Compositional Transformation and Impurity-Mediated Optical Transitions in Co-Evaporated Cu₂AgBiI₆ Thin Films for Photovoltaic Applications

Quaternary copper-silver-bismuth-iodide compounds represent a promising new class of wide-bandgap (2 eV) semiconductors for photovoltaic and photodetector applications. In this study, vapor phase co-evaporation is utilized to fabricate Cu2AgBiI6 thin films and photovoltaic devices. The findings show that the properties of vapor-deposited films are highly dependent upon processing temperature, exhibiting increased pinhole density and transforming into a mixture of quaternary, binary, and metallic phases depending on the post-deposition annealing temperature. This change in phase is accompanied by an enhancement in photoluminescence (PL) intensity and charge-carrier lifetime, along with the emergence of an additional absorption peak at high energy (approximate to 3 eV). Generally, increased PL is a desirable property for a solar absorber material, but this change in PL is ascribed to the formation of CuI impurity domains, whose defect-mediated optical transition dominates the emission properties of the thin film. Via optical pump terahertz probe spectroscopy, it is revealed that CuI impurities hinder charge-carrier transport in Cu2AgBiI6 thin films. It is also revealed that the predominant performance limitation in Cu2AgBiI6 materials is the short electron-diffusion length. Overall, the findings pave the way for potential solutions to critical issues in copper-silver-bismuth-iodide materials and indicate strategies to develop environmentally compatible wide-bandgap semiconductors.