Surface Defect-Passivation in SnO2 Electron Transport Layers by Caesium Iodide for High-Carrier Dynamics in Perovskite Solar Cells

Perovskite solar cell (PSC) performance is critically limited by surface defects and interfacial energy losses at the tin oxide (SnO2) electron transport layer (ETL). This study investigates the surface modification of SnO2 using cesium iodide (CsI) and cesium fluoride (CsF) to understand their influence on perovskite crystallization and interfacial carrier dynamics. Photovoltaic characterization reveals that the device response is strongly dependent on the chosen halide. Modification with CsI significantly enhances device performance, achieving a champion power conversion efficiency (PCE) of 21.77%, compared to 20.13% for the pristine baseline and 17.05% for the CsF-treated device. The superior performance of the CsI-modified device is driven by reduced trap density and an order of magnitude increase in carrier mobility. Structural and spectroscopic analyses demonstrate that CsI effectively passivates oxygen vacancies via strong interfacial electronic interactions, extending the bulk carrier lifetime to 34.96 ns and promoting the growth of highly compact perovskite crystals with larger domains (30.0 nm) and minimized dislocation density. Furthermore, an analysis of ambient-processing effects clarifies an inherent trade-off between short-circuit current density and fill factor (FF). These results establish that tailored halide engineering of the ETL surface is an effective strategy for mitigating interfacial recombination and advancing high-performance PSCs.