Carrier Diffusion Lengths in Hybrid Perovskites: Processing, Composition, Aging and Surface Passivation Effects

Considering the many possibilities for all three ions in the hybrid halide perovskite structure, hundreds of distinct compositions have already been reported. Such com-positional changes can alter the carrier diffusion length - a key parameter for solar cell performance. Given the large compo-sitional and processing parameter space, a rapid and simple technique for directly measuring diffusion length is needed. Here we use a laser grating technique to screen the diffusion length in perovskite materials. First, we control the processing conditions and the resulting grain size to increase the diffusion length in pure CH₃NH₃PbI₃. Next, we partially replace iodide with bromide and show that surprisingly, the diffusion length increases after aging for 1 month in air. Finally, we use a 4 nm Al₂O₃ layer to passivate the surface of CH₃NH₃PbBr₃, leading to a remarkable increase in diffusion length from 201 nm to 532 nm. The correlation that we have established between material processing and diffusion length offers guidance in how to improve materials for devices.