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

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 Yb3+ 2F5/2 -> F-2(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.