Studies On The Nature Of Defects And Mechanisms Of Passivation In Organometal Halide Perovskites Revealed By First-Principles Calculations

In the last few years,organometal halide perovskites(OMHPs),especially methylammonium lead triiodide(MAPbI₃,CH₃NH₃Pb I₃),have been investigated substantially for the development of solar photovoltaic and light-emitting devices on account of their easy preparation and high performance.The power conversion efficiency of OMHPs-based solar cell has increased rapidly and reached 25.2%for now.In spite of the remarkable progress,there are still some restrictive problems need to be solved to realize the further commercialization.One of the main detrimental factors is the existence of all kinds of crystal defects.In general,these defects can introduce defect levels in the band gap of OMHPs and trap charge carriers followed by nonradiative recombination,which substantially influence photovoltaic processes such as electron-hole generation,separation and recombination,and eventually reduce the efficiency and performance of the devices.However,the formation of such crystal defects is still uncontrollable due to lack of knowledge on these defects.Thus,understanding the nature of the surface defects and their functions is crucial for fully understanding the photophysical processes in these materials and better design of the materials and devices.In this thesis,we have systemically studied the electronic structure of OMHPs,the basic properties of bulk defects and surface defects in MAPbI₃,and the passivation mechanism of surface defects under O₂ environment by the use of first-principles calculations based on density functional theory(DFT).And our significant results are summarized as follows:1.To accurately understand the nature of crystal defects in OMHPs,we should first make clear the essential features of the electronic structure of OMHPs.The experimental recognition and characterization of the three-dimensional(3D)perovskite is relatively easy.However,it is extremely difficult to obtain pure phases with specific number of layers especially when the layer number is large,which limits the tunable wavelength range and hinders the fundamental understanding of the physical properties of two-dimensional(2D)perovskite.Based on it,we calculated the geometric structure and electronic structure of 3D tetragonal MAPbI₃ and 2D perovskite with 1-4 layers((OA)₂(MA)_n-1Pb_nI_3n+1,n=1,2,3,4)in the use of PBE functional.We found that both the 3D and 2D perovskites we studied were the direct gap semiconductor,whose valence band maximum(VBM)and conduction band minimum(CBM)are located at the?point.And their VBM consists mainly of the p orbital of iodine and the s orbital of lead while CBM consists mainly of the p orbital of lead.Such results show that the introduction of the long alkyl chains can change the dimension of perovskite and increase the band gap without affecting the orbital composition of band edges.Moreover,for 2D perovskite,both experimental and theoretical band gap energies show an exponential function against the number of layers(n),and the band gap may gradually decrease until close to MAPbI₃ with the increase of n,which suggests that the band gap of 2D perovskite is determined not only by quantum confinement effect,but other factors including chemical components also give significant contribution.The above results will not only improve our understanding of the electronic structure of OMHPs,but also provide a theoretical basis for the future study of defect properties.2.Many unusual photoluminescence(PL)behaviors have been observed in MAPbI₃ and crystal defects are believed to play important roles in most of these PL behaviors.However,experimental characterization of the chemical nature of these defects is very difficult because the concentration of the defects is extremely low.Therefore,all these unusual PL behaviors are still not fully understood due to the unknown nature of the corresponding defects.According to the two distinct phenomena(PL enhancement and PL decline)observed by experimental cooperators,we proposed a mechanism in which the PL behaviors of MAPbI₃ can be regulated by the transformation between different valence states of bulk defects.Through the defect energy level calculations of chosen defective systems,we can note that the energy levels of Pb and I interstitials(Pb₄)and I₄))are different for different valence states.For Pb₄),only the neutral charge state is deep level defect,while only the+1 charge state of I₄)has deep energy level in the band gap,which should have efficient PL quenching ability.All the other states of both Pb₄)and I₄)are defects with shallow level,which have little effect on the PL of the materials.To understand the properties of the charge transition for a certain defect further,its thermodynamic transition levels were studied.We can see that Pb₄)and I₄)have a negative-U behavior,implying that these two defects are not stable at+1 and neutral charge states,respectively.So,the transformation between different valence states for Pb₄)and I₄)must be(2+/0)and(+/-),respectively.Combined with the defect formation energy,we finally confirm that the transformations from Pb⁺²₄)to Pb⁰₄)and from I⁺¹₄)to I^-1₄)are responsible for the PL decline and PL enhancement,respectively.Our work helps us to identify the chemical nature of the defects and the nature of the crystal defects shows the direction to further understand the role of defects in the photophysical processes.3.The PL intensity and device efficiency are also reported to be very sensitive to the atmosphere environment in relation to the surface defects,among which O₂ plays an important role.Although many experiments have proved the interaction between O₂and certain surface defects,the detailed mechanism and the structure of the defects are still unclear.Therefore,we employed the Heyd-Scuseria-Ernzerhof(HSE)hybrid functional with a fraction of Hartree-Fock exchange including the spin-orbit coupling(SOC)to systematically investigated the electronic properties of the surface defects of MAPbI₃ before or after the adsorption of O₂ in the first time.According to the change of defect energy levels,the adsorption energies of O₂,and the change in the bond length of O₂ after adsorption,we found that the main carrier traps on the MAPbI₃ surface were the vacancy of I on the vacant termination and the interstitial of Pb on the flat termination.The adsorption of O₂ on the defect site can efficiently passivate the vacancy of I and the interstitial of Pb by tuning the energy level away from the midgap.Meanwhile,based on the results from the charge density difference,we can point out that the origin of the passivation mechanism by O₂ is that certain electrons can be transferred from a nearby Pb atom with low coordination to oxygen molecule located at the defect site,which turns O₂ into superoxide species or peroxide species.More interestingly,these two defects are easily formed under a Pb-rich condition.Thus,we suggest that MAPbI₃ samples prepared under the Pb-rich condition are more likely to observe the PL enhancement induced by oxygen passivation.Identification the nature of the surface defects by atmosphere effect could be a general method to investigate the crystal defects in other perovskite materials.