Structure, Morphology, and Photovoltaic Implications of Halide Alloying in Lead-Free Cs3Sb2ClxI9-x 2D-Layered Perovskites

Compositional tuning is a major driving force behind the excellent optoelectronic properties observed in typical Pb-based perovskites. For lead-free perovskite derivatives, a challenge to understand the connection between compositional tuning and intrinsic optoelectronic properties, hence a barrier toward boosting their performance, comes from the fact that multiple crystalline substructures can form based on composition, film processing, or both. Especially with lower dimensional (0D, 1D, and 2D) substructures, the particular polymorph present in the film can be a greater determinant of optoelectronic properties than the composition itself. Herein, a simple method to alloy the halide site in all-inorganic lead-free Cs3Sb2I9 films is reported while maintaining a consistent 2D-layered substructure, as a means to independently study the photovoltaic implications of halide substitution. A broad suite of spectroscopy and device measurements is used to identify an optimal stoichiometric substitution of chloride for iodide (approximate to 8 mol%, measured) that balances both intrinsic and bulk optoelectronic properties to achieve a top power conversion efficiency of 2.2%. This work underscores the importance of controlling substructure while investigating the impacts of compositional tuning for the development of lead-free perovskites and more broadly validates the approach toward realizing lead-free alternative perovskite solar technologies.