Efficient near-infrared emission from lead-free ytterbium-doped cesium bismuth halide perovskites

Abstract

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 photoluminescence data reported to date are confusing and contradictory. Here we resolve the literature discrepancies and show that Cs₃Bi₂Br₉ thin films deposited by physical vapor deposition show absorption and emission peaks at 433 and 472 nm, respectively. Peak location and lineshapes of blue-shifted (390–440 nm) photoluminescence previously reported and attributed to quantum confinement in Cs₃Bi₂Br₉ nanocrystal dispersions could be reproduced in BiBr₃ solutions in different solvents but without Cs₃Bi₂Br₉ nanocrystals present. This suggests that high quantum yield photoluminescence reported in this wavelength range may be originating from unreacted precursors and impurities in nanocrystal dispersions rather than from Cs₃Bi₂Br₉ quantum dots. The addition of Yb that can substitute up to 50% of the Bi in Cs₃Bi₂Br₉ leaves the Cs₃Bi₂Br₉ structure unchanged and results in near-infrared Yb^{3+ 2}F_5/2 → ²F_7/2 emission (1.25 eV) with 14.5% quantum yield. This 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 involves quantum cutting. This would make lead-free Cs₃Bi₂Br₉ a potential quantum cutting material for solar spectrum shaping to increase solar cell efficiency.