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Silver–Bismuth Halide Double Salts for Lead‐Free Photovoltaics: Insights from Symmetry‐Based Modeling

Ag/Bi halide double salts, also called rudorffites, constitute a promising path to achieve low‐cost, high‐efficiency lead‐free optoelectronic devices, in particular solar cells. These materials present tunable gaps within the visible range, high short‐circuit currents, and interesting efficiencies in outdoor and indoor devices. Herein, a combination of symmetry analysis and first‐principles calculations is used to explore the structural, electronic, and optical properties of prototypical rudorffites solar cell absorbers AgBiI4 and Ag3BiI6. The challenges to model those ternary materials are first established. It is shown that Ag/Bi double salts cannot be modeled as random alloys. Second, a Wyckoff position splitting method is developed leading to the generation of model structures, which are unique for each compound, and agrees with experimental structural data. These structures are used to establish the optoelectronic properties of AgBiI4 and Ag3BiI6. Finally, the ideal photovoltaic performance of the materials is predicted through the spectral limited maximum efficiency approach. It is shown that AgBiI4 presents a slightly higher potential for solar cell applications, and the realized devices are mostly limited by the low open‐circuit voltage. The structural models developed here can help unveiling complex properties of the materials and explore substitutional engineering within halide double salts.