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

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 x AgBiI 4+ x thin films and scrutinizes the effects of x on the phase composition, dimensionality, optoelectronic properties, and photovoltaic performance. Formation of pure 3D CuAgBiI 5 at x = 1, 2D Cu 2 AgBiI 6 at x = 2, and a mix of the two at 1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu 2 AgBiI 6 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 CuAgBiI 5 . These differences are mirrored in the power conversion efficiencies of the CuAgBiI 5 and Cu 2 AgBiI 6 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 Cu 2 AgBiI 6 layer thickness to match the carrier diffusion length of ≈40–50 nm. Nonencapsulated Cu 2 AgBiI 6 solar cells display storage stability over 240 days.