Short Photoluminescence Lifetimes in Vacuum-Deposited CHNHPbI Perovskite Thin Films as a Result of Fast Diffusion of Photogenerated Charge Carriers.

It is widely accepted that a long photoluminescence (PL) lifetime in metal halide perovskite films is a crucial and favorable factor, as it ensures a large charge diffusion length leading to a high power conversion efficiency (PCE) in solar cells. It has been recently found that vacuum-evaporated CHNHPbI (eMAPI) films show very short PL lifetimes of several nanoseconds. The corresponding solar cells, however, have high photovoltage (>1.1 V) and PCEs (up to 20%). We rationalize this apparent contradiction and show that eMAPI films are characterized by a very high diffusion coefficient , estimated from modeling the PL kinetics to exceed 1 cm/s. Such high values are favorable for long diffusion length as well as fast transport of carriers to film surfaces, where they recombine nonradiatively with surface recombination velocity ∼ 10 cm/s. Possible physical origins leading to the high values are also discussed.