Investigation Of Optoelectronic Properties Of Ruddlesden-Popper Perovskites Using First-Principles Theoretical Methods

Researchers are actively exploring lead-free perovskite materials to overcome the stability and environmental issues associated with lead halide perovskites.Lead-free perovskites have attracted widespread attention due to their lower toxicity and environmental friendliness.Many lead-free perovskite materials have been developed and have exhibited promising optoelectronic properties.However,lead-free perovskite materials still have some limitations.Firstly,the power conversion efficiency of current lead-free perovskites is relatively low,posing a large gap compared to lead halide perovskites.Secondly,the long-term stability and weather resistance of lead-free perovskites remain to be addressed.More research and innovation are still needed to overcome these challenges and realize the widespread application of lead-free perovskites in the field of optoelectronics.Against this backdrop,this dissertation systematically investigates two key physical properties that influence the photovoltaic conversion efficiency in the Ruddlesden-Popper type perovskite Y₂Ti₂O₅S₂:light absorption and charge carrier transport.Firstly,this type of perovskite has been experimentally proven to possess remarkable stability and has even been applied in photocatalytic total water splitting,indicating its superior stability compared to lead halide perovskites.Secondly,it has been experimentally measured to have a similarly low charge carrier recombination rate as lead halide perovskites.Using first-principles calculations,this work confirms the two-dimensional transport characteristics of charge carriers within this material and,from the perspective of excited-state dynamics,verifies the electron-hole separate transport properties of Y₂Ti₂O₅S₂.The dynamic features of wave functions during molecular dynamics processes are studied in detail.Based on the analysis of wave function overlap,the optical transition pathways are investigated,revealing the distribution of wave functions responsible for effective light absorption in this material.This dissertation focuses on the following aspects of the material:With this background,this dissertation systematically investigates two major physical properties that influence the photovoltaic conversion efficiency in Ruddlesden-Popper type perovskite Y₂Ti₂O₅S₂:light absorption and charge carrier transport.Firstly,this type of perovskite has been experimentally proven to be stable and has even been applied in photocatalytic water splitting,demonstrating its superior stability compared to lead halide perovskites.Secondly,it has been experimentally measured to have the same low charge carrier recombination rate as lead halide perovskites.Using first-principles calculations,this dissertation confirms the two-dimensional transport characteristics of charge carriers in this material and,from the perspective of excited state dynamics,verifies the electron-hole spatially separated transport property of Y₂Ti₂O₅S₂.The dynamic features of wave functions during molecular thermal dynamics processes are extensively studied.Based on the investigation of wave function overlap,the pathways for optical transitions that contribute to the actual light absorption in this material are identified.Consequently,focusing on the experimentally observed photoelectric properties of this material,namely high efficiency,long carrier lifetime,and low light absorption,systematic studies are conducted as follows:1.A high-speed carrier transport pathway has been discovered in X₂Ti₂O₅S₂（X=Sc,Y,La）materials,explaining the relatively efficient photocatalytic water splitting efficiency of Y₂Ti₂O₅S₂.By analyzing the electron localization function（ELF）,a continuous distribution region of 0.2<ELF<0.7 is found in the perovskite layer of X₂Ti₂O₅S₂,extending within the xy plane.By comparing the carrier concentrations in the bulk phase and the 2D phase within the high-speed transport pathway,the influence of carrier concentration on carrier recombination is demonstrated.Additionally,through the analysis of the ELF formula,it is suggested that the ELF value can also describe the correlation of carrier recombination,providing further evidence for the rationality of the high-speed transport pathway.2.In order to further elucidate the efficient water splitting and long carrier lifetime observed in experiments,the electron-phonon coupling that plays a decisive role in carrier lifetime is investigated.By eliminating the electron-phonon coupling in the Y₂Ti₂O₅S₂ target layer（rock-salt layer）,the overall electron-phonon coupling of the material is reduced,leading to an extended carrier lifetime.In this dissertation,by analyzing the distribution and overlap characteristics of wave functions and combining them with the phonon-induced spectral effects of the electron-phonon coupling,the location of wave function distribution that predominantly contributes to the electron-phonon coupling is identified as the rock-salt layer.By means of atomic substitution or addition,the distribution of wave functions is fine-tuned to eliminate the electron-phonon coupling in the rock-salt layer.Consequently,the electron-phonon coupling within the material is reduced to a level similar to that of lead halide perovskites,resulting in a times extension of the carrier lifetime.3.Due to the weak light absorption of the material,the reasons for the weak light absorption are elucidated from the perspective of electronic transitions.The optical transition pathways of such materials are determined,and a significant enhancement in light absorption is achieved by modifying or strengthening these pathways.By analyzing the degree of overlap of the band edge wave functions in the material,the primary spatial location for electronic optical transitions is identified as the region between the Y and S atoms in the rock-salt layer.The weak wave function overlap between Y and S explains the low light absorption of Y₂Ti₂O₅S₂.A substantial improvement in optical transitions and light absorption is achieved by enhancing and modifying the wave function overlap in the rock-salt layer,leading to a significant increase in light absorption and an extension of the carrier lifetime.Furthermore,this section also calculates the optical transitions and carrier lifetimes of other sulfur-containing compounds such as Cu₂SnS₃ to identify the optimal configuration for this class of materials.