Contactless Visualization of Fast Charge Carrier Diffusion in Hybrid Halide Perovskite Thin Films

Organic-inorganic metal halide perovskite solar cells have recently attracted considerable attention with reported device efficiencies approaching those achieved in polycrystalline silicon. Key for an efficient extraction of photogenerated carriers is the combination of low nonradiative relaxation rates leading to long carrier lifetimes and rapid diffusive transport. The latter, however, is difficult to assess directly with reported values varying widely. Here, we present an experimental approach for a contactless visualization of the charge carrier diffusion length and velocity in thin films based on time-resolved confocal detection of photoluminescence at varying distances from the excitation position. Our measurements on chloride-treated methylammonium lead iodide thin films, the material for which the highest solar cell efficiencies have been reported, reveal a charge carrier diffusion length of 5.5-7.7 mu m and a transport time of 100 ps for the first micrometer corresponding to a diffusion constant of about 5-10 cm(2) s(-1), similar to GaAs thin films.