Vapor-Compatible Molecular Passivation of Thermally Evaporated Lead-Free Perovskites for Indoor Photovoltaics

Tin-based halide perovskites (THPs) have emerged as promising lead-free light absorbers for indoor photovoltaics (IPVs). However, surface-defect-mediated recombination and Sn 2+ oxidation remain major limitations, especially in vapor-based sequential deposition of perovskite films that requires solid-state conversion and careful control in stoichiometry. Herein, we address this challenge using a surface passivation strategy via thermal evaporation to modulate interfacial defects of THP thin films. We found that thermally evaporated 1,3-propanediammonium iodide (PDAI 2 ) on the perovskite surface significantly reduces Sn 2+ oxidation and compensates halide-deficient surface sites, thereby suppressing trap-assisted recombination and improving the band alignment at the electron-contact interface. Consequently, the best-performing devices based on the optimized films can achieve a power conversion efficiency (PCE) of 18.36% under 1000 lux indoor light illumination. Under AM 1.5 G illumination, an open-circuit voltage ( V oc ) of 0.85 V is also obtained, which is higher than those of the previous reports using thermally evaporated THP. This work highlights the importance of surface defect passivation for improving the device performance and provides insight into a scalable pathway for lead-free perovskites in efficient IPV applications.