First-principles Investigation On The Structural And Photoelectronic Properties Of ABX₃ Perovskites

Environmental pollution and energy shortage are obstacles at present that hinder the social development of human being.As a clean and efficent renewable energy source,solar energy could relieve the environment stress and meet the increasing energy demand of social development.Therefore,it is important to take use of solar energy efficiently.Solar cells are one kind of optoelectronic devices that could utilize solar energy.The photoelectric conversion material is key core of solar cells,which could transform solar energy into electric energy by utilizing photovoltaic effect,and thus directly detemines the photoelectric conversion efficiency of solar cells.Perovskite solar cells（PSCs）arouse wide concern based on the third generation of dye sensitized solar cells（DSSCs）due to the advantages of high efficiency,low cost,and simple manufacture technology.PSCs get fast development recently with the highest photoelectric conversion efficiency of 22.1%.In this thesis,various of ABX₃(A=Cs⁺,MA（CH₃NH₃⁺）,FA（HC（NH₂）₂⁺）,MO（CH₃C（NH₂）₂⁺）,and GA（C（NH₂）₃⁺）;B=Pb²⁺,Sn²⁺,and Ge²⁺;X=Cl^?,Br^?,and I^?)perovskites models were built,including mixed perovskites,Ge-based perovskites,and formamidinium lead halide perovskites.The microstructures,density of states,band structures,and absorption spectra of perovskites were calculated and analysed by using first-principles theory,with the aim to establish relationship between microstructures and photoelectric properties,elucidate photoelectric conversion mechanism,and screen efficient photoelectric conversion perovskite materials.This thesis could provide theoretical explanation and design guidance for experimental investigation.The structural,electronic and optical properties of organic/inorganic hybrid perovskites MAPb_xSn_1-xI₃（x=0?1）and FAPbI_xCl_3-x（x=0–3）were investigated by the first-principles theory.For MAPb_xSn_1-xI₃ perovskites,the MA cations have tiny contribution to the band adge,but provide⁰.7 e to Pb/SnI₃ framework as the donor.There exists a combined covalent and ionic interaction between Pb/Sn and I ions.The valence band maximum（VBM）is mainly contributed by the I 5p orbitals with the overlapping of Pb 6s（Sn 5s）orbitals,while the conduction band minimum（CBM）is dominated by Pb 6p（Sn 5p）orbitals.With the increase of Sn/Pb ratio,the adsorption peaks are red shifted due to the decrease of the bandgap.CH₃NH₃SnI₃ perovskite shows a great potential to absorb light in the visible region.For FAPbI_xCl_3-x perovskites,the FA cations lie along[0 0 1]direction in the trigonal FAPbX₃（X=Cl^?,Br^?,and I^?）.The direction is slightly shifted owing to the distortion of PbX₆（X=Cl^?,I^?）octahedrons in the mixed FAPbI_xCl_3-x.FAPbI_xCl_3-x are direct band-gap semiconductors,with the direct bandgap nature at Z（0,0,0.5）symmetry point.The VBM are composed of antibonding orbitals of I 5p（Cl 3p）and Pb 6s orbitals,and the CBM is composed of Pb 6p orbital.The Pb–I bond distances（0.315⁰.334 nm）are larger than Pb–Cl bond distances（0.282⁰.302 nm）.As the I/Cl ratio increases,the lattice parameters increase,band gaps decrease,and the absorption spectrums are red shifted.FAPbI₃ has an ideal band gap of 1.53eV and exhibits the superior absorption spectrum,especially in the range of 300 nm to 500 nm,showing great potential as the photoelectric conversion material.The structural and photoelectronic properties of trigonal and hexagonal FAPbI₃perovskites were investigated by using first-principles theory.Results showed that the crystal deformation from trigonal to hexagonal crystal would distort the PbI₃ framework,change the covalent/ionic Pb–I bonds,and eventually alter the semiconductor bandgaps.The trigonal and hexagonal FAPbI₃ perovskites are both direct-bandgap semiconductors.The direct-bandgap nature of trigonal crystal locates at Z（0,0,0.5）symmetry point with the ideal bandgap of¹.50eV;the direct-bandgap nature of hexagonal crystal locates atΓ（0,0,0）symmetry point with the wide bandgap of².50 eV.For both crystals,the main contributions to the VBM are I 5p orbitals with a little overlapping of Pb 6s orbitals,and the main components of the CBM are Pb6p orbitals.The FA cations do not directly participate into the electron transition process,just acting as charge donors to supply PbI₃ framework with more than 0.7 e.There exists both covalent and ionic interactions between Pb and I ions.Compared with the hexagonal crystal,the trigonal FAPbI₃ possesses smaller electron and hole effective masses.It exhibits dramatic red shifted absorption spectrum and a better absorption efficiency than hexagonal FAPbI₃ and tetragonal MAPbI₃ perovskites.Bonding energy analyses showed that the hexagonal FAPbI₃was more stable than the trigonal crystal,and interaction between FA and PbI₃ framework was stronger than that between MA and PbI₃ framework.Our results could provide theoretical guidence for the experimental design and synthesis of FAPbI₃ perovskite solar cells.The crystal configuration,electronic structure,charge-carrier transport,and optical properties of Ge-based MAGeX₃ perovskites（X=Cl^–,Br^–,and I^–）and AGeI₃（A=Cs⁺,MA,FA,MO,and GA）were investigated by using first-principles theory.Results showed that the increase in Ge–X bonds（from Cl^?to I^?）in MAGeX₃ increased the volumes,weakened the covalent coupling of Ge–X,lowered the bandgaps,reduced the electron and hole effective masses,and red shifted the absorption spectra.Different A cations in the AGeI₃ systems altered the package of perovskite crystals and thus significantly influenced the electronic and optical properties of those perovskites.Electronic property analyses revealed that the VBM of AGeI₃perovskites were mainly contributed by the I 5p and Ge 4s orbitals,whereas the CBM were dominated by Ge 4p orbitals.In AGeI₃ perovskites,the bandgap increased and absorption spectrum blue shifted in the sequence of Cs⁺→MA→FA→MO→GA.Our results highlighted the effects of A and X on the photoelectronic properties of Ge-based perovskites.