Electron transfer processes at the interfaces dictate the factors that improve the photovoltaic parameters, such as open-circuit voltage (V oc ) and short-circuit current (J sc ), of a dye-sensitized solar cell device, besides selection of a set of suitable anode, dye, electrolyte, and cathode materials. An inefficient charge injection process at the dye−TiO 2 interface and charge recombination at the TiO 2 −dye/ electrolyte interface have detrimental effects on improving both J sc and V oc . Hence, tailoring the factors that govern the improvement of J sc and V oc will be an ideal approach to get the desired sensitizers with good device efficiencies. Squaraines are far-red-active zwitterionic dyes and have a high molar extinction coefficient along with unique aggregation properties due to the large dipole moment associated with them. Here, we report a series of unsymmetrical squaraine dyes, SQS1 to SQS6, with systematic variation of alkyl groups at the sp 3 -C and N-atoms of the indoline unit that is away from the anchoring group to control the dye−dye interactions on the TiO 2 surface. The branched alkyl groups help in modulating the self-assembly of sensitizers on the TiO 2 surface, besides passivating the surface that helps avoid the charge recombination processes. Light harvesting efficiency and cyclic voltammetry studies of dye-sensitized TiO 2 electrodes indicate that the aggregation and charge hopping process between the dye molecules can be modulated, respectively, by systematically increasing the number of carbon atoms in the alkyl groups. Such a variation in the branched alkyl group helps enhance V oc from 672 (SQS1) to 718 mV (SQS6) and J sc from 7.95 (SQS1) to 12.22 mA/cm 2 (SQS6), with the device efficiency ranging from 3.82% to 6.23% without any coadsorbent. Dye SQS4 achieves the highest efficiency of 7.1% (V oc = 715 mV, J sc = 13.05 mA/cm 2 ) with coadsorbent chenodeoxycholic acid (CDCA) using an iodine (I − /I 3 − ) electrolyte compared to its analogues. An analysis of the incident photon-to-current efficiency profiles indicates that the major contribution to photocurrent generation is from the aggregated squaraine dyes on TiO 2 .

Organic dyes possessing conjugated π-framework forms closely packed monolayers on photoanode in dye-sensitized solar cell (DSSC), because of the limitation to control the orientation and the extend of intermolecular π-π interaction, self-aggregation of dyes leads to reduced cell performance. In this report, a series of homodimeric (D-D and D-D) and heterodimeric (D-D and D-D) donor/acceptor (D/A) dyes containing spiroBiProDOT π-spacer were designed and synthesized by utilizing Pd-catalyzed direct arylation reaction and correlates the device performance with monomeric dyes (D and D). Both the thiophenes (π-spacer) of spiroBiProDOT were functionalized with same or different donor groups which led to homodimeric and heterodimeric chromophores in a single sensitizer. The homodimeric spiro-dye D-D showed higher power conversion efficiency (PCE), of 7.6% with a V and J of 0.672 V and 16.16 mA/cm, respectively. On the other hand, the monomeric D exhibited a PCE of 3.2% (V of 0.64 V and J of 7.2 mA/cm), which is lower by 2.4 fold compared to dimeric analogue. The spiro-unit provides flexibility between the incorporated chromophores to orient on TiO due to four sp-centers, which arrest the molecular motions after chemisorption. This study shows a new molecular approach to incorporate two chromophores in the dimeric dye possessing complementary absorption characteristics toward panchromatic absorption. The attenuated charge recombination at TiO/Dye/redox couple interface in case of D-D, owing to better passivation of TiO surface, was elucidated through impedance analysis. The FT-IR spectrum of D-D adsorbed on TiO film indicated both the carboxylic units were involved in chemisorption which makes strong coupling between dye and TiO.

The performance of photoelectrochemical solar cells using heteroleptic Ru dyes paired with Co3+/2+ electrolytes is worse than their organic-dye-based analogues. Previous studies point to the importance of minimizing intermolecular interactions between the Ru dyes and the Co3+/2+ complexes by bulky groups on the dyes or the redox mediators. However, the performance boosting additive 4-tert-butylpyridine (t-BPy) is often added to the electrolyte for electron lifetime studies, which masks the intrinsic effect of the molecular structure of the redox-active molecules. Here, electron lifetime studies including four cobalt mediators decorated with alkyl chains and two Ru dyes with/without nonyl chains have been performed in the absence and presence of t-BPy. A synergistic effect of alkyl chain substitution on Ru dyes and Co3+/2+ complex mediators is revealed in the absence of t-BPy. The electron lifetime increases dramatically when both the dyes and the mediators are decorated with alkyl chains. The synergistic effect is explained by the size of the free space between the dyes compared to the size of the redox mediators. Adding alkyl chains to the Ru dyes decreases the free space between the dyes, so that the bulky Co3+/2+ complexes cannot approach the TiO2 surface, increasing the recombination distance. Although alkyl chains successfully diminish intermolecular interactions leading to higher charge separation efficiency and increased electron lifetime, the fill factor is limited by the slow diffusion of the bulky mediators, especially in the presence of t-BPy. The exceptionally long electron lifetime achieved due to the synergistic effect of alkyl chain barriers paves the way for t-BPy free photoelectrochemical solar cells.

Light absorbing sensitizer is the heart of the third generation photovoltaic technologies such as dye-sensitized solar cells. Despite possessing large extinction coefficients for the metal-free organic dyes, the light-harvesting efficiency is limited with the narrow absorption profile in either visible or NIR regions of the solar spectrum. Though monolayer formation of organic sensitizers on semiconducting metal oxide surface leads to dye–dye interaction that leads to broadening the absorption profile, the contribution of the aggregated state to the power conversion process is found out to be poor. Here, we have engineered far-red active π-extended cis-configured unsymmetrical squaraine dyes with carboxylic acid and cycnoacetic acid as anchoring groups with controlled aggregation and panchromatic light absorption by including the steric and electronic factors. The presence of sp3-C (out-of-plane) and N-alkyl (in-plane) groups played an important role in modulating the assembly of the dyes on the TiO2 electrode; the cis-squaraine unit helps to extend the conjugation in the NIR region besides enhancing the visible transition and the incident photon-to-current conversion efficiency profile that showed the importance of anchoring groups for the panchromatic response. Dye PSQ10 showed solar-to-electric conversion with an onset of 850 nm with the device performance of 7.0% (V oc = 0.581 V, J sc = 17.06 mA/cm2, FF = 70%).