Thermally Induced Surface Self-Passivation in Tin Perovskite Solar Cells

Due to the band offset, there is significant interfacial recombination between the tin-based perovskite and C₆₀, which exhibits excellent electron transport capability. Here, we introduce a novel strategy leveraging surface self-passivation through controlled thermal decomposition to reduce the level of interfacial recombination substantially. By carefully tuning the annealing temperature (70 vs 100 °C) and organic cation composition (diethylammonium (DEA) vs methylamine (MA)), we achieve selective surface restructuring and SnI₂ formation, effectively suppressing interfacial recombination at the perovskite/C₆₀ interface. Detailed characterization using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) confirmed the formation of a stable SnI₂ passivation layer. At the same time, photoluminescence and quasi-Fermi level splitting (QFLS) analyses revealed a notable reduction in the interfacial recombination losses. Consequently, this surface self-passivation strategy significantly improved the power conversion efficiency (PCE) by approximately 1%, primarily through an open-circuit voltage (V_OC) increase of around 50 mV. Our findings underscore the critical role of interface engineering and thermal control in advancing the efficiency of Sn-based perovskite solar cells.