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 Cs2AgInCl6, Cs2AgInCl6, and Cs2AgInCl6, 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.

Automated summary

This study reports the successful formation of Cs2AgInCl6 thin films through reactive physical vapor deposition, and doping these films with Yb3+. The spectra of the films show orange and near-infrared emissions at 630 nm and 990 nm, respectively. The Cs2AgInCl6 band gap is determined to be 3 eV based on excitation and absorbance spectra. The study also demonstrates that up to 12% Yb doping and 20% NIR photoluminescence quantum yield can be achieved in the films.