Reactive Physical Vapor Deposition of Yb-Doped Lead-Free Double Perovskite Cs₂AgBiBr₆ with 95% Photoluminescence Quantum Yield

Yb-doped Cs2AgBiBr6 is a promising lead-free halide double perovskite that can be used as a downconverting coating on silicon solar cells to redshift UV and blue photons to the near-infrared where the quantum efficiencies are larger. Herein, we show that photoluminescence quantum yield (PLQY) of Yb-doped Cs2AgBiBr6 thin films synthesized via physical vapor deposition depends strongly on how the substrate temperature changes during deposition, which determines the amount of Bi incorporated into the film. Yb-doped Cs2AgBiBr6 films with PLQY as high as 95% were deposited with excess BiBr3 and by ramping substrate temperature during the deposition. Ramping the substrate temperature reduces BiBr3 loss from the film by promoting reactions that form Cs2AgBiBr6. As a result, the films formed have high PLQY and retain 93% of their initial PLQY values after 1 month.