Direct Observation of Long Electron-Hole Diffusion Distance in CH3NH3PbI3 Perovskite Thin Film

In high performance perovskite based solar cells, CH3NH3PbI3 is the key material. We carried out a study on charge diffusion in spin-coated CH3NH3PbI3 perovskite thin film by transient fluorescent spectroscopy. A thickness-dependent fluorescent lifetime was found. By coating the film with an electron or hole transfer layer, [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) or 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene (Spiro-OMeTAD) respectively, we observed the charge transfer directly through the fluorescence quenching. One-dimensional diffusion model was applied to obtain long charge diffusion distances in thick films, which is similar to 1.7 mu m for electrons and up to similar to 6.3 mu m for holes. Short diffusion distance of few hundreds of nanosecond was also observed in thin films. This thickness dependent charge diffusion explained the formerly reported short charge diffusion distance (similar to 100 nm) in films and resolved its confliction to thick working layer (300-500 nm) in real devices. This study presents direct support to the high performance perovskite solar cells and will benefit the devices' design.