Unsymmetrical squaraine dyes for dye-sensitized solar cells: Position of the anchoring group controls the orientation and self-assembly of sensitizers on the TiO₂ surface and modulates its flat band potential

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.