Anion/cation substitution in lead-free double-perovskite films: for band-gap optimization

During the past few years, halide double perovskites have been extensively explored for designing eco-friendly and stable perovskite-family absorber materials. In this work, thin films of Cs₂AgBiBr₆ double perovskites were successfully fabricated with the aim of obtaining a lead-free system. Optical studies confirmed the large band gap of 2.33 eV of Cs₂AgBiBr₆ films. Post-synthetic vapor treatment of Cs₂AgBiBr₆ thin films, with tin (IV) iodide (SnI₄; SI), was performed to engineer their optical response. Structural and optical studies confirmed the phase purity of the various SI-treated films. X-ray diffraction studies further showed a systematic shift toward lower 2θ values, which signified the expansion of lattice parameters on SI substitution in the Cs₂AgBiBr₆ structure. The as-prepared pristine and SI-treated films showed good coverage with a reasonably large grain size. Furthermore, the optical studies revealed a 0.47 eV reduction in the band gap of SI-treated films, as opposed to a small band-gap change of approximately 0.22 eV when the pristine Cs₂AgBiBr₆ film was treated with cesium iodide (CsI). This showed the role of the combined effects of charge-balancing defects and compositional substitution in band-gap lowering in Cs₂AgBiBr₆. The controlled doping in lead-free double perovskites for improved optical properties might help in strengthening their use for future optoelectronic applications.