The relationship between the chemical composition of lead halide perovskite materials and their crystal and electronic structure is not yet sufficiently understood, despite its fundamental importance. Here, we determine the crystal and electronic structure of cesium lead bromide (CsPbBr3) while deliberately varying the cesium content. At substoichiometric concentrations of cesium, there are large variations in the frontier electronic structure of CsPbBr3 with only small variations in Cs content. We observe a critical point after which large variations in the chemical composition of CsPbBr3 result in comparably small changes in valence and conduction band energies. This behavior is starkly different from that of traditional semiconductors, such as InGaAs and GaInP, and demonstrates an impressive energetic tolerance of CsPbBr3 to large changes in its stoichiometry. This observation helps us to understand why a broad range of relatively uncontrolled, simple processing methodologies can deliver highly functional metal halide perovskite thin films.
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