Sterically Controlled Interfacial Charge-Transfer Mechanisms in Unsymmetrical Squaraine Dyes for Suppressed Aggregation and Enhanced Performance in High-Efficiency Dye-Sensitized Solar Cells.

Steric regulation at the dye/semiconductor interface critically governs charge recombination and interfacial energetics in dye-sensitized solar cells (DSSCs) but remains poorly defined for unsymmetrical squaraine sensitizers. Here, two π-extended unsymmetrical squaraine dyes (AQ1 and AQ2) are designed to clarify how alkyl-chain-induced steric modulation influences aggregation, surface packing, and intramolecular charge-transfer (ICT) dynamics. Both dyes show strong visible-near-infrared absorption and suitable the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) alignment for electron injection into TiO2. In AQ2, branched alkyl substituents provide enhanced steric shielding at the TiO2 interface, suppressing aggregation, modifying interfacial dipoles, and increasing Rrec. Importantly, efficiency gains arise mainly from steric control of interfacial energetics and recombination kinetics rather than aggregation suppression alone. Consequently, AQ2-based DSSCs deliver 7.41% power conversion efficiency with higher Rrec = 11.64 Ω and longer electron lifetime (τ = 8.58 ms) than AQ1, despite lower dye loading. Electrochemical impedance spectroscopy (EIS) and density functional theory (DFT) corroborate suppressed back-electron transfer.