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

The double halide perovskite Cs 2 AgInCl 6 , first synthesized in 2017, is a potential lead-free alternative to CsPbCl 3 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 Cs 2 AgInCl 6 thin films through reactive physical vapor deposition (PVD) via co-evaporation of CsCl, AgCl, and InCl 3 for the first time in a solvent-, ligand- and oxygen-free environment. We also doped these films with Yb 3+ by co-evaporating YbCl 3 . 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 Yb 3+ dopant and absorbance spectra, we determine the Cs 2 AgInCl 6 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 Cs 2 AgInCl 6 persists, shifted only slightly (10 s of nm), in Cs 2 AgBiCl 6 , Cs 2 AgBiBr 6 , and Cs 2 NaBiCl 6 , 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. Ytterbium doped and undoped Cs 2 AgInCl 6 films deposited by co-evaporation of metal halide salts luminesce in the visible and near infrared when excited with ultraviolet photons above its bandgap.