Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites
We use scanning photocurrent microscopy and time resolved microwave conductivity to measure the diffusion of holes and electrons in a series of lead bromide perovskite single crystals, APbBr3, with A = methylammonium (MA), formamidinium (FA) and Cs. We find that the diffusion length of holes (LDh+ ~ 10 – 50 μm) is on average an order of magnitude longer than that of electrons (LDe− ~ 1 – 5 μm), regardless of the A-type cation or applied bias. Furthermore, we observe a weak dependence of LD across the A-cation series MA > FA > Cs. When considering the role of the halide, we find that the diffusion of holes in MAPbBr3 is comparable to that in MAPbI3 but the electron diffusion length is up to five times shorter. This study shows that the disparity between hole and electron diffusion is a ubiquitous feature of lead halide perovskites. As with organic photovoltaics, this imbalance will likely become an important consideration in the optimization of lead halide perovskite solar cells.