In Situ Formation of Pseudohalide Anions Induced by Humid Air and Light Passivates Formamidinium-Based Halide Perovskites

Metal halide perovskites based on formamidinium (FA), or FA-rich compositions have shown great promise for high-performance photovoltaics. A deeper understanding of the impact of ambient conditions (e.g., moisture, oxygen, and illumination) on the possible reactions of FA-based perovskite films and their processing sensitivities has become critical for further advances toward commercialization. Herein, we investigate reactions that take place on the surface of the FA0.7Cs0.3, mixed Br/I wide bandgap perovskite thin films in the presence of humid air and ambient illumination. The treatment forms a surface layer containing O, OH, and N-based anions. We propose the latter originates from formamidine trapped at the perovskite/oxide interface reacting further to cyanide and/or formamidinate-an understudied class of pseudohalides that bind to Pb. Optimized treatment conditions improve photoluminescence quantum yield owing to both reduced surface recombination velocity and increased bulk carrier lifetime. The corresponding perovskite solar cells also exhibit improved performance. Identifying these reactions opens possibilities for better utilizing cyanide and amidinate ligands, species that may be expected during vapor processing of FA-based perovskites. Our work also provides new insights into the self-healing or self-passivating of MA-free perovskite compositions where FA and iodide damage could be partially offset by advantageous reaction byproducts.