Role of Methylammonium Orientation in Ion Diffusion and Current-Voltage Hysteresis in the CH₃NH₃PbI₃ Perovskite

Hybrid organic-inorganic perovskites, and particularly CH₃NH₃PbI₃ (MAPbI₃), have emerged as a new generation of photovoltaic devices due to low cost and superior performance. The performance is strongly influenced by current-voltage hysteresis that arises due to ion migration, and the challenge remains how to suppress the ion migration and hysteresis. Our first-principles calculations demonstrate that the energy barriers to diffusion of the I^-, MA⁺, and Pb²⁺ ions are greatly affected by dipole moments of the MA species. The energy barriers of the most mobile I^- ion range from 0.06 to 0.65 eV, depending on MA orientation. The positively charged MA⁺ and Pb²⁺ ions diffuse along the dipole direction, while the negatively charged I^- ion strongly prefers to diffuse against the dipole direction. By influencing ion migration, the arrangement of MA molecules can effectively modulate the current-voltage hysteresis intensity. The current work contributes to the fundamental understanding of the microscopic mechanism of ion migration in MAPbI₃ and suggests means to suppress the hysteresis effect and optimize perovskite performance. By demonstrating in detail how the arrangement of the organic molecules can efficiently influence ion migration and, hence, amplitude of the current-voltage hysteresis, our results suggest that the hysteresis effect can be suppressed and the long-term performance of perovskites can be improved, if the organic molecules are arranged and stabilized in an antiferroelectric order.