Effect of Grain Boundaries on Charge Transport in Methylammonium Lead Iodide Perovskite Thin Films

Methylammonium lead iodide (MAPbI₃) has attracted great interest as an organic-inorganic hybrid perovskite for photovoltaic applications. Vacancy-mediated ion migration is one of the dominant carrier transport mechanisms in MAPbI₃. Our previous work clarified the nature of migrating species and their moderating effect on electronic transport. However, to develop strategies to mitigate ion migration and its impact thereof, it is important to know whether the migration is homogeneous or controlled by microstructural features, such as grain boundaries. In this work, we implement temperature-dependent pulsed voltage-current measurements of MAPbI₃ thin films with different grain sizes under dark conditions and distinguish the electromigration of iodine vacancies and methylammonium vacancies. Upon increasing the grain size, the total accumulated charge decreases, whereas the activation energies increase. This is consistent with the high grain boundary density in small-grained films responsible for facilitating charge transport. These results suggest that one viable strategy to decrease ion migration would be to engineer the grain boundaries of MAPbI₃.