Excitation Intensity-Dependent Terminal Halide Photosegregation Stoichiometries in Formamidinium/Cesium Lead Iodide/Bromide [FACsPb(I_1–xBr_x)₃] Thin Films

Abstract

A thermodynamic band gap-based model for mixed-halide perovskite photosegregation currently explains numerous features of the phenomenon. This includes excitation intensity (I exc ) thresholds for photosegregation as well as I exc -dependent photosegregation rates and rate constants. An intriguing prediction of the model involves I exc -dependent terminal halide stoichiometries (x terminal ), following photosegregation. Rather than suggest a common terminal value, e.g., x terminal = 0.2, for methylammonium lead iodide/bromide [MAPb(I 1−x Br x ) 3 ], the model predicts I exc -dependent x terminal . This, in principle, allows for the controlled tuning of mixed-halide perovskite photoresponses. More important, though, is the opportunity to study this response to develop deeper insight into the origin of nearly ubiquitous photosegregation in lead-based, mixed-halide perovskites. Here, we demonstrate I excdependent x terminal in formamidinium/cesium, mixed-halide [FACsPb(I 1−x Br x ) 3 ] perovskites. We show that modifications to theory, which account for photosegregated domain subpopulations and photocarrier funneling efficiencies, lead to good agreement between measured and predicted I exc -dependent x terminal values. I-rich phase fractions increase with I exc and result in asymptotic x terminal versus I exc . This addresses an open discrepancy between experiment and theory to advance a detailed understanding of light-induced instabilities in mixed-halide perovskites.L ight-induced anion photosegregation is a nearly ubiquitous property of lead-based, mixed-halide perovskites under illumination. 1−3 It is observed as unwanted, yet reversible, changes to the optical and electronic response of these materials when subjected to above-gap excitation. First seen in methylammonium lead iodide/bromide [MAPb-(I 1−x Br x ) 3 ], 4 the phenomenon has since been reported in other mixed-halide perovskites such as formamidinium lead iodide/bromide [FAPb(I 1−x Br x ) 3 ], 5 methylammonium lead chloride/bromide [MAPb(Cl 1−x Br x ) 3 ], 6 cesium lead iodide/ bromide [CsPb(I 1−x Br x ) 3 ] 3 and in stable, mixed-cation systems such as (MA,FA,Cs)Pb(I 1−x Br x ) 3 . 7−9 As such, the phenomenon represents an impediment to proposed uses of mixed-halide perovskites in applications such as tandem solar cells. 9,10 Mitigating mixed-halide perovskite photosegregation is required to bring to fruition this promising material system. Empirical approaches, developed to suppress the phenomenon, 11−14 include replacing the A cation of mixed-halide perovskites, e.g., replacing MA + with Cs + , to suppress photosegregation. Qualitatively, this stems from substituting MA + 's well-known chemical and thermal instabilities 15 with the generally robust nature of Cs + . 7−9 For APb(I 1−x Br x ) 3 (A = FA + , MA + , or Cs + ), empirical photostability decreases in the following order: CsPb(I 1−x Br x ) 3 > FAPb(I 1−x Br x ) 3 > MAPb-(I 1−x Br x ) 3 . 5,16,17