Photostability of Formamidinium-Based Mixed-Halide Perovskites

Light-induced anion photosegregation is a near-ubiquitous response of lead-based, mixed-halide perovskites to illumination. It is observed as unwanted, yet reversible, changes of the optical and electronic response of these materials when subjected to above-gap excitation and represents an impediment to proposed uses of mixed-halide perovskites in applications such as tandem solar cells.<br/>Photosegregation sensitivity depends on the nature of the A-cation (methylammonium (MA⁺), formamidinium (FA⁺) and Cs⁺). For APb(I_1-xBr_x)₃ photostability rank order follows CsPb(I_1-xBr_x)₃&gt;FAPb(I_1-xBr_x)₃&gt;MAPb(I_1-xBr_x)₃. Qualitatively, this stems from well-known chemical and thermal instabilities of MA⁺as well as to the generally robust nature of all-inorganic Cs-based perovskites.<br/>Here, we assess the relative proclivity of formamidinium-based mixed-halide perovskites such as formamidinium/methylammonium [(FA,MA)Pb(I_1-xBr_x)₃], formamidinium/cesium [(FA,Cs)Pb(I_1-xBr_x)₃], and formamidinium/methylammonium/cesium [(FA,MA,Cs)Pb(I_1-xBr_x)₃] iodide/bromide perovskites to photosegregate under illumination. Goals of the study are to establish their photosegregation propensity and, where possible, compare observations to a recently developed model for light-induced, mixed-halide photosegregation. The model currently explains numerous features of the phenomenon, including the existence of excitation intensity (<i>I</i>_exc) thresholds, <i>I</i>_exc-dependent photosegregation rates/rate constants, and temperature insensitive, as well as carrier diffusion length-dependent terminal halide stoichiometries.