Non-Adiabatic Dynamics Reveals Carrier Separation at Grain-boundaries in CsPbBr₃ Perovskite

Lead halide perovskites are known to have outstanding optoelectronic properties making them ideal candidates for applications in electronic devices. One such property is long electron-hole recombination time to which two mechanisms are often suggested: the electric fields from ferroelectric domains and localization from large polaron formation leading to electron-hole separation. However, despite extensive research, the exact mechanism remains under debate. A simulation of large polarons is challenging since the simulation-box needs to be large enough to accommodate the polaron. Also, accurate description of electronic structure is necessary. To that end, we employed time-dependent tight-binding model parameterized to the DFT band structure. Calculated temperature dependency of electron mobilities based on non-adiabatic dynamics are in good agreement with previous theory and experiment. Our TD-TB model allowed us to simulate large enough system, where we observed the natural formation of nano-domains with twinning grain boundaries consistent with experimental reports on CsPbBr­₃ single crystals. The polarization planes forming at the grain boundaries serve to localize electrons on opposite sides of the grain boundaries. The effect of the twinning grain-boundaries is identified as a new mechanism to explain long electron-hole recombination time in halide perovskites.