(757g) Physical Vapor Deposition of Yb-Doped Cesium Lead Halide Perovskites

Inorganic halide perovskites have attracted attention with potential applications in high-efficiency solar cells. Recently, ytterbium-doped CsPbCl_3-xBr_x (x<1) perovskite showed efficient quantum cutting, a process wherein photons absorbed at high energies (e.g., > 2.5 eV) generate two photons with energies ~ 1.25 eV close to the silicon bandgap energy (1.1 eV). The mechanism for this quantum cutting process is not well understood, but the leading theories suggest that Pb vacancy defects in the perovskite efficiently trap excitons and transfer energy to two nearby Yb³⁺ ions, which subsequently emit from the Yb³⁺²F_5/2→ ²F_7/2 transition.¹ A thin layer of this material on a silicon solar cell can convert blue photons to two near-infrared (NIR) photons, decreasing energy losses due to the relaxation of the high-energy charge carriers to the band edges. In this way, silicon solar cell efficiencies can surpass the Quessier limit. The majority of reports on ytterbium-doped perovskites rely on nanocrystal or solution-based synthesis to make the materials.^2,3

We are exploring physical vapor deposition, a solvent-free technique, to synthesize halide-perovskite thin films. Specifically, we deposited CsPbBr₃ and Yb-doped CsPbCl_3-xBr_x films by co-evaporating CsCl, PbCl_2, CsBr, PbBr₂, and YbBr₃ and controlling the flux of each using quartz crystal microbalances. Films were characterized with x-ray diffraction, scanning electron microscopy, optical absorbance, photoluminescence, and photoluminescence quantum yield measurements. Physical vapor deposition (PVD) by coevaporation is a scalable solventless approach to forming high purity large-grained polycrystalline films. Specifically, we investigated the effects of deposition temperature, between 26 ^oC and 162 ^oC, and post-deposition annealing, between 250 ^oC and 350 ^oC, on the structure, texturing, and morphology of orthorhombic CsPbBr₃ films formed by PVD.

CsPbBr₃ has two phase transitions as the temperature is increased, one from orthorhombic to tetragonal at 88°C and the other from tetragonal to cubic at 130°C. All films, regardless of the stable phase at the deposition temperature, transform to orthorhombic upon cooling to room temperature. Films deposited at temperatures below the tetragonal to cubic transition show significant orientation along the o-<202> direction, while films deposited below the tetragonal to cubic transition show orientation along the o-<121> direction. This texturing favors the growth of high cation density planes of the stable phase at the deposition temperature. Annealing these films grows the grains and further aligns the grains in the preferred orientation determined by the substrate temperature during deposition. The orthorhombic <202> texturing also dominates after annealing as long as the films have <202> aligned grains before annealing. Films annealed at 350°C showed significantly greater visible photoluminescence than films annealed at 250°C and 300°C and films that were not annealed.

Yb-doped CsPbCl_3-xBr_x films annealed at 350 ^oC for 2 hours emitted NIR photoluminescence at ~990 nm (Yb³⁺²F_5/2→ ²F_7/2 transition, 1.25 eV) with quantum yields (PLQY) exceeding 60%, when excited with photons with energies above the CsPbCl_3-xBr_x bandgap (e.g., > 2.5 eV). The PLQY depended strongly on the annealing environment. Surprisingly, films annealed in nitrogen-filled glove box had the lowest PLQY. Films annealed in the air had higher PLQY while films annealed in glove box first and then in the air had the highest. We hypothesize that grain growth in the glove box followed by oxygen passivation of remaining defects in the air is responsible for the high PLQYs in these films.

1. S. Erickson, M. J. Crane, T. J. Milstein, D. R. Gamelin, J. Phys. Chem. C, 2019, 123, 12474-12484.
2. M. Kroupa, J. Y. Roh, T. J. Milstein, S. E. Creutz, D. R. Gamelin, ACS Energy Lett., 2018, 3, 2390-2395.
3. J. Milstein, D. M. Kroupa, D. R. Gamelin, Nano Lett., 2018, 18, 3792-3799.