Células solares sensibilizadas por corante:fundamentos e status atual

Construção e funcionamento de DSSCs

Substrato transparente e condutor

Eletrodo de Trabalho (WE)

Fotossensibilizador ou corante

1. 1.
   Em primeiro lugar, o corante deve ser luminescente.
   
2. 2.
   Em segundo lugar, o espectro de absorção do corante deve cobrir as regiões ultravioleta-visível (UV-vis) e região do infravermelho próximo (NIR).
   
3. 3.
   O orbital molecular mais ocupado (HOMO) deve estar localizado longe da superfície da banda de condução de TiO ₂ e o orbital molecular mais baixo desocupado (LUMO) deve ser colocado o mais próximo da superfície do TiO ₂ , e subsequentemente deve ser maior em relação ao TiO ₂ potencial de banda de condução.
   
4. 4.
   O HOMO deve ser inferior ao dos eletrólitos redox.
   
5. 5.
   A periferia do corante deve ser hidrofóbica para aumentar a estabilidade de longo prazo das células, pois resulta em contato direto minimizado entre o eletrólito e o ânodo; caso contrário, distorção induzida por água do corante do TiO ₂ superfície pode aparecer, o que pode reduzir a estabilidade das células.
   
6. 6.
   Para evitar a agregação do corante sobre o TiO ₂ superfície, co-absorventes como ácido quenodesoxicólico (CDCA) e grupos de ancoragem como alcoxi-silil [24], ácido fosfórico [25] e grupo de ácido carboxílico [26, 27] foram inseridos entre o corante e TiO ₂ . Isso resulta na prevenção da agregação do corante e, portanto, limita a reação de recombinação [28] entre o eletrólito redox e os elétrons no TiO ₂ nanocamada, bem como resulta na formação de ligação estável.
   

Eletrólito

1. 1.
   O par Redox deve ser capaz de regenerar o corante oxidado de forma eficiente.
   
2. 2.
   Deve ter estabilidade química, térmica e eletroquímica de longo prazo.
   
3. 3.
   Não deve ser corrosivo com componentes DSSC.
   
4. 4.
   Deve ser capaz de permitir a difusão rápida de portadores de carga, aumentar a condutividade e criar contato efetivo entre os eletrodos de trabalho e os contra-eletrodos.
   
5. 5.
   O espectro de absorção de um eletrólito não deve se sobrepor ao espectro de absorção de um corante.
   

Contador Eletrodo (CE)

Princípio de funcionamento

1. 1.
   Em primeiro lugar, a luz incidente (fóton) é absorvida por um fotossensibilizador e, portanto, devido à absorção de fótons, os elétrons são promovidos do estado fundamental (S ⁺ / S) para o estado excitado (S ⁺ / S *) do corante, onde a absorção para a maior parte do corante está na faixa de 700 nm que corresponde à energia do fóton quase cerca de 1,72 eV.
   
2. 2.
   Agora, os elétrons excitados com uma vida útil de intervalo de nanossegundos são injetados na banda de condução de TiO nanoporoso ₂ eletrodo que fica abaixo do estado excitado do corante, onde o TiO ₂ absorve uma pequena fração dos fótons solares da região do UV [43]. Como resultado, o corante é oxidado.
   $$ {\ mathrm {S}} ^ {+} / \ mathrm {S} + \ mathrm {h} \ upnu \ rightarrow {\ mathrm {S}} ^ {+} / {\ mathrm {S}} ^ { \ ast} $$$$ {\ mathrm {S}} ^ {+} / {\ mathrm {S}} ^ {\ ast} \ rightarrow {\ mathrm {S}} ^ {+} / \ mathrm {S} + {\ mathrm {e}} ^ {-} \ \ left ({\ mathrm {TiO}} _ 2 \ right) $$
3. 3.
   Esses elétrons injetados são transportados entre TiO ₂ nanopartículas e difundir em direção ao contato traseiro (óxido condutor transparente [TCO]). Através do circuito externo, os elétrons alcançam o contra-eletrodo.
   
4. 4.
   Os elétrons no contra-eletrodo reduzem I ^- ₃ para I ^- ; assim, a regeneração do corante ou a regeneração do estado fundamental do corante ocorre devido à aceitação de elétrons de I ^- mediador redox de íon e I ^- é oxidado a I ^- ₃ (estado oxidado).
   $$ {\ mathrm {S}} ^ {+} / {\ mathrm {S}} ^ {\ ast} + {\ mathrm {e}} ^ {-} \ rightarrow {\ mathrm {S}} ^ {+ } / \ mathrm {S} $$
5. 5.
   Novamente, o mediador oxidado (I ^- ₃ ) se difunde em direção ao contra-eletrodo e se reduz a íon I.
   $$ {{\ mathrm {I}} ^ {-}} _ 3+ {2 \ mathrm {e}} ^ {-} \ rightarrow {3 \ mathrm {I}} ^ {-} $$

Avaliação do desempenho da célula solar sensibilizada com corante

Limitações dos dispositivos

Limitação em relação à estabilidade dos dispositivos

Limitação em relação à estabilidade extrínseca (estabilidade do material selante)

Limitação em relação à estabilidade intrínseca

Diferentes maneiras de aumentar a eficiência dos DSSCs

1. 1.
   Para aumentar a eficiência dos DSSCs, o corante oxidado deve ser firmemente reduzido ao seu estado fundamental original após a injeção de elétrons. Em outras palavras, o processo de regeneração (que ocorre na faixa dos nanossegundos [56]) deve ser rápido em comparação ao processo de oxidação do corante [o processo de recombinação (0,1 a 30 μs)]. Como o potencial do mediador redox (I ^- íon) afeta fortemente a fotovoltagem máxima, portanto, o potencial do par redox deve estar próximo ao estado fundamental do corante. Para realizar este processo repetido viável, cerca de 210 mV de força motriz é necessária (ou cerca de 0,6 V [56]).
   
2. 2.
   Aumentando a porosidade do TiO ₂ nanopartículas, a absorção máxima do corante ocorre em WE.
   
3. 3.
   Reduzir ou proibir a formação da corrente escura depositando uma camada fina uniforme ou sob a camada de TiO ₂ nanopartículas sobre a placa de vidro de condução. Assim, o eletrólito não tem contato direto com o FTO ou contra contato e, portanto, não é reduzido pelos elétrons coletores, o que restringe a formação da corrente escura.
   
4. 4.
   Prevenção do aprisionamento de TiO nanoporoso ₂ nanopartículas por moléculas de TBP ou por um solvente eletrolítico. Assim, é necessária uma sensibilização uniforme do WE por um sensibilizador. Se toda a superfície do TiO nanoporoso ₂ eletrodo não é uniformemente coberto pelo sensibilizador, então os pontos nus de TiO nanoporoso ₂ pode ser capturado por moléculas de TBP ou por um solvente eletrolítico.
   
5. 5.
   A co-sensibilização é outra forma de otimizar o desempenho do DSSC. Na co-sensibilização, dois ou mais corantes sensibilizantes com diferentes faixas de espectro de absorção são misturados. para ampliar a faixa de resposta do espectro [57].
   
6. 6.
   Promovendo o uso de diferentes materiais na fabricação de eletrodos como nanotubos, nanofios de carbono, grafeno; usando eletrólitos variados em vez de um líquido, como eletrólito em gel e eletrólitos quase sólidos; fornecer diferentes pré-pós-tratamentos para o eletrodo de trabalho, como pré-tratamento de anodização e TiCl ₄ tratamento; usando diferentes tipos de CEs [14] e desenvolvendo sensibilizadores hidrofóbicos, o desempenho, bem como a eficiência dessas células podem ser tremendamente melhorados.
   
7. 7.
   Ao inserir fosforescência ou cromóforos luminescentes, como a aplicação de óxidos dopados com terras raras no DSSC [58,59,60], revestindo uma camada luminescente no vidro do fotoanodo [60,61,62], ou seja, usando o fenômeno plasmônico [ 63] e adicionando corantes de relé de energia (ERDs) ao eletrólito [57, 64, 65].
   

Melhorias anteriores e posteriores nos DSSCs

Eletrodos de trabalho e contador

Electrolyte

Liquid Electrolyte

V _OC e 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 TiO₂ -conduction band, thus resulting in an augmentation in the value of V _OC of the cell. This suppresses the dye aggregation over the TiO₂ 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 ⁻ /SCN₂ , Br ^- /Br₂ , and SeCN ⁻ /SeCN₂ 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 TiO₂ 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 TiO₂ 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 SiO₂ 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 °C in darkness and 1000 h at 60 °C, 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 ocorre. 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 TiO₂ surface, the pyridine ring induced electron density into the TiO₂ 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 TiO₂ , 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 TiO₂ superfície.

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 TiO₂ -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 TiO₂ 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 η 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 SiO₂ or Al₂ O ₃ can be applied or coated around the TiO₂ 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 °C. 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 TiO₂ layer up to 2 μm and thus leads to incomplete light harvesting efficiency (LHE) of dye. Also, a high recombination rate between TiO₂ 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 TiO₂ 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 )

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

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 TiO₂ -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 η  = 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.

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]

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 Grätzel 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.

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 × 10 ⁴ M ^{- 1} cm ^{- 1} and 1.33 × 10 ⁴ M ^{- 1} cm ^{- 1} , respectivamente. 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 η  = 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 TiO₂ chemically which results in the partial transformation of protons of the anchoring group to the surface of the TiO₂ . 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 TiO₂ surface and the downward shift in the Fermi level of TiO₂ . 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 TiO₂ . 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. Grätzel 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 η  = 10.5% [158] and η  = 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 η  = 11.2% for the respective device [155]. Despite having almost the same structure to the N3 dye, the higher value of η for N719 was accredited to the change in the counterions, as they altered the speed of adsorption onto the porous TiO₂ 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-TiO₂ layer, and the antireflective filming, Grätzel group reported η  = 11.3% [32] for the device containing the dyes C101 and η  = 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 e 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 TiO₂ 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.

Molecular structure of Ruthenium complex based dye sensitizers

Molecular structure of C101 and Z991 sensitizers

π-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 TiO₂ 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 π-systems to enhance the absorption and the cell efficiency, simultaneously. Thus, the reason behind the π-system extension in dyes is to create sensitizers with higher molar extinction coefficients (ε ), 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 TiO₂ superfície. 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. O ε  = 1.69 × 10 ⁴ M ^{- 1} cm ^{- 1} for the Z910 dye [164], ε  = 1.82 × 10 ⁴ M ^{- 1} cm ^{- 1} for the K19 dye [145], and ε  = 1.89 × 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 °C (in darkness) and 60 °C 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 TiO₂ (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 (λ _máximo ) with a ε  = 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 ε  = 17,400 M ^− 1 cm ^{- 1} , respectivamente. Thus, due to the substitution of 4, 4′-bis (carboxylvinyl)-2, 2′-bipyridine by 4,4′- dinonyl-2,2′-bipyridine, ε 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 e V _OC  = 640 mV and 7.81% with J _SC  = 16 mA/cm ² e V _OC  = 666 mV [169], respectively. Grätzel 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 °C 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 °C 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 η 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 TiO₂ was prone to hydrolyze, if water gets adsorbed on the TiO₂ surface [172], thus resulting in usually lower absorption spectrum in these sensitizers due to the smaller conjugated π-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 °C in darkness and at 55 °C under illumination without any degradation [173]. It has been found that by coadsorption of decylphosphonic acid on the TiO₂ 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 TiO₂ . 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 η  = 8% accompanied by good stability under prolonged light soaking for about 1000 h at AM 1.5 and 55 °C [175]. Triethoxysilane [176] and boronic acid are some other anchoring groups. π-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=NC₆ H ₄ COOH (1) and FcCH=NCH₂ CH ₂ OH (2), where Fc = C₅ H ₄ FeC₅ 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 TiO₂ superfície. 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 TiO₂ 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].

Effect of dye protonation on photocurrent-voltage characteristics of nanocrystalline TiO₂ 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]

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.

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 TiO₂ 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 TiO₂ 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 TiO₂ 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 η  = 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., λ _abs at 383 nm and 485 nm obtained for 2TPA-R, are almost the same as those observed for 2TPA (λ _abs at 388 nm) and TPA-R (λ _abs at 476 nm) in CH₂ Cl ₂ solution (2 × 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 TiO₂ surface with a suitable structure and effective intramolecular E_n T and ICT processes, is also required for synthesis.

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]

The construction of most of the organic dyes is based on the donor- acceptor (D-A)-like structure linked through a π-conjugated bridge (D–π–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 π 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 TiO₂ layer, where the π-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 π-conjugation due to the shift of the both HOMO and LUMO energy levels, thus, D–π–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–π–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 π-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, π-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 π-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–π–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 η  = 4.93% [219]. Wang and co-workers designed organic dyes based on thienothiophene as π 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, Grätzel et al. found the efficient electron injection rates of 200 fs from C343 into the conduction band of the TiO₂ , 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 π-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 π-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 ɛ  = 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 °C during thermal analysis.

Molecular Structure of metal-free organic dyes

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, η  = 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 π-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 η  = 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 TiO₂ (− 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 TiO₂ electrode (as shown in Fig. 19) [230]. The less efficiency of other dyes was contributed by the intermolecular π-stacking and aggregation processes in these dyes, proceeding on the photoanode surface.

Absorption spectra of 2TPA, TPA-R, and 2TPA-R in CH₂ Cl ₂ solutions [184]

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 TiO₂ 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–π–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].

Molecular structure of a Coumarin and b indole

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 η  = 10.06%) > LD3p > LD2 > LD3a > LD1. The higher value of η e 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 η  = 12.3% was achieved by Yella et al. for DSSC with cobalt as the mediator [236].

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

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 TiO₂ nanoparticle films to determine their adsorption on the TiO₂ 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 TiO₂ 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.

Energy level diagram of LG-11 to LG-14 porphyrins, electrolyte and TiO₂ ( 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 )

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 π–π 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–π–A organic dye can be improved by simply binding donor substitutions on the π-linker of the dye [240]. Thus, Prachumrak and co-workers have synthesized three new molecularly engineered D–π–A dyes, namely T2–4, comprising TPA as a donor, terthiophene containing different numbers of TPA substitutions as a π-conjugated linker and cyanoacrylic acid as an acceptor [240]. To minimize the electron recombination between redox electrolyte and the TiO₂ surface as well as an increase the electron correction efficiency, the introduction of electron donating TPA substitutes on the π-linker of the D–π–A dye can play a favorable game, leading to improved V _OC e 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 π 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 TiO₂ superfície. 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 TiO₂ 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 (τ ) = 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 (λ _abs  = 476 nm with ε  = 45,900 M ^− 1 cm ^{- 1} in ACN) than those for the furan counterpart (λ _abs  = 439 nm with ε  = 33,000 M ^− 1 cm ^{- 1} in ACN). However, the performance of the solar cells based on the furan counterpart (ɳ_max  = 7.36%) was better as compared to the one based on D5 (ɳ_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 TiO₂ superfície. 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 (η  = 1.9%) and T2–4 (η  = 2.4%) dyes with low IPCE values for rhodanine-3-acetic acid as an anchoring group, as compared to T2–1 (η  = 5.5%) and T2–2 (η  = 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 μm transparent and 1.5 μm scattering TiO₂ electrode and used an electrolyte composed of 0.6 M butylmethylimidazolium iodide (BMII), 0.03 M I₂ , 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 η  = 5.4% and V _OC  = 0.706 V, J _SC  = 7.14 mAcm ^− 2 , FF = 55.6, and η  = 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–π–A-type and D–D–π–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 π 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 η  = 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 TiO₂ 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 TiO₂ /CuInS₂ -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.

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 (TiO₂ ) surface, which stimulates the electron transfer from the sensitizer (anthocyanin molecules) to the conduction band of porous semiconducting (TiO₂ ) 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].

Molecular structures of LD porphyrins

Flavonoid :Flavonoid is an enormous compilation of natural dyes which shows a carbon framework (C₆ –C₃ –C₆ ) 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 TiO₂ 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 × 10 ^− 4 and 28.3 × 10 ^− 4 (Ω.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 α 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 TiO₂ photoelectrode with roselle extract (η  = 0.046%) and papaw leaves (η  = 0.022%), respectively and found better efficiency for roselle extract-sensitized cell because of the broader absorption of the roselle extract onto TiO₂ [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 e 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 η  = 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. O eu - V characteristics of DSSCs employing different natural dyes are shown in Table 6.

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 (α811nm = 1.5 × 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 π-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.

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