Defects And Their Interaction With Polarization In Metal Halide Perovskite Materials

Metal halide perovskite solar cells(PSCs)are emerging as a promising candidate for next-generation photovoltaic technology,the power conversion efficiencies of which have been improved from 3.8%to over 25%in the past decade.Despite the significant improvement in performance,the current-voltage hysteresis and the long-term stability remain the major issues impeding commercialization of PSCs.Both the hysteresis and stability are related with the migration of ionic defects in the perovskite mate-rials.Besides,the presence of ferroelectric polarization in hybrid organic-inorganic perovskites could also be responsible for the hysteresis effect.In this dissertation,by combining the first-principles calculations and theoretical analysis,we systematically study influence of the diffusion of ionic defects on the carrier transport,as well as the interaction between the formation of surface vacancy defects and polarization in metal halide perovskites.The first chapter briefly reviews the research status of the defect and polarization properties of metal halide perovskites.The second chapter introduces the simulation method and theoretical foundation of the first-principles calculations used in this dissertation.The third chapter introduces the influence of ionic migration on carrier transport.Based on a simplified drift-diffusion model,we quantitatively explore the contributing effect of ionic defect migration on the hysteresis of PSCs.We demonstrate that when the scanning voltage is positively larger than the presetting voltage,the photo-generated carriers in perovskite need to overcome an energy barrier to transfer toward their selective contacts.The energy barrier increases with the scanning voltage approximately linearly,with a slope depending not only on the properties of perovskite but also on those of the contact materials.The fourth chapter introduces the effect of defect diffusion on the interface band structure.By using the first-principles calculations,we find that the band offset of the MAPb I₃/Ti O₂interface can be changed by the distribution of ionic defects,and thus vary with the presetting voltage.As the number of negatively charged defects in perovskite accumulated at the interface decreases,the band offsets and thus the electron extraction at the interface are more favorable when preset at a higher voltage than at a lower voltage.The results obtained here,in company with those in chapter three,reveal that the defect migration contributes the current-voltage hysteresis through not only carrier transfer in the bulk perovskite,but also carrier extraction at the interface between perovskite and contact materials.The fifth chapter introduces the interaction between surface defects and polariza-tion.By using the first-principles calculations,we study the influence of polarization on the formation of vacancy defects at the MAPb I₃surface.We find that the formation en-ergies of the vacancy defects at tetragonal MAPb I₃(110)surface are highly related to the surface polarizations.The positive total polarization and local polarization of MA fa-cilitate the formation of surface MA vacancies,while the negative total polarization and local polarization of MAI are favorable for the formation of surface iodine vacancies.A generic model is proposed to understand the calculation results quantitatively based on the Coulomb interactions between the charged defect and the polarization-induced electrostatic field.The sixth chapter introduces the stress mechanism of the asymmetric quantum-confined Stark effect in CsPbI₃ nanocrystals.In order to explain the blueshift in the photoluminescence peak of single excitons observed in the experiments,we study the surface-stress-induced polarization in CsPbI₃by using the first-principles calculations.We find that the surface stress could induce a hydrostatic tensile strain,which further leads to crystal symmetry reduction in both cubic and orthorhombic phase of CsPbI₃.The non-centrosymmetric structure possesses a polarization of severalC/cm²,consis-tent with the experimental results.The last chapter summarizes the main conclusions of this dissertation,closing with some perspectives for future research work.