Methoxy-substituted aromatic SAM Co-adsorbents for interfacial modification and performance enhancement in dye-sensitized solar cells

Abstract In this study, two aromatic self-assembled monolayer (SAM) molecules, 4-(5'-phenyl-2,2'-bithen-5-yl)benzoic acid (ZE-Ph) and its methoxy-functionalized derivative 4-[5'-(4-methoxyphenyl)-2,2'-bithen-5-yl]benzoic acid (ZE-OMe), were synthesized and employed as co-adsorbents to enhance the performance of dye-sensitized solar cells (DSSCs). Owing to its planar structure and para-substituted methoxy group, ZE-OMe demonstrated stronger binding affinity and more homogeneous coverage on the titanium dioxide (TiO 2 ) surface. The current density-voltage (J–V) measurements revealed that the device modified with ZE-OMe achieved the highest performance, with a short-circuit current density (J sc ) of 12.2 mA/cm 2 , open-circuit voltage (V oc ) of 680 mV, fill factor (FF) of 61.2%, and power conversion efficiency (PCE) of 5.0%. In contrast, the corresponding values for ZE-Ph were J sc = 9.6 mA/cm 2 , V oc = 660 mV, FF = 54.3%, and PCE = 3.4%. X-ray photoelectron spectroscopy (XPS) analysis confirmed the successful incorporation of the methoxy group and its interaction with the TiO 2 surface, as evidenced by the presence of C–O and O = C–O components. Electrochemical impedance spectroscopy (EIS) revealed that the ZE-OMe–modified DSSC exhibits reduced resistive losses (R s = 19.0 Ω; R 1 = 2.6 Ω; R 2 = 20.3 Ω; R 3 = 15.3 Ω) together with a prolonged electron lifetime, indicating improved interfacial kinetics and suppressed loss pathways, which collectively account for its superior photovoltaic performance. These results highlight the critical role of molecular structure and surface chemistry in determining DSSC performance and identify ZE-OMe as an effective SAM molecule for interface engineering in high-efficiency solar cells.