Solution Processable Direct Bandgap Copper‐Silver‐Bismuth Iodide Photovoltaics: Compositional Control of Dimensionality and Optoelectronic Properties

Abstract The search for lead‐free alternatives to lead‐halide perovskite photovoltaic materials resulted in the discovery of copper(I)‐silver(I)‐bismuth(III) halides exhibiting promising properties for optoelectronic applications. The present work demonstrates a solution‐based synthesis of uniform Cu_xAgBiI_4+x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI₅ at x = 1, 2D Cu₂AgBiI₆ at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu₂AgBiI₆ has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, a valence band level at ‐5.25 eV, improved carrier lifetime, and higher recombination resistance as compared to CuAgBiI₅. These differences are mirrored in the power conversion efficiencies of the CuAgBiI₅ and Cu₂AgBiI₆ solar cells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The latter value is the highest reported for this class of materials owing to the favorable film morphology provided by the hot‐casting method. Future performance improvements might emerge from the optimization of the Cu₂AgBiI₆ layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu₂AgBiI₆ solar cells display storage stability over 240 days.