Carbazole based organic dyes as effective photosensitizers: A comprehensive analysis of their structure‐property relationships

Abstract

The present work describes the effect of structural modification of carbazole‐based photosensitizers carrying carboxylic acid as a common anchoring functionality, on the photovoltaic parameters of newly fabricated DSSCs. In this study, we have selected our previously reported three carbazole‐based derivatives, viz. S_1‐3 having different structural designs, that is, D‐π‐A (S₁), D‐D‐π‐A (S₂), and A‐π‐D‐π‐A (S₃) with different donor units and π‐spacers, but an identical cyanoacetic acid anchoring unit. We have evaluated their optical, electrochemical, and photovoltaic behaviors in order to explore their structure‐property relationships. Also, the theoretical investigations were performed to obtain a deeper understanding of their HOMO‐LUMO levels, charge distribution in FMOs, directional flow of electrons within the push‐pull type sensitizers, and optical behavior. Finally, the DSSCs were constructed by employing these dyes as sensitizers without any co‐absorbents and the performance of the devices was evaluated by using illuminated current‐voltage characteristics. Among the tested dyes, di‐anchoring S₃ exhibited improved PCE of 3.77 % due to its strong adsorption on the TiO₂ surface that resulted in superior V_OC of the cell. While the S₂ containing electron‐releasing anisole as an auxiliary donor exhibited better J_SC value leading to the optimum PCE of 3.73 % which is comparable to that of S₃. Obviously, these results validate the role of the π‐spacer and additional donor of the sensitizers on the overall performance of the DSSCs.

INTRODUCTION

The production of electrical energy from the cleanest and eco-friendly renewable resources is considered to be one of the most challenging scientific problems in the present world. Among the different technologies used for energy generation, the dye-sensitized solar cell (DSSC) that belongs to the family of third-generation photovoltaics, has garnered much attention as the most prospective and potential solar cell since its innovation by Gräztel group.^[] Among the different components of DSSCs, sensitizers play a key role in overall performance of the device by initiating photon harvesting and commencing the electrochemical processes for a current generation.^[] In reality, the dye as a photosensitizer serves as a vital element in DSSC, so its molecular geometry should be properly designed to achieve a broader panchromatic absorption profile, good stability as well as suitable frontier molecular orbitals (FMOs) energy levels.^[]

To date, many organometallic Ru-II complexes, zinc-porphyrins, as well as several aromatic/heteroaromatic metal-free dyes have been explored as potential sensitizers for mesoporous TiO₂ based DSSCs.^[] Among these dyes, the Ru-II complexes were exploited as first-rate sensitizers due to their excellent photo-electrochemical behavior and stability. Also, they have been shown to possess optimum performance beyond 10 %. But, the high cost of noble metals, poor availability, purification problems, and environmental hazards have made further developments limited in this area.^[] Conversely, the organic dyes offer several merits over the Ru-II-based complexes, which include good design versatility, tunable photophysical and electrochemical behavior, a large molar extinction coefficient, and reduced production cost. However, the performances of DSSCs sensitized with organic dyes were still lower than that of devices based on Ru-II complexes, mainly due to their narrow absorption bands and instability.^[] Though considerable efforts have been made to molecular engineering of dyes to apprehend panchromatic sensitizers that harvest almost all solar spectrum, satisfying results in the performance are yet to come up using a single dye.^[] In this context, the donor-acceptor strategy, with the insertion of appropriate π-spacer, that is, D-π-A is the most widely used approach to achieve desired properties in the organic sensitizers.

Encouraged by these outcomes, several research groups including ours have explored numerous organic dyes based on electron-donating units such as thiophene, aniline, triphenylamine, carbazole, indole, and so on carrying effective anchoring units like cyanoacetic acid, barbituric acid, phosphonic acid, sulphonic acid, benzonitriles, pyridines, and so on as potential photo-sensitizers in DSSCs.^[] Among the aforementioned systems, the heteroaromatic carbazole moiety is considered to be one of the most widely used electron donors in the design of the photosensitizers, mainly due to the fact that it has very good hole carrying ability with wide energy bandgap and allows facile substitution for hydrogen at different positions along with decent thermal and photochemical stability.^[] Further, the benzene and its analogs are used efficiently as the π-spacer in the molecular engineering of the dyes due to their tunable spectroscopic and electrochemical behaviors, good charge transporting ability and increased overall stability and rigidity in the resulting dyes.^[] Further, the cyanoacetic acid and carboxyl groups have been reported to be effective electron acceptor/anchoring units for the design of new D-π-A type sensitizers and such dyes have been shown to display superior power conversion efficiency (PCE).^[] Considering aforesaid facts, several new organic photosensitizers based on heteroaromatic carbazole donor have been designed and well investigated in the literature. For example, Ramkumar et al. (2012) reported a D-(π-A)₂ configured organic dye (Car-th-CN) centered on electron-rich carbazole as donor connected to cyanoacetic acid unit via cyanovinyl thiophene as effective photosensitizer for DSSC application which yielded PCE of 4.04 % on fabrication.^[] Later, a carbazole-based dye (SK3) with D-π-A architecture was reported by Soni and co-workers in 2015. Interestingly, this dye displayed PCE of 9.0 % with a cobalt based redox shuttle, while a PCE of 7.1 % has been obtained with triiodide electrolyte when employed it as a photosensitizer in DSSCs.^[] Recently, Yan et al. (2020) designed and investigated three triphenylethylene (TPE) decorated carbazole based photosensitizers (JY 60–62) and TPE free carbazole centred dye CVHTC was selected as the reference. The dye JY-60 reached the best PCE of 6.4 % surpassing the reference dye CVHTC effeciency by 30 %.^[]

Highlights

• Extensively studied our previously reported carbazole-based dyes (S_1-3) as sensitizers
• Performed their in-depth optical, electrochemical, theoretical, and photovoltaic studies
• Conducted a comprehensive analysis of their structure-property relationships
• The unsymmetrical di-anchored A-π-D-π-A (S₃) dye displayed higher PCE due to better V_OC
• The D-D-π-A (S₂) dye showed PCE value almost comparable to S₃, but improved J_SC

Although many reports have been evolved for a detailed structure–property relationship of several heterocycle-based sensitizers, there are very limited references accessible for cyanoacetic acid anchored carbazole-based dyes, addressing the structural diversity and its effect on photovoltaic parameters. Keeping this in view, herein we describe a comparative account of their optical, electrochemical, computational, and photovoltaic studies, including EIS analysis of three interesting carbazole-based dyes with different design architectures, viz. D-π-A (S₁), D-D-π-A (S₂), and A-π-D-π-A (S₃) as active sensitizers in DSSCs.^[] The effect of their molecular structures on the above-said properties has been discussed in-depth. Figure  depicts the molecular structures of the carbazole-based chromophores, S_1-3 under study.

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RESULTS AND DISCUSSION

Optical and electrochemical studies

The UV-Vis. absorption spectra of S_1-3 recorded in chloroform solution at room temperature using Analytica Jena SPECORD S600 UV-Vis. absorption spectrophotometer are presented in Figure  and their respective absorption data are given in Table . In their spectra, the dyes S_1-3 show two well-resolved characteristic absorption peaks at the region of 278–332 nm and 408–421 nm, respectively. The absorption peak observed in the former region arises due to π-π* electronic transition. While the intense absorption peak witnessed in the latter region corresponds to the intermolecular charge transfer (ICT) from carbazole donor to the cyanoacetic acid acceptor moiety. It is interesting to note that the unsymmetrical di-anchoring dye S₃ carrying phenylene and vinylene π-spacers show abathochromic shift in comparison to the other two dyes.

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TABLE 1 Optical and electrochemical characterization data of dyes S_1-3

Dyes	S1	S2	S3
λabs (nm)	412	416	421
λem (nm)	546	546	548
Stokes shift (nm)	134	130	127
EOX (V)	0.78	0.91	1.36
E0-0, Optical (eV)	2.61	2.67	2.63
GSOP (eV)	–5.48	–5.61	–6.06
ESOP (eV)	–2.87	–2.94	–3.43
∆Ginj (eV)	1.33	1.26	0.77
∆Greg (eV)	0.28	0.41	0.86
∆Grec (eV)	1.28	1.41	1.86

Further, their photoluminescence emission spectra were recorded in chloroform solution using Jasco FP-6200 spectrophotometer. Figure  portrays the normalized fluorescence emission spectra of the dyes S_1-3 in CHCl₃ solutions and their resultant spectral parameters are tabulated in Table . In Figure , it has been observed that the dyes S_1-3 display a strong single emission band in the range of 546–548 nm. Further, their Stokes shift values were calculated from their normalized photophysical data (Table ). Also, their E_0-0 (optical band gaps) were estimated from the intersection point of normalized absorption, and emission spectra and the corresponding values are summarized in Table 

In addition, the UV-Visible measurements over TiO₂ film were recorded on a Cary 300 spectrophotometer. A corresponding blank TiO₂ film was used as reference to obtain the UV-visible spectrum of the dye adsorbed on the TiO₂ film. Both have the same thickness. We used integrated sphere to correct fir reflectance and scattering. The obtained absorption spectra of S_1-3 dyes adsorbed on the surface of TiO₂ are portrayed in Figure . From the spectra, it is clear that the absorption peaks of S_1-3 dyes adsorbed on the surface of TiO₂ films have been broadened and redshifted in comparison to the corresponding peaks of pure samples in the solution medium. Generally, the observed blue-shift corresponds to H-aggregation (parallel orientations), while red-shift relates to J-aggregation (tilted orientations) of a fluorescent dye. Here, the formation of aforesaid J-aggregates can be attributed to the interaction between the carboxylate group of the dye molecules and Ti⁴⁺ ions of the film.^[] Such interactions facilitate the lowering of the π* energy level of the dye molecule and thus bringing about broadening of its absorption spectrum. Such broadening of the absorption profile is a prerequisite for a dye to act as an efficient sensitizer in DSSCs as it may lead to enhancement in photocurrent density of the devices.^[] Further, the absorbance profile of the sensitizers S_1-3 follows the order S₃ > S₂ > S₁ which is in good agreement with the obtained J_SC values (Table ).

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The HOMO/GSOP and LUMO/ESOP values are usually used to gauge the prospects of dyes for processes in the cell such as active injection of accumulated charges as well as their regeneration. Further, the highest occupied molecular orbital/ground state oxidation potential (HOMO/GSOP) and lowest unoccupied molecular orbital/excited state oxidation potential (LUMO/ ESOP) values of the dyes S_1-3 were determined from cyclic voltammetry (CV) studies and the estimated values are tabulated in Table . The GSOP and ESOP energy levels of dyes as estimated in volts (V) were converted to electron volt (eV) against NHE according to the Equations () and (). 1 $\begin{equation}{\rm{GSOP}} = - \left[ {E_{{\rm{onset}}}^{{\rm{oxd}}} + {\rm{4}}{\rm{.7\;eV}}} \right]\end{equation}$2 $\begin{equation}{\rm{ESOP}} = \left[ {{\rm{GSOP}} + {E_{0 - 0}}} \right]{\rm{\;eV}}\end{equation}$

In the equation, $${\rm{E}}_{{\rm{onset}}}^{{\rm{oxd}}}$$ is the oxidation potential onset of CV curve and $${{\rm{E}}_{{\rm{0 - 0}}}}$$ corresponds to optical energy band-gap.

The estimated GSOP/HOMO levels of dyes S_1-3 are –5.48 (S₁), –5.61 (S₂), and –6.06 eV (S₃) versus SHE. However, the assessed LUMO/ESOP levels of dyes S_1-3 were found to be –2.87 (S₁), –2.94 (S₂), and–3.43 eV (S₃) versus SHE. Based on the calculated HOMO/GSOP and LUMO/ESOP values, the energy level diagram for the dyes S_1-3 with respect to the Nernst potential of CB edge of the TiO₂ (–4.2 eV) and I^–/I₃^–electrolyte system (–5.2 eV) against the SCE (standard calomel electrode) has been constructed and it is shown in Figure . Further, the thermodynamic parameters, viz. ΔG_inj (Gibbs free energy for electrons injection), ∆G_reg (Gibbs free energy for dye regeneration), and ΔG_rec (Gibbs free energy for charge recombination) were calculated from the energy level diagram using Equations () to (), and the corresponding values are listed in Table . 3 $\begin{equation}{\rm{\Delta \;}}{G_{inj}} = E_{OX}^*\; - {E_{CB}}\end{equation}$4 $\begin{equation}{\rm{\Delta \;}}{G_{rec}} = {E_{CB}}\; - {E_{OX}}\;\end{equation}$5 $\begin{equation}\Delta \;{G_{reg}} = {E_{I_3^{ - \;}/{I^ - }}}\; - {E_{OX\;}}\end{equation}$

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From the obtained results, it is quite clear that all the dyes possess appropriate thermodynamic driving forces (negative values showing spontaneity) for effective injection of charge as well as dye-regeneration.^[] Therefore, the dyes can be employed as potential photosensitizers as they satisfy all the stringent electrochemical prerequisites for the appropriate movement of charges through the complete photo-electronic conversion cycle in the cell.

Computational studies

The adiabatic quantum calculations such as DFT and TD-DFT are considered to be the most dependable computational approaches in the field of organic photovoltaics, for simulating conjugated organic molecules.^[] Theoretical studies are generally undertaken to gain deeper insight into the molecular geometry and electron distribution in the sensitizer as well as its optical behavior in the excited state. In the present work, the Turbomole V7.2 software package was used to perform all the simulations. To begin with, dye structure was first optimized using semi-empirical Austin Model 1 (AM1) using MOPAC in T-Mole 4.2. Here, all the DFT simulations were executed at B3LYP/def-TZVPP level for the isolate dyes S_1-3 in the gas phase, to obtain the optimized geometry and electron distributions in their FMOs (frontier molecular orbitals) energy levels.^[] Figure  shows the optimized molecular geometries of S_1-3 dyes and their subsequent charge distributions at their corresponding FMO levels as acquired from the T-mole 3D visualizer. Correspondingly, the theoretically obtained HOMO, LUMO, and bandgap values of the dyes are shown in Table . From the results (Table ), the HOMOs of the organic materials S_1-3 are ‒5.82 eV (S₁), ‒5.74 eV (S₂), and ‒6.26 eV (S₃), whereas their LUMOs are ‒2.93 eV (S₁), ‒2.50 eV (S₂), and ‒2.90 eV (S₃). Also, it has been noticed that the trend of the theoretically calculated FMO energy values of the dyes is almost comparable with experimentally determined values. As predicted, the optimized 3-D structures of S_1-3 evidently showcased the effective charge separation in their FMOs levels. From Figure , it is clear that the HOMOs of the dye S_1-3 are distributed evenly over the entire π-conjugated system, i.e., carbazole scaffold. While, at their LUMOs, the electron cloud is noticeably shifted from the carbazole donor to the electron-accepting cyanoacetic acid unit at different extent mainly due to dissimilar structural architectures. From the results, it is quite evident that the electron clouds in all the dyes are effectively delocalized from donor to acceptor unit, favoring the promising electron movements. But, in the case of S₃, the electron cloud has completely moved towards the cyanoacetic acid unit that has linked directly to the phenylene π-spacer rather than the vinylene unit.

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TABLE 2 Computational analysis data

Dyes	HOMO (eV)	LUMO (eV)	E0-0, Optical (eV)
S1	–5.82	–2.93	2.89
S2	–5.74	–2.50	3.24
S3	–6.26	–2.90	3.36

In the present study, the molecular electrostatic potential (MESP) analysis was executed to predict certain molecular properties of the organic dyes. Figure  shows the electrostatic potential (ESP) maps of S_1-3 dyes, obtained using their optimized geometries on the corresponding molecular surface. Here, the generated ESP maps illustrate the charge distributions in the molecules three-dimensionally. As a rule, the areas of electrostatic potential in terms of color increase in the order of blue > green > yellow > orange > red.^[] However, the areas of low potential (red color) and the areas of high potential (blue color) are characterized by abundance and dearth of electrons, respectively. From Figure , it is evident that the low potential was observed for all the three dyes on the electron-accepting carboxylic acid (-COOH) group, thus indicating the abundance of electrons in the red region of the molecule. The yellow and green color regions imply intermediate potential. Also, the density of states (DOS) plots obtained for isolate dyesS_1-3 are portrayed in Figures S4-S6. Generally, the density of states is an average over the time and space domains occupied by the system, and it describes the contribution of different orbitals over a different range of energy. The contribution of p-orbital was seen to be predominately over the s-orbital with respect to increasing energy. Here, p-orbital contribution of the O (oxygen) atom of the anchoring unit in the dye facilitates strong binding on the surface of TiO₂. Interestingly, from the figure, it is evident that all the dyes exhibit a similar type of density of states indicating that S_1-3 possess all the prerequisites to act as good photo-sensitizers.

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Further, in order to probe the electronic excitations of dyes S_1-3 under the influence of time-dependent perturbations, the TD-DFT studies were carried out. Generally, the TD-DFT simulations significantly depend on the adiabatic approximation and it states that at any point of time, the XC functional depends only on the instantaneous density. Consequently, the XC functional applied for optimized geometry of dyes derived from ground-state DFT simulations, that is, B3LYP density functional can be used in the aforesaid simulations. It is well documented in the literature that the precision of the assimilated results broadly depends on the employed basis set and exchange functions.^[] The simulated absorption spectra of the S_1-3 obtained at B3LYP/def TZVP level are depicted in Figures S1-S3. As evidenced by Figures S1-S3, the simulated absorption spectra of S_1-3 show two distinct peaks corresponding to π-π* transitions and ICT phenomena, which are well in agreement with the experimentally obtained UV-Vis. absorption spectra. These results revealed that the basis set and (XC) functional chosen are quite appropriate for the above-said calculations.

Photovoltaic studies

With the drive to establish the structure-performance relationship of the organic dyes, the DSSCs were fabricated by using the dyes S_1-3 as sensitizers on the surface of mesoporous TiO₂ electrode.^[] The detailed experimental procedure followed for the fabrication of the cell is given in the Supporting Information. Also, the EIS analysis was carried out for all the fabricated devices in order to comprehend the photovoltaic performance of the cells.^[]

The J-V and IPCE characteristic curves acquired for the S_1-3 sensitized devices are shown in Figures  and their subsequent photovoltaic parameters data are tabulated in Table . The experimental results point out that the cell-based on unsymmetrical di-anchored dye (S₃) carrying two cyanoacetic acid moieties showcase the highest PCE (3.77 %) and superior V_OC (0.623 V) when compared to other cells. The light-harvesting ability (J_SC) of tested sensitizers decreases in the order of S₂ (8.96 mA.cm^–2) > S₃ (8.66 mA.cm^–2) > S₁ (7.38 mA.cm^–2) according to the conjugation length prevailing in their structures. From the obtained photovoltaic parameters (Table ), it is apparent that the DSSC based on S₃ displays the highest photovoltaic performance and their obtained PCE (η) values are in the declining order of S₃ (3.77 %) > S₂ (3.73 %) > S₁ (3.07 %). Generally, the J_SC values of the devices are directly associated with the IPCE values/quantum efficiency. The %IPCE of the devices fabricated with S_1-3 follows the decreasing order of S₂ (55%) > S₃ (51%) > S₁ (48%). Interestingly, the J_SC values of the fabricated devices follow the trend obtained from the IPCE response of the cell. Thus, the higher J_SC value obtained for the S₂ sensitized cell can be interpreted based on their yield in quantum efficiency. These results prominently suggest that the position of the phenylene moiety in the dye structure has a pronounced effect on the photovoltaic parameters.

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TABLE 3 Photovoltaic data of devices

Dyes	JSC (mA.cm–2)	VOC (V)	FF (%)	η (%)	τeff (ms)
S1	7.38	0.593	70.02	3.07	0.29
S2	8.96	0.594	70.03	3.73	0.30
S3	8.66	0.623	69.87	3.77	0.95

Generally, the photovoltage (V_OC) values of DSSCs are known to be affected by the charge-recombination dynamics occurring in the cell. The experimentally obtained Nyquist and Bode distinctive plots of fabricated devices using S_1-3 as sensitizers are depicted in Figures . In Nyquist characteristic plots (Figure ), a major semicircle for each fabricated device was observed which reflects the charge transfer recombination resistance (R_CT) at the TiO₂/dye/electrolyte interface, that is, the recombination kinetics between electrons in TiO₂ conduction band and $$I_3^ - $$ species from the electrolyte.^[] Generally, the R_CT corresponds to the diameters of the largest semicircle. However, the other two smaller semicircles corresponding to recombination resistance at the Pt/electrolyte interface and impedance corresponding to the Warburg diffusion process of $${I^ - }$$/$$I_3^ - $$ in the electrolyte may be hidden by the larger semicircle.^[] In the Nyquist plot (Figure ), the radii of the intermediate semicircle of the fabricated DSSCs were found to be in the order of S₃ > S₂ > S₁, suggesting that the di-anchoring S₃ has a substantially higher charge transfer resistance and slower electron recombination than S₁ and S₂. According to the results, the presence of an extra electron acceptor (bi-anchor) unit decreases electron recombination at the TiO₂/dye/electrolyte interfaces, which may account for the higher system performance. From the results, we can infer that the appearance of a larger semicircle in the middle frequency region clearly indicates the higher R_CT value, implying a lower dark current as well as a reduced rate of electron recombination process and thereby leading to a higher open-circuit voltage. Further, the lifetime in CB edge of TiO₂ (τ_eff) of the cells was determined from the Bode plots (Figure ) and the estimated values of devices sensitized with S_1-3 dyes were found to be S₃ (0.95 ms) > S₂ (0.30 ms) $$ \cong \;$$S₁ (0.29 ms) that replicates the order of their V_OC values (Table ).^[] The obtained results indicate that the molecular geometry of the dyes plays a vital role in the suppression of dark current/back reaction in sensitized DSSCs.

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CONCLUSION

In summary, we described the effect of different structural designs of carbazole-based dyes, viz. D-π-A (S₁), D-D-π-A (S₂), and unsymmetrical di-anchoring A-π-D-π-A (S₃) on their various physical properties. These dyes bearing different donor units and π-spacers, but identical cyanoacetic acid anchor, were employed as photosensitizers in the DSSC devices. A comprehensive account of their optical, electrochemical, computational, and photovoltaic studies has been highlighted. As observed, the cell sensitized with di-anchored dye (S₃) carrying two cyanoacetic acid units showcased the highest PCE of 3.77 % along with superior V_OC of 0.623 V, mainly due to its good anchoring ability. While the dye carrying the electron-releasing anisole unit as auxiliary donor showed the closest efficiency of 3.73 % due to its excellent J_SC value of 8.96 mA/cm². Overall, they demonstrated good photovoltaic performance in the devices due to favorable structural features. Eventually, these results provide a better understanding and deeper insight into how an appropriate sensitizer can be selected for further enhancement of photovoltaic properties by optimizing the various fundamental processes in the cell.

ACKNOWLEDGMENTS

The authors are greatly indebted to National Institute of Technology Karnataka (NITK), Surathkal, India, and North Carolina State University (NCSU), Raleigh, USA for providing necessary laboratory facilities.

CONFLICT OF INTEREST

The authors declare that they have no conflict of interest.

DATA AVAILABILITY STATEMENT

Data available on request due to privacy / ethical restrictions.

References

B. O'Regan, M. Grätzel, Nature 1991, 353, 737.

M. Grätzel, Acc. Chem. Res. 2009, 42, 1788.

L. Yu, J. Xi, H. T. Chan, T. Su, W. K. Chan, D. L. Phillips, J. Phys. Chem. C 2013, 117, 2041.

P. Liu, B. Xu, Y. Hua, M. Cheng, K. Aitola, K. Sveinbjörnsson, J. Zhang, G. Boschloo, L. Sun, L. Kloo, J. Power Sources 2017, 344, 11.

J. S. Panicker, B. Balan, S. Soman, V. C. Nair, Solar Energy 2016, 139, 547.

Y. S. Yen, H. H. Chou, Y. C. Chen, C. Y. Hsu, J. T. Lin,J. Mater. Chem. 2012, 22, 8734.

A. Mishra, M. K. R. Fischer, P. Bäuerle, Angew. Chemie ‐ Int. Ed., 2008, 48, 2474.

J. Gong, J. Liang, K. Sumathy, Renew. Sust. Energ. Rev. 2012, 16, 5848.

J. M. Cole, G. Pepe, O. K. Al Bahri, C. B. Cooper, Chem. Rev. 2019, 119, 7279.

G. Chen, H. Sasabe, T. Igarashi, Z. Hong, J. Kido, J. Mater. Chem. A 2015, 3, [eLocator: 14517].

V. Sugathan, E. John, K. Sudhakar, Renew. Sust. Energ. Rev. 2015, 52, 54.

M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry‐Baker, E. Mueller, P. Liska, N. Vlachopoulos, M. Graetzel, J. Am. Chem. Soc. 1993, 115, 6382.

G. Wang, Y. Wu, W. Ding, G. Yu, Z. Hu, H. Wang, S. Liu, Y. Zou, C. Pan, J. Mater. Chem. A 2015, 3, [eLocator: 14217].

H. Lee, J. Kim, D. Y. Kim, Organic Electronics 2018, 52, 103.

S. Wanwong, W. Sangkhun, J. Wootthikanokkhan, RSC Advances 2018, 8, 9202.

H. Li, Y. Wu, Z. Geng, J. Liu, D. Xu, W. Zhu, J. Mater. Chem. A 2014, 2, [eLocator: 14649].

Y. Ooyama, Y. Harima, Eur. J. Org. Chem. 2009, 18, 2903.

M. Liang, J. Chen, Chem. Soc. Rev. 2013, 42, 3453.

P. Naik, R. Su, M. R. Elmorsy, A. El‐Shafei, A. V. Adhikari, J. Energy Chem. 2018, 27, 351.

P. Naik, M. R. Elmorsy, R. Su, D. D. Babu, A. El‐Shafei, A. V. Adhikari, Sol. Energy, 2017, 153, 600 .

J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi, J. Am. Chem. Soc. 2014, 136, 5722.

K. S. Keremane, I. M. Abdellah, P. Naik, A. El‐Shafei, A. V. Adhikari, Phys. Chem. Chem. Phys. 2020, 22, [eLocator: 23169].

P. Qin, X. Yang, R. Chen, L. Sun, T. Marinado, T. Edvinsson, J. Phys. Chem. C 2007, 111, 1853.

H. Cheema, A. Peddapuram, R. E. Adams, L. McNamara, L. A. Hunt, N. Le, J. Org. Chem. 2017, 82, [eLocator: 12038].

F. De Angelis, Acc. Chem. Res. 2014, 47, 3349.

W. Zhang, P. Heng, H. Su, T. Ren, L. Wang, J. Zhang, J. Phys. Chem. C 2018, 122, [eLocator: 25219].

A. Venkateswararao, K. R. J. Thomas, C. P. Lee, C. T. Li, K. C. Ho, ACS Appl. Mater. Interfaces 2014, 6, 2528.

K. S. Keremane, R. Rao, A. V. Adhikari, Photochem. Photobiol. 2020, 97, 289.

K. S. Keremane, A. V. Adhikari, Electrochemical Science Advances, 2021, [DOI: https://dx.doi.org/10.1002/elsa.202000036]

X. Shi, Y. Yang, L. Wang, Y. Li, J. Phys. Chem. C 2019, 123, 4007..

T. Wei, X. Sun, X. Li, Y. Xie, ACS Appl. Mater. Interfaces 2015, 7, [eLocator: 21956].

S. Ramkumar, S. Manoharan, S. Anandan, Dyes Pigm. 2012, 94, 503.

S. S. Soni, K. B. Fadadu, J. V. Vaghasiya, B. G. Solanki, K. K. Sonigara, A. Singh, D. Das, P. K. Iyer, J. Mater. Chem. A 2015, 3, [eLocator: 21664].

Y.Q. Yan, Y.Z. Zhu, P.P. Dai, J. Han, M. Yan, J. Y. Zheng, Sol. Energy, 2020, 207, 428.

P. Naik, R. Su, M. R. Elmorsy, D. D. Babu, A. El‐Shafei, A. V. Adhikari, J. Photochem. Photobiol. A 2017, 345, 63.

P. Naik, R. Su, M. R. Elmorsy, A. El‐Shafei, A. V. Adhikari, Dyes Pigm. 2018, 149, 177.

P. Naik, R. Su, M. R. Elmorsy, A. El‐Shafei, A. V. Adhikari, Photochem. Photobiol. Sci. 2018, 17, 302.

W. Wu, J. Yang, J. Hua, J. Tang, L. Zhang, Y. Long, J. Mater. Chem. 2010,20, 1772.

D. Cao, J. Peng, Y. Hong, X. Fang, L. Wang, H. Meier, Org. Lett., 2011, 13, 1610.

G. Li, K. J. Jiang, Y. F. Li, L. M. Yang, J. Phys. Chem. C 2008, 112, [eLocator: 11591].

P. Qu, G. J. Meyer, Langmuir 2001,17, 6720.

G. Oskam, B. V. Bergeron, G. J. Meyer, P. C. Searson, J. Phys. Chem. B 2001, 105, 6867.

Y. Cao, T. Hughes, D. Giesen, M. D. Halls, A. Goldberg, T. R. Vadicherla, M. Sastry, B. Patel, W. Sherman, A. L. Weisman, R. A. Friesner, J. Comput. Chem. 2016,37, 1425.

C. J. Tan, C. S. Yang, Y. C. Sheng, H. W. Amini, J. Phys. Chem. C 2016, 120, [eLocator: 21272].

P. Naik, A. Planchat, Y. Pellegrin, F. Odobel, A. V. Adhikari, Sol. Energy 2017, 157, 1064.

P. K. M. Lokhande, K. K. Sonigara, M. M. Jadhav, D. S. Patil, S. S. Soni, N. Sekar, ChemistrySelect, 2019, 4, 4044.

V. V. Divya, C. H. Suresh, New J. Chem. 2020, 44, 7200.

Y. Wang, L. Xu, X. Wei, X. Li, New J. Chem. 2014, 38, 3227.

J. Bisquert, J. Phys. Chem. Chem. Phys. 2003, 5, 5360.

F. Fabregat‐Santiago, J. Bisquert, G. Garcia‐Belmonte, G. Boschloo, A. Hagfeldt, Sol. Energy Mater. Sol. Cells, 2005,87, 117.

S. Chang, H. Wang, Y. Hua, Q. Li, X. Xiao, W.‐K. Wong, W. Y. Wong, X. Zhu, T. Chen, J. Mater. Chem. A,1, 2013, 1, [eLocator: 11553].

M. Xu, X. Hu, Y. Zhang, X. Bao, A. Pang, J.‐K. Fang, ACS Appl. Energy Mater., 2018, 1, 2200.

S. Li, Y. Lin, W. Tan, J. Zhang, X. Zhou, J. Chen, Z. Chen, Int. J. Miner. Metall. Mater. 2010, 17, 92.

A. S. Nair, R. Jose, Y. Shengyuan, S. Ramakrishna, J. Colloid Interface Sci. 2011, 353, 39.

Q. Wang, J. E. Moser, M. Grätzel, J. Phys. Chem. B 2005, 109, [eLocator: 14945].

M. V. Vinayak, T. M. Lakshmykanth, M. Yoosuf, S. Soman, K. R. Gopidas, Sol.Energy 2016, 124, 227.