  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-C61-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 ~1.7 um for
electrons and up to ~6.3 um 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 (~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.
