| With the rapid development of society,the demand for energy is increasing rapidly.The limitations of traditional fossil energy and the problems of environmental pollution and climate warming caused by a large amount of consumption have led to the urgent demand for sustainable energy.In 2009,perovskite solar cells based on methylammonium iodine lead MAPb I3(MA=CH3NH3+),ignited researchers' enthusiasm for ABX3 perovskite materials.However,the toxicity of lead in MAPb I3 and the low light response range are two key factors that restrict the further development of MAPb I3 in solar cells.In order to solve above two key problems.The first-principle calculation method based on density functional theory are carried out to study the structure of MAPb I3.The purpose is to find lead-free perovskite solar cell materials by substitutional doping,and realize the adjustment of MAPb I3 band gap at the same time.The main creative achievements are as follows:First,the tensile and compressive strain can be used to adjust the band gap of MAPb I3.The band gap of MAPb I3 can be widened by tensile strain,and the band gap of MAPb I3 can be narrowed when the compression strain is applied.The orientation of organic cations in MAPb I3 have a slight influence on the structure and band gap of MAPb I3.The vacancy defect of different atoms in MAPb I3 has an slight effect on the structure and physical properties of MAPb I3.The main results are as follows: when C atoms appear vacancy defects,MAPb I3 is still semiconductor,MAPb I3 appears as a metallic property when the vacancy defect of Pb atoms occurs.In addition,when N vacancies,H atoms connected with N or C appear vacancies,MAPb I3 appears spin polarization.At this time MAPb I3 has magnetic properties.Second,replacing the Pb atoms in MAPb I3 with ten transition metal atoms,new perovskite compounds exhibit various physical properties in their ground state.Among them,MAZn I3 and MACo I3 are the first gapless semiconductor and spin gapless semiconductor in perovskite,respectively.Other compounds,such as MASc I3,MATi I3,MACr I3,MAFe I3 and MANi I3,are ferromagnetic half-metals.MAVI3 and MAMn I3 are ferromagnetic semiconductors.Under the action of compression strain and tensile strain,the physical properties of the eight ferromagnetic compounds have changed with the strain.In particular,it is worth noting that MACo I3 and MANi I3 have undergone phase transition from metal to half-metal to spin gapless semiconductor to semiconductor within the strain-10% to 10%.Third,it is found that the mixed perovskites obtained by different Ti doped MAPb I3,have very rich physical properties.Cubic MATi0.25Pb0.75I3,tetragonal MATi0.25Pb0.75I3,orthorhombic MATi0.25Pb0.75I3 and cubic MATi0.75Pb0.25I3 are magnetic half-metals,tetragonal MATi0.75Pb0.25I3 is a magnetic metal,orthorhombic MATi0.75Pb0.25I3 is a magnetic semiconductor.The mixed perovskite compounds obtained by Mn doped MAPb I3 are all magnetic semiconductors.With the increase of Mn doping concentration,the band gaps of mixed perovskite in cubic phase,tetragonal phase and orthogonal phase decrease.Fourth,it is found that three new lead-free perovskite compounds Cs2 Sn Cl6,Cs2 Sn Br6 and Cs2 Sn I6 exhibit good mechanical stability in equilibrium lattice constants,and all of them have high melting temperature.The HSE06 hybrid functional can be used to obtain the semiconductor bandgap that is in agreement with the experimental results.The three kinds of materials show good light absorption characteristics in the ultraviolet region.With the increase of the number of halogen atoms,the absorption spectra of them have a red shift. |
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