin (from red cabbage) in
2017 [308]. They prepared two different dyes at different volumes,
i.e., anthocyanin dye at a volume of 10 ml and combination dyes at a
volume of 8 ml (anthocyanin): 2 ml (N719 synthetic dye),
respectively. They observed an enhancement in conversion efficiency
up to 125%, because individually the anthocyanin dye achieved a
conversion efficiency of 0.024% whereas for the combination dye
0.054% conversion efficiency was achieved. This enhancement was
considered due to the higher light absorption. Thus, greater photon
absorption took place and the electrons in excited state were also
increased to enhance the photocurrent. Thus, cocktail dyes are also
developing as a new trend in DSSCs. Chang et al. achieved a e = 1.47%
when chlorophyll dye (from wormwood) and anthocyanin dye (from purple
cabbage) as natural dyes were mixed together at volume ratio of 1:1 [
309], whereas the individual dyes showed lower conversion
efficiencies. Puspitasari et al. fabricated different DSSCs by mixing
the three different natural dyes as turmeric, mangosteen, and
chlorophyll. The highest efficiency of 0.0566% was attained for the
mixture of the three dyes, where the absorbance peak of the mixed
dyes was observed at 300 nm and 432 nm [106]. Similarly, Lim and
co-workers have achieved a 0.085% of efficiency when mixing the
chlorophyll and xanthophyll dyes together [310]. In 2018, Konno et
al. studied the PV characteristics of DSSCs by mixing different dyes
and observed highest n = 3.03% for the combination dye "D358 + D131,"
respectively [311]. Figure 24 shows the IPCE of mixed pigments and
single pigments.

Fig. 22
figure 22

UV-vis spectra and in insert Q-band magnification for CPI, CPICu, and
CPIZn incorporated into the TiO[2] films [237]

Full size image

An approach used to enhance the performance of DSSCs is plasmonic
effect. Surface plasmon resonance (SPR) is resonant oscillation of
conduction electrons at the interface between negative and positive
permittivity material stimulated by incident light. In 2013,
Gangishetty and co-workers synthesized core-shell NPs comprising a
triangular nanoprism core and a silica shell of variable thickness.
SPR band centered at ~ 730 nm was observed for the nanoprism Ag
particles, which overlapped with the edge of the N719 absorption
spectrum very well. They found the incorporation of the nanoprism Ag
particles into the photoanode of the DSSCs yielded a 32% increase in
the overall PCE [312]. Hossain et al. used the phenomenon of
plasmonic with different amounts of silver nanoparticles (Ag NPs)
coated with a SiO[2] layer prepared as core shell Ag@SiO[2]
nanoparticles (Ag@SiO[2] NPs) and studied the effect of SiO[2]
-encapsulated Ag nanoparticles in DSSCs. They found the highest PCE
of 6.16% for the photoanode incorporated 3 wt% Ag@SiO[2]; the optimal
PCE was 43.25% higher than that of a 0 wt% Ag@SiO[2] NP photoanode [
313]. However, a simultaneous decrease in the efficiency with further
increases in the wt% ratio, i.e., for 4 wt% Ag@SiO[2] and 5 wt%
Ag@SiO[2], was observed. This decrease for the excess amounts of
Ag@SiO[2] NPs was attributed to three reasons: (i) reduction in the
effective surface area of the films, (ii) absorption of less amount
of the dye, and (iii) an increase in the charge-carrier recombination
[314]. After analyzing the nyquist plots (as shown in Fig. 25), they
have found a decreased diameter of Z[2] monotonically as the Ag@SiO
[2] NP content increased to 3 wt% and R2 decreased from 10.4 to
6.64 O for the conventional DSSC to the 3 wt% Ag@SiO[2] NPs
containing DSSC. Jun et al. used quantum-sized gold NPs to create
plasmonic effects in DSSCs [315]. They fabricated the TiO[2]
photoanode by incorporating the Au nanoparticles (Au NPs) with an
average diameter of 5 nm into the commercial TiO[2] powder (average
diameter 25 nm) and used N749 black dye as a sensitizer. Thus, due to
the SPR effect, the efficiency for the DSSC (incorporating Au NPs)
was enhanced by about 50% compared to that without Au nanoparticles.
Effect of incorporating green-synthesized Ag NPs into the TiO[2]
photoanode has been investigated in 2017 [316]. Uniform Ag NPs
synthesized by treating silver ions with Peltophorum pterocarpum
flower extract at room temperature showed the Ag NPs as
polycrystalline in nature with face centered cubic lattice with an
approximate size in the range of 20-50 nm [316]. The PCE of the
device was improved from 2.83 to 3.62% with increment around 28%
after incorporation of the 2 wt% of the Ag NPs due to the plasmonic
effect of the modified electrode. Bakr et al. have fabricated Z907
dye-sensitized solar cell using gold nanoparticles prepared by pulsed
Nd:YAG laser ablation in ethanol at wavelength of 1064 nm [63]. The
addition of synthesized Au NPs to the Z907 dye increased the
absorption of the Z907 dye, thus achieving n = 1.284% for the cell
without Au NPs and 2.357% for the cell incorporating the Au NPs.
Recently, in 2018, a novel 3-D transparent photoanode and scattering
center design was applied as to increase the energy conversion
efficiency from 6.3 to 7.2% of the device [317] because the
plasmonics plays an important role in the absorption of light and
thus, the application is developing at a very fast pace and grabbing
a lot of attention worldwide in the last few years. Recently, a study
on incorporation of Mn^2+ into CdSe quantum dots was carried out by
Zhang and group [318]. An improved efficiency from 3.4% (CdS/CdSe) to
4.9% (CdS/Mn-CdSe) was achieved for the device upon the addition of
Mn^2+ into CdSe because when Mn^2+ is doped into the CdSe (as shown
in Fig. 26), the QDs on the surface of the film became compact and
the voids among the particles were small, thus reducing the
recombination of photogenerated electrons. Also with the loading of
Mn^2+ into the CdSe, the size of the QD clusters was increased.
However, in QDSCs (quantum dot-sensitized solar cells), there is an
inefficient transfer of electrons through the mesoporous
semi-conductor layer [319], because their application on a commercial
level is still far off. Thus, Surana et al. reported the assembling
of CdSe QDs, tuned for photon trapping at different wavelengths in
order to achieve an optimum band alignment for better charge transfer
in QDSC [319]. TiO[2] hollow spheres (THSs) synthesized by the
sacrifice template method was reported as a scattering layer for a
bi-layered photoanode for DSSCs by Zhang and co-workers [320]. They
used the mixture of multi-walled carbon nanotubes with P25 as an
under layer and THSs as an overlayer for the photoanode which showed
good light scattering ability. The cross-sectional FESEM images
revealed the disordered mecroporous network for the scattering layer
containing THSs which was supposed to be responsible for the enhanced
light absorption and the transfer of electrolyte. Thus, n = 5.13% was
achieved for P25/MWNTs-THSs, whereas 4.49% of efficiency was reported
for a pure P25 photoanode-based DSSC. Also, the electron lifetime (t
[e]) estimated for pure P25 by Bode phase plots of EIS spectra was
5.49 ms; however, 7.96 ms was shown for P25/MWNTs-THSs.

Fig. 23
figure 23

Chemical structures of a anthocyanin, b flavonoid, c b,b-carotene,
and d chlorophyll

Full size image
Fig. 24
figure 24

IPCE of mixed pigment and single pigments, where single pigment were
Eosin Y, D131, and D358 and mixed pigments were D358 and Eosin Y;
D358 and D131; D131 and Eosin Y [311]

Full size image
Fig. 25
figure 25

a Nyquist plots obtained from the EIS of DSSCs with varying Ag@SiO[2]
content (inset shows the equivalent circuit). b R2 ohm with respect
to the Ag@SiO[2] NPs content [313]

Full size image
Fig. 26
figure 26

SEM images of a CdS/CdSe and b CdS/Mn:CdSe QD sensitization on TiO^2
surface. c TEM image of CdS/Mn:CdSe QDs [318]

Full size image

John and group reported the synthesis and application of ZnO-doped
TiO[2] nanotube/ZnO nanoflake heterostructure as a photoanode in
DSSCs for the first time in 2016 [321]. They used different
characterization techniques to investigate the layered structure of
the novel nanostructure. The Rutherford backscattering spectroscopy
revealed that during the doping process, a small percentage of Zn was
doped into TONT in addition to the formation of ZnO nanoflakes on the
top, which led to a preferential orientation of the nanocrystallites
in the tube on annealing. Back in 2017, Zhang et al. reported paper
on low-dimensional halide perovskite and their applications in
optoelectronics due to the ~ 100% of photoluminescence quantum yields
of perovskite quantum dots [322]. The main emphasis of their paper
was on the study of halide perovskites and their versatile
application, i.e., in optoelectronics in spite of PV applications
only. The main role of perovskite nanoparticles in solar cells is
being applied as sensitizers. Similarly, in the queue of developing
highly efficient DSSCs, Chiang and co-workers fabricated DSSCs based
on PtCoFe nanowires with rich {111} facets exhibiting superior I^- 
[3] reduction activity as a counter electrode, which surpassed the
previous PCE record of the DSSCs using Ru(II)-based dyes [323].
Recently, in accordance with enhancing the charge collection
efficiencies (e[coll]) as well as PCE of DSSCs, Kunzmann et al.
reported a new strategy of fabricating low-temperature (lt)-sintered
DSSC and demonstrated the highest efficacy reported for lt-DSSC to
date [324]. They have integrated TiO[2]-Ru(II) complex (TiO[2]_Ru_IS)
-based hybrid NPs into the photoelectrode. Due to a better charge
transport and a reduced electron recombination, devices with
single-layer photoelectrodes featuring blends of P25 and TiO[2]_Ru_IS
give rise to a 60% e[coll] relative to a 46% e[coll] for devices with
P25-based photoelectrodes. Further, for usage of a multilayered
photoelectrode architecture with a top layer based on TiO[2]_Ru_IS
only, devices with an even higher e[coll] (74%) featuring a e = 8.75%
and stabilities of 600 h were shown. The two major rewards obtained
for such devices were the dye stability due to its amalgamation into
the TiO[2] anatase network and, secondly, the enhanced charge
collection yield due to its significant resistance towards electron
recombination with electrolytes.

Conclusions

The main aim of this study was to put a comprehensive review on new
materials for photoanodes, counter electrodes, electrolytes, and
sensitizers as to provide low-cost, flexible, environmentally
sustainable, and easy to synthesize DSSCs. However, a brief
explanation has been given to greater understand the working and
components of DSSCs. One of the important emphases in this article
has been made to establish a relation between the photosensitizer
structure, the interfacial charge transfer reactions, and the device
performance which are essential to know as to develop new metal and
metal-free organic dyes. In terms of low stability offered by DSSCs,
two major issues, i.e., low intrinsic stability and the sealing of
the electrolytes (extrinsic stability), have been undertaken in this
study. To fulfill huge demand of electricity and power, we have two
best possible solutions: this demand should be compensated either by
the nuclear fission or by the sun. Even so, the nuclear fission
predicted to be the best alternative has great environmental issues
as well as a problems associated with its waste disposal. Thus, the
second alternative is better to follow. DSSCs are developed as a
cheap alternative but the efficiency offered by DSSCs in the field is
not sufficient. Thus, we have to do a wide research on all possible
aspects of DSSCs. We proposed to develop DSCCs based on different
electrodes viz. graphene, nanowires, nanotubes, and quantum dots; new
photosensitizers based on metal complexes of Ru or Os/organic
metal-free complexes/natural dyes; and new electrolytes based on
imidazolium salts/pyridinium salts/conjugated polymers, gel
electrolytes, polymer electrolytes, and water-based electrolytes. In
summary, so far, extensive studies have been carried out addressing
individual challenges associated with working electrode, dye, and
electrolytes separately; hence, a comprehensive approach needs to be
used where all these issues should be addressed together by choosing
appropriate conditions of electrolyte (both in choice of material and
structure), optimum dye, and the most stable electrolyte which
provides better electron transportation capability.

In terms of their commercial application, a DSSC needs to be
sustainable for > 25 years in building-integrated modules to avoid
commotion of the building environment for repair or replacement and a
lifespan of 5 years is sufficient for portable electronic chargers
integrated into apparel and accessories [325]. However, DSSCs are
being quite bulky due to their sandwiched glass structure, but the
flexible DSSCs (discussed elsewhere) that can be processed using
roll-to-roll methods may came as an alternative but then has to
compromise with the shorter lifespan. Although the stability and
lifetime of a DSSC most probably depend on the encapsulation and
sealing as discussed above. Apart from the usage of expensive glass
substrates in the case of modules and panels, one of the biggest
hurdles is to manufacture glass that is flat at the 10 mm length
scale over areas much larger than 30 x 30 cm^2 [326] and the
humidity. Another challenge is to choose which metal interconnects in
the cells that are more or less corroded to the electrolyte, and high
degree of control over cell-to-cell reproducibility is required to
achieve same current and/or voltage for all the cells in the module.
If the abovementioned challenges would be overcome, then there is no
roadblock for the commercial applications of DSSCs, which has been
restricted up to an amicable extent. G24i has introduced a DSC module
production of 25 MW capacity in 2007 in Cardiff, Wales (UK), with
extension plans up to 200 MW by the end of 2008 (http://www. g24i.
com), and afterwards, many DSSC demonstration modules are now
available. However, the maximum outdoor aging test of DSSCs is
reported for 2.5 years up to now [327].

Abbreviations

ACN:

    Acetonitrile

Ag NPs:

    Silver nanoparticles

AM 1.5:

    Air mass 1.5

Au NPs:

    Gold nanoparticles

BODIPY:

    Boradiazaindacene

BPI:

    4,5-Bis(4-methoxyphenyl)-1H-imidazole

C:

    Carbon

CBZ:

    Carbazole

CDCA:

    Chenodeoxycholic acid

CdSe:

    Cadmium selenide

CNF:

    Carbon nanofiber

CNT:

    Carbon nanotube

CPEs:

    Conjugated polymer electrolytes

CuBr:

    Copper bromide

CuI:

    Copper iodide

CuSCN:

    Copper thiocyanate

CV:

    Cyclic voltammetry

DPP:

    Diketopyrrolopyrrole

DSSCDB:

    Dye-sensitized solar cell database

DSSCs:

    Dye-sensitized solar cells

EC:

    Ethylene carbonate

EIS:

    Electron impedance spectroscopy

EPFL:

    Ecole Polytechnique Federale de Lausanne

ERDs:

    Energy relay dyes

FF:

    Fill factor

FRET:

    Fluorescence (Forster) resonance energy transfer

FTO:

    Fluorine-doped tin oxide

GA:

    Glucuronic acid

GBL:

    g-Butyrolactone

GuSCN:

    Guanidinium thiocyanate

HfO[2] :

    Hafnium oxide

HOMO:

    Highest occupied molecular orbital

HTMs:

    Hole transport materials

IL:

    Ionic liquid

IMVS:

    Intensity-modulated photovoltage spectroscopy

IPCE:

    Incident photon to current conversion efficiency

Ir:

    Iridium

ITO:

    Indium-doped tin oxide

J [SC] :

    Short circuit current

LCs:

    Liquid crystals

LHE:

    Light harvesting efficiency

LUMO:

    Lowest unoccupied molecular orbital

MePN:

    3-Methoxypropionitrile

MLCT:

    Metal to ligand charge transfer

nc:

    Nanocrystalline

NHE:

    Normal hydrogen electrode

NIR:

    Near-infrared region

NMBI:

    N-Methylbenzimidazole

NMP:

    N-Methylpyrrolidine

NPs:

    Nanoparticles

Os:

    Osmium

PANI:

    Polyaniline

PC:

    Propylene carbonate

PCE:

    Power conversion efficiency

PEO:

    Poly(ethylene oxide)

PET:

    Polyethylene terephthalate

P [max] :

    Maximum power output

POZ:

    Phenoxazine

Pt:

    Platinum

PTZ:

    Phenothiazine

PV:

    Photovoltaic

QDs:

    Quantum dots

QSSE:

    Quasi-solid-state electrolyte

RT:

    Room temperature

Ru:

    Ruthenium

SCE:

    Saturated calomel electrode

SCs:

    Solar cells

SPR:

    Surface plasmon resonance

SSE:

    Solid-state electrolyte

TBP:

    4-Tert-butylpyridine

TCO:

    Transparent conducting oxide

TiO[2] :

    Titanium dioxide

TPA:

    Triphenylamine

UV-vis:

    Ultraviolet-visible

V [OC] :

    Open circuit voltage

WE:

    Working electrode

ZnO:

    Zinc oxide

ZnS:

    Zinc sulfide

e :

    Efficiency

References

 1. Tributsch H, Calvin M (1971) Electrochemistry Of Excited
    Molecules: Photo-Electrochemical Reactions Of Chlorophylls.
    Photochem Photobiol 14:95-112.

    CAS  Article  Google Scholar 

 2. Tsubomura H, Matsumura M, Nomura Y, Amamiya T (1976) Dye
    sensitised zinc oxide: aqueous electrolyte: platinum photocell.
    Nature 261:402-403.

    CAS  Article  Google Scholar 

 3. O'Regan B, Gratzel M (1991) A low-cost, high-efficiency solar
    cell based on dye-sensitized colloidal TiO[2] films. Nature
    353:737-740.

    Article  Google Scholar 

 4. Nazeeruddin K, Baranoff E, Gratzel M (2011) Dye-sensitized solar
    cells: A brief overview. Sol Energy 85:1172-1178.

    CAS  Article  Google Scholar 

 5. Altobello S, Bignozzi C (2004) A, Caramori S, Larramona G, Quici
    S, Marzanni G, Lakhmiri R; Sensitization of TiO[2] with ruthenium
    complexes containing boronic acid functions. J Photochem
    Photobiol A Chem 166:91-98.

    CAS  Article  Google Scholar 

 6. Kunzmann A, Valero SE, Sepulveda A, Rico-Santacruz M, Lalinde ER,
    Berenguer J, Garcia-Martinez JM, Guldi D, Serrano ED, Costa R
    (2018) Hybrid Dye-Titania Nanoparticles for Superior
    Low-Temperature Dye-Sensitized Solar Cells. Adv Energy Mat
    8:121-212.

    Article  CAS  Google Scholar 

 7. Snaith HJ (2010) Estimating the Maximum Attainable Efficiency in
    Dye-Sensitized Solar Cells. Adv Funct Mater 20:13-19.

    CAS  Article  Google Scholar 

 8. Frank AJ, Kopidakis N, De Lagemaat JV (2004) Electrons in
    nanostructured TiO2 solar cells: Transport, recombination and
    photovoltaic properties. Coord Chem Rev 248:1165-1179.

    CAS  Article  Google Scholar 

 9. Anandan S (2007) Recent improvements and arising challenges in
    dye-sensitized solar cells. Sol Energy Mater Sol Cells
    91:843-846.

    CAS  Article  Google Scholar 

10. Anandan S, Madhavan J, Maruthamuthu P, Raghukumar V, Ramakrishnan
    VT (2004) Synthesis and characterization of naphthyridine and
    acridinedione ligands coordinated ruthenium (II) complexes and
    their applications in dye-sensitized solar cells. Sol Energy
    Mater Sol Cells 81:419-428.

    CAS  Article  Google Scholar 

11. Bose S, Soni Vand Genwa KR (2015) Recent Advances and Future
    Prospects for Dye Sensitized Solar Cells: A Review. Int J Sci Res
    Pub:5(4).

12. Web reference [available online at http://www.sta.com.au/
    downloads/ DSC%20Booklet.pdf].

13. Shalini S, Balasundaraprabhu R, Satish Kumar T, Prabavathy N,
    Senthilarasu S, Prasanna S (2016) Status and outlook of
    sensitizers/dyes used in dye sensitized solar cells (DSSC): a
    review: Sensitizers for DSSC. Int J Energy Res 40:1303-1320.

    CAS  Article  Google Scholar 

14. Wu J, Lan Z, Lin J, Huang M, Huang Y, Fan L, Luo G, Lin Y, Xie Y,
    Wei Y (2017) Counter electrodes in dye-sensitized solar cells.
    Chem Soc Rev 46:5975-6023.

    CAS  Article  Google Scholar 

15. Kakiage K, Aoyama Y, Yano T, Oya K, Fujisawab J, Hanaya M (2015)
    Highly-efficient dye-sensitized solar cells with collaborative
    sensitization by silyl-anchor and carboxy-anchor dyes. Chem
    Commun 51:15894-15897.

    CAS  Article  Google Scholar 

16. Yeoh ME, Chan KY (2017) Recent advances in photo-anode for
    dye-sensitized solar cells: a review. Int J Energy Res
    41:2446-2467.

    Article  Google Scholar 

17. Fan K, Yu J, Ho W (2017) Improving photoanodes to obtain highly
    efficient dye-sensitized solar cells: a brief review. Mater Horiz
    4:319-344.

    CAS  Article  Google Scholar 

18. Mehmood U (2014) Rahman S, Harrabi K, Hussein IA, Reddy BVS,
    Recent advances in dye sensitized solar cells. Advances in
    Materials Science and Engineering Article ID 974782:1-12.

    Google Scholar 

19. Andualem A, Demiss S (2018) Review on Dye-Sensitized Solar Cells
    (DSSCs). Edelweiss Appli Sci Tech 2:145-150.

    Google Scholar 

20. Grant FA (1959) Properties of Rutile (Titanium Dioxide). Rev Mod
    Phys 31:646-674.

    CAS  Article  Google Scholar 

21. Bickley RI (1978) Chem Phys Solids Surf 7:118.

    Article  Google Scholar 

22. Tennakone K, Kumara GRRA, Kottegoda IRM, VPS P (1999) An
    efficient dye-sensitized photoelectrochemical solar cell made
    from oxides of tin and zinc. Chem Comm 1:15-16.

    Article  Google Scholar 

23. Sayama K, Sugihara H, Arakawa H (1998) Photoelectrochemical
    Properties of a Porous Nb[2]O[5] Electrode Sensitized by a
    Ruthenium Dye. Chem Mater 10:3825-3832.

    CAS  Article  Google Scholar 

24. Fung AKM, Chiu B (2003) K W, Lam M H W; Surface modification of
    TiO2 by a ruthenium(II) polypyridyl complex via silyl-linkage for
    the sensitized photocatalytic degradation of carbon tetrachloride
    by visible irradiation. Water Res 37:1939-1947.

    CAS  Article  Google Scholar 

25. Zaban A, Ferrere S, Gregg BA (1998) Relative Energetics at the
    Semiconductor/Sensitizing Dye/Electrolyte Interface. J Phys Chem
    102:452-460.

    CAS  Article  Google Scholar 

26. Nazeeruddin MK, Kay A, Rodicio I, Humphry-Baker R, Mueller E,
    Liska P, Vlachopoulos N, Graetzel M (1993) Conversion of light to
    electricity by cis-X2bis(2,2'-bipyridyl-4,4'-dicarboxylate)
    ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-,
    and SCN-) on nanocrystalline titanium dioxide electrodes. J Am
    Chem Soc 115:6382-6390.

    CAS  Article  Google Scholar 

27. Hagberg DP, Yum JH, Lee H, De Angelis F, Marinado T, Karlsson KM,
    Humphry-Baker R, Sun L, Hagfeldt A, Gratzel M, Nazeeruddin MK
    (2008) Molecular engineering of organic sensitizers for
    dye-sensitized solar cell applications. J Am Chem Soc
    130:6259-6266.

    CAS  Article  Google Scholar 

28. Neale NR, Kopidakis N, van de Lagemaat J, Gratzel M, Frank AJ
    (2005) Effect of a Coadsorbent on the Performance of
    Dye-Sensitized TiO[2] Solar Cells: Shielding versus Band-Edge
    Movement. J PhysChem B 109:23183-23189.

    CAS  Google Scholar 

29. Ferrere S, Zaban A, Gregg BA (1997) Dye Sensitization of
    Nanocrystalline Tin Oxide by Perylene Derivatives. J Phys Chem B
    101:4490-4493.

    CAS  Article  Google Scholar 

30. Oskam G, Bergeron BV, Meyer GJ, Searson PC (2001) Pseudohalogens
    forDye sensitized TiO[2] photoelectrochemical cell. J Phys Chem B
    105:6867-6873.

    CAS  Article  Google Scholar 

31. Nusbaumer H, Moser JE (2001) Zakeeruddin SM et al; Co^II(dbbip)
    [2] ^2+ complex rivals tri-iodide/iodide redox mediator in dye
    sensitized photovoltaic cells. J Phys Chem B 105:10461-10464.

    CAS  Article  Google Scholar 

32. Gao F, Wang Y, Shi D, Zhang J, Wang M, Jing X, Humphry-Baker R,
    Wang P, Zakeeruddin SM, Gratzel M (2008) Enhance the optical
    absorptivity of nanocrystalline TiO[2] film with high molar
    extinction coefficient ruthenium sensitizers for high performance
    dye sensitized solar cells. J Am Chem Soc 130(32):10720-10728.

    CAS  Article  Google Scholar 

33. Wu J, Lan Z, Hao S, Li P, Lin J (2008) Huang M et al; Progress on
    the electrolytes for Dye-sensitized solar cells. Pure Appl Chem
    80:2241-2258.

    CAS  Article  Google Scholar 

34. Toivola M, Ahlskog F, Lund P (2006) Industrial sheet metals for
    nanocrystalline dye-sensitized solar cell structures. Sol Energy
    Mater Sol Cells 90:2881-2893.

    CAS  Article  Google Scholar 

35. Kay HA, Gratzel M (1996) Low cost photovoltaic modules based on
    dye sensitized nanocrystalline titanium dioxide and carbon
    powder. Sol Energy Mater Sol Cells 44:99-117.

    CAS  Article  Google Scholar 

36. Wang M, Anghel AM, Marsan B, Cevey Ha NL, Pootrakulchote N,
    Zakeeruddin SM, Gratzel M (2009) CoS Supersedes Pt as Efficient
    Electrocatalyst for Triiodide Reduction in Dye-Sensitized Solar
    Cells. J Am Chem Soc 131:15976-15977.

    CAS  Article  Google Scholar 

37. Fakharuddin A, Jose R, Brown TM, Fabregat-Santiago F, Bisquert J
    (2014) A perspective on the production of dye-sensitized solar
    modules. Energy Environ Sci 7:3952-3981.

    CAS  Article  Google Scholar 

38. Gratzel M and Moser JE; Solar Energy Conversion, Electron
    Transfer in chemistry (Ed.: V. Balzani) Energy and the
    environment (Ed.: I. Gould) (2001) 5:589-644.

39. Wang H-J, Chen CP, Jeng RJ (2014) Polythiophenes Comprising
    Conjugated Pendants for Polymer Solar Cells: A Review. Materials
    7(4):2411-2439.

    CAS  Article  Google Scholar 

40. Gratzel M (2005) Solar Energy Conversion by Dye-Sensitized
    Photovoltaic Cells. Inorg Chem 44:6841-6851.

    Article  CAS  Google Scholar 

41. Cai H, Tanga Q, Heb B, Lia R, Yu L (2014) Bifacial dye-sensitized
    solar cells with enhanced rear efficiency and power output.
    Nanoscale 6:15127-15133.

    CAS  Article  Google Scholar 

42. Liu J, Tang Q, He B, Yu L (2015) Cost-effective, transparent iron
    selenide nanoporous alloy counter electrode for bifacial
    dye-sensitized solar cell. J Power Sources 282:79-86.

    CAS  Article  Google Scholar 

43. Kusama H, Orita H, Sugihara H (2008) TiO[2] Band Shift by
    Nitrogen-Containing Heterocycles in Dye-Sensitized Solar Cells: a
    Periodic Density Functional Theory Study. Langmuir 24:4411-4419.

    CAS  Article  Google Scholar 

44. Murakami TN, Gratzel M (2008) Counter electrode for DSC:
    application of functional materials as catalysts. Inorg Chim Acta
    361:572-580.

    CAS  Article  Google Scholar 

45. Murakami Iha NY, Garcia CG, Bignozzi CA (2003) Dye-sensitized
    photoelectrochemical cells. In: Nalwa HS (ed) Handbook of
    photochemistry and photobiology. American Scientific Publishers,
    Stevenson Ranch, California, USA, pp 49-82.

    Google Scholar 

46. Kruger J, Plass R, Cevey L, Piccirelli M, Gratzel M (2001) High
    efficiency solid-state photovoltaic device due to inhibition of
    interface charge recombination. Appl Phys Lett 79:2085-2087.

    CAS  Article  Google Scholar 

47. Spath M, Sommeling PM, van Roosmalen JAM, Smit HJP, van der Burg
    NPG, Mahieu DR, Bakker NJ, Kroon JM (2003) Reproducible
    Manufacturing of Dye-Sensitized Solar Cells on a Semi-automated
    Baseline. Prog Photovolt Res Appl 11:207-220.

    CAS  Article  Google Scholar 

48. Dai S, Wang KJ, Weng J et al (2005) Design of DSC panel with
    efficiency more than 6%. Sol Energy Mater Sol Cells 85:447-455.

    CAS  Article  Google Scholar 

49. Green MA, Emery K, King DL, Hishikawa Y, Warta W (2006) Solar
    Cell Efficiency Tables (Version 28). Prog Photovolt Res Appl
    14:455-461.

    Article  Google Scholar 

50. Kumara GRA, Kaneko S, Konno A, Okuya M, Murakami K,
    Onwona-agyeman B, Tennakone K (2006) Large area dye-sensitized
    solar cells: material aspects of fabrication. Prog Photovolt Res
    Appl 14:643-651.

    CAS  Article  Google Scholar 

51. Schoppa M (2006) Untersuchung der Versiegelung von
    Farbstoffsolarzellen. Diplomarbeit, TU-Berlin.

    Google Scholar 

52. Sommeling PM, Spath M, Smit HJP, Bakker NJ, Kroon JM; \ (2004) J
    Photochem Photobiol A 164:137-144.

    CAS  Article  Google Scholar 

53. Hinsch A, Kroon JM, Kern JMR, Uhlendorf I, Holzbock J, Meyer A,
    Ferber J (2001) Long term stability of dye sensitized solar
    cells. Prog Photovolt Res Appl 9:425-438.

    CAS  Article  Google Scholar 

54. Wang P, Zakeeruddin SM, Baker RH, Moser JE, Gratzel M (2003)
    Molecular-Scale interface engineering of TiO[2] nanocrystals:
    Improving the efficiency and stability of dye sensitized solar
    cells. Adv Mater 15:2101-2104.

    CAS  Article  Google Scholar 

55. Wang P, Klein C, Baker RH, Zakeeruddin SM, Gratzel M (2005)
    Stable >= 8% efficient nanocrystalline dye-sensitized solar cell
    based on an electrolyte of low volatility. Appl Phys Lett
    86:123508.

    Article  CAS  Google Scholar 

56. Bauer C, Boschloo G, Mukhtar E, Hagfeldt A (2002) Interfacial
    Electron-Transfer Dynamics in Ru(tcterpy)(NCS)[3]-Sensitized TiO
    [2] Nanocrystalline Solar Cells. J Phys ChemB 106:12693-12704.

    CAS  Article  Google Scholar 

57. Hardin BE, Hoke ET, Armstrong PB, Yum J-H, Comte P et al (2009)
    Increased light harvesting in dye-sensitized solar cells with
    energy relay dyes. Nat Photonics 3:406-411.

    CAS  Article  Google Scholar 

58. Wang J, Wu J, Lin J, Huang M, Huang Y et al (2012) Application of
    Y[2]O[3]:Er^3+ Nanorods in Dye-Sensitized Solar Cells. Chem Sus
    Chem 5:1307-1312.

    CAS  Article  Google Scholar 

59. Li L, Yang Y, Fan R, Jiang Y et al (2014) A simple modification
    of near-infrared photon-to-electron response with fluorescence
    resonance energy transfer for dye-sensitized solar cells. J Power
    Sources 264:254-261.

    CAS  Article  Google Scholar 

60. Chander N, Khanb AF, Komarala VK (2015) Improved stability and
    enhanced efficiency of dye sensitized solar cells by using
    europium doped yttrium vanadate down-shifting nanophosphor. RSC
    Adv 5:66057-66066.

    CAS  Article  Google Scholar 

61. Wang TH, Huang TW, Tsai YC, Chang YW, Liao CS (2015) A
    photoluminescent layer for improving the performance of
    dye-sensitized solar cells. Chem Commun 51:7253-7256.

    CAS  Article  Google Scholar 

62. Han DM, Song HJ, Han CH, Kim YS (2015) Enhancement of the outdoor
    stability of dye-sensitized solar cells by a spectrum conversion
    layer with 1,8-naphthalimide derivatives. RSC Adv 5:32588-32593.

    CAS  Article  Google Scholar 

63. Bakr NA, Ali AK, Jassim SM (2017) Fabrication and Efficiency
    Enhancement of Z907 Dye Sensitized Solar Cell Using Gold
    Nanoparticles. J Advan Phys 6(3):370-374.

    Article  Google Scholar 

64. Gao R, Cui Y, Liu X, Wang L (2014) Multifunctional Interface
    Modification of Energy Relay Dye in Quasi-solid Dye-sensitized
    Solar Cells. Sci Rep 4:5570.

    CAS  Article  Google Scholar 

65. Rahman MM, Kob MJ, Lee JJ (2015) Novel energy relay dyes for high
    efficiency dye-sensitized solar cells. Nanoscale 7:3526-3531.

    CAS  Article  Google Scholar 

66. Hagfeldt A, Gratzel M (1995) Light-Induced Redox Reactions in
    Nanocrystalline Systems. Chem Rev 95:49-68.

    CAS  Article  Google Scholar 

67. Kalyanasundaram K, Gratzel M (1998) Applications of
    functionalized transition metal complexes in photonic and
    optoelectronic devices. Coord Chem Rev 77:347-414.

    Article  Google Scholar 

68. Aye MM, Win TT, Maung YM, Soe KKK (2014) Binary Oxide
    Photoelectrode with Coffee Natural Dye Extract for DSSC
    Application. Int J Innov Sci Res 8(2):276-282.

    Google Scholar 

69. Hu X, Huang K, Fang D, Liu S (2011) Enhanced performances of dye
    sensitized solar cells based on graphite-TiO[2] composites. Mater
    Sci Eng B 176(5):431.

    CAS  Article  Google Scholar 

70. Lee KM, Hu CW, Chen HW, Ho KC (2008) Incorporating carbon
    nanotube in a low-temperature fabrication process for
    dye-sensitized TiO2 solar cells. Sol Energy Mater Sol Cells
    92:1628-1633.

    CAS  Article  Google Scholar 

71. Muduli S, Lee W, Dhas V, Mujawar S, Dubey M, Vijayamohanan K, Han
    SH, Ogale S (2009) Enhanced Conversion Efficiency in
    Dye-Sensitized Solar Cells Based on Hydrothermally Synthesized
    TiO[2]-MWCNT Nanocomposites. Appl Mater Interface 1:2030-2035.

    CAS  Article  Google Scholar 

72. Lee W, Lee J, Min SK et al (2009) Effect of single-walled carbon
    nanotubes in PbS/TiO2 quantum dots-sensitized solar cells. Mater
    Sci Eng B 156:48-51.

    CAS  Article  Google Scholar 

73. Sun SR, Gao L, Liu YQ et al (2010) Enhanced dye-sensitized solar
    cells using grapheme-Tio2 photoanode prepared by heterogeneous
    coagulation. Appl Phys Lett 96:083113-083115.

    Article  CAS  Google Scholar 

74. Sharma GD, Daphnomili D, Gupta KSV, Gayathri T et al (2013)
    Enhancement of power conversion efficiency of dye-sensitized
    solar cells by co-sensitization of zinc-porphyrin and
    thiocyanate-free ruthenium(II)-terpyridine dyes and graphene
    modified TiO[2] photoanode. RSC Adv 3:22412-22420.

    CAS  Article  Google Scholar 

75. Tathavadekar M, Biswal M, Agarkar S, Giribabu L, Ogale S (2014)
    Electronically and catalytically functional carbon cloth as a
    permeable and flexible counter electrode for dye sensitized solar
    cell. Electrochim Acta 123:248s.

    Article  CAS  Google Scholar 

76. Jiang QW, Lia GR, Gao XP; Highly ordered TiN nanotube arrays as
    counter electrodes for dye-sensitized solar cells (2009) Chem
    Commun 0:6720-6722.

77. Li GR, Wang F, Jiang QW, Gao XP, Shen PW (2010) Carbon nanotubes
    with titanium nitride as a low-cost counter-electrode material
    for dye-sensitized solar cells. Angew Chem Int Ed 49:3653-3656.

    CAS  Article  Google Scholar 

78. Sun H, Zhanga L, Wang ZS (2014) Single-crystal CoSe[2] nanorods
    as an efficient electrocatalyst for dye-sensitized solar cells. J
    Mater Chem A 2:16023-16029.

    CAS  Article  Google Scholar 

79. Park JT, Chi WS, Kim SJ, Lee D, Kim JH (2014) Mesoporous TiO[2]
    Bragg Stack Templated by Graft Copolymer for Dye-sensitized Solar
    Cells. Sci Rep 4:5505.

    CAS  Article  Google Scholar 

80. Lee KS, Lee Y, Lee JY, Ahn JH, Park JH (2012) Flexible and
    platinum-free dye sensitized solar cells with
    conducting-polymer-coated graphene counter electrodes. Chem Sus
    Chem 5:379-382.

    CAS  Article  Google Scholar 

81. Banerjee A, Upadhyay KK, Bhatnagar S, Tathavadekar M et al (2014)
    Nickel cobalt sulfide nanoneedle array as an effective
    alternative to Pt as a counter electrode in dye sensitized solar
    cells. RSC Adv 4:8289-8294.

    CAS  Article  Google Scholar 

82. Calogero G, Calandra P, Irrera A, Sinopoli A, Citroa I, Marco GD
    (2011) A new type of transparent and low cost counter-electrode
    based on platinum nanoparticles for dye-sensitized solar cells.
    Energy & Environmental Science 4(5):1838-1844.

    CAS  Article  Google Scholar 

83. Anothumakkool B, Game O, Bhange SN, Kumari T, Ogale SB, Kurungot
    S (2014) Enhanced catalytic activity of
    polyethylenedioxythiophene towards tri-iodide reduction in DSSCs
    via 1-dimensional alignment using hollow carbon nanofibers.
    Nanoscale 6:10332-10339.

    CAS  Article  Google Scholar 

84. Huang J, Yao N, Fu K, Deng X, Zhang S, Hou P, Xu X (2018)
    Enhanced Dye-Sensitized Solar Cell Efficiency by Insertion of a H
    [3]PW[12]O[40] Layer Between the Transparent Conductive Oxide
    Layer and the Compact TiO[2] Layer. Sci Adv Mater 10(6):867-871.

    CAS  Article  Google Scholar 

85. Li GR, Song J, Pan GL, Gao XP (2011) highly Pt-like
    electrocatalytic activity of transition metal nitrides for
    dye-sensitized solar cells. Energy Environ Sci 4:1680-1683.

    CAS  Article  Google Scholar 

86. Gokhale R, Agarkar S, Debgupta J, Shinde D, Lefez B et al (2012)
    Laser synthesized super-hydrophobic conducting carbon with
    broccoli-type morphology as a counter-electrode for
    dye-sensitized solar cells. Nanoscale 4:6730-6734.

    CAS  Article  Google Scholar 

87. Naphade R, Tathavadekar M, Jog JP, Agarkar S, Ogale S (2014)
    Plasmonic light harvesting of dye sensitized solar cells by
    Au-nanoparticle loaded TiO2 nanofibers. J Mater Chem A 2:975-984.

    CAS  Article  Google Scholar 

88. Wu WQ, Xu YF, Su CY et al (2014) Ultra-long anatase TiO[2]
    nanowire arrays with multi-layered configuration on FtO glass for
    high-efficiency dye-sensitized solar cells. Energy Environ Sci 7
    (2):644-649.

    CAS  Article  Google Scholar 

89. Wu WQ, Lei BX, Rao HS, Xu YF, Wang YF, Su CY, Kuang DB (2013)
    Hydrothermal Fabrication of Hierarchically Anatase TiO2 Nanowire
    arrays on FTO Glass for Dye-sensitized Solar Cells. Sci Rep
    3:1352.

    Article  CAS  Google Scholar 

90. Sharma SS, Soni S, Sharma K et al (2018) Study of Photovoltaic
    Parameters of Dye-Sensitized Solar Cells Based on Ti(acac)[2]oiPr
    [2] Treated Working Electrode. Adv Sc Eng Med 10:1051-1054.

    Article  Google Scholar 

91. Zhao YL, Yao DS, Song CB, Zhu L, Song J, Gu XQ, Qiang YH (2015)
    CNT-G-TiO2 layer as a bridge linking TiO2 nanotube arrays and
    substrates for efficient dyesensitized solar cells. RSC Adv
    5:43805-43809.

    CAS  Article  Google Scholar 

92. Qiu YC, Chen W, Yang SH (2010) Double-layered photoanodes from
    variable-size anatase TiO2 nanospindles: a candidate for
    high-efficiency dye-sensitized solar cells. Angew Chem 122
    (21):3757-3761.

    Article  Google Scholar 

93. Zhao YL, Song DM, Qiang YH, Gu XQ, Zhu L, Song CB (2014)
    Dye-sensitized solar cells based on TiO2 hollow spheres/TiO2
    nanotube array composite films. Appl Surf Sci 309:85-89.

    CAS  Article  Google Scholar 

94. Zhou Y, Xia C, Hu X, Huang W, Aref AA, Wang B, Liu Z, Sun Y, Zhou
    W, Tang Y (2014) Dye-sensitized solar cells based on
    nanoparticle-decorated ZnO/SnO2core/shell nanoneedle arrays. Appl
    Surf Sci 292:111-116.

    CAS  Article  Google Scholar 

95. Huang Y, Wu H, Yu Q, Wang J, Yu C, Wang J, Gao S, Jiao S, Zhang
    X, Wang P (2018) Single-Layer TiO2 Film Composed of Mesoporous
    Spheres for High-Efficiency and Stable Dye-Sensitized Solar
    Cells. ACS Sustain Chem Eng 6(3):3411-3418.

    CAS  Article  Google Scholar 

96. Maheswari D, Venkatachalam P (2015) Fabrication of High
    Efficiency Dye-Sensitised Solar Cell with Zirconia-Doped TiO2
    Nanoparticle and Nanowire Composite Photoanode Film. Aust J Chem
    68:881-888.

    CAS  Article  Google Scholar 

97. Shejale KP, Sharma RK, Roy MS, Kumar M (2014) Carbon coated
    stainless steel as counter electrode for dye sensitized solar
    cells. AIP Conference Proceedings 1620:218-222.

    Article  CAS  Google Scholar 

98. Swathi KE, Jinchu I, Sreelatha KS, Sreekala CO, Menon SK (2018)
    Effect of microwave exposure on the photo anode of DSSC
    sensitized with natural dye. IOP Conf Ser: Mater Sci Eng
    310:012141.

    Article  Google Scholar 

99. Guo M, Yao Y, Zhao F, Wang S, Xiao J (2017) An
    In2.77S4@conductive carbon composite with superior
    electrocatalytic activity for dye-sensitized solar cells, J.
    Photochem. Photobiol. A. 332:87-91.

    CAS  Article  Google Scholar 

100. Tsai CH, Huang WC, Hsu YC, Shih CJ, Teng IJ, Yu YH (2016) Poly
    (o-methoxyaniline) doped with an organic acid as cost-efficient
    counter electrodes for dye-sensitized solar cells. Electrochim
    Acta 213:791-801.

    CAS  Article  Google Scholar 

101. Liu I, Hou YC, Lia CW, Lee YL (2017) Highly electrocatalytic
    counter electrodes based on carbon black for cobalt(III)/(II)
    -mediated dye-sensitized solar cells. J Mater Chem A 5:240-249.

    CAS  Article  Google Scholar 

102. Tsai CH, Fei PH, Chen CH (2015) Investigation of Coral-Like Cu2O
    Nano/Microstructures as Counter Electrodes for Dye-Sensitized
    Solar Cells. Materials 8:5715-5729.

    CAS  Article  Google Scholar 

103. Sim K, Sung SJ, Jo HJ, Jeon DH, Kim DH, Kang JK (2013)
    Electrochemical Investigation of High-Performance Dye Sensitized
    Solar Cells Based on Molybdenum for Preparation of Counter
    Electrode. Int J Electrochem Sci 8:8272-8281.

    CAS  Google Scholar 

104. Maiaugree W, Lowpa S, Towannang M, Rutphonsan P, Tangtrakarn A
    et al (2015) A dye sensitized solar cell using natural counter
    electrode and natural dye derived from mangosteen peel waste. Sci
    Rep 5:15230.

    CAS  Article  Google Scholar 

105. Kudin KN, Ozbas B, Schniepp HC, Prud'homme RK, Aksay IA, Car R
    (2008) Raman Spectra of Graphite Oxide and Functionalized
    Graphene Sheets. Nano Lett 8:36-41.

    CAS  Article  Google Scholar 

106. Puspitasari N, SSN A, Yudoyono G, Endarko E (2017) Effect of
    Mixing Dyes and Solvent in Electrolyte Toward Characterization of
    Dye Sensitized Solar Cell Using Natural Dyes as The Sensitizer.
    IOP Conf. Series: Materials Science and Engineering 214:012022.

    Article  Google Scholar 

107. Komiya R, Han L, Yamanaka R, Islam A, Mitate T (2004) Highly
    efficient quasi-solid state dye-sensitized solar cell with ion
    conducting polymer electrolyte. J. Photochem. Photobiol. A
    164:123-127.

    CAS  Article  Google Scholar 

108. Wang H, Li H, Xue B, Wang Z, Meng Q, Chen L (2005) Solid-State
    Composite Electrolyte LiI/3-Hydroxypropionitrile/SiO2 for
    Dye-Sensitized Solar Cells. J Am Chem Soc 127:6394-6401.

    CAS  Article  Google Scholar 

109. Kumara GRA, Konno A, Shiratsuchi K, Tsukahara J, Tennakone K
    (2002) Dye-Sensitized Solid-State Solar Cells: Use of Crystal
    Growth Inhibitors for Deposition of the Hole Collector. Chem
    Mater 14:954-955.

    CAS  Article  Google Scholar 

110. Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J,
    Spreitzer H, Gratzel M (1998) Solid-state dye-sensitized
    mesoporous TiO2 solar cells with high photon-to-electron
    conversion efficiencies. Nature 395:583-585.

    CAS  Article  Google Scholar 

111. Wu JH, Hao SC, Lan Z, Lin JM, Huang ML, Huang YF, Fang LQ, Yin
    S, Sato T (2007) A Thermoplastic Gel Electrolyte for Stable
    Quasi-Solid-State Dye-Sensitized Solar Cells. Adv Funct Mater
    17:2645-2652.

    CAS  Article  Google Scholar 

112. Chen K, Liu C, Huang H et al (2013) Polyvinyl butyral-based thin
    film polymeric electrolyte for dyesensitized solar cell with
    long-term stability. Int J Electrochem Sci 8:3524-3539.

    CAS  Google Scholar 

113. Iwata S, Shibakawa S, Imawaka N, Yoshino K (2018) Stability of
    the current characteristics of dye-sensitized solar cells in the
    second quadrant of the current-voltage characteristics. Energy
    Reports 4:8-12.

    Article  Google Scholar 

114. Wang P, Wenger B, Baker RH, Moser JE, Teuscher J et al (2005)
    Charge Separation and Efficient Light Energy Conversion in
    Sensitized Mesoscopic Solar Cells Based on Binary Ionic Liquids.
    J Am Chem Soc 127:6850-6856.

    CAS  Article  Google Scholar 

115. Murakoshi K, Kogure R, Wada Y, Yanagida S (1998) Fabrication of
    solid-state dye-sensitized TiO2 solar cells combined with
    polypyrrole. Sol Energy Mater Sol Cells 55:113-125.

    CAS  Article  Google Scholar 

116. MacFarlane DR, Golding J, Forsyth S, Forsytha M, Deacon GB
    (2001) Low viscosity ionic liquids based on organic salts of the
    dicyanamide anion. Chem Comm 24:1430-1431.

    Article  CAS  Google Scholar 

117. Kuang D, Wang P, Ito S, Zakeeruddin SM, Gratzel M (2006) Stable
    mesoscopic dye-sensitized solar cells based on tetracyanoborate
    ionic liquid electrolyte. J Am Chem Soc 128:7732-7733.

    CAS  Article  Google Scholar 

118. Moudam O, Villarroya-Lidon S (2014) High-Efficiency Glass and
    Printable Flexible Dye-Sensitized Solar Cells with Water-Based
    Electrolytes. J Solar Energy Article ID 426785:7.

    Google Scholar 

119. Cheng M, Yang X, Li S, Wanga X, Sun L (2012) Efficient
    dye-sensitized solar cells based on an iodine-free electrolyte
    using L-cysteine/L-cystine as a redox couple. Energy Environ Sci
    5:6290-6293.

    CAS  Article  Google Scholar 

120. Huang CY, You CF, Cheng CE, Lei BC et al (2016) Liquid
    crystal-doped liquid electrolytes for dye-sensitized solar cell
    applications. OPTICAL MATERIALS EXPRESS 6(4):1024-1031.

    CAS  Article  Google Scholar 

121. Schlichthorl G, Huang SY, Sprague J, Frank AJ (1997) Band Edge
    Movement and Recombination Kinetics in Dye-Sensitized
    Nanocrystalline TiO2 Solar Cells: A Study by Intensity Modulated
    Photovoltage Spectroscopy. J Phys Chem B 101:8141-8155.

    Article  Google Scholar 

122. Kusama H, Konishi Y, Sugihara H, Arakawa H (2003) Influence of
    alkylpyridine additives in electrolyte solution on the
    performance of dye-sensitized solar cell. Sol Energy Mater Sol
    Cells 80:167-179.

    CAS  Article  Google Scholar 

123. Wang P, Zakeeruddin SM, Comte P, Charvet R, Baker RH, Gratzel M
    (2003) Enhance the Performance of Dye-Sensitized Solar Cells by
    Co-grafting Amphiphilic Sensitizer and Hexadecylmalonic Acid on
    TiO2 Nanocrystals. J Phys Chem B 107:14336-14341.

    CAS  Article  Google Scholar 

124. Kopidakis N, Neale NR, Frank AJ (2006) Effect of an Adsorbent on
    Recombination and Band-Edge Movement in Dye-Sensitized TiO2 Solar
    Cells: Evidence for Surface Passivation. J Phys Chem B
    110:12485-12489.

    CAS  Article  Google Scholar 

125. Hodes G, Cahen D (2012) All-Solid-State,
    Semiconductor-Sensitized Nanoporous Solar Cells. Acc Chem Res
    45:705-713.

    CAS  Article  Google Scholar 

126. Tennakone K, Senadeera GKR, De Silva DBRA, Kottegoda IRM (2000)
    Highly stable dye-sensitized solid-state solar cell with the
    semiconductor 4CuBr 3 S(C4 H 9 ) 2 as the hole collector. Appl
    Phys Lett 77:2367.

    CAS  Article  Google Scholar 

127. Bandara J, Weerasinghe H (2005) Solid-state dye-sensitized solar
    cell with p-type NiO as a hole collector. Sol Energy Mater Sol
    Cells 85:385-390.

    CAS  Article  Google Scholar 

128. Yun S, Qin Y, Uhl AR, Vlachopoulos N, Yin M et al (2018)
    New-generation integrated devices based on dye-sensitized and
    perovskite solar cells. Energy Environ Sci 11:476-526.

    CAS  Article  Google Scholar 

129. Schmidt-Mende L, Zakeeruddin SM, Gratzel M (2005) Efficiency
    improvement in solid-state-dye-sensitized photovoltaics with an
    amphiphilic Ruthenium-dye. Appl Phys Lett 86:13504-13506.

    Article  CAS  Google Scholar 

130. Schmidt-Mende L, Gratzel M (2006) TiO2 pore-filling and its
    effect on the efficiency of solid-state dye-sensitized solar
    cells. Thin Solid Films 500:296-301.

    CAS  Article  Google Scholar 

131. Vannikov AV, Grishina AD, Novikov SV (1994) Electron transport
    and electroluminescence in polymer layers. Russ Chem Rev
    63:103-123.

    Article  Google Scholar 

132. Billingham NC, Calvert PD (1989) Electrically conducting
    polymers-a polymer science viewpoint. In: In Conducting Polymers/
    Molecular Recognition, Advances in polymer science, Volume 90.
    Springer-Verlag, Berlin, pp 1-104.

    Google Scholar 

133. Ishikawa M, Kawai M, Ohsawa Y (1991) Synthesis and properties of
    electrically conducting polytriphenylamines. Synth Met
    40:231-238.

    CAS  Article  Google Scholar 

134. Tanaka H, Tokito S, Taga Y, Okada A (1996) Novel
    hole-transporting materials based on triphenylamine for organic
    electroluminescent devices. Chem Comm:2175-2176.

135. Stergiopoulos T, Arabatzis IM, Katsaros G, Falaras P (2002)
    Binary Polyethylene Oxide/Titania Solid-State Redox Electrolyte
    for Highly Efficient Nanocrystalline TiO2 Photoelectrochemical
    Cells. Nano Lett 2:1259-1261.

    CAS  Article  Google Scholar 

136. Bai S, Bu C, Tai Q, Liang L, Liu Y et al (2013) Effects of Bis
    (imidazolium) Molten Salts with Different Substituents of
    Imidazolium Cations on the Performance of Efficient
    Dye-Sensitized Solar Cells. ACS Appl Mater Interfaces
    5:3356-3361.

    CAS  Article  Google Scholar 

137. Wang L, Zhang H, Wang C, Ma T (2013) Highly Stable Gel-State
    Dye-Sensitized Solar Cells Based on High Soluble Polyvinyl
    Acetate. ACS Sustain Chem Eng 1:205-208.

    CAS  Article  Google Scholar 

138. Sun KC, Sahito IA, Noh JW, Yeo SY, Im JN et al (2016) Highly
    efficient and durable dye-sensitized solar cells based on a
    wet-laid PET membrane electrolyte. J Mater Chem A 4:458-465.

    CAS  Article  Google Scholar 

139. Padinger F, Rittberger RS, Sariciftci NS (2003) Effects of
    Postproduction Treatment on Plastic Solar Cells. Adv Funct Mater
    13(1):85-88.

    CAS  Article  Google Scholar 

140. Ravirajan P, Haque SA, Durrant JR, Poplavskyy D, Bradley DDC,
    Nelson J (2004) Hybrid nanocrystalline TiO2 solar cells with a
    fluorene-thiophene copolymer as a sensitizer and hole conductor.
    J Appl Phys 95:1473-1480.

    CAS  Article  Google Scholar 

141. Jeon S, Jo Y, Kim KJ, Jun Y, Han CH (2011) High Performance
    Dye-Sensitized Solar Cells with Alkylpyridinium Iodide Salts in
    Electrolytes. ACS Appl Mater Interfaces 3(2):512-516.

    CAS  Article  Google Scholar 

142. Lee RH, Liu JK, Ho JH, Chang JW, Liu BT, Wang HJ, Jeng RJ (2011)
    Synthesis of a Series of Quaternized Ammonium Iodide-Containing
    Conjugated Copolymer Electrolytes and Their Application in
    Dye-sensitized Solar Cells. Polym Int 60:483-492.

    CAS  Article  Google Scholar 

143. Nazeeruddin MK, Zakeeruddin SM, Baker RH, Jirousek M, Liska P
    (1999) Acid-Base Equilibria of (2,2'-Bipyridyl-4,4'-dicarboxylic
    acid)ruthenium(II) Complexes and the Effect of Protonation on
    Charge-Transfer Sensitization of Nanocrystalline Titania. Inorg
    Chem 38:6298-6305.

    CAS  Article  Google Scholar 

144. Nazeeruddin MK, Pechy P (1705-1706) Gratzel M; Efficient
    panchromatic sensitization of nanocrystallineTiO2 films by a
    black dye based on atrithiocyanato-ruthenium complex (1997) Chem
    Commun.

    Google Scholar 

145. Wang P, Klein C, Baker RH, Zakeeruddin SM, Gratzel M (2005) A
    High Molar Extinction Coefficient Sensitizer for Stable
    Dye-Sensitized Solar Cells. J Am Chem Soc 127:808-809.

    CAS  Article  Google Scholar 

146. Chen CY, Wang M, Li JY, Pootrakulchote N, Alibabaei L et al
    (2009) Highly Efficient Light-Harvesting Ruthenium Sensitizer for
    Thin-Film Dye-Sensitized Solar Cells. ACS Nano 3:3103-3109.

    CAS  Article  Google Scholar 

147. Klein C, Nazeeruddin MK, Liska P, Censo DD, Hirata N, Palomares
    E, Durrant JR, Gratzel M (2005) Engineering of a Novel Ruthenium
    Sensitizer and Its Application in Dye-Sensitized Solar Cells for
    Conversion of Sunlight into Electricity. Inorg Chem 44:178-180.

    CAS  Article  Google Scholar 

148. Tomkeviciene A, Puckyte G, Grazulevicius JV, Degbia M, Tran-Van
    F et al (2012) Diphenylamino-substituted derivatives of
    9-phenylcarbazole as glass-forming hole-transporting materials
    for solid state dye sensitized solar cells. Synth Met
    162:1997-2004.

    CAS  Article  Google Scholar 

149. Unger EL, Morandeira A, Persson M, Zietz B, Ripaud E et al
    (2011) Contribution from a hole-conducting dye to the
    photocurrent in solid-state dye-sensitized solar cells. Phys Chem
    Chem Phys 13:20172-20177.

    CAS  Article  Google Scholar 

150. Xie ZB, Midya A, Loh KP, Adams S, Blackwood DJ, Wang J, Zhang
    XJ, Chen ZK (2010) Highly Efficient Dye-Sensitized Solar Cells
    Using Phenothiazine Derivative Organic Dyes. Prog Photovolt
    18:573-581.

    CAS  Article  Google Scholar 

151. Gratzel M, cells D-s s (2003) J Photochem and Photobio C:
    Photochem Rev 4(2):145-153.

    CAS  Article  Google Scholar 

152. Takahashi Y, Arakawa H, Sugihara H et al (2000) Highly efficient
    polypyridyl-ruthenium II sensitizers with chelating oxygen donor
    ligands:b-diketonato-bis(dicarboxybipyridine) ruthenium. Inorg
    Chim Acta 310:169-174.

    Article  Google Scholar 

153. Anandan S, Latha S, Maruthamuthu P (2002) Syntheses of mixed
    ligands complexes of Ru(II) with 4,4'-dicarboxy-2,2'-bipyridine
    and substituted pteridinedione and the use of these complexes in
    electrochemical photovoltaic cells. J Photochem Photobiol A
    150:167-175.

    CAS  Article  Google Scholar 

154. Islam A, Sugihara H, Hara K, Singh LP, Katoh R et al
    Sensitization of nanocrystalline TiO2 film by ruthenium (II)
    diimine dithiolate complexes (2001) J. Photochem. Photobiol. A
    145:135-141.

    CAS  Article  Google Scholar 

155. Nazeeruddin MK, Angelis FD, Fantacci S, Selloni A, Viscardi G et
    al (2005) Combined Experimental and DFT-TDDFT Computational Study
    of Photoelectrochemical Cell Ruthenium Sensitizers. J Am Chem Soc
    127:16835-16847.

    CAS  Article  Google Scholar 

156. Nazeeruddin MK, Baker RH, Liska P, Gratzel M (2003)
    Investigation of Sensitizer Adsorption and the Influence of
    Protons on Current and Voltage of a Dye-Sensitized
    Nanocrystalline TiO2 Solar Cell. J Phys Chem B 107:8981-8987.

    CAS  Article  Google Scholar 

157. Nazeerudding MK, Pechy P, Renouard T, Zakeeruddin SM,
    Humphrey-Baker R, Comte P et al (2001) Engineering of Efficient
    Panchromatic Sensitizers for Nanocrystalline TiO2-Based Solar
    Cells. J Am Chem Soc 123:1613-1624.

    Article  CAS  Google Scholar 

158. Wang ZS, Yamaguchi T, Sugihara H, Arakawa H (2005) Significant
    Efficiency Improvement of the Black Dye-Sensitized Solar Cell
    through Protonation of TiO2 Films. Langmuir 21:4272-4276.

    CAS  Article  Google Scholar 

159. Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han L (2006)
    Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%. J
    Appl Phys 45:L638-L640.

    CAS  Article  Google Scholar 

160. Venkatraman V, Raju R, Oikonomopoulos SP, Alsberg BK (2018) The
    dye-sensitized solar cell database. J Cheminformatics 10:18.

    Article  Google Scholar 

161. El-Shazly AN, Shalan AE, Rashad MM, Abdel-Aal EA, Ibrahima IA,
    El-Shahat MF (2018) Solid-state dye-sensitized solar cells based
    on Zn[1-x]Sn[x]O nanocomposite photoanodes. RSC Adv
    8:24059-24067.

    CAS  Article  Google Scholar 

162. Gratzel M (2009) Acc. Chem. Res 42:1788 the 12.3% conversion
    efficiency was presented by Professor Gratzel in "International
    Symposium on Innovative Solar Cells 2009" (The University of
    Tokyo, Japan, 2nd-3rd, March, 2009).

163. Al-Rawashdeh NAF, Albiss BA, Yousef MHI (2018) Graphene-Based
    Transparent Electrodes for Dye Sensitized Solar Cells. IOP Conf.
    Ser.: Mater. Sci. Eng 012019:305.

    Google Scholar 

164. Gratzel M (2004) Conversion of sunlight to electric power by
    nanocrystalline dye-sensitized solar cells. J Photochem Photobiol
    A 164:3-14.

    CAS  Article  Google Scholar 

165. Nazeeruddin MK, Wang Q, Cevey L, Aranyos V, Liska P et al (2006)
    DFT-INDO/S Modeling of New High Molar Extinction Coefficient
    Charge-Transfer Sensitizers for Solar Cell Applications.
    InorgChem 45:787-797.

    CAS  Google Scholar 

166. Wang P, Zakeeruddin SM, Moser JE, Baker RH, Comte P, Aranyos V
    et al (2004) Stable New Sensitizer with Improved Light Harvesting
    for Nanocrystalline Dye-Sensitized Solar Cells. Adv Mater 16
    (20):1806-1811.

    CAS  Article  Google Scholar 

167. Nazeeruddin MK, Bessho T, Cevey L, Ito S, Klein C et al A high
    molar extinction coefficient charge transfer sensitizer and its
    application in dye-sensitized solar cell (2007) J. Photochem.
    Photobiol. A 185:331-337.

    CAS  Article  Google Scholar 

168. Kuang D, Ito S, Wenger B, Klein C, Moser JE et al High Molar
    Extinction Coefficient Heteroleptic Ruthenium Complexes for Thin
    Film Dye-Sensitized Solar Cells (2006) J. Am. chem. Soc
    128:4146-4154.

    CAS  Article  Google Scholar 

169. Nazeeruddin M, Klein C, Liska P, Gratzel M (2005) Synthesis of
    novel ruthenium sensitizers and their application in
    dye-sensitized solar cells. Coord Chem Rev 249:1460-1467.

    CAS  Article  Google Scholar 

170. Jiang KJ, Masaki N, Xia JB, Noda S, Yanagida S (2006) A novel
    ruthenium sensitizer with a hydrophobic
    2-thiophen-2-yl-vinyl-conjugated bipyridyl ligand for effective
    dye sensitized TiO2 solar cells. Chem Commun 23:2460-2462.

    Article  CAS  Google Scholar 

171. Chen CY, Wu SJ, Wu CG, Chen JG, Ho KC (2006) A ruthenium complex
    with superhigh light-harvesting capacity for dye-sensitized solar
    cells. Angew Chem Int Ed 45:5822-5825.

    CAS  Article  Google Scholar 

172. Zakeeruddin SM, Nazeeruddin MK, Baker RH, Pechy P, Quagliotto P
    et al (2002) Design, Synthesis, and Application of Amphiphilic
    Ruthenium Polypyridyl Photosensitizers in Solar Cells Based on
    Nanocrystalline TiO2 Films. Langmuir 18:952-954.

    CAS  Article  Google Scholar 

173. Wang P, Zakeeruddin SM, Moser JE, Nazeeruddin MK, Sekiguchi T,
    Gratzel M (2003) A stable quasi-solid-state dye-sensitized solar
    cell with an amphiphilic ruthenium sensitizer and polymer gel
    electrolyte. Nat Mat 2:402-407.

    CAS  Article  Google Scholar 

174. Galoppini E (2004) Linkers for anchoring sensitizers to
    semiconductor nanoparticles. Coord Chem Rev 248:1283-1297.

    CAS  Article  Google Scholar 

175. Wang P, Klein C, Moser JE, Humphry-Baker R, Cevey-Ha NL et al
    (2004) Amphiphilic Ruthenium Sensitizer with 4,4'-Diphosphonic
    Acid-2,2'-bipyridine as Anchoring Ligand for Nanocrystalline Dye
    Sensitized Solar Cells. J Phys Chem B 108:17553-17559.

    CAS  Article  Google Scholar 

176. Ford WE, Rodgers MAJ (1994) Interfacial electron transfer in
    colloidal SnO2 hydrosols photosensitized by electrostatically and
    covalently attached Ru(II) polypyridine complexes. J Phys Chem B
    98:3822.

    CAS  Article  Google Scholar 

177. Chauhan R, Trivedi M, Bahadur L, Kumar A (2011) Application of
    p-Extended Ferrocene with Varied Anchoring Groups as
    Photosensitizers in TiO2-Based Dye-Sensitized Solar Cells
    (DSSCs). Chem Asian J 6:1525-1532.

    CAS  Article  Google Scholar 

178. Chen WC, Nachimuthu S, Jiang JC (2017) Revealing the influence
    of Cyano in Anchoring Groups of Organic Dyes on Adsorption
    Stability and Photovoltaic Properties for Dye-Sensitized Solar
    Cells. Sci Rep 7:4979.

    Article  CAS  Google Scholar 

179. Hagfeldt A, Gratzel M (2000) Molecular Photovoltaics. Acc Chem
    Res 33:269-277.

    CAS  Article  Google Scholar 

180. Aghazada S, Nazeeruddin MK (2018) Ruthenium Complexes as
    Sensitizers in Dye-Sensitized Solar Cells. Inorganics 6:52.

    Article  Google Scholar 

181. Ito S, Zakeeruddin SM, Humphry-Baker R, Liska P, Charvet R et al
    (2006) High-Efficiency Organic-Dye- Sensitized Solar Cells
    Controlled by Nanocrystalline-TiO2 Electrode Thickness. Adv Mater
    18:1202-1205.

    CAS  Article  Google Scholar 

182. Robertson N (2006) Optimizing dyes for dye-sensitized solar
    cells. Angew Chem Int Ed 45:2338-2345.

    CAS  Article  Google Scholar 

183. Qin P, Yang X, Chen R, Sun L (2007) Influence of p-Conjugation
    Units in Organic Dyes for Dye-Sensitized Solar Cells. J Phys Chem
    C 111:1853-1860.

    CAS  Article  Google Scholar 

184. Tian H, Yang X, Pan J, Chen R, Liu M, Zhang Q, Hagfeldt A, Sun L
    (2008) A Triphenylamine Dye Model for the Study of Intramolecular
    Energy Transfer and Charge Transfer in Dye-Sensitized Solar
    Cells. Adv Funct Mater 18:3461-3468.

    CAS  Article  Google Scholar 

185. Ninq Z, Zhang Q, Wu W, Pei H, Liu B, Tian H (2008) Starburst
    Triarylamine Based Dyes for Efficient Dye-Sensitized Solar Cells.
    J Org Chem 73:3791-3797.

    Article  CAS  Google Scholar 

186. Li Q, Lu L, Zhong C, Shi J, Huang Q, Jin X, Peng T, Qin J, Li Z
    (2009) New Indole-Based Metal-Free Organic Dyes for
    Dye-Sensitized Solar Cells. J Phys Chem B 113:14588-14595.

    CAS  Article  Google Scholar 

187. Ooyama Y, Harima Y (2009) Molecular Designs and Syntheses of
    Organic Dyes for Dye-Sensitized Solar Cells. Eur J Org
    Chem:2903-2934.

    Article  CAS  Google Scholar 

188. Fischer MKR, Wenger S, Wang M, Mishra A, Zakeeruddin SM, Gratzel
    M, Bauerle P (2010) D-p-A Sensitizers for Dye-Sensitized Solar
    Cells: Linear vs Branched Oligothiophenes. Chem Mater
    22:1836-1845.

    CAS  Article  Google Scholar 

189. Mishra A, Fischer MK, Bauerle P (2009) Metal-free organic dyes
    for dye-sensitized solar cells: from structure: property
    relationships to design rules. Angew Chem Int Ed 48:2474-2499.

    CAS  Article  Google Scholar 

190. Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M et al (2010)
    Efficient Dye-Sensitized Solar Cells with an Organic
    Photosensitizer Featuring Orderly Conjugated
    Ethylenedioxythiophene and Dithienosilole Blocks. Chem Mater
    22:1915-1925.

    CAS  Article  Google Scholar 

191. Fuse S, Sugiyama S, Maitani MM, Wada Y, Ogomi Y, Hayase S, Katoh
    R et al (2014) Elucidating the Structure-Property Relationships
    of Donor-p-Acceptor Dyes for Dye-Sensitized Solar Cells (DSSCs)
    through Rapid Library Synthesis by a One-Pot Procedure. Chem - A
    Eu J, Special Issue: European Young Chemists 20(34):10685-10694.

    CAS  Article  Google Scholar 

192. Wang C, Li J, Cai SY, Ning Z, Zhao D, Zhang Q, Su JH (2012)
    Performance improvement of dye-sensitizing solar cell by
    semi-rigid triarylamine-based donors. Dyes Pigments 94:40-48.

    CAS  Article  Google Scholar 

193. Chen R, Yanga X, Tiana H, Sun L (2007) Tetrahydroquinoline dyes
    with different spacers for organic dye-sensitized solar cells. J
    Photochem Photobiol A: Chemistry 189:295-300.

    CAS  Article  Google Scholar 

194. Chen R, Yang X, Tian H, Wang X, Hagfeldt A, Sun L (2007) Effect
    of Tetrahydroquinoline Dyes Structure on the Performance of
    Organic Dye-Sensitized Solar Cells. Chem Mater 19:4007-4015.

    CAS  Article  Google Scholar 

195. Campbell WM, Jolley KW, Wagner P, Wagner K, Walsh PJ et al
    (2007) Highly Efficient Porphyrin Sensitizers for Dye-Sensitized
    Solar Cells. J Phys Chem C 111:11760-11762.

    CAS  Article  Google Scholar 

196. Thomas KRJ, Lin JT, Hsuc YC, Ho KC (2005) Organic dyes
    containing thienylfluorene conjugation for solar cells. Chem
    Commun:4098-4100.

197. Tian H, Yang X, Chen R, Zhang R, Hagfeldt A, Sun L (2008) Effect
    of Different Dye Baths and Dye-Structures on the Performance of
    Dye-Sensitized Solar Cells Based on Triphenylamine Dyes. J Phys
    Chem C 112:11023-11033.

    CAS  Article  Google Scholar 

198. Hara K, Sayama K, Ohga Y, Shinpo A, Sugab S, Arakawa H (2001) A
    coumarin-derivative dye sensitized nanocrystalline TiO2 solar
    cell having a high solar-energy conversion efficiency up to 5.6%.
    Chem Commun:569-570.

199. Wang ZS, Cui Y, Hara K, Dan-oh Y, Kasada C, Shinpo A (2007) A
    High-Light-Harvesting-Efficiency Coumarin Dye for Stable
    Dye-Sensitized Solar Cells. Adv Mater 19:1138-1141.

    CAS  Article  Google Scholar 

200. Horiuchi T, Miura H, Sumioka K, Uchida S (2004) High Efficiency
    of Dye-Sensitized Solar Cells Based on Metal-Free Indoline Dyes.
    J Am Chem Soc 126:12218-12219.

    CAS  Article  Google Scholar 

201. Liu B, Zhu W, Zhang Q, Wu W, Xu M, Ning Z, Xiea Y, Tian H;
    Conveniently synthesized isophoronedyes for high efficiency
    dye-sensitized solar cells: tuning photovoltaic performance by
    structural modification of donor group in donor-p-acceptor system
    (2009) Chem Commun 1766-1768.

202. Choi H, Baik C, Kang SO, Ko J, Kang MS, Nazeeruddin MK, Gratzel
    M (2008) Highly efficient and thermally stable organic
    sensitizers for solvent-free dye-sensitized solar cells. Angew
    Chem Int Ed 47:327-330.

    CAS  Article  Google Scholar 

203. Wang ZS, Koumura N, Cui Y, Takahashi M, Sekiguchi H et al (2008)
    Hexylthiophene-Functionalized Carbazole Dyes for Efficient
    Molecular Photovoltaics: Tuning of Solar-Cell Performance by
    Structural Modification. Chem Mater 20:3993-4003.

    CAS  Article  Google Scholar 

204. Kim MJ, Yu YJ, Kim JH, Jung YS, Kay KY, Ko SB, Lee CR, Jang IH,
    Kwon YU, Park NG (2012) Tuning of spacer groups in organic dyes
    for efficient inhibition of charge recombination in
    dye-sensitized solar cells. Dyes Pigments 95:134-141.

    Article  CAS  Google Scholar 

205. Kim SH, Kim HW, Sakong C, Namgoong J, Park SW et al (2011)
    Effect of Five-Membered Heteroaromatic Linkers to the Performance
    of Phenothiazine-Based Dye-Sensitized Solar Cells. Org Lett
    13:5784-5787.

    CAS  Article  Google Scholar 

206. Tian H, Yang X, Cong J, Chen R, Liu J, Hao Y, Hagfeldt A, Sun L
    (2009) Tuning of phenoxazine chromophores for efficient organic
    dye-sensitized solar cells. Chem Commun:6288-6290.

207. Stathatos E, Lianos P (2001) Synthesis of a Hemicyanine Dye
    Bearing Two Carboxylic Groups and Its Use as a Photosensitizer in
    Dye-Sensitized Photoelectrochemical Cells. Chem Mater
    13:3888-3892.

    CAS  Article  Google Scholar 

208. Sayama K, Tsukagoshi K, Mori T, Hara K, Ohga Y, Shinpou A, Abe
    Y, Suga S, Arakawa H (2003) Efficient sensitization of
    nanocrystalline TiO2 films with cyanine and merocyanine organic
    dyes. Sol Energy Mater Sol Cells 80:47-71.

    CAS  Article  Google Scholar 

209. Rocca D, Gebauer R, De Angelis F, Nazeeruddin MK, Baroni S
    (2009) Time-dependent density functional theory study of
    squaraine dye-sensitized solar cells. Chem Phys Lett 475:49-53.

    CAS  Article  Google Scholar 

210. Li C, Yum JH, Moon SJ, Herrmann A, Eickemeyer F, Pschirer NG Erk
    P. Schoeneboom J et al; An Improved Perylene Sensitizer for Solar
    Cell Applications (2008) Chem SusChem 1:615-618.

211. Li C, Yang X, Chen R, Pan J, Tian H, Zhu H, Wang X, Hagfeldt A,
    Sun L (2007) Anthraquinone dyes as photosensitizers for
    dye-sensitized solar cells. Sol Energy Mater Sol Cells
    91:1863-1871.

    CAS  Article  Google Scholar 

212. Kumaresan D, Thummel RP, Bura T, Ulrich G, Ziessel R (2009)
    Color Tuning in New Metal-Free Organic Sensitizers (Bodipys) for
    Dye-Sensitized Solar Cells. Chem Eur J 15:6335-6339.

    CAS  Article  Google Scholar 

213. Tanaka K, Takimiya K, Otsubo T, Kawabuchi K, Kajihara S, Harima
    Y (2006) Development and Photovoltaic Performance of
    Oligothiophene-sensitized TiO2 Solar Cells. Chem Lett 35:592-593.

    CAS  Article  Google Scholar 

214. Ohshita J, Matsukawa J, Hara M, Kunai A, Kajiwara S, Ooyama Y,
    Harima Y, Kakimoto M (2008) Attachment of
    Disilanylene-Oligothienylene Polymers on TiO2 Surface by
    Photochemical Cleavage of the Si-Si Bonds. Chem Lett 37:316-317.

    CAS  Article  Google Scholar 

215. Jia X, Zhang W, Lu X, Wang ZS, Zhou G (2014) Efficient
    quasi-solid-state dye-sensitized solar cells based on organic
    sensitizers containing fluorinated quinoxaline moiety. J Mater
    Chem 2:19515-19525.

    CAS  Article  Google Scholar 

216. Xu M, Wenger S, Bala H, Shi D, Li R, Zhou Y, Zakeeruddin SM,
    Gratzel M, Wang P (2009) Tuning the Energy Level of Organic
    Sensitizers for High-Performance Dye-Sensitized Solar Cells. J
    Phys Chem C 113:2966-2973.

    CAS  Article  Google Scholar 

217. Kim S, Lee JK, Kang SO, Ko J, Yum JH, Fantacci S, De Angelis F,
    Censo DD, Nazeeruddin MK, Gratzel M (2006) Molecular Engineering
    of Organic Sensitizers for Solar Cell Applications. J Am Chem Soc
    128:16701-16707.

    CAS  Article  Google Scholar 

218. Hagberg DP, Marinado T, Karlsson KM, Nonomura K, Qin P, Boschloo
    G, Brinck T, Hagfeldt A, Sun L (2007) Tuning the HOMO and LUMO
    energy levels of organic chromophores for dye sensitized solar
    cells. J Org Chem 72:9550-9556.

    CAS  Article  Google Scholar 

219. Yu X, Ci Z, Liu T, Feng X, Wang C, Ma T, Bao M (2014) Influence
    of different electron acceptors in organic sensitizers on the
    performance of dye-sensitized solar cells. Dyes Pigments
    102:126-132.

    CAS  Article  Google Scholar 

220. Xu M, Li R, Pootrakulchote N, Shi D, Guo J, Yi Z, Zakeeruddin
    SM, Gratzel M, Wang P (2008) Energy-Level and Molecular
    Engineering of Organic D-p-A Sensitizers in Dye-Sensitized Solar
    Cells. J Phys Chem C 112:19770-19776.

    CAS  Article  Google Scholar 

221. Marinado T, Hagberg DP, Hedlund M, Edvinsson T, Johansson EMJ,
    Boschloo G, Rensmo H, Brinck T, Sun L, Hagfeldt A (2009)
    Rhodanine dyes for dye-sensitized solar cells : spectroscopy,
    energy levels and photovoltaic performance. Phys Chem Chem Phys
    11:133-141.

    CAS  Article  Google Scholar 

222. Liang M, Xu W, Cai F, Chen P, Peng B, Chen J, Li Z (2007) New
    Triphenylamine-Based Organic Dyes for Efficient Dye-Sensitized
    Solar Cells. J Phys Chem C 111:4465-4472.

    CAS  Article  Google Scholar 

223. Rehm JM, McLendon GL, Nagasawa Y, Yoshihara K, Moser J, Gratzel
    M (1996) Femtosecond Electron-Transfer Dynamics at a Sensitizing
    Dye-Semiconductor (TiO2) Interface. J Phys Chem B 100:9577-9578.

    Article  Google Scholar 

224. Pastore M and De Angelis F; Aggregation of Organic Dyes on TiO2
    in Dye-Sensitized Solar Cells Models: An ab Initio Investigation.
    (2010) ACS Nano 4:556-562.

    Article  CAS  Google Scholar 

225. Giribabu L, Singh VK, KumarCV SY, Reddy VG, Reddy PY (2011)
    Organic-Ruthenium(II) Polypyridyl Complex Based Sensitizer for
    Dye-Sensitized Solar Cell Applications. Advances in
    optoelectronics Article ID 294353:8.

    Google Scholar 

226. Horiuchi T, Miuraa H, Uchida S (2003) Highly-efficient
    metal-free organic dyes for dye-sensitized solar cells. Chemistry
    Communications 24:3036-3037.

    Article  CAS  Google Scholar 

227. Ito S, Miura H, Uchida S, Takata M, Sumioka K, Liska P, Comte P,
    Pechyb P, Gratzel M (2008) High-conversion-efficiency organic
    dye-sensitized solar cells with a novel indoline dye. Chem
    Commun:5194-5196.

228. Wu Y, Marszalek M, Zakeeruddin SM, Zhang Q, Tian H, Gratzel M,
    Zhu W (2012) High-conversion-efficiency organic dye-sensitized
    solar cells: molecular engineering on D-A-p-A featured organic
    indoline dyes. Energy Environ Sci 5:8261-8272.

    CAS  Article  Google Scholar 

229. Suzuka M, Hayashi N, Sekiguchi T, Sumioka K, Takata M, Hayo N,
    Ikeda H, Oyaizu K, Nishide H (2016) A Quasi-Solid State DSSC with
    10.1% Efficiency through Molecular Design of the
    Charge-Separation and -Transport. Sci Rep 6:28022.

    CAS  Article  Google Scholar 

230. Irgashev RA, Karmatsky AA, Kim GA, Sadovnikov AA, Emets VV,
    Grinberg VA et al (2017) Novel push-pull thieno[2,3-b]
    indole-based dyes for efficient dye-sensitized solar cells
    (DSSCs). Arkivoc part iv:34-50.

    Article  Google Scholar 

231. Tachibana Y, Haque SA, Mercer IP, Durrant JR, Klug DR (2000)
    Electron Injection and Recombination in Dye Sensitized
    Nanocrystalline Titanium Dioxide Films: A Comparison of Ruthenium
    Bipyridyl and Porphyrin Sensitizer Dyes. J Phys Chem B
    104:1198-1205.

    CAS  Article  Google Scholar 

232. Wang Q, Campbell WM, Bonfantani EE, Jolley KW, Officer DL, Walsh
    PJ, Gordon K, Humphry-Baker R, Nazeeruddin MK, Gratzel M (2005)
    Efficient Light Harvesting by Using Green Zn-Porphyrin-Sensitized
    Nanocrystalline TiO2 Films. J Phys Chem B 109:15397-15409.

    CAS  Article  Google Scholar 

233. Kalyanasundaram K, Vlachopoulos N, Krishnan V, Monnier A,
    Gratzel M (1987) Sensitization of titanium dioxide in the visible
    light region using zinc porphyrins. J Phys Chem B 91:2342-2347.

    CAS  Article  Google Scholar 

234. Krishna NV, Krishna JVS, Singh SP, Giribabu L, Islam A, Bedja I
    (2017) Bulky Nature Phenanthroimidazole-Based Porphyrin
    Sensitizers for Dye-Sensitized Solar Cell Applications. J Phys
    Chem C 121:25691-25704.

    Article  CAS  Google Scholar 

235. Wang CL, Chang YC, Lan CM, Lo CF, Diau EWG, Lin CY (2011)
    Enhanced light harvesting with p-conjugated cyclic aromatic
    hydrocarbons for porphyrin-sensitized solar cells. Energy Environ
    Sci 4:1788-1795.

    CAS  Article  Google Scholar 

236. Yella A, Lee HW, Tsao HN, Yi C, Chandiran AK, Nazeeruddin MK,
    Diau EW, Yeh CY, Zakeeruddin SM, Gratzel M (2011)
    Porphyrin-sensitized solar cells with cobalt (II/III)-based redox
    electrolyte exceed 12 percent efficiency. Science 334:629-634.

    CAS  Article  Google Scholar 

237. Giovannetti R, Zannotti M, Alibabaei L, Ferraro S; Equilibrium
    and Kinetic Aspects in the Sensitization of Monolayer Transparent
    TiO2 Thin Films with Porphyrin Dyes for DSSC Applications (2014)
    Int J Photoenergy Article 834269:9 pages.

238. Liang M, Lu M, Wang QL, Chen WY, Han HY, Sun Z, Xue S (2011)
    Efficient Dye-Sensitized Solar Cells with Triarylamine Organic
    Dyes Featuring Functionalized-Truxene Unit. J Power Sources
    196:1657-1664.

    CAS  Article  Google Scholar 

239. Liu WH, Wu IC, Lai CH, Lai CH, Chou PT, Li YT, Chen CL, Hsu YY,
    Chi Y (2008) Simple organic molecules bearing a
    3,4-ethylenedioxythiophene linker for efficient dye-sensitized
    solar cells. Chem Commun 7:5152-5154.

    Article  CAS  Google Scholar 

240. Prachumrak N, Sudyoadsuk T, Thangthong A, Nalaoh P,
    Jungsuttiwong S, Daengngern R, Namuangruk S, Pattanasattayavonga
    P, Promarak V (2017) Improvement of D-p-A organic dye-based
    dye-sensitized solar cell performance by simple triphenylamine
    donor substitutions on the p-linker of the dye. Mater Chem Front
    1:1059-1072.

    CAS  Article  Google Scholar 

241. Hagberg DP, Edvinsson T, Marinado T, Boschloo G, Hagfeldt A, Sun
    L (2006) A novel organic chromophore for dye-sensitized
    nanostructured solar cells. Chem Commun:2245-2247.

242. Baheti A, Tyagi P, Justin Thomas KR, Hsu YC, T'suen Lin J (2009)
    Simple Triarylamine-Based Dyes Containing Fluorene and Biphenyl
    Linkers for Efficient Dye-Sensitized Solar Cells. J Phys Chem C
    113(20):8541-8547.

    CAS  Article  Google Scholar 

243. Lu M, Liang M, Han HY, Sun Z, Xue S (2011) Organic Dyes
    Incorporating Bis-hexapropyltruxeneamino Moiety for Efficient
    Dye-Sensitized Solar Cells. J Phys Chem C 115:274-281.

    CAS  Article  Google Scholar 

244. Lin JT, Chen PC, Yen YS, Hsu YC, Chou HH, Yeh MCP (2008) Organic
    Dyes Containing Furan Moiety for High-Performance Dye-Sensitized
    Solar Cells. Org Lett 11:97-100.

    Article  CAS  Google Scholar 

245. Simon SJC, FGL Parlane, WB Swords, CW Kellett, C Du et al;
    Halogen Bonding Promotes Higher Dye-Sensitized Solar Cell
    Photovoltages (2016) J. Am. Chem. Soc138:10406-10409.

    CAS  Article  Google Scholar 

246. Tsao HN, Burschka J, Yi C, Kessler F, Nazeeruddina MK, Gratzel M
    (2011) Influence of the interfacial charge-transfer resistance at
    the counter electrode in dye-sensitized solar cells employing
    cobalt redox shuttles. Energy Environ Sci 4:4921-4924.

    CAS  Article  Google Scholar 

247. Manfredi N, Trifiletti V, Melchiorre F, Giannotta G, Biagini P,
    Abbotto A (2018) Performance enhancement of a dye-sensitized
    solar cell by peripheral aromatic and heteroaromatic
    functionalization in di-branched organic sensitizers. New J Chem
    42:9281-9290.

    CAS  Article  Google Scholar 

248. Qiu XP, Zhou HP, Zhang XF, Xu TH, Liu XL, Zhao YY, Lu R (2008)
    Synthesis of phenothiazine-functionalized porphyrins with high
    fluorescent quantum yields. Tetrahedron Lett 49:7446-7449.

    CAS  Article  Google Scholar 

249. Tian H, Yang X, Cong J, Chen R, Teng C, Liu J, Hao Y, Wang L,
    Sun L (2010) Effect of different electron donating groups on the
    performance of dye-sensitized solar cells. Dyes Pigments
    84:62-68.

    CAS  Article  Google Scholar 

250. Zhu XQ, Dai Z, Yu A, Wu S, Cheng JP (2008) Driving Forces for
    the Mutual Conversions between Phenothiazines and Their Various
    Reaction Intermediates in Acetonitrile. J Phys Chem B
    112:11694-11707.

    CAS  Article  Google Scholar 

251. Tian H, Yang X, Chen R, Pan Y, Li L, Hagfeldt A, Sun L (2007)
    Phenothiazine derivatives for efficient organic dye-sensitized
    solar cells. J Chem Commun:3741-3743.

252. Wu W, Yang J, Hua J, Tang J, Zhang L, Longa Y, Tian H (2010)
    Efficient and stable dye-sensitized solar cells based on
    phenothiazine sensitizers with thiophene units. J Mater Chem
    20:1772.

    CAS  Article  Google Scholar 

253. Boldrini CL, Manfredi N, Perna FM, Trifiletti V, Capriati V,
    Abbotto A (2017) Dye-Sensitized Solar Cells that use an Aqueous
    Choline Chloride-Based Deep Eutectic Solvent as Effective
    Electrolyte Solution. 5:345-353.

254. Wagner J, Pielichowski J, Hinsch A, Pielichowski K, Bogdal D,
    Pajda M, Kurek SS, Burczyk A (2004) New carbazole-based polymers
    for dye solar cells with hole-conducting polymer. Syn Metals
    146:159-165.

    CAS  Article  Google Scholar 

255. Li J, Dierschke F, Wu J, Grimsdalea AC, Mullen K (2006) Poly
    (2,7-carbazole) and perylene tetracarboxydiimide: a promising
    donor/acceptor pair for polymer solar cells. J Mater Chem
    16:96-100.

    CAS  Article  Google Scholar 

256. Promarak V, Saengsuwan S, Jungsuttiwong S, Sudyoadsuk T, Keawin
    T (2007) Synthesis and characterization of N-carbazole end-capped
    oligofluorenes. Tetrahedron Lett 48:89-93.

    CAS  Article  Google Scholar 

257. Hwang SW, Chen Y (2002) Photoluminescent and Electrochemical
    Properties of Novel Poly(aryl ether)s with Isolated
    Hole-Transporting Carbazole and Electron-Transporting
    1,3,4-Oxadiazole Fluorophores. Macromolecules 35:5438-5443.

    CAS  Article  Google Scholar 

258. Koumura N, Wang ZS, Miyashita M, Uemura Y, Sekiguchi H, Cui Y,
    Mori A, Mori S, Hara K (2009) Substituted carbazole dyes for
    efficient molecular photovoltaics: long electron lifetime and
    high open circuit voltage performance. J Mater Chem 19:4829-4836.

    CAS  Article  Google Scholar 

259. Liu B, Wang R, Mi W, Lia X, Yu H (2012) Novel branched coumarin
    dyes for dye-sensitized solar cells: significant improvement in
    photovoltaic performance by simple structure modification. J
    Mater Chem 22:15379-15387.

    CAS  Article  Google Scholar 

260. YeM WX, Wang M, Iocozzia J, Zhang N, Lin C, Lin Z (2015) Recent
    advances in dye-sensitized solar cells: from photoanodes,
    sensitizers and electrolytes to counter electrodes. Mater Today
    18(3):155-162.

    Article  CAS  Google Scholar 

261. Karlsson KM, Jiang X, Eriksson SK, Gabrielsson E, Rensmo H,
    Hagfeldt A, Sun L (2011) Phenoxazine Dyes for Dye-Sensitized
    Solar Cells: Relationship Between Molecular Structure and
    Electron Lifetime. Chem Eur J 17:6415-6424.

    CAS  Article  Google Scholar 

262. Qu S, Wu W, Hua J, Kong C, Long Y, Tian H (2010) New
    Diketopyrrolopyrrole (DPP) Dyes for Efficient Dye-Sensitized
    Solar Cells. J Phys Chem C 114:1343-1349.

    CAS  Article  Google Scholar 

263. Singh SP, Roy MS, Justin Thomas KR, Balaiah S, Bhanuprakash K,
    Sharma GD (2012) New Triphenylamine-Based Organic Dyes with
    Different Numbers of Anchoring Groups for Dye-Sensitized Solar
    Cells. J Phys Chem C 116:5941-5950.

    CAS  Article  Google Scholar 

264. Alexander H, Thon SM, Hoogland S, Voznyy O, Zhitomirsky D,
    Debnath R, Levina L, Rollny LR et al (2012) Hybrid passivated
    colloidal quantum dot solids. Nat Nanotechnol 7:577-582.

    Article  CAS  Google Scholar 

265. Li T-L, Leea YL, Teng H (2012) High-performance quantum
    dot-sensitized solar cells based on sensitization with CuInS2
    quantum dots/CdS heterostructure. Energy Environ Sci 5:5315-5324.

    CAS  Article  Google Scholar 

266. Chuang CH, Brown PR, Bulovic V, Bawendi MG (2014) Improved
    performance and stability in quantum dot solar cells through band
    alignment engineering. Nat Mater 13:796-801.

    CAS  Article  Google Scholar 

267. Saad MA, Samsi NS, Hassan OH, Yahya MZA, Taib MFM, Ali AMM,
    Zakaria R; Studies of the absorbance peak on the N719 dye
    influence by combination between Cadmium Selenide (CdSe)QDs and
    Zinc Sulfide(ZnS)QDs (2018) 2nd International Conference on
    Engineering and Technology for Sustainable Development, ICET4SD
    2017 - Yogyakarta, Indonesia, MATEC Web of Conferences 154:01040.

268. Cherepy NJ, Smestad GP, Gratzel M, Zhang JZ (1997) Ultrafast
    Electron Injection: Implications for a Photoelectrochemical Cell
    Utilizing an Anthocyanin Dye-Sensitized TiO2 Nanocrystalline
    Electrode. J Phys Chem B 101:9342-9351.

    CAS  Article  Google Scholar 

269. Hao S, Wu J, Huang Y, Lin J (2006) Natural Dyes as
    Photosensitizers for Dye-Sensitized Solar Cell. Sol Energy
    80:209-214.

    CAS  Article  Google Scholar 

270. Sutthanut K, Sripanidkulchai B, Yenjai C, Jay M (2007)
    Simultaneous identification and quantitation of 11 flavonoid
    constituents in Kaempferia parviflora by gas chromatography. J
    Chromatogr A 1143:227-233.

    CAS  Article  Google Scholar 

271. Kishiomoto S, Maoka T, Sumitomo K, Ohmiya A (2005) Analysis of
    Carotenoid Composition in Petals of Calendula (Calendula
    officinalis L.). Biosci Biotechnol Biochem 69:2122-2128.

    Article  Google Scholar 

272. Ahliha AH, Nurosyid F, Supriyanto A, Kusumaningsih T (2018)
    Optical properties of anthocyanin dyes on TiO2 as
    photosensitizers for application of dye-sensitized solar cell
    (DSSC). IOP Conf Series: Materials Science and Engineering
    012018:333.

    Google Scholar 

273. Narayan MR (2012) Review: Dye sensitized solar cells based on
    natural photosensitizers. Renew Sust Energ Rev 16:208-215.

    CAS  Google Scholar 

274. Narayan M, Raturi A (2011) Investigation of some common Fijian
    flower dyes as photosensi-tizers for dye sensitized solar
    cellsabstract. Appl Sol Energy 47:112.

    Article  Google Scholar 

275. Ludine NA, Al-Alwani Mahmoud AM, Mohamad AB, Kadhum AAH, Sopian
    K, Karim NSA (2014) Review on the development of natural dye
    photosensitizer for dye-sensitized solar cells. Renew Sust Energ
    Rev 31:386-396.

    Article  CAS  Google Scholar 

276. Suryana R (2013) Khoiruddin, Supriyanto A; Beta-Carotene Dye of
    Daucus carota as Sensitizer on Dye-Sensitized Solar Cell. Mater
    Sci Forum 737:15-19.

    Article  CAS  Google Scholar 

277. Ruiz-Anchondo T, Glossman-Mitnik D (2009) Computational
    Characterization of the b b, -Carotene Molecule. J Mol Struct
    THEOCHEM 913:215-220.

    CAS  Article  Google Scholar 

278. Eli D, Ahmad M, Maxwell I, Ezra D, Francis A, Sarki S, Roselle
    D-s s c u n d e f (2016) (Hibiscus sabdariffa) flowers and pawpaw
    (Carica papaya) leaves as sensitizers. J Energy Nat Resour 5
    (1):11-15.

    Article  Google Scholar 

279. Haryanto DA, Landuma S, Purwanto A (2014) Fabrication of
    dye-sensitized solar cell (DSSC) using annato seeds (Bixa
    orellana Linn). AIP Conf Proc 1586:104.

    CAS  Article  Google Scholar 

280. Hemalatha KV, Karthick SN, Raj CJ, Hong NY, Kim SK, Kim HJ
    (2012) Performance of Kerria japonica and Rosa chinensis flower
    dyes as sensitizers for dye-sensitized solar cells. Spectrochim
    Acta A Mol Biomol Spectrosc 96:305-309.

    CAS  Article  Google Scholar 

281. Sawhney N, Raghav A, Satapathi S (2017) Utilization of Naturally
    Occurring Dyes as Sensitizers in Dye Sensitized Solar Cells. IEEE
    Journal of Photovoltaics 7(2):539-544.

    Article  Google Scholar 

282. Boyo AO, Abdulsalami IO, Oluwa T, Oluwole SO, Umar A (2013)
    Development of Dye Sensitized Solar Cells Using Botuje Green
    Leaves (Jathopha Curcas Linn). Sci J Phys Article ID sjp 182:4.

    Google Scholar 

283. Muhammed Thanzeel MA, Paulose P, George G, John J (2016)
    Dye-sensitized solar cell using natural dyes extracted from
    Bougainvillea and bottle Brush flower. Int J Eng Trends Technol
    (IJETT) 33(1).

284. Munawaroh H, Fadillah G, Saputri LNMZ, Hanif QA, Hidayat R,
    Wahyuningsih S (2016) The co-pigmentation of anthocyanin isolated
    from mangosteen pericarp (Garcinia Mangostana L.) as Natural Dye
    for DyeSensitized Solar Cells (DSSC). IOP Conf. Ser.: Mater. Sci.
    Eng 107:012061.

    Article  Google Scholar 

285. Argazzi R, Larramona G, Contado C, Bignozzi CA (2004)
    Preparation and Photoelectrochemical Characterization of a Red
    Sensitive Osmium Complex Containing
    4,4',4''-Tricarboxy-2,2':6',2''-Terpyridine and Cyanide Ligands. J
    Photochem Photobiol A 164:15-21.

    CAS  Article  Google Scholar 

286. Geary EAM, Yellowlees LJ, Jack LA, Oswald IDH, Parsons S, Hirata
    N, Durrant JR, Robertson N (2005) Synthesis, structure, and
    properties of [Pt(II)(diimine)(dithiolate)] dyes with 3,3'-,
    4,4'-, and 5,5'-disubstituted bipyridyl: applications in
    dye-sensitized solar cells. Inorg Chem 44:242-250.

    CAS  Article  Google Scholar 

287. Ferrere S (2002) New photosensitizers based upon [Fe^II(L)[2]
    (CN)[2]] and [Fe^IIL[3]], where L is substituted 2,2'-bipyridine.
    Inorg Chim Acta 329:79-92.

    CAS  Article  Google Scholar 

288. Kuang D, Klein C, Snaith HJ, Moser JE, Humphry-Baker R, Comte P,
    Zakeeruddin SM, Gratzel M (2006) Ion Coordinating Sensitizer for
    High Efficiency Mesoscopic Dye-Sensitized Solar Cells: Influence
    of Lithium Ions on the Photovoltaic Performance of Liquid and
    Solid-State Cells. Nano Lett 6(4):769-773.

    CAS  Article  Google Scholar 

289. Kuang D, Klein C, Ito S, Moser JE, Humphry-Baker R, Zakeeruddin
    SM, Gratzel M (2007) High molar extinction coefficient
    ion-coordinating ruthenium sensitizer for efficient and stable
    mesoscopic dye-sensitized solar cells. Adv Funct Mater 17
    (1):154-160.

    CAS  Article  Google Scholar 

290. Kuang D, Klein C (2008) Snaith , Humphry-Baker R, Zakeeruddin
    SM, Gratzel M; A new ion-coordinating ruthenium sensitizer for
    dye sensitized solar cells. Inorg Chim Acta 361(3):699-706.

    CAS  Article  Google Scholar 

291. Jang SR, Lee C, Choi H, Ko JJ, Lee J, Vittal R, Kimal KJ (2006)
    Oligophenylenevinylene-Functionalized Ru(II)-bipyridine
    Sensitizers for Efficient Dye-Sensitized Nanocrystalline TiO2
    Solar Cells. Chem Mater 18(23):5604-5608.

    CAS  Article  Google Scholar 

292. Kuang D, Klein C, Ito S, Moser JE, Humphry- Baker R, Evans N,
    Duriaux F, Gratzel C, Zakeeruddin SM, Gratzel M (2007)
    High-Efficiency and Stable Mesoscopic Dye-Sensitized Solar Cells
    Based on a High Molar Extinction Coefficient Ruthenium Sensitizer
    and Nonvolatile Electrolyte. Adv Mater 19(8):1133-1137.

    CAS  Article  Google Scholar 

293. Chen CY, Wu SJ, Li JY, Wu CG, Chen JG, Ho KC (2007) A new route
    to enhance the light-harvesting capability of ruthenium complexes
    for dye sensitized solar cells. Adv Mater 19(22):3888-3891.

    CAS  Article  Google Scholar 

294. Song HK, Park YH, Han CH, Jee JG (2009) Synthesis of ruthenium
    complex and its application in dye-sensitized solar cells. J Ind
    Eng Chem 15(1):62-65.

    Article  CAS  Google Scholar 

295. Paek S, Baika C, Kang MS, Kang H, Koa J (2010) New type of
    ruthenium sensitizers with a triazole moiety as a bridging group.
    J Organomet Chem 695(6):821-826.

    CAS  Article  Google Scholar 

296. Chang WC, Chen HS, Li TY, Hsu NM, Tingare YS, Li CY, Liu YC, Su
    C, Li WR (2010) Highly Efficient N-Heterocyclic Carbene/
    Pyridine-Based Ruthenium Sensitizers: Complexes for
    Dye-Sensitized Solar Cells. Angew Chem 49(44):8161-8164.

    CAS  Article  Google Scholar 

297. Chen HS, Chang WC, Su C, Li TY, Hsu NM, Tingare YS, Li CY, Shieb
    JH, Li WR (2011) Carbene-based ruthenium photosensitizers. Dalton
    Trans 40(25):6765-6770.

    CAS  Article  Google Scholar 

298. Islam A, Singh SP, Yanagida M, Karim MR, Han L (2011)
    Amphiphilic Ruthenium(II) Terpyridine Sensitizers with Long Alkyl
    Chain Substituted b-Diketonato Ligands: An Efficient
    Coadsorbent-Free Dye-Sensitized Solar Cells. Int J Photoenergy
    Article ID 757421:7.

    Google Scholar 

299. Kisserwan H, Ghaddar TH (2010) Enhancement of photovoltaic
    performance of a novel dye, "T18", with ketene thioacetal groups
    as electron donors for high efficiency dye-sensitized solar
    cells. Inorg Chim Acta 363(11):2409-2415.

    CAS  Article  Google Scholar 

300. Hallett AJ, Jones JE (2011) Purification-free synthesis of a
    highly efficient ruthenium dye complex for dye-sensitised solar
    cells (DSSCs). Dalton Trans 40(15):3871-3876.

    CAS  Article  Google Scholar 

301. Yen YS, Chen YC, Hsu YC, Chou HH, Lin JT, Yin DJ (2011)
    Heteroleptic ruthenium sensitizers that contain an ancillary
    bipyridine ligand tethered with hydrocarbon chains for efficient
    dye-sensitized solar cells. Chemistry 17(24):6781-6788.

    CAS  Article  Google Scholar 

302. Bessho T, Yoneda E, Yum JH, Guglielmi M, Tavernelli I, Imai H,
    Rothlisberger U, Nazeeruddin MK, Gratzel M (2009) New paradigm in
    molecular engineering of sensitizers for solar cell applications.
    J Am Chem Soc 131(16):5930-5934.

    CAS  Article  Google Scholar 

303. Wu KL, Hsu HC, Chen K, Chi Y, Chung MW, Liu WH, Chou PT (2010)
    Development of thiocyanate-free, charge-neutral Ru(II)
    sensitizers for dye-sensitized solar cells. Chem Commun 46
    (28):5124-5126.

    CAS  Article  Google Scholar 

304. Margulis GY, Lim B, Hardin BE, Unger EL, Yum JH, Feckl JM,
    Fattakhova-Rohlfing D, Bein T et al (2013) Highly soluble energy
    relay dyes for dye-sensitized solar cells. Phys Chem Chem Phys
    15:11306-11312.

    CAS  Article  Google Scholar 

305. Forster T (1959) 10th Spiers Memorial Lecture. Transfer
    mechanisms of electronic excitation. Discuss Faraday Soc 27:7-17.

    Article  Google Scholar 

306. Siegers C, Hohl-Ebinger J, Zimmermann B, Wurfel U, Mulhaupt R,
    Hinsch A, Haag R (2007) A Dyadic Sensitizer for Dye Solar Cells
    with High Energy-Transfer Efficiency in the Device. ChemPhysChem
    8:1548-1556.

    CAS  Article  Google Scholar 

307. Lin Y-J, Chen JW, Hsiao PT, Tung YL, Chang CC, Chen CM (2017)
    Efficiency improvement of dye-sensitized solar cells by in situ
    fluorescence resonance energy transfer. J Mater Chem A
    5:9081-9089.

    CAS  Article  Google Scholar 

308. Pratiwi DD, Nurosyid F, Supriyanto A, Suryana R (2017)
    Efficiency enhancement of dye-sensitized solar cells (DSSC) by
    addition of synthetic dye into natural dye (anthocyanin). IOP
    Conf Series: Materials Science and Engineering Article ID
    012012:176.

    Google Scholar 

309. Chang H, Kao MJ, Chen TL, Chen CH, Cho KC, Lai XR (2013)
    Characterization of Natural Dye Extracted from Wormwood and
    Purple Cabbage for Dye-Sensitized Solar Cells. J Photoenergy
    Article ID 159502:8.

    Google Scholar 

310. Lim A, Manaf NH, Tennakoon K, Chandrakanthi RLN, Lim LBL, Sarath
    Bandara JMR, Ekanayake P (2015) Higher Performance of DSSC with
    Dyes from Cladophora sp. as Mixed Cosensitizer through
    Synergistic Effect. J Biophys Article ID 510467:8.

    Google Scholar 

311. Zhang L, Konno A (2018) Development of Flexible Dye-sensitized
    Solar Cell Based on Predyed Zinc Oxide Nanoparticle. Int J
    Electrochem Sci 13:344-352.

    CAS  Article  Google Scholar 

312. Gangishetty MK, Lee KE, Scott RWJ, Kelly TL (2013) Plasmonic
    Enhancement of Dye Sensitized Solar Cells in the
    Red-to-near-Infrared Region using Triangular Core-Shell Ag@SiO2
    Nanoparticles. ACS Appl Mater Interfaces 5:11044-11051.

    CAS  Article  Google Scholar 

313. Hossain MA, Park J, Yoo D, Baek YK, Kim Y, Kim SH, Lee D (2016)
    Surface Plasmonic Effects on Dye-Sensitized Solar Cells by
    SiO2-Encapsulated Ag Nanoparticles. Curr Appl Phys 16:397-403.

    Article  Google Scholar 

314. Ihara M, Kanno M, Inoue S (2010) Photoabsorption-Enhanced Dye
    Sensitized Solar Cell by Using Localized Surface Plasmon of
    Silver Nanoparticles Modified with Polymer. Phys E Low Dimens
    Syst Nanostruct 42:2867-2871.

    CAS  Article  Google Scholar 

315. Jun HK, Careemb MA, Arof AK (2016) Plasmonic effects of quantum
    size gold nanoparticles on dye-sensitized solar cell. Materials
    Today: Proceedings 3S:S73-S79.

    Article  Google Scholar 

316. Saravanan S, Kato R, Balamurgan M, Kaushik S, Soga T (2017)
    Efficiency improvement in dye sensitized solar cells by the
    plasmonic effect of green synthesized silver nanoparticles. J
    Sci: Advanced Materials and Devices 2:418-424.

    Google Scholar 

317. Sim YH, Yun MJ, Cha SI, Seo SH, Lee DY (2018) Improvement in
    Energy Conversion Efficiency by modification of Photon
    Distribution within the Photoanode of Dye-Sensitized Solar Cells.
    ACS Omega 3:698-705.

    CAS  Article  Google Scholar 

318. Zhang C, Liu S, Liu X, Deng F, Xiong Y, Tsai FC (2018)
    Incorporation of Mn2+ into CdSe quantum dots by chemical bath
    co-deposition method for photovoltaic enhancement of quantum
    dot-sensitized solar cells. R Soc Open Sci 171712:5.

    Google Scholar 

319. Surana K, Mehra RM, Bhattacharya B (2018) Quantum Dot Solar
    Cells with size tuned CdSe QDs exhibiting 1.51V. Materialstoday
    proceedings 5(3):9108-9113.

    Google Scholar 

320. Zhang P, Hu Z, Wang Y, Qin Y, Li W, Wang J (2016) A Bi-layer
    Composite Film Based on TiO2 Hollow Spheres, P25, and
    Multi-walled Carbon Nanotubes for Efficient Photoanode of
    Dye-sensitized Solar Cell. Nano-Micro Letter 8(3):232-239.

    Article  CAS  Google Scholar 

321. Aijo John K, Naduvath J, Mallick S, Pledger JW, Remillard SK, De
    Young PA et al (2016) Electrochemical Synthesis of Novel Zn-Doped
    TiO2 Nanotube/ZnO Nanoflake Heterostructure with Enhanced DSSC
    Efficiency. Nano-Micro Letter 8(4):381-387.

    Article  CAS  Google Scholar 

322. Zhang J, Yang X, Deng H, Qiao K, Farooq U, Ishaq M, Yi F, Liu H,
    Tang J, Song H (2017) Low-Dimensional Halide Perovskites and
    Their Advanced Optoelectronic Applications. Nano-Micro Letter
    9:36.

    Article  CAS  Google Scholar 

323. Chiang CC, Hung CY, Chou SW, Shyue JJ, Cheng KY, Chang PJ, Yang
    YY, Lin CY, Chang TK et al (2017) PtCoFe Nanowire Cathodes Boost
    Short-Circuit Currents of Ru(II)-Based Dye-Sensitized Solar Cells
    to a Power Conversion Efficiency of 12.29%. Adv Funct Mat 28
    (3):1703282.

    Article  CAS  Google Scholar 

324. Kunzmann A, Valero S, Sepulveda AE, Rico-Santacruz M, Lalinde E,
    Berenguer JR, Garcia-Martinez J et al (2018) Hybrid Dye-Titania
    Nanoparticles for Superior Low-Temperature Dye-Sensitized Solar
    Cells. Adv Energy Mater 8(12):1702583.

    Article  CAS  Google Scholar 

325. Baxter JB (2012) Commercialization of dye sensitized solar
    cells. J Vac Sc Technol A: Vaccum, Surafces, and Films 30
    (2):020801.

    Article  CAS  Google Scholar 

326. Desilvestro H, Bertoz M, Tulloch S, Tulloch GE (2010) Packaging,
    Scale-up, and Commercialization of Dye Solar Cells. In:
    Kalyanasundaram K (ed) Dye-sensitized solar cells. CRC Press,
    Lausanne.

    Google Scholar 

327. Berginc M, Krasovec UO, Topic M (2014) Outdoor ageing of the
    dye-sensitized solar cell under different operation regimes.
    Krasovec Sol Energy Mater Sol Cells 120:491-499.

    CAS  Article  Google Scholar 

328. Tong Z, Peng T, Sun W, Liu W, Guo S, Zhao XZ (2014) Introducing
    an Intermediate Band into Dye-Sensitized Solar Cells by W6+
    Doping into TiO2 Nanocrystalline Photoanodes. J Phys Chem C 118
    (30):16892-16895.

    CAS  Article  Google Scholar 

329. Latini A, Cavallo C, Aldibaja FK, Gozzi D et al (2013)
    Efficiency Improvement of DSSC Photoanode by Scandium Doping of
    Mesoporous Titania Beads. J Phys Chem C 117(48):25276-25289.

    CAS  Article  Google Scholar 

330. Bakhshayesh AM, Farajisafiloo N (2015) Efficient dye-sensitised
    solar cell based on uniform In-doped TiO2 spherical particles.
    Applied Physics A 120(1):199-206.

    CAS  Article  Google Scholar 

331. Subramanian A, Wang HW (2012) Effects of boron doping in TiO2
    nanotubes and the performance of dye-sensitized solar cells. Appl
    Surf Sci 258(17):6479-6484.

    CAS  Article  Google Scholar 

332. Song J, Yang HB, Wang X, Khoo SY, Wong CC, Liu XW, Li CM (2012)
    Improved utilization of photogenerated charge using
    fluorine-doped TiO(2) hollow spheres scattering layer in
    dye-sensitized solar cells. ACS Appl Mater Interfaces 4
    (7):3712-3717.

    CAS  Article  Google Scholar 

333. Tabari-Saadi Y, Mohammadiet MR (2015) Efficient dye-sensitized
    solar cells based on carbon-doped TiO2 hollow spheres and
    nanoparticles. J Mater Sci Mater Electron 26(11):8863-8876.

    CAS  Article  Google Scholar 

334. Lin J, Chen J, Chen X (2011) High-efficiency dye-sensitized
    solar cells based on robust and both-end-open TiO2 nanotube
    membranes. Nanoscale Res Lett 6:475.

    Article  CAS  Google Scholar 

335. Song CB, Qiang YH, Zhao YL, Gu XQ, Zhu L, Song CJ, Liu X (2014)
    Dye-sensitized Solar Cells Based on Graphene-TiO2 Nanoparticles/
    TiO2 Nanotubes Composite Films. Int J Electrochem Sci
    9:8090-8096.

    Google Scholar 

336. Patle LB, Chaudhari AL (2016) Performance of DSSC with Cu Doped
    TiO2 Electrode Prepared by Dip Coating Technique. Int J Sci Eng
    Res 7(8):1004-1009.

    Google Scholar 

337. Mao X, Zhou R, Zhang S, Ding L, Wan L, Qin S, Chen Z, Xu J, Miao
    S (2016) High Efficiency Dye-sensitized Solar Cells Constructed
    with Composites of TiO2 and the Hot-bubbling Synthesized
    Ultra-Small SnO2 Nanocrystals. Sci Rep 6:19390.

    CAS  Article  Google Scholar 

338. Llanos J, Brito I, Espinoza D, Sekar R, Manidurai P (2018) A
    down-shifting Eu3+-doped Y2WO6/TiO2 photoelectrode for improved
    light harvesting in dye-sensitized solar cells. R Soc open sci
    5:171054.

    CAS  Article  Google Scholar 

339. Liu X, Yuan R, Liu Y, Zhu S, Lin J, Chen X (2016) Niobium
    pentoxide nanotube powder for efficient dye-sensitized solar
    cells. New J Chem 40:6276-6280.

    CAS  Article  Google Scholar 

340. Sharma SS, Sharma K, Sharma V; Nanographite-TiO2 photoanode for
    dye sensitized solar cells (2016) AIP Conference Proceedings
    1728:020515.

341. Yang Q, Yang P, Duan J, Wang X, Wang L, Wang Z, Tang Q (2016)
    Ternary platinum alloy counter electrodes for high-efficiency
    dye-sensitized solar cells. Electrochim Acta 190:85-91.

    CAS  Article  Google Scholar 

342. Ibrayev N, Seliverstova E, Dzhanabekova R, Serikov T (2018)
    Photovoltaic properties of DSSC with composite counter electrodes
    based on Pt and SLGO. Mater Sci Eng 289:012009.

    Google Scholar 

343. Tang Q, Zhang H, Meng Y, He B, Yu L (2015) Dissolution
    Engineering of Platinum Alloy Counter Electrodes in
    Dye-Sensitized Solar Cells. Angew Chem Int Ed 54:11448-11452.

    CAS  Article  Google Scholar 

344. Dao VD, Choi Y, Yong K, Larina LL, Shevaleevskiy O, Choi HS
    (2015) A facile synthesis of bimetallic AuPt nanoparticles as a
    new transparent counter electrode for quantum-dot-sensitized
    solar cells. J Power Sources 274:831-838.

    CAS  Article  Google Scholar 

345. Chen X, Liu J, Qian K, Wang J (2018) Ternary composites of
    Ni-polyaniline-graphene as counter electrodes for dye-sensitized
    solar cells. RSC Adv 8:10948-10953.

    CAS  Article  Google Scholar 

346. Hou W, Xiao Y, Han G, Fu D, Wu R (2016) Serrated, flexible and
    ultrathin polyaniline nanoribbons: An efficient counter electrode
    for the dye-sensitized solar cell. J Power Sources 322:155-162.

    CAS  Article  Google Scholar 

347. Kukunuri S, Karthicka SN, Sampath S (2015) Robust, metallic
    Pd17Se15 and Pd7Se4 phases from a single source precursor and
    their use as counter electrodes in dye sensitized solar cells. J
    Mater Chem A 3:17144-17153.

    CAS  Article  Google Scholar 

348. Yang P, Tang Q (2016) A branching NiCuPt alloy counter electrode
    for .high-efficiency dye-sensitized solar cells. Appl Surf Sci
    362:28-34.

    CAS  Article  Google Scholar 

349. Wang G, Zhang J, Kuang S, Zhang W (2016) Enhanced
    Electrocatalytic Performance of a Porous g-C3N4/Graphene
    Composite as a Counter Electrode for Dye-Sensitized Solar Cells.
    Chem Eur J 22:11763-11769.

    CAS  Article  Google Scholar 

350. Balamurugan J, Thanh TD, Kim NH, Lee JH (2016) Nitrogen-Doped
    Graphene Nanosheets with FeN Core-Shell Nanoparticles as
    High-Performance Counter Electrode Materials for Dye-Sensitized
    Solar Cells. Adv Mater Interfaces 3:1500348.

    Article  CAS  Google Scholar 

351. Liang J, Li J, Zhu H, Han Y, Wang Y, Wang C, Jin Z, Zhang G, Liu
    J (2016) One-step fabrication of large-area ultrathin MoS2
    nanofilms with high catalytic activity for photovoltaic devices.
    Nanoscale 8:16017-16025.

    CAS  Article  Google Scholar 

352. Qian X, Li H, Shao L, Jiang X, Hou L (2016) Morphology-Tuned
    Synthesis of Nickel Cobalt Selenides as Highly Efficient Pt-Free
    Counter Electrode Catalysts for Dye-Sensitized Solar Cells. ACS
    Appl Mater Interfaces 8:29486-29495.

    CAS  Article  Google Scholar 

353. Li H, Qian X, Zhu C, Jiang X, Shao L, Hou L (2017) Template
    synthesis of CoSe2/Co3Se4nanotubes: tuning of their crystal
    structures for photovoltaics and hydrogen evolution in alkaline
    medium. J Mater Chem A 5:4513-4526.

    CAS  Article  Google Scholar 

354. Jia J, Wu J, Dong J, Bao Q, Fan L, Lin J (2017) Influence of
    deposition voltage of cobalt diselenide preparation on the film
    quality and the performance of dye-sensitized solar cells. Sol
    Energy 151:61-67.

    CAS  Article  Google Scholar 

355. Tsai CH, Hsiao YC, Chuang PY (2018) Investigation of
    Electrochemically Deposited and Chemically Reduced Platinum
    Nanostructured Thin Films as Counter Electrodes in Dye-Sensitized
    Solar Cells. Coatings 8:56.

    Article  CAS  Google Scholar 

356. Zhou H, Yin J, Nie Z, Yang Z, Li D, Wang J, Liu X, Jin C, Zhang
    X, Ma T (2016) Earth-abundant and nano-micro composite catalysts
    of Fe3O4@reduced graphene oxide for green and economical
    mesoscopic photovoltaic devices with high efficiencies up to 9%.
    J Mater Chem A 4:67-73.

    CAS  Article  Google Scholar 

357. Li CT, Lee CT, Li SR, Lee CP, Chiu IT, Vittal R, , Wu NL, Sun
    SS, Ho KC; Composite films of carbon black nanoparticles and
    sulfonated-polythiophene as flexible counter electrodes for
    dye-sensitized solar cells (2016) J Power Sources 302:155-163.

    CAS  Article  Google Scholar 

358. Arbab AA, Sun KC, Sahito IA, Qadir MB, Choi YS, Jeong SH (2016)
    A Novel Activated-Charcoal-Doped Multiwalled Carbon Nanotube
    Hybrid for Quasi-Solid-State Dye-Sensitized Solar Cell
    Outperforming Pt Electrode. ACS Appl Mater Interfaces
    8:7471-7482.

    CAS  Article  Google Scholar 

359. Shi Z, Lu H, Liu Q, Cao F, Guo J, Deng K, Li L (2014) Efficient
    p-type dye-sensitized solar cells with all-nano-electrodes:
    NiCo2S4 mesoporous nanosheet counter electrodes directly
    converted from NiCo2O4 photocathodes. Nanoscale Res Lett 9:608.

    Article  CAS  Google Scholar 

360. Duan Y, Tang QW, Liu J, He B, Yu L (2014) Transparent Metal
    Selenide Alloy Counter Electrodes for High-Efficiency Bifacial
    Dye-Sensitized Solar Cells. Angew Chem 126:14797-14802.

    Article  Google Scholar 

361. Cui X, Xu W, Xiea Z, Wang Y (2016) High-performance
    dye-sensitized solar cells based on Ag-doped SnS2 counter
    electrodes. J Mater Chem A 4:1908-1914.

    CAS  Article  Google Scholar 

362. Wu CS, Chang TW, Teng H, Lee YL (2016) High performance carbon
    black counter electrodes for dye-sensitized solar cells. Energy
    115:513-518.

    CAS  Article  Google Scholar 

363. Ruess R, Nguyen THQ, Schlettwein D (2018) Metal Complexes as
    Redox Shuttles in Dye-Sensitized Solar Cells Based on
    Electrodeposited ZnO: Tuning Recombination Kinetics and
    Conduction Band Energy. J Electrochem Soc 165(4):H3115.

    CAS  Article  Google Scholar 

364. Guo H, Zhu Y, Li W, Zheng H, Wu K, Ding K, Ruan B et al.;
    Synthesis of highly effective Pt/carbon fiber composite counter
    electrode catalyst for dye-sensitized solar cells (2015)
    Electrochim Acta 176:997-1000.

    CAS  Article  Google Scholar 

365. Sie CZW, Ngaini Z (2017) Incorporation of Kojic Acid-Azo Dyes on
    TiO2 Thin Films for Dye Sensitized Solar Cells Applications. J
    Solar Energy Article ID 2760301:10.

    Google Scholar 

366. Yu C, Liu Z, Meng X, Lu B, Cuia D, Qiu J (2016) Nitrogen and
    phosphorus dual-doped graphene as a metal-free high-efficiency
    electrocatalyst for triiodide reduction. Nanoscale 8:17458-17464.

    CAS  Article  Google Scholar 

367. Jeon IY, Kim HM, Choi IT, Lim K, Ko J, Kim JC, Choi HJ et al
    (2015) High performance dye-sensitized solar cells using
    edge-halogenated graphene nanoplatelets as counter electrodes.
    Nano Energy 13:336-345.

    CAS  Article  Google Scholar 

368. Li YY, Li CT, Yeh MH, Huanga KC, Chena PW, Vittala R, Hoab KC
    (2015) Graphite with different structures as catalysts for
    counter electrodes in dye-sensitized solar cells. Electrochim
    Acta 179:211-219.

    CAS  Article  Google Scholar 

369. Kumar R, Nemala SS, Mallick S, Bhargava P (2017) High efficiency
    dye sensitized solar cells made by carbon derived sucrose. Opt
    Mater 64:401-405.

    CAS  Article  Google Scholar 

370. Zhang J, Long H, Miralles SG, Bisquert J, Fabregat-Santiago F,
    Zhang M (2012) The combination of a polymer-carbon composite
    electrode with a high-absorptivity ruthenium dye achieves an
    efficient dye-sensitized solar cell based on a thiolate-disulfide
    redox couple. Phys Chem Chem Phys 14:7131-7136.

    CAS  Article  Google Scholar 

371. Xu X, Huang D, Cao K, Wang M, Zakeeruddin SM, Gratzel M (2013)
    Electrochemically reduced graphene oxide multilayer films as
    efficient counter electrode for dye-sensitized solar cells. Sci
    Rep 3:1489.

    Article  CAS  Google Scholar 

372. Yang J, Ganesan P, Teuscher J, Moehl T, Kim YJ, Yi C et al
    (2014) Influence of the Donor Size in D-p-A Organic Dyes for
    Dye-Sensitized Solar Cells. J Am Chem Soc 136:5722-5730.

    CAS  Article  Google Scholar 

373. Mathew S, Yella A, Gao P, Humphry-Baker R, Curchod BF,
    Ashari-Astani N, Tavernelli I, Rothlisberger U, Nazeeruddin MK,
    Gratzel M (2014) Dye-sensitized solar cells with 13% efficiency
    achieved through the molecular engineering of porphyrin
    sensitizers. Nat Chem 6:242-247.

    CAS  Article  Google Scholar 

374. Ahmad S, Bessho T, Kessler F, Baranoff E, Frey J, Yi C, Gratzel
    M, Nazeeruddin MK (2012) A new generation of platinum and iodine
    free efficient dye-sensitized solar cells. Phys Chem Chem Phys
    14:10631-10639.

    CAS  Article  Google Scholar 

375. Lee KM, Chen PY, Hsu CY, Huang JH, Ho WH, Chen HC, Ho KC (2009)
    A high-performance counter electrode based on poly
    (3,4-alkylenedioxythiophene) for dye-sensitized solar cells. J
    Power Sources 188:313-318.

    CAS  Article  Google Scholar 

376. Ahmad S, Yum JH, Butt HJ, Nazeeruddin MK, Gratzel M (2010)
    Efficient Platinum-Free Counter Electrodes for Dye-Sensitized
    Solar Cell Applications. J Chem Phys Chem 11:2814.

    CAS  Article  Google Scholar 

377. Wang H, Feng Q, Gong F, Li Y, Zhou G, Wang ZS (2013) In situ
    growth of oriented polyaniline nanowires array for efficient
    cathode of Co(III)/Co(II) mediated dye-sensitized solar cell. J
    Mater Chem A 1:97-104.

    Article  Google Scholar 

378. Chiang CH, Chen SC, Wu CG (2013) Preparation of highly
    concentrated and stable conducting polymer solutions and their
    application in high-efficiency dye-sensitized solar cell. Org
    Electron 14:2369-2378.

    CAS  Article  Google Scholar 

379. He J, Wu W, Hua J, Jiang Y, Qu S, Li J, Longa Y, Tian H (2011)
    Bithiazole-bridged dyes for dye-sensitized solar cells with high
    open circuit voltage performance. Mater Chem 21:6054-6062.

    CAS  Article  Google Scholar 

380. Burschka J, Dualeh A, Kessler F, Baranoff E, Cevey-Ha NL, Yi C,
    Nazeeruddin MK, Gratzel M (2011) Tris(2-(1H-pyrazol-1-yl)
    pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors
    and Its Application in Highly Efficient Solid-State
    Dye-Sensitized Solar Cells. J Am Chem Soc 133:18042-18045.

    CAS  Article  Google Scholar 

381. Juozapavicius M, O'Regan BC, Anderson AY, Grazulevicius JV,
    Mimaite V (2012) Efficient dye regeneration in solid-state
    dye-sensitized solar cells fabricated with melt processed hole
    conductors. Org Electron 13:23-30.

    CAS  Article  Google Scholar 

382. Leijtens T, Ding IK, Giovenzana T, Bloking JT, McGehee MD,
    Sellinger A (2012) Hole transport materials with low glass
    transition temperatures and high solubility for application in
    solid-state dye-sensitized solar cells. ACS Nano 6:1455-1462.

    CAS  Article  Google Scholar 

383. Senadeera GKR, Jayaweera PVV, Perera VPS, Tenna-kone K (2002)
    Solid-state dye-sensitized photocell based on pentacene as a hole
    collector. Sol Energy Mater Sol Cells 73:103-108.

    Article  Google Scholar 

384. Metri N, Sallenave X, Plesse C, Beouch L, Aubert PH, Goubard F,
    Chevrot C, Sini G (2012) Processable Star-Shaped Molecules with
    Triphenylamine Core as Hole-Transporting Materials: Experimental
    and Theoretical Approach. J Phys Chem C 116:3765-3772.

    CAS  Article  Google Scholar 

385. Wei D (2010) Dye Sensitized Solar Cells. Int J Mol Sci
    11:1103-1113.

    CAS  Article  Google Scholar 

386. Funaki T, Yanagida M, Onozawa-Komatsuzaki N, Kawanishi Y, Kasuga
    K, Sugihara H (2009) A 2-quinolinecarboxylate-substituted
    ruthenium(II) complex as a new type of sensitizer for
    dye-sensitized solar cells. Inorg Chim Acta 362:2519-2522.

    CAS  Article  Google Scholar 

387. Polo AS, Itokazu MK, Iha NYM (2004) Metal complex sensitizers in
    dye-sensitized solar cells. Coordination Coord Chem Rev
    248:1343-1361.

    CAS  Article  Google Scholar 

388. Bessho T, Constable EC, Graetzel M, Redondo AH, Housecroft CE,
    Kylberg W, Nazeeruddin MK, Neuburgerb M, Schaffner S (2008) An
    element of surprise--efficient copper-functionalized
    dye-sensitized solar cells. Chem Commun 32:3717-3719.

    Article  CAS  Google Scholar 

389. Jayaweera PM, Palayangoda SS, Tennakone K (2001) Nanoporous TiO2
    solar cells sensitized with iron (II) complexes of
    bromopyrogallol red ligand. J Photochem and Phtobio A:Chem
    140:173-177.

    CAS  Article  Google Scholar 

390. Gratzel M; Photovoltaic performance and long-term stability of
    dye-sensitized meosocopic solar cells (2006) CR Chimie 9:578-583.

    Article  CAS  Google Scholar 

391. Onozawa-Komatsuzaki N, Yanagida M, Funaki T, Kasuga K, Sayama K,
    Sugihara H (2011) Near IR dye-sensitized solar cells using a new
    type of ruthenium complexes having 2,6-bis (quinolin-2-yl)
    pyridine derivatives. Sol Ene Mat & Sol Cells 95:310-314.

    CAS  Article  Google Scholar 

392. Onozawa-Komatsuzaki N, Yanagida M, Funaki T, Kasuga K, Sayama K,
    Sugihara H (2009) Near IR sensitization of nanocrystalline TiO2
    witha new ruthenium complex having a 2,6-bis
    (4-carboxyquinolin-2-yl) pyridine ligand. Inorg Chem Comn
    12:1212-1215.

    CAS  Article  Google Scholar 

393. White RC, Benedetti JE, Goncalves AD, Romao W, Vaz BG, Eberlin
    MN, Carlo RD, Correia Marco A, De Paoli MA, Nogueira AF (2011)
    Synthesis, Characterization and Introduction of a New
    Ion-Coordinating Ruthenium Sensitizer Dye in Quasi-Solid State
    TiO2 Solar Cells. J. Photochem. and Photobio. A: Chem.
    222:185-191.

    CAS  Article  Google Scholar 

394. Sivakumar R, Marcelis ATM, Anandan S (2009) Synthesis and
    characterization of novel heteroleptic ruthenium sensitizer for
    nanocrystalline dye-sensitized solar cells. J Photochem and
    Photobio A: Chem 208:154-158.

    CAS  Article  Google Scholar 

395. Sahin C, Ulusoy M, Zafer C, Ozsoy C, Varlikli C, Dittrich T,
    Cetinkaya B, Icli S (2010) The synthesis and characterization of
    2-(2'-pyridyl)benzimidazole heteroleptic ruthenium complex:
    Efficient sensitizer for molecular photovoltaics. Dyes and Pigts
    841:88-94.

    Article  CAS  Google Scholar 

396. Argazzi R, Larramona G, Contado C, Bignozzi CA (2003)
    Preparation and photoelectrochemical characterization of a red
    sensitive osmium complex containing
    4,4',4''-tricarboxy-2,2':6',2''-terpyridine and cyanide ligands. J
    Photochem and Photobio A: Chem 164:15-21.

    Article  CAS  Google Scholar 

397. Zabri H, Odobel F, Altobello S, Caramori S, Bignozzi CA (2004)
    Efficient osmium sensitizers containing 2,2'-bipyridine-4,4'-
    bisphosphonic acid ligand. J Photochem and Phtobio A: Chem
    166:99-106.

    CAS  Article  Google Scholar 

398. Islam A, Sugihara H, Hara K, Singh LP, Katoh R, Yanagida M,
    Takahashi Y, Murata S, Arakawa H (2000) New platinum(II)
    polypyridyl photosensitizers for TiO2 solar cells. New J Chem
    24:343-345.

    CAS  Article  Google Scholar 

399. Wang ZS, Hara K, Dan-oh Y, Kasada C, Shinpo A, Suga S, Arakawa
    H, Sugihara H (2005) Photophysical and (Photo)electrochemical
    Properties of a Coumarin Dye. J Phys Chem B 109:3907-3914.

    CAS  Article  Google Scholar 

400. Mao M, Zhang XL, Wu GH (2018) Novel Imidazole Substituted
    Bodipy-Based Organic Sensitizers in Dye-Sensitized Solar Cells.
    Int J Photoenergy Article ID 2061472:9.

    Google Scholar 

401. Campbell WM, Burrell AK, Officer DL, Jolley KW (2004) Porphyrins
    as light harvesters in the dye-sensitised TiO2 solar cell. Jolley
    Coord Chem Rev 248:1363-1379.

    CAS  Article  Google Scholar 

402. Nazeeruddin MK, Humphry-Baker R, Officer DL, Campbell WM,
    Burrell AK, Gratzel M (2004) Application of metalloporphyrins in
    nanocrystalline dye-sensitized solar cells for conversion of
    sunlight into electricity. Langmuir 20:6514-6517.

    CAS  Article  Google Scholar 

403. Chang H, Lo Y (2010) Pomegranate Leaves and Mulberry Fruit as
    Natural Sensitizers for Dye-Sensitized Solar Cells. Sol Energy
    84:1833-1837.

    CAS  Article  Google Scholar 

404. Patrocinio AOT, Mizoguchi SK, Paterno LG, Garcia CG, Iha NYM;
    Efficient and low cost devices for solar energy conversion:
    Efficiency and stability of some natural-dye-sensitized solar
    cells (2009) Synth Met 159(21-22):2342-2344.

    Article  CAS  Google Scholar 

405. Teresita RA, Norma F, Daniel GM (2010) Natural Carotenoids as
    Nanomaterial Precursors for Molecular Photovoltaics: A
    Computational DFT Study. Molecules 15:4490-4510.

    Article  CAS  Google Scholar 

406. Gokilamani N, Muthukumarasamy N, Thambidurai M, Ranjitha A,
    Velauthapillai D (2013) Utilization of natural anthocyanin
    pigments as photosensitizers for dye-sensitized solar cells. J
    Sol-Gel Sci Technol 66:212-219.

    CAS  Article  Google Scholar 

407. Alaba AK (2012) Utilization of Natural Morinda lucida as
    photosensitizers for dyesensitized solar cell. Arch Appl Sci Res
    4(1):419-425.

    CAS  Google Scholar 

408. Al-Bat'hi SAM, Alaei I, Sopyan I (2013) Natural photosensitizers
    for dye sensitized solar cells. Int J Renew Energy Res 3(1):138.

    Google Scholar 

409. Mansa RF, Govindasamy G, Farm YY, Bakar HA, Dayou J, Sipaut CS
    (2014) Hibiscus flower as a natural dye sensitizer for a
    dye-sensitized solar cell. J Phys Sci 25(2):85-96.

    CAS  Google Scholar 

410. Arifin Z, Soeparman S, Widhiyanuriyawan D, Suyitno S (2017)
    Performance Enhancement of Dye-Sensitized Solar Cells Using a
    Natural Sensitizer. Int J Photoenergy Article ID 2704864:5.

    Google Scholar 

411. Ali RAM, Nayan N (2010) Fabrication and analysis of
    dye-sensitized solar cell using natural dye extracted from dragon
    fruit. Int J of Integrated Eng (Issue on Electrical and
    Electronic Engineering) 2(3):55-62.

    Google Scholar 

412. Gokilamani N, Muthukumarasamy N, thambidurai M, Ranjitha A,
    Velauthapillai D, Senthil TS, Balasundaraprabhu R (2013)
    Dye-sensitized solar cells with natural dyes extracted from rose
    petals. J Mat Sci Mat Electron 24(9).

    CAS  Google Scholar 

413. Aduloju KA, Shitta MB, Simiyu J Effect of extracting solvents on
    the stability and performances of dye-sensitized solar cell
    prepared using extract from Lawsonia Inermis. Fundamental J
    Modern Phys 1(2):261-268.

414. Jaafar H, Ain MF, Ahmad ZA (2017) Performance of E. conferta and
    G. atroviridis fruit extracts as sensitizers in dye-sensitized
    solar cells (DSSCs). Ionics 24(3):891-899.

    Article  CAS  Google Scholar 

415. Hosseinnezhad M, Moradian S, Gharanjig K (2015) Fruit extract
    dyes as photosensitizers in solar cells. Curr Sci 109(5):953.

    CAS  Article  Google Scholar 

416. Ashok A, Mathew SE, Shivaram SB, Shankarappa SA, Nair SV,
    Shanmugam M (2018) Cost effective natural photo-sensitizer from
    upcycled jackfruit rags for dye sensitized solar cells. J Sci
    Advanced Materials and Devices 3(2):213-220.

    Article  Google Scholar 

417. Karki IB, Nakarmi JJ, Mandal PK, Chatterjee S (2014)
    Dye-sensitized solar cells sensitized with natural dye extracted
    from Indian Jamun. BIBECHANA 11(1):34-39.

    Google Scholar 

418. Kumara NTRN, Ekanayake P, Lim A, Iskandar M, Ming LC; Study of
    the Enhancement of Cell Performance of Dye Sensitized Solar Cells
    Sensitized With Nephelium lappaceum (F: Sapindaceae) (2013) J
    Solar Energy Engineering 135(3):031014 (5 pages).

    Article  CAS  Google Scholar 

419. Susanti D, Nafi M, Purwaningsih H, Fajarin R, Kusuma GE (2014)
    The Preparation of Dye Sensitized Solar Cell (DSSC) from TiO2 and
    Tamarillo Extract. Procedia Chemistry 9:3-10.

    CAS  Article  Google Scholar 

420. Saha S, Das P, Chakraborty AK, Sarkar S, Debbarma R (2016)
    Fabrication of DSSC with nanoporous TiO2 film and Kenaf Hibiscus
    dye as sensitizer. Int J Renew Energy Res 6(2):620.

    Google Scholar 

Download references

Acknowledgements

The authors would like to acknowledge the Department of Science &
Technology, SERB Division, Govt. of India (Award # SR/FTP/PS-112/2012
Dated 1.11.2013), for providing financial support to carry out
research on dye-sensitized solar cells (DSSCs).

Funding

This work was supported by Science and Engineering Research Board,
Department of Science & Technology, Govt. of India (SR/FTP/PS-112/
2012).

Availability of Data and Materials

It is a review article that gives a comprehensive study about the
materials including the photoanode, sensitizer, electrolyte and
counter electrode, device architecture, and fabricating techniques
used in the fabrication of dye-sensitized solar cells (DSSCs). It
emphasizes the role of the sensitizer and the strategies to improve
the performances of the dye as well as some recent development aiming
to answer specific issues till date.

Author information

Authors and Affiliations

 1. Department of Physics, Bhagwant University, Ajmer, 305004, India

    Khushboo Sharma

 2. School of Materials Science and Engineering, Nanyang
    Technological University, Singapore, 639798, Singapore

    Vinay Sharma

 3. Department of Physics, Govt. Women Engineering College, Ajmer,
    305002, India

    S. S. Sharma

Authors

 1. Khushboo Sharma
    View author publications

    You can also search for this author in PubMed Google Scholar

 2. Vinay Sharma
    View author publications

    You can also search for this author in PubMed Google Scholar

 3. S. S. Sharma
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

SSS developed the concept. VS collected the study materials required
for the preparation of the manuscript entitled "Dye Sensitized Solar
Cells: Fundamentals and Current Status". SSS is a supervisor of one
author. KS drafted the article and SSS polished the content to
present form. All authors reviewed the paper. All authors read and
approved the final manuscript.

Corresponding author

Correspondence to S. S. Sharma.

Ethics declarations

Authors' Information

Khushboo Sharma is a Research Scholar in Physics at Bhagwant
University, Ajmer, India, and currently working in the field of
dye-sensitized solar cells. She did her Master's degree in 2013 from
MDS University, Ajmer, India. She had been working as a project
fellow for project on "Development of New Materials for
Dye-Sensitized Solar Cells" of Department of Science and Technology,
SERB Division, New Delhi at Government Women Engineering College,
Ajmer, India.

Vinay kumar Sharma did his M. Tech in Material Science from the
Centre for Converging Technologies, University of Rajasthan, Jaipur,
India. Presently, he is working at the School of Materials Science
and Engineering, Nanyang Technological University, Singapore. Vinay
does his research in Materials Physics, Solid State Physics and
Materials Science. His current project is on "Magnetocaloric effect
in iron based systems".

Shyam S. Sharma is a faculty in Physics at the Govt. Women
Engineering College, Ajmer, India. He obtained his Ph.D. in 2010 at
the University of Rajasthan, Jaipur, India, in the field of Organic
Solar Cells. His research interest is in the area of organic
semiconductor materials and devices for electronic and optoelectronic
technology. He has about 50 scientific publications in international
journals and proceedings of international and national conferences,
and has published a book on Synthesis and characterization of organic
photovoltaic cells. He has been honored for his research work with an
Innovative Engineer Award from United Engineers Council. He is a life
member of the Indian Physics Association (IPA), Indian Association of
Physics Teacher (IAPT), Material Research Society of India (MRSI),
and The Indian Science Congress Association. He is also associated
with the Material Research Society of Singapore, Synchrotron
Radiation Center, Italy, and UGC-DAE CSR, Indore. Presently, he is
the Chief Coordinator of the World Bank funded project TEQIP
(Technical Education Quality Improvement Programme) Phase-III in his
institute.

Competing Interests

The authors declare that they have no competing interests.

Publisher's Note

Springer Nature remains neutral with regard to jurisdictional claims
in published maps and institutional affiliations.

Rights and permissions

Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestricted
use, distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide
a link to the Creative Commons license, and indicate if changes were
made.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Sharma, K., Sharma, V. & Sharma, S.S. Dye-Sensitized Solar Cells:
Fundamentals and Current Status. Nanoscale Res Lett 13, 381 (2018).
https://doi.org/10.1186/s11671-018-2760-6

Download citation

  * Received: 22 September 2017

  * Accepted: 17 October 2018

  * Published: 28 November 2018

  * DOI: https://doi.org/10.1186/s11671-018-2760-6

Share this article

Anyone you share the following link with will be able to read this
content:

Get shareable link

Sorry, a shareable link is not currently available for this article.

Copy to clipboard

Provided by the Springer Nature SharedIt content-sharing initiative

Keywords

  * Dye-sensitized solar cells (DSSCs)
  * Photoanode
  * Counter electrode
  * Electrolytes
  * Metal and metal-free organic dyes
  * Efficiency
  * Stability

Download PDF

Advertisement

Advertisement
[s11671-018]

  * Support and Contact
  * Jobs
  * Language editing for authors
  * Scientific editing for authors
  * Leave feedback

  * Terms and conditions
  * Privacy statement
  * Accessibility
  * Cookies

Follow SpringerOpen

  * SpringerOpen Twitter page
  * SpringerOpen Facebook page

By using this website, you agree to our Terms and Conditions,
California Privacy Statement, Privacy statement and Cookies policy.
Manage cookies/Do not sell my data we use in the preference centre.

Springer Nature

(c) 2022 BioMed Central Ltd unless otherwise stated. Part of Springer
Nature.