Stability and efficiency improvement of perovskite solar cells by surface hydroxyl defect passivation of SnO2 layer with 4-fluorothiophenol

Abstract

Chemical bath-deposited SnO₂ has been widely used as an electron transport layer for high-efficiency perovskite solar cells (PSCs). However, the solution-processed SnO₂ has –OH groups on its surface due to proton-rich condition, which degrades the stability and efficiency of PSCs. In this study, we report on an effective way to reduce the density of surface hydroxyl groups on SnO₂ and thereby improve the stability and efficiency of SnO₂-based PSCs. Post-treatment of as-prepared SnO₂ layer with halogen-substituted thiophenol, specifically 4-fluorothiophenol, results in substantial reduction of surface hydroxyl groups as confirmed by X-ray photoelectron spectroscopy and X-ray absorption near edge structure. Hydroxyl groups are found to act as defect sites since interfacial charge transfer and carrier lifetime of perovskite are improved by post-treatment. As a result, power conversion efficiency (PCE) is enhanced from 21% to 23% by substitution of surface hydroxyl with 4-fluorothiophenol. Moreover, a stability test for 500 h reveals that 90% of initial PCE is maintained for perovskite deposited on the surface-modified SnO₂, while a significant degradation by more than 40% is observed for that formed on untreated SnO₂. Pinholes generated at the perovskite/SnO₂ interface are reduced by surface modification with 4-fluorothiophenol, which is responsible for the improved stability.