Surface Passivation for Halide Optoelectronics: Comparing Optimization and Reactivity of Amino-Silanes with Formamidinium

Amino-silane-based surface passivation schemes are gaining attention in halide perovskite optoelectronics, with varying levels of success. We compare surface treatments using (3-aminopropyl)trimethoxysilane (APTMS) and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPTMS), applied via room-temperature vacuum deposition, to the perovskite FA_0.78Cs_0.22Pb(I_0.85Br_0.15)₃ (FA = formamidinium). Both molecules improve thin-film photoluminescence properties and photovoltaic device performance, although their effectiveness depends strongly on deposition time. We show AEAPTMS has a wider, more robust processing window and yields higher performance under optimized conditions. In contrast, overexposure, particularly with APTMS, reduces performance, with notable reductions in photoluminescence lifetime and absorbance. To probe the underlying chemistry, we employ nuclear magnetic resonance (NMR) spectroscopy and depth-resolved time-of-flight secondary ion mass spectrometry (ToF-SIMS), demonstrating that both amino-silanes react with formamidinium (FA⁺) cations in solution and in the solid state. This work underscores the importance of optimizing deposition conditions to balance effective passivation with potential performance loss and elucidates previously unrecognized reactive chemistry between amino-silane passivating agents and halide perovskites.