Vapor Deposition of Yb-doped CsPbCl₃ via Evaporation of YbCl₃ and Mechanosynthesized CsPbCl₃

A potential alternative to silicon-perovskite tandem solar cells for surpassing the Shockley-Quessier Limit is high-efficiency downconversion of blue and UV photons to near-infrared by a single quantum cutting layer deposited beneath the glass on the silicon solar cell. Yb-doped CsPbCl₃ has attracted significant attention as a quantum-cutting material because of CsPbCl₃’s high absorption coefficient and near-infrared (1.25 eV) photoluminescence quantum yields (PLQY) approaching 200%.^1-5However, most of these results have been obtained with nanocrystal dispersions suffering from sub-bandgap absorption and scattering, and it has been difficult to scale up. Therefore, a large-scale, proven scalable deposition method such as vapor deposition is desired for integration into solar panel manufacturing. The development of vapor deposition for the deposition of Yb-doped CsPbCl₃ is at its nascent stages, and surpassing 100% PLQY and achieving uniform emission has presented substantial challenges, with only one yet-to-be-reproduced report of 183% PLQY via a single source evaporation from Yb-doped CsPb(Cl_1−xBr_x)₃.⁶ Here, we present the mechanosynthesis of CsPbCl₃ powders via ball milling of CsCl and PbCl₃ and CsPbCl₃ co-evaporation onto glass substrates with YbCl₃to synthesize thin films of Yb-doped CsPbCl₃. Film thickness (typically 180-210 nm) and stoichiometry were controlled via the evaporation fluxes of the precursors using quartz crystal microbalances. PLQY is sensitive to the mechanosynthesis parameters through the phase purity of the starting powder. We obtain phase pure CsPbCl₃ under mechanosynthesis conditions that favor maintenance of high temperatures during ball milling and after annealing at 400^○C. We investigated various synthesis parameters, such as Yb concentration, and post-deposition annealing conditions, such as the time, temperature, and environment, such as annealing in air versus in a nitrogen-filled glovebox. CsPbCl₃ evaporates congruently in the 450-480^○C range and forms CsPbCl₃ films. Yb can be incorporated via coevaporation yielding films with PLQY ranging from 0% to greater than 50%. PLQY is sensitive to temperature, and its temperature dependence varies with the annealing environment. PLQY peaks at around 300^○C for air-annealed films, decreasing precipitously thereafter, with XRD showing impurity phases at 400^○C. In contrast, PLQY from films annealed in the nitrogen-filled glovebox remains high at 400^○C.<br/><br/><br/>¹ Ferro, S. Wobben, M and Ehrler, B. Rare-earth quantum cutting in metal halide perovskites – a review. Mater. Horiz., 2021, 8, 1072<br/>² Crane, M. J., Kroupa, D. M. & Gamelin, D. R., Detailed-balance analysis of Yb³⁺:CsPb(Cl_1−xBr_x)₃ quantum-cutting layers for high-efficiency photovoltaics under real-world conditions. Energy Environ Sci, 2019, 12, 2486–2495.<br/>³ Kroupa, D., Crane, M. & Gamelin, D., in Physical Chemistry of Semiconductor Materials and Interfaces XVIII (eds. Congreve, D., Bronstein, H. A., Nielsen, C. & Deschler, F.) 17 (SPIE, 2019). doi:10.1117/12.2528622.<br/>⁴ Roh, J. Y. D. et al. 2020. Yb³⁺ Speciation and Energy-Transfer Dynamics in Quantum-Cutting Yb³⁺-Doped CsPbCl₃ Perovskite Nanocrystals and Single Crystals. Physical Review Materials, 2020, 4,105405.<br/>⁵ Kroupa, D. M. et al. Quantum-Cutting Ytterbium-Doped CsPb(Cl_1−xBr_x)₃ Perovskite Thin Films with Photoluminescence Quantum Yields over 190%. ACS Energy Letters, 2018, 3, 2390-2395.<br/>⁶M. J. Crane, et al., Single-Source Vapor Deposition of Quantum-Cutting Yb³⁺:CsPb(Cl_1-xBr_x)₃ and Other Complex Metal-Halide Perovskites, ACS Appl. Energy Mater., 2019, 2, 4560–4565.