Benign Interfacial Iodine Vacancies in Perovskite Solar Cells

Methylammonium lead iodide (MAPbI 3 ) perovskite solar cells (PSCs) have become the forefront of photovoltaic technologies and attracted intense attention worldwide. MAPbI 3 perovskites are mostly in the form of thin films in high-performance MAPbI 3 PSCs, and charge transfer and other critical electronic dynamic processes take place at the interfaces of PSCs. The iodine vacancy V I is thought to play a major role in arousing severe hysteresis in photocurrent-photovoltage scan, which limits industrialization of PSCs. However, the surface and interfacial V I properties of MAPbI 3 PSCs have not been systematically studied. We utilize first-principles method and nonadiabatic electron dynamics simulations to study the structural and electronic properties of V I at various sites of freestanding MAPbI 3 film and the MAPbI 3 /TiO 2 heterojunction. We show that the surface and interfacial V I are more stable than bulk, in agreement with accumulation of V I at grain boundaries observed in experiments. The migration of V I in the perovskite layer under electric field during voltage scans contributes to the anomalous hysteresis in PSCs. V I at Pb–I layer and MA–I layer are quite different: V I at MA–I layer are more stable, while V I defect states at Pb–I layer are more local and weakly covalent bonded. V I promotes both electronic injection and recombination rates, but overall reduces the power conversion efficiencies (PCE) of PSCs. Nevertheless, interfacial V I is found to be the least harmful to the PCE of PSCs comparing with random sites in the bulk, contributing to the high PCE of MAPbI 3 PSCs.