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.