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

摘要

The search for lead-free alternatives to lead-halide perovskite photovoltaicmaterials resulted in the discovery of copper(Ⅰ)-silver(Ⅰ)-bismuth(Ⅲ) halidesexhibiting promising properties for optoelectronic applications. The presentwork demonstrates a solution-based synthesis of uniform CuxAgBiI4+x thinfilms and scrutinizes the effects of x on the phase composition, dimensionality,optoelectronic properties, and photovoltaic performance. Formationof pure 3D CuAgBiI_5 at x = 1, 2D Cu2AgBiI6 at x = 2, and a mix of the two at1 < x < 2 is demonstrated. Despite lower structural dimensionality, Cu_2AgBiI_6has broader optical absorption with a direct bandgap of 1.89 ± 0.05 eV, avalence band level at -5.25 eV, improved carrier lifetime, and higher recombinationresistance as compared to CuAgBiI_5. These differences are mirroredin the power conversion efficiencies of the CuAgBiI_5 and Cu2AgBiI6 solarcells under 1 sun of 1.01 ± 0.06% and 2.39 ± 0.05%, respectively. The lattervalue is the highest reported for this class of materials owing to the favorablefilm morphology provided by the hot-casting method. Future performanceimprovements might emerge from the optimization of the Cu_2AgBiI_6 layerthickness to match the carrier diffusion length of ≈40–50 nm. NonencapsulatedCu_2AgBiI_6 solar cells display storage stability over 240 days.