Physical Vapor Deposition of Yb-doped Double Perovskite Halide Cs₂AgIn(Br, Cl)₃

Inorganic double perovskite halides A₂B⁺B³⁺X₆ (A = Cs, Rb, B⁺= Ag, Na, B³⁺= In, Sb, etc. and X = Br, Cl) have been receiving attention as an alternative to the extensively studied APbX₃ because of their potential to eliminate Pb while preserving some of their attractive optical and electronic properties. For instance, downconversion and nearly perfect quantum cutting with photoluminescence quantum yields (PLQY) approaching 200% have been achieved with Yb-doped CsPb(Cl, Br)₃,¹ but the search for an as efficient perovskite without Pb continues. Quantum cutting down conversion phosphors are receiving increasing attention, as overlayers for solar cells, owing to their capacity of red-shifting photons from sunlight to those of lower energies and enhancing the efficiencies of solar cells by reducing charge recombination and energy losses as hot carriers relax to band edges. Yb³⁺ stands out among rare earth metals as an ideal downconverter because it can emit at ~1.25 eV, just above the bandgap of 1.1 eV of crystalline silicon, and can potentially quantum cut one incoming photon in the UV range to two NIR photons when embedded in a host with bandgap &gt; 2.5 eV. We have been exploring double all inorganic halide perovskites as a host for Yb³⁺. A₂B⁺B³⁺X₆ structure has been reported to be stable to heat, moisture, and exposure to light.² Specifically, we use physical vapor deposition (PVD) as a solvent-free, ligand-free technique with precise control over parameters such as film thicknesses and composition and as a synthesis technique that can overcome limitations such as poor precursor solubilities in solvents. This talk will cover the physical vapor deposition of undoped and Yb-doped Cs₂AgSbBr₆ and Cs₂AgInCl₆ thin films as well as undoped Cs₂AgInBr₆. The latter, Cs₂AgInBr₆, has been difficult to synthesize using other methods. We show that while cubic Cs₂AgInBr₆can be formed at high temperatures, it appears to be unstable at room temperature, ultimately decomposing into more stable products such as Cs₃In₂Br₉, Cs₂AgBr₃, CsAgBr₂, InBr₃ and AgBr, likely depending on local composition. Specifically, co-deposited precursors CsBr, AgBr, and InBr₃ react at and above ~100 ^oC to form cubic Cs₂AgInBr₆. This structure remains stable long enough to be studied when cooled down to room temperature. However, it ultimately decomposes. In contrast, Cs₂AgInCl₆can be formed via codeposition of the precursors easily and is stable. XRD confirmed the formation of Cs₂AgInCl₆, which can be doped with Yb up to 12% without impurity phases. So far, NIR PLQY values of 28.1% have been achieved when doped with 4% of Yb. The major obstacle in depositing Cs₂AgSbBr₆ lay in the volatility of the Sb precursor, SbBr₃. PLQY values could be improved to ~14% either by increasing the amount of SbBr₃ flux or adopting a sandwich deposition structure where SbBr₃ is placed between two layers of CsBr and co-deposited CsBr, AgBr and YbBr₃. In contrast, Cs₂AgInCl₆ could be deposited and formed easily owing to the higher evaporation temperature of InCl₃. These films could be doped with Yb by coevaporating YbCl₃ with the other precursors. Achieving high PLQY requires annealing at 300 ^oC for 1 hour. .<br/>1. Crane, M. J. <i>et al.</i> Single-Source Vapor Deposition of Quantum-Cutting Yb³⁺ :CsPb(Cl_{1– x} Br_x )₃ and Other Complex Metal-Halide Perovskites. <i>ACS Appl. Energy Mater.</i> <b>2</b>, 4560–4565 (2019).<br/>2. Tang, H. <i>et al.</i> Lead-Free Halide Double Perovskite Nanocrystals for Light-Emitting Applications: Strategies for Boosting Efficiency and Stability. <i>Adv. Sci.</i> <b>8</b>, 2004118 (2021).