Concentration-Dependent Interfacial Engineering with a ZE-2OMe Co-adsorbent for Enhanced DSSC Performance

Controlling charge transport and recombination processes at the titanium dioxide (TiO2)/dye/electrolyte interface is a key strategy for improving the efficiency of dye-sensitized solar cells (DSSCs). This study systematically examines the concentration-dependent effects of a methoxy-functionalized aromatic coadsorbent, 4-5′-(3,5-dimethoxyphenyl)-2,2’-bithien-5-ylbenzoic acid (ZE-2OMe). Low-concentration ZE-2OMe primarily improves the TiO2/dye interface by mitigating interfacial loss pathways and passivating surface defect states, thereby enhancing charge collection and suppressing recombination. Optical analyses confirm suppressed nonradiative recombination, reflected by increased photoluminescence (PL) intensity and a time-resolved photoluminescence (TRPL) lifetime that extends from 2.84 ns in the control to 4.87 ns at 0.01 mM. XPS confirms carboxylate anchoring of ZE-2OMe on TiO2 and shows S 2p doublets, evidencing robust immobilization of the coadsorbent layer. Notably, the incident-photon-to-current efficiency (IPCE) spectra show no additional contribution in the ZE-2OMe absorption window, and dye loading does not increase upon coadsorbent addition, indicating that the short-circuit current density (Jsc) enhancement is primarily driven by interfacial/electronic regulation rather than complementary light harvesting. Electrochemical impedance spectroscopy (EIS) demonstrates that at 0.01 mM, the reductions in R1 = 5.90 Ω, R2 = 70.1 Ω, and the series resistance (Rs = 21.8 Ω) indicate more efficient charge transport and suppressed recombination at the interfaces. The J–V measurements show that the photovoltaic conversion efficiency (PCE) increases from 3.5% (control) to 5.4% at 0.01 mM ZE-2OMe, while higher concentrations lead to reduced performance.