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 TiO 2 surface. All of the dyes functionalized with hydrophobic alkyl groups at sp 3 -C and N atoms of the indoline units that is far away from the TiO 2 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 TiO 2 surface and minimizing the interfacial charge recombination process of electrons from TiO 2 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 TiO 2 and ZrO 2, 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 TiO 2 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 TiO 2 .