Benign Interfacial Iodine Vacancies in Perovskite Solar Cells

Methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs) have become the forefront of photovoltaic technologies and attracted intense attention worldwide. MAPbI3 perovskites are mostly in the form of thin films in high-performance MAPbI3 PSCs, and charge transfer and other critical electronic dynamic processes take place at the interfaces of PSCs. The iodine vacancy VI 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 VI properties of MAPbI3 PSCs have not been systematically studied. We utilize first-principles method and nonadiabatic electron dynamics simulations to study the structural and electronic properties of VI at various sites of freestanding MAPbI3 film and the MAPbI3/TiO2 heterojunction. We show that the surface and interfacial VI are more stable than bulk, in agreement with accumulation of VI at grain boundaries observed in experiments. The migration of VI in the perovskite layer under electric field during voltage scans contributes to the anomalous hysteresis in PSCs. VI at Pb–I layer and MA–I layer are quite different: VI at MA–I layer are more stable, while VI defect states at Pb–I layer are more local and weakly covalent bonded. VI promotes both electronic injection and recombination rates, but overall reduces the power conversion efficiencies (PCE) of PSCs. Nevertheless, interfacial VI 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 MAPbI3 PSCs.