Imaging the long diffusion lengths of photo-generated carriers in mixed halide perovskite films

Organometal halide perovskite has emerged as a promising photovoltaic material due to its low-cost synthesis process and outstanding performance. Though long diffusion length of photo-generated carriers plays a very important role in its success, a direct measurement of carrier diffusion lengths is still lacking. We fabricated highly crystalline CH$_{3}$NH$_{3}$PbI$_{3-x}$Cl$_{x}$ thin film devices on SiO$_{2}$/Si substrate with either nickel or gold as contact electrodes and parylene as encapsulation layer. By performing spatially scanned photocurrent imaging measurement with a local illumination spot, we show that photo-generated carriers in the prepared perovskite have very long diffusion lengths, with hole diffusion length $L_{h}= 22\pm 7\mu m$ and electron diffusion length $L_{e}= 8\pm 4\mu m$. Our work provides scanning photocurrent microscopy as a powerful tool to directly extract hole and electron diffusion lengths at the same time, and may be further used to elucidate the vastly different carrier diffusion lengths (on order of $100nm$ to $100\mu m$) in organometal halide perovskites prepared by different methods.