Investigation On The Regulation Of Composition,Dimensionality And Surface Defects Of Cesium Lead Halide Perovskites

Cesium lead halide perovskite showing great application prospects in light emitting,photovoltaic and other fields due to excellent photoelectric properties have drawn a tremendous amount of attention and extensive research.However,the stability of perovskite and device efficiency need to be further improved.Although,some strageties have been applied such as composition engineering,dimensionality engineering and surface defect engineering,and their effect and formation mechanism need to be further studied.Our work focus on the regulation of dimensionality and composition on the photoelectric properties,the formation and passivation of surface defects and ion migration suppression of inorganic perovskite.The main works are as follows:（1）Dimension effect of mixed-halide perovskite materialsDielectric constant（ε₀）and band gap（E_g）are key parameters affecting the carrier transport and recombination,and the performance of optoelectric device.Composition and dimension regulation are two effective strategies to improve the efficiency of inorganic perovskite devices,but the regulation mechanism of them onε₀ and E_g of Cs Pb X₃（X=Cl,Br,I）perovskite is unclear,and the relationship between them is still unknown.In order to explore the influence mechanism of dimension effect on the photoelectric properties of mixed halide perovskite,the geometric structures,mixing enthalpies and dielectric properties of mixed halide perovskites with different halogen substitution sites in different dimensions have been studied.And all the results show that,I-based perovskite substituted by Br and Br-based perovskite substituted by Cl can be realized under all substitution ratios and positions of mixed halide perovskite materials in different dimensions.Dimension and composition have significant effects on band gap and band bowing parameters,showing a synergistic effect,and the effect of dimension on electronic properties is greater than that of composition.From the effect of Pb-X（X=Cl,Br,I）bond lengths and polarized born effective charge onε₀ and E_g,it is found thatε₀ and 1/E_g² have a good linear relationship in different dimensions.This work is helpful to further understand the dielectric effect of perovskite with different dimensions and screen suitable perovskite materials for different optoelectronic devices.（2）Multifunctional organic molecules passivate surface defects of perovskite and enhance charge transferIn the process of rapid solution preparation,a large number of defects inevitably occur on the perovskite surface,which will increase the carrier non-radiative recombination and reduce the efficiency and stability.The formation mechanism of defects on different surfaces of Cs Pb I₂Br and the passivation strategy of deep level defects need to be studied deeply.In this work,we have calculated the phase diagram of Cs Pb I₂Br bulk phase structure,obtained the formation energy of different defect types in different surfaces,and revealed the deep level defect types and formation mechanism.By calculating the formation energy and energy level of surface defects,vacancies with high defect formation energies belong to shallow level defects.Meanwhile,antisite Pb_Br and interstitial Br_i,I_i with low defect formation energies are deep level defects,which is the main factor leading to the reduction of radiative recombination efficiency.Combining with the characteristics of deep level defects,we proposed an appropriate surface passivation strategies:1)passivators should have suitable spatial configurations to form strong interaction with different surface defects,2)passivators should have both donor and accept electron ability to balance the redundant charge at the defects.Based on these rules,four multifunctional organic molecules with electron donor and acceptor groups were designed,which can form strong interaction with surface defects,neutralize the excess charge on the surface,reduce or eliminate deep-level defects,and promote the charge transfer between the surface and molecules effectively.This study reveals the formation mechanism of various defect types on the perovskite surface and puts forward the organic molecular design strategies for passivating deep level defects,which provides guidance for the design of passivators to improve the efficiency and stability of perovskite.（3）Suppression of ion migration on perovskite surface by organic amine ionsIon migration of mixed-halide perovskite leads to phase segregation,decreases open circuit voltage,resulting in low efficiency and unstability of optoelectronic devices.Surface defect is the main factor inducing ion migration,which can provide migration channels for ions.In this work,the mechanism of ions migration are studied and found that ion migration is affected by surface structure,ion radius,steric hindrance,Coulomb interaction of surrounding atoms,and electric field.By analyzing the migration barriers of different types of ions on different surfaces.We found the Pb I₂-terminated surface is stable,and ion migration rarely occur,while the halogen ions on the Cs I-terminated surface with I and Br vacancies are more likely to migrate.The larger the ion radius,the greater steric hindrance,the stronger Coulomb interaction between atoms,the larger the migration barrier is.With built-in electric field,the ion migration barrier will be reduced and the ion migration will be intensified.In order to suppress the halide ions migration,four different organic amine caions were introduced,including butylamine,phenylethylamine,thiophenylethylamine and phenylethylamine.The results show that amine ions lead to the change of X-Pb-X（X=I,Br）bond angles between adjacent octahedrons,enhance the interface interaction,reduce band edge states and increase band gap.Compared with flexible chain butyl amine ions,conjugated organic amine ions can supress ion migration effectively,and the ion migration barrier increases with chain length.This work systematically studied the effect of all inorganic perovskite cations on ion migration and pointed out the mechanism of organic cations supression ion migration,providing theoretical guidance for the design of high-efficiency passivator.In this doctoral thesis,the linear relationship between the two key parameters of band gap and dielectric constant is systematically studied,which provides a theoretical support for the selection of reasonable cesium lead halide perovskite materials in the design of optoelectronic devices.The formation mechanism of surface defects is clarified,which makes up for the lack of understanding of the surface defects properties in experiment.According to the passivation stragtegies of deep level defects by multifunctional organic molecules are proposed,and the enhancement of charge transfer and the supression of ion migration are realized.The mechanism of ion migration of mixed halogen perovskite is revealed,and the strategy of supressing ion migration is put forward.This doctoral thesis is expected to provide guidance for the design of efficient and stable perovskite optoelectronic devices.