Physical vapor deposition of Yb-doped Cs₂AgSbBr₆ films

Ytterbium-doped halide double perovskites of the form Cs₂AgBX₆ (B = Bi, Sb, In and X = Cl, Br) are being explored as potential lead-free UV/blue to near-infrared (NIR) downconversion materials. Of the various B and X combinations, Cs₂AgSbBr₆ has only recently been synthesized and never yet deposited as a film using vapor deposition. Here, we report the deposition of undoped and Yb-doped Cs₂AgSbBr₆ thin films via thermal evaporation from CsBr, AgBr, SbBr₃, and YbBr₃. Film composition control is difficult due to the high volatility of SbBr₃. We explored various strategies, including co-evaporation and sequential deposition of layers, followed by postdeposition annealing. The formation of Cs₂AgSbBr₆ was confirmed with x-ray diffraction and optical absorption, although impurity phases such as Cs₂AgBr₃ were often present because the films easily became Sb-deficient due to volatile SbBr₃ leaving the film. NIR photoluminescence quantum yields of up to 12% were achieved with this material for the first time. The optical properties and reported bandgap transitions are critically reviewed and assessed in light of new optical absorption data from thin films. The Cs₂AgSbBr₆ film has an indirect bandgap at 1.95 ± 0.05 eV followed by a direct transition at 2.5 ± 0.05 eV.