Effects of Bi and Sb ion incorporation on the optoelectronic properties of mixed lead–tin perovskites†

Doping with small densities of foreign ions is an essential strategy for tuning the optoelectronic properties of semiconductors, but the effects of doping are not well-understood in halide perovskites. We investigate the effect of Bi³⁺ and Sb³⁺ doping in lead–tin perovskites. Films doped with small amounts of BiI₃ and SbI₃ show greatly increased non-radiative recombination at precursor doping concentrations as low as 1 ppm for Bi³⁺ and 1000 ppm for Sb³⁺. We rationalize such behaviour by density functional theory (DFT) simulations, showing that these metal ions can be incorporated in the perovskite crystal by introducing deep trap levels in the band gap. Having found that very small amounts of Bi³⁺ greatly reduce the optoelectronic quality of lead–tin perovskite films, we investigate the presence of Bi impurities in perovskite precursor chemicals and find quantities approaching 1 ppm in some. In response, we introduce a facile method for removing Bi³⁺ impurities and demonstrate removal of 100 ppm Bi from a perovskite ink. This work demonstrates how the incorporation of small concentrations of foreign metal ions can severely affect film quality, raising the importance of precursor chemical purity.