First-principles Studies On Interface Electronic Properties Induced By Vacancies In Perovskite Materials

Perovskite,with general chemical formula of ABX₃,exhibits many interest-ing and intriguing properties,therefore possesses a variety of applications in many field.Interface electronic properties play a very important role in determining the performance and efficiencies of perovskite materials.Besides,there is always a certain amount of X-vacancy in perovskite materials.Sometimes it affects the performance of perovskite-based devices,but sometimes it can be used to adjust the emergent phenomenon.Here we studied the properties of iodine vacancy in MAPbI₃ perovskite solar cells,and the oxygen vacancy induced magnetoelectric coupling effect in ferroelectric PbTiO₃/SrTiO₃ heterostructure,respectively.Methylammonium lead iodide MAPbI₃ perovskite solar cells?PSCs?have become the forefront of photovoltaic technologies and attracted intense attention worldwide.MAPbI₃ perovskite 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 s-tudied.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 d-ifferent: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.First-principles calculations predict the emergence of magnetoelectric cou-pling mediated by two-dimensional electron gas?2DEG?at the ferroelectric PbTiO₃/SrTiO₃ heterostructure.Free electrons endowed by naturally existing oxygen vacancies in SrTiO₃ are driven to the heterostructure interface under the polarizing field of ferroelectric PbTiO₃ to form a 2DEG.The electrons are cap-tured by interfacial Ti atoms,which surprisingly exhibits ferromagnetism even at room temperature with a small critical density of ??15.5 ?C/cm2.The ferroelectricity-controlled ferromagnetism mediated by interfacial 2DEG shows strong magnetoelectric coupling strength,enabling convenient control of mag-netism by electric field and vice versa.The PbTiO₃/SrTiO₃ heterostructure is cheap,easily-grown and controllable,promising future applications in low cost spintronics and information storage at ambient condition.