Investigation Of Vacancy Defect In CsSnX₃（X=Br,Cl） Based On First Principles

Global economic development continues to increase the demand for energy,and the promotion of the concept of"green and sustainable development".The promotion and utilization of renewable energy have become the focus of scientific researchers.Solar cells have attacted more and more researchers’interests and attention.Because they meet the requirement of clean and energy-saving,and flexible fabrication methods.Perovskites have set off a revolution in the field of renewable energy,owing to their excellent optoelectronic properties（such as high photoelectric conversion efficiency,adjustable band gap,high absorption rate,etc.）.Current works demonstrate that lead-based perovskites have the highest efficiency,but the mental toxicity limits their further development.The emergence of tin-based perovskites offers the possibility to fabricate non-toxic photovoltaic devices with excellent optoelectronic properties.The properties of materials are closely linked to the performance of devices,which is inevitably affected by defects and impurities in perovskite.Therefore,it is essential to investigate the nature of defects systematically.In this work,we adopt the first-principles calculation method to calculate the optoelectronic properties and defective properties in Cs Sn X₃（X=Br,Cl）perovskites.The conclusions are summarized in the following.Firstly,we investigate the effect of the different atomic vacancies on the optoelectronic properties in Cs Sn Br₃perovskites.The results show that V_Cs and V_Sndefects introduce defect states in the depths of the valence band,and increase the band gap value;V_Br defect introduces a defect state,and narrows band gap.It can be concluded that V_Cs and V_Sn defects lead to P-type properties of perovskites,while V_Brdefect leads to N-type perovskites.Optical analysis shows that the peak of the dielectric function and absorption coefficient,in V_Sn and V_Br defective systems,decreases significantly in the low-energy region.Above all,the self-doping of defect state produced by V_Sn defect makes its absorption spectrum blue-shift.However,the variation trend almost coincides with the pristine perovskites in V_Br system.The second part of the paper mainly analyzes the effect of deep level defects on the photoelectric properties of Cs Sn Cl3 perovskite.Firstly,we adopt the different exchange-correlation functionals（PBE and HSE06）to calculate Cs Sn Cl₃perovskite.The results show that the value of band gap（2.81 e V）is close to the experimental value by using HSE06（HF=72.5%）.Next,we further investigate the properties of photoelectricity and deep-level defect in perovskites.The results demonstrates that the V_Cl defect induces the formation of a deep-level defect state in the band gap,which was narrowed obviously,and the light absorption coefficient increases.Due to the similar chemical properties,the overlap between defect states and CBM is larger.In addition,molecular dynamics analysis shows that the generation of V_Cl defect perturbs the Sn-Cl lattice vibrations.In our work,we explain the effect mechanism of different defects on the photoelectric properties of perovskites microscopically.It is found that vacancy defects can regulate the optical properties of perovskite materials.This has important theoretical value for studying the properties of perovskite and its application in the field of photoelectric devices.