Dynamical Imaging of Surface Photo-potentials in Hybrid Lead Iodide Perovskite Films under High Optical Irradiance and the Role of Selective Contacts

To leverage the outstanding photonic qualities of lead halide hybrid perovskites under high optical irradiance, their reliability and temporal stability must be assessed. Time-resolved scanning electron microscopy unveils that when illuminating the free surface of methylammonium lead iodide (MAPbI3) films at 500 Sun in vacuum, a giant photopotential locally develops in tens of seconds, differently evolving depending on charge-selective substrates. It is reversible on timescales of minutes in the case of hole-transporting poly(3,4-ethylenedi- oxythiophene) polystyrene sulfonate, while more persistent effects occurs, on the timescale of hours, in the case of electron-transporting TiO2. In addition, films grown on TiO2 show irreversible decay of photoluminescence measured in situ and photoinduced alteration of the work function at some grain bounda- ries. Different responses at high irradiance are ascribable to contact-dependent and light-induced spatial redistribution of charged defects, either ions or local- ized dipoles. It is also clear that photoexcited charges play different roles in the photochemistry of systems, depending on selective contacts and they are likely to mediate diverse photoassisted redox reaction paths. The TiO2 layer may act as a photocatalyst, leading to MAPbI3 degradation.