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 .

A series of donor–acceptor–donor (D–A–D) based unsymmetrical squaraine dyes have been synthesized with indoline and aniline as donor units. The sp3-C, N atoms of the indoline and N atom of the aniline donors are appended with alkyl groups systematically to control the self-assembly of dyes on TiO2 and to passivate the photoanode that helps in improving the open circuit voltage (V oc) of the dye-sensitized solar cell (DSSC) device. The resulting dyes, AK1–4, possess strong absorptions between 529 and 541 nm with extinction coefficients of 1.44–1.84 × 105 M–1 cm–1. Photophysical and electrochemical studies revealed that the lowest unoccupied molecular orbital and highest occupied molecular orbital of the dyes are aligned with the conduction band of TiO2 and the redox function of the electrolyte for the efficient charge injection and dye regeneration processes, respectively. A DSSC device performance for AK4 of 7.93% (V oc of 815.88 mV and short circuit current density (J sc) of 12.4 mA/cm2) is obtained, with an incident photon to current conversion efficiency (IPCE) response of 92% at 473 nm and 86% at 566 nm without sensitizing with coadsorbent. IPCE studies revealed the equal contribution for the photocurrent generation from the H-aggregated structures. Further, such a high V oc and DSSC efficiency have been observed first time for this class of zwitterionic dyes with the I–/I3 – electrolyte without sensitizing with coadsorbent. The importance of introducing the alkyl groups at the indoline as well as aniline donor units for the synergistic boosting of both V oc and J sc is compared with the model dye AK1. Complementary light absorbing dyes SQ1 and SQS4 possess absorption at 643 nm with extinction coefficients of 1.4 and 3.0 × 105 M–1 cm–1, respectively. A cosensitized device performance of 9.36% is obtained for the AK4:SQS4 (1:1) combination in the I–/I3 –electrolyte. The IPCE profile indicates a good response over 450–700 nm with 94% at 621 nm. The importance of aggregated structures from both the dyes AK4 and SQS4 for photocurrent generation is realized.

Metal-free near-infrared (NIR) active unsymmetrical squaraine dyes, RSQ1 and RSQ2, with benzodithiophene (BDT) π-spacer and cyanoacrylic acid acceptor were synthesized by utilizing palladium catalyzed direct (hetero)arylation reaction. Methyl and 2-ethylhexyl groups were strategically placed at the BDT unit for RSQ1 and RSQ2, respectively, to investigate the effect of alkylated π-spacer on dye aggregation on the TiO surface and recombination reactions at TiO/dye/electrolyte interface. These dyes have strong absorption (ε > 10 M cm) in near-infrared (NIR) region and exhibit similar optical and electrochemical properties as they have same conjugated framework. RSQ2 performed better than RSQ1 owing to its higher open-circuit voltage (V) and fill factor (ff) in spite of having comparable short-circuit current density (J). The panchromatic incident photon-to-current conversion efficiency (IPCE) response was also observed for both the dyes. RSQ2 showed power conversion efficiency (PCE) of 6.72% with short-circuit current density (J) of 18.53 mA/cm, open circuit voltage (V) of 0.538 V, and fill factor (ff) of 67.4%, without any coadsorbent. Attenuation of the charge recombination for RSQ2 was revealed by electrochemical impedance analysis (EIS) and open-circuit potential decay transients (OCVD), which attributes to its higher V and ff in comparison to RSQ1.

A high molar extinction coefficient with sharp absorption properties from the range of visible to near-infrared regions makes squaraine dyes very attractive and potential chromophores for dye-sensitized solar cell (DSSC) applications. Here, we report a series of alkyl groups and glycolic chain [triethylene glycol (TEG)]-wrapped amphiphilic indoline-based unsymmetrical squaraine dyes, where the number of carbon atoms in the alkyl groups and TEG was systematically changed by incorporating the alkyl groups and TEG within the dye molecule for controlling the self-assembly of the dyes on the TiO2 surface and to improve the interfacial properties at the dye-TiO2/electrolyte interface. Due to the same skeletal characteristics, ASQ dyes showed similar photophysical and electrochemical properties in solution. Photovoltaic characterization of ASQ dyes was carried out in nonaqueous and aqueous electrolytes to evaluate the effect of alkyl groups and TEG on nonaqueous and aqueous DSSC device performances. V OC, J SC, and photovoltaic efficiencies were systematically enhanced by increasing the number of carbon atoms of alkyl groups into the dye molecules for nonaqueous DSSCs. Furthermore, addition of chenodeoxycholic acid (CDCA) decreased the charge recombination processes and resulted in enhanced efficiency, V OC, and J SC (enhanced incident photon-to-current conversion efficiency performance) compared to that without CDCA. The ASQ4 dye gave the highest nonaqueous DSSC efficiency of 6.39%, a V OC of 711 mV, and a J SC of 12.18 mA/cm2 with 2 equiv of CDCA in the ASQ dye series. Furthermore, increased carbon atoms in the alkyl groups showed a detrimental effect on the aqueous DSSC efficiency due to the mismatch in the polarity at the dye-TiO2/electrolyte interface, being the reason for the decreased device efficiencies from ASQ2 to ASQ4 dyes. The ASQ2 dye gave the highest aqueous DSSC efficiency of 2.36%, a V OC of 611 mV, and a J SC of 4.75 mA/cm2 without CDCA in the ASQ dye series.

A series of metal-free indoline-based donor-π-acceptor (D-π-A) visible-light active organic dyes, where an indoline unit wrapped with alkyl or alkyl and glycolic (TEG) groups is used as a donor, a mono- or bi-thiophene unit is used as a π-spacer, and cyanoacrylic acid acts as an acceptor unit, have been designed and synthesized for dye-sensitized solar cell (DSSC) application. These dyes showed three absorption bands in a dichloromethane solution: the first and second transitions are observed in the UV (337–385 nm) and visible (477–514 nm) regions and correspond to π–π* and intramolecular charge transfer (ICT) transition, respectively, whereas the third absorption band obtained in the near-IR (629–758 nm) region has been observed for the first time for a D-π-A type dye architecture. Furthermore, the ICT transition band for AKT dyes showed negative solvatochromism with increasing polarity of solvents. Photovoltaic characterizations for AKT dyes have been systematically studied with a cobalt electrolyte to evaluate the effect of thiophene incorporation and alkyl group substitution by the TEG group on the DSSC device performance. Here, AKT2 showed the highest DSSC efficiency of 3.16% with V OC of 0.680 V and J SC of 6.24 mA/cm2 without chenodeoxycholic acid (CDCA) in the AKT dye series, whereas DSSC efficiencies for AKT dyes were not affected much upon further addition of CDCA. Furthermore, AKT dyes were co-sensitized with a complementary optical responsive indoline-based SQS4 dye and showed enhanced efficiency. The combination of AKT2/SQS4 dyes with a ratio of 1:1 showed a maximum of 5.20% co-sensitized solar cell efficiency compared to other dye combinations with the cobalt electrolyte.

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%).

The position of the anchoring group is systematically changed with a series of alkyl group wrapped donor–acceptor–donor (D–A–D) based squaraine dyes, 4-SQ to 7-SQ, for the use in dye-sensitized solar cells (DSSCs). By this approach, the orientation as well as the self-assembly of the sensitizers can be controlled on the semiconducting TiO2 surface. All of the dyes functionalized with hydrophobic alkyl groups at sp3-C and N atoms of the indoline units that is far away from the TiO2 surface to control the self-assembly of dyes and passivate the surface. Controlling both the orientation as well as the self-assembly of the sensitizers synergistically enhances the V oc of the DSSC device by imparting the dipole moment on the TiO2 surface and minimizing the interfacial charge recombination process of electrons from TiO2 to the oxidized electrolyte, respectively. Further, the presence of a meta-carboxyl group with respect to the N atom of the indoline donor unit for the dyes 4-SQ and 6-SQ makes them nonconductive for the charge injection process, which sheds light on the importance of through-space electron transfer for the device performance. Emission from the relaxed twisted state was found to be a deactivation pathway for 4-SQ on TiO2 and ZrO2, which revealed the importance of structural factors that promote spatial interaction between the sensitizer and metal oxide surface. Computational studies showed the systematic changes in the dipole moment for the sensitizers 4-SQ, 5-SQ, and 6-SQ upon anchoring to the TiO2 surface. The DSSC device performance varied with the position of anchoring groups in the sensitizers. The DSSC device performance of 5-SQ indicates a J sc value of 11.35 mA cm–2, V oc of 0.698 V, and ff of 77% corresponding to a power conversion efficiency of 6.08% in the presence of 3 equiv of coadsorbent CDCA, which is nearly 1.5 times higher than 6-SQ (V oc 0.7 V, J sc 7.76 mA cm–2, ff 76%, and η 4.14%) and 2.6 times higher than 4-SQ (V oc 0.658 V, J sc 4.42 mA cm–2, ff 78%, and η 2.28%). IPCE studies revealed the importance of orientation for the charge injection and self-assembly of dyes, as devices with 5-SQ and 6-SQ as a sensitizer showed 94 and 77% response at 578 nm, respectively, which correspond to the aggregated structure of the dye. Mott–Schottky and IPCE experiments showed that the orientation of sensitizers could modulate the V oc due to the shift in the flat band potential of TiO2.

Alkyl group wrapped donor−acceptor−donor (D-A-D) based unsymmetrical squaraine dyes SQ1, SQ5, and SQS4 were used to evaluate the effect of sensitizing solvents on dye-sensitized solar cell (DSSC) efficiency. A drastic change in DSSC efficiency was observed when the photo-anodes were sensitized in acetonitrile (bad solvent when considering dye solubility) and chloroform (good solvent) with an Iodolyte (I − /I 3 − ) electrolyte. The DSSC device sensitized with squaraine dyes in acetonitrile showed better photovoltaic performance with enhanced photocurrent generation and photovoltage compared to the device sensitized in chloroform. In a good sensitizing solvent, dyes with long hydrophobic alkyl chains are deleterious forming aggregates on the TiO 2 surface, which results in an incident photon-to-current conversion efficiency (IPCE) response mostly from monomeric and dimeric structures. Meanwhile, a bad sensitizing solvent facilitates the formation of well-packed self-assembled structures on the TiO 2 surface, which are responsible for a broad IPCE response and high device efficiencies. The photoanode sensitized in the bad sensitizing solvent showed enhanced V OC values of 642, 675, and 699 mV; J SC values of 6.38, 11.1, and 11.69 mA/cm 2 ; and DSSC device efficiencies of 3.0, 5.63, and 6.13% for the SQ1, SQ5, and SQS4 dyes in the absence of a coadsorbent (chenodeoxycholic acid (CDCA)), respectively, which were further enhanced by CDCA addition. Meanwhile, the photoanode sensitized in the good sensitizing solvent showed relatively low photovoltaic V OC values of 640, 652, and 650 mV; J SC values of 5.78, 6.79, and 6.24 mA/cm 2 ; and device efficiencies of 2.73, 3.35, and 3.20% for SQ1, SQ5, and SQS4 in the absence of CDCA, respectively, which were further varied with equivalents of CDCA. The best DSSC device efficiencies of 6.13 and 3.20% were obtained for SQS4 without CDCA, where the dye was sensitized in acetonitrile (bad) and chloroform (good) sensitizing solvents, respectively.