Sterically Controlled Interfacial Charge-Transfer Mechanisms in Unsymmetrical Squaraine Dyes for Suppressed...

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 R_rec. 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 R_rec = 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.