Physical vapor deposition of Yb-doped Cs2AgSbBr6 films

Abstract

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.