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_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.