Optical properties of ytterbium-doped and undoped Cs₂AgInCl₆ thin films deposited by co-evaporation of chloride salts

The double halide perovskite Cs₂AgInCl₆, first synthesized in 2017, is a potential lead-free alternative to CsPbCl₃ for optoelectronic applications, but its discovery is recent, there are knowledge gaps and controversies in its optical properties, and its formation as a thin film for practical use remains difficult due to the poor solubility of the precursors used in solution synthesis. Herein, we report the formation of Cs₂AgInCl₆ thin films through reactive physical vapor deposition (PVD) via co-evaporation of CsCl, AgCl, and InCl₃ for the first time in a solvent-, ligand- and oxygen-free environment. We also doped these films with Yb³⁺ by co-evaporating YbCl₃. Consistent with previous reports, the undoped films emit orange luminescence peaking at 630 nm, while Yb-doped films emit near-infrared (NIR) luminescence at 990 nm. Using the excitation spectrum of the orange and NIR emissions from the Yb³⁺ dopant and absorbance spectra, we determine the Cs₂AgInCl₆ band gap to be 3 eV. Up to 12% (of Ag or In lattice positions) Yb could be incorporated into the film without disrupting its cubic crystal structure or stability. The highest NIR photoluminescence quantum yield was 20%, achieved in films doped with 8% Yb. We show that the emission at 630 nm, previously assigned to self-trapped excitons in Cs₂AgInCl₆ persists, shifted only slightly (10 s of nm), in Cs₂AgBiCl₆, Cs₂AgBiBr₆, and Cs₂NaBiCl₆, double perovskites with significantly different band gaps, suggesting that at least some of this emission intensity is due to a common defect, possibly In and Bi antisite defects, and not necessarily only due to self-trapped excitons.