Impact of Crystal Surface on Photoexcited States in Organic-Inorganic Perovskites

Despite their outstanding photovoltaic performance, organic-inorganic perovskite solar cells still face severe stability issues and limitations in their device dimension. Further development of perovskite solar cells therefore requires a deeper understanding of loss mechanisms, in particular, concerning the origin and impact of trap states. Here, different surface properties of submicrometer sized CH3NH3PbI3 particles are studied as a model system by photoluminescence spectroscopy to investigate the impact of the perovskite crystal surface on photoexcited states. Comparison of single crystals with either isolating or electron-rich surface passivation indicates the presence of positively charged surface trap states that can be passivated in case of the latter. These surface trap states cause enhanced nonradiative recombination at room temperature, which is a loss mechanism for solar cell performance. In the orthorhombic phase, the origin of multiple emission peaks is identified as the recombination of free and bound excitons, whose population ratio critically depends on trap state properties. The dynamics of exciton trapping at 50 K are observed on a time-scale of tens of picoseconds by a simultaneous population decrease and increase of free and bound excitons, respectively. These results emphasize the potential of surface passivation to further improve the performance of perovskite solar cells.