Molecular Engineering of Hole Transporting Material with Thioether Spacer for Inverted Perovskite Solar Cell

Abstract

Phosphonic acid–based hole transporting materials (HTMs) have gained considerable attention in inverted perovskite solar cells (PSCs) owing to their ability to form robust layer and facilitate efficient interfacial charge extraction. In this study, the effect of incorporating a thioether moiety into the alkyl spacer of carbazole-based HTMs was investigated to explore its impact on energy level alignment and interfacial interactions. Three structurally related HTMs—MeO- ETE-PACz, MeO-5PACz, and Me-4PACz—were evaluated in PSC devices. MeO-ETE-PACz, featuring a sulfur-containing linker, demonstrated improved short-circuit current density (Jsc) and fill factor (FF) compared to MeO-5PACz, which shares the same head group. Ultraviolet photoelectron spectroscopy (UPS) confirmed that MeO-ETE-PACz exhibited the most favorable energy level alignment with the perovskite valence band maximum, resulting in a minimal energy offset for hole extraction. X-ray photoelectron spectroscopy (XPS) measurements showed shift in Pb 4f binding energies upon HTM deposition, suggesting an electronic interaction between the thioether linker and undercoordinated Pb²⁺ sites. Furthermore, external quantum efficiency (EQE) spectra revealed enhanced photocurrent generation in the short-wavelength region for MeO-ETE-PACz–based devices, which correlates with the improved energetic alignment. The findings highlight the critical role of linker design in tuning interfacial energy alignment and improving the photovoltaic characteristics of phosphonic acid-based hole transport materials.