Vapor Deposition and Optical Properties of Cs₂AgBiCl₆ Thin Films

Halide perovskites, particularly those containing lead, have revolutionized the field of optoelectronics. However, lead toxicity necessitates the pursuit of environmentally friendly, lead-free alternatives. Bismuth and silver-based halide double perovskites, such as Cs2AgBiCl6, have emerged as promising candidates. Previous studies on Cs2AgBiCl6 have yielded inconsistencies regarding the bandgap of the material and the origin of its characteristic visible orange emission. We address these disparities by thoroughly characterizing Cs2AgBiCl6 thin films deposited via evaporation and subsequent reaction of the constituent metal halides in a high-vacuum physical vapor deposition chamber. Rigorous optical characterization reveals an indirect bandgap of 2.68 eV and a direct transition at 2.9 eV. From transient PL lifetime measurements, fitted using a physically based kinetic mathematical model of exciton decay channels, we propose that the oft-observed orange emission comprises self-trapped exciton and radiative defect emissions, with fractional contributions depending on the material synthesis conditions. These clarifications bridge knowledge gaps and open avenues for fine-tuning Cs2AgBiCl6 films with potential optical, electronic, and optoelectronic applications.