Theoretical Study On Excited States And Light Absorption Properties Of Organic-inorganic Hybrid Perovskites

With the rapid development of modern industry,the energy crisis forces us to find a renewable energy alternative to fossil energy.In just ten years,the photoelectric conversion efficiency of perovskite solar cells has reached 25.2%.Therefore,perovskite solar cells are regarded as one of the ways to solve the energy crisis in the 21st century.Although the photoelectric properties of perovskite can be qualitatively measured by visible light absorption spectrum and energy band structure,there is a lack of a quantitative method to measure the photoelectric properties,so as to realize the rapid screening of perovskite materials and guide perovskite solar cell optimization.Therefore,based on the photovoltaic effect,this paper divides the photoelectric conversion process of perovskite into photon energy absorption process and electronic excitation process,explores the influence of structure on light absorption process and electronic excitation process,and explores comprehensive evaluation indicators to measure perovskite optoelectronic properties.In order to explore the nature of the light absorption process of perovskites,the crystal structures of 81 perovskites were constructed.Their band structure,exciton binding energy,static permittivity,and absorption spectrum were calculated.The main conclusions are as follows:Firstly,the A-site organic cations mainly adjust the size of the band gap by changing the lattice size of the BX3 inorganic framework.For example,the band gap of MAPbI3,KCPbI3 and TAPbI3 are 1.48 eV,2.31 eV and 2.68 eV,respectively.As the period of the X-site halogen element gradually increases,the band gap of the perovskite gradually decreases.Secondly,as the dimension of perovskite decreases,its static permittivity increases,such as εstd-yyMAPbBr2I=13.42,εstd-yyMCPbBr2I=14.47,εstd-yyMAPbBr2I=21.30,which indicates that the dielectric shielding effect increases.And due to the enhancement of the Coulomb interaction force between electrons and holes,the exciton binding energy calculated by the fitting formula will also increase accordingly.Thirdly,based on the visible light absorption spectrum,an evaluation index called light absorption energy is proposed for the first time in this paper to quantitatively measure the light absorption ability of perovskite,which has an e-index relationship with the band gap,E=71.93×e-gap/1.23-3.56.In order to investigate the electronic excitation process of perovskite,this paper systematically studies the electronic excitation of perovskite from the ground state to the first singlet state based on time-dependent density functional theory,combined with electron-hole analysis and inter-fragment charge transfer method.process.The main research contents are as follows:In this paper,the accuracy performance of six functionals in excited state calculation is tested,and compared with two double-hybrid functionals,it is found that M06-2X is the most suitable functional for perovskite excited state calculation.The error is only about 0.3 eV.Secondly,36 kinds of perovskite cluster structures were constructed and optimized in this paper.Combined with the analysis of electron-hole distribution,the results showed that electrons and holes were mainly distributed on the B-site and X-site atoms of the inorganic framework(BX3).As the metal element at the B site and the halogen element at the X site become heavier,the exciton binding energy decreases from 6.64 eV to 4.88 eV,and the separation effect of electrons and holes becomes more and more excellent.Finally,combined with the radar chart analysis method,the exciton binding energy(calculated by the Coulomb formula),excitation energy,net charge transfer,electron and hole overlap index,and hole-electron centroid distance are used as characteristic indicators,and they are normalized.It is plotted in a radar chart to visually describe the electronic excitation properties of perovskites.On this basis,a comprehensive evaluation index(S)for the electronic excitation process of perovskite is proposed for the first time in this paper.The electronic excitation ability of perovskite is well reflected.