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.