(253i) Efficient Near-Infrared Emission from Lead-Free Cesium Bismuth Halide Perovskites Doped with Ytterbium

All-inorganic metal halide perovskites have attracted significant attention for applications as solar cells, light-emitting diodes, and photodetectors because they have strong and tunable absorptions and emissions. Lead-based perovskites have been the focus of many studies because they perform well in these applications, but lead is toxic. Bismuth-based halide perovskites are non-toxic alternatives to widely researched lead-containing halide perovskites for optoelectronics. Cesium bismuth bromide, in particular, may have good optical properties, but optical absorption and photoluminescence (PL) data reported to date and their interpretations vary significantly: there are almost as many different optical absorption spectra published for Cs₃Bi₂Br_9­ as there are articles on this material.¹ For instance, the wavelengths of published Cs₃Bi₂Br₉ UV-visible PL peaks vary significantly, from 389 to 478 nm. While one may expect blue shifts due to quantum confinement in nanocrystals, there are no clear trends in published data where the wavelength decreases with decreasing Cs₃Bi₂Br₉ nanocrystal size. In this talk, we will resolve the literature discrepancies and show that Cs₃Bi₂Br₉ thin films deposited by physical vapor deposition (PVD) show absorption and emission peaks at 433 and 472 nm, respectively. Peak location and lineshapes of blue-shifted absorption and emission previously reported and attributed to quantum confinement in Cs₃Bi₂Br₉ nanocrystal could be reproduced in BiBr₃ solutions in different solvents even without any Cs₃Bi₂Br_9­. This suggests that high photoluminescence quantum yield (PLQY) and blue-shifted emissions reported below 433 nm may be originating from unreacted precursors and impurities in nanocrystal dispersions rather than from Cs₃Bi₂Br₉.

To avoid contamination from unreacted precursors and undesired phases, we use PVD, a solvent-free technique, to synthesize halide perovskitethin films. Specifically, we synthesized Cs₃Bi₂Br₉ by coevaporating CsBr and BiBr₃ in a ratio of 3:2 while controlling their fluxes using independent quartz crystal microbalances. Typical 300-500 nm thin films are characterized by a battery of techniques, including x-ray diffraction, Raman spectroscopy, scanning electron microscopy, optical absorption, PL, and absolute PLQY measurements.

Inspired by recent reports of very efficient quantum cutting^2,3 in Yb-doped CsPb(Cl_1-xBr_x)₃,we doped Cs₃Bi₂Br_9­ films with Yb. Quantum cutting is a specific mechanism of energy transfer wherein photons absorbed at high energies (>2.5 eV in this case, i.e., blue and UV) generate two NIR photons (e.g., 1.25 eV). Quantum cutting has the potential to help increase the silicon solar cell efficiencies above 33%, the Shockley-Quessier limit. The addition of Yb that can substitute up to 50% of the Bi in Cs₃Bi₂Br₉ leaves the Cs₃Bi₂Br₉ structure unchanged and results in NIR Yb³⁺²F_5/2→ ²F_7/2 emission (1.25 eV) with 14.5% quantum yield. While still lower than Yb-doped CsPb(Cl_1-xBr_x)₃, a PLQY of 14.5 is promising because Cs₃Bi₂Br₉ is otherwise not emissive: the highest reliable visible PLQY in literature is 0.2%. Despite this, energy transfer from Cs₃Bi₂Br₉ to Yb appears to compete efficiently with non-radiative recombination and results in a 14.5% quantum yield. Moreover, NIR emission decreases sharply when the perovskite host's bandgap is reduced below 2.5 eV, twice the Yb³⁺ emission energy, by substituting bromine with iodine, raising the possibility that the emission mechanism may involve quantum cutting. This also raises the tantalizing possibility that Cs₃Bi₂Br₉ could be a potential lead-free quantum cutting material for solar spectrum shaping to increase solar cell efficiency. Moreover, Cs₃Bi₂Br_9­ is an alternative material to study the nature of defects necessary for quantum cutting. In contrast to CsPb(Cl_1-xBr_x)₃, where a defect complex involving two Yb³⁺ ions and a Pb vacancy is implicated, Yb³⁺ can substitute for isovalent Bi³⁺ in Cs₃Bi₂Br₉­.

1. N. Tran, I. J. Cleveland and E. S. Aydil. J. Mater. Chem. C., 2020, 8, 10456-10463.
2. J. Milstein, D. M. Kroupa, and D. R. Gamelin. Nano Lett., 2018, 18, 3792-3799.
3. J. Crane, D. M. Kroupa, J. Y. Roh, R. Y. Anderson, M. D. Smith, and D. R. Gamelin. ACS Appl. Energy Mater., 2019, 2, 4560-4565.