Leveraging 4D-USEM in Probing Charge Carriers at the Surface of Single Crystal Perovskites

Perovskite single crystals offer revolutionary light-harvesting technology, showcasing exceptional optoelectronic properties and device performance. Achieving stable perovskite single crystals with enhanced optical properties and device performance is crucial for guiding material scientists, chemists, and engineers in designing superior crystals with very low defect concentration, if any. Despite the extensive investigations on their photo-induced charge-carriers dynamics, most of the time-resolved techniques focus mainly on bulk properties rather than surface characteristic which plays a crucial role for their optoelectronic-device performance. Thus, understanding charge carrier and surface defects at atomic level and femtoseconds scale is necessary. Herein, four-dimensional ultrafast scanning electron microscopy (4D-USEM) have been utilized to probing the photo-generated carrier transport at the first few nanometers of the top surface In this technique, a pulsed primary electron beam generated optically by a UV excitation pulse from a cooled Schottky field-emitter tip produces secondary electrons that are sensitive to the localization of the charge carrier on surfaces and interfaces of the photoactive materials. Additionally, density functional theory (DFT) was conducted to unfold the reasons behind the formation of both defect centers and ions migration. In this work, four different samples of single crystal perovskites were investigated; MAPbI₃(001), MAPbI₃(100), FA_0.6MA_0.4PbI₃ (FA-rich) and FA_0.4MA_0.6PbI₃ (MA-rich). Our time-resolved results revealed that the orientations and compositions strongly affect charge carrier behavior. For example, MAPbI₃ (100) and MA-rich samples displayed a shorter carrier lifetime and exhibited a dark image-contrast in the SUEM experiments. Whereas the lifetime and concentration of defects were notably improved and reduced, respectively in case of MAPbI₃ (001) and FA-rich samples. These findings shed the light on the importance of understanding the role of orientation/termination and the cation’s selection on the charge carrier behavior at the first few nanometers to assist engineering high-performance optoelectronic devices based on single crystal perovskites.