The Effect Of Pressure And Local Atomic Structure On The Photoelectric Properties Of CuInS₂ From First-principles Calculations

CuInS₂（CIS）as a kind of light absorbing layer material has a direct band gap with an excellent energy matching to the solar spectrum.CIS has three types of polymorphic modifications: stable at room temperature chalcopyrite（CP-CIS）,metastable zincblende（ZB-CIS）and wurtzite（WZ-CIS）structures.Before the zincblende（ZB-CIS）and wurtzite（WZ-CIS）structures were found,people pay more attention to the chalcopyrite structure.This thesis used first-principles calculations based on density functional theory to calculate the photovoltaic material CIS up to100 GPa to clarify its possible structural phase transitions and its photoelectric properties.I confirmed a structural phase transition of CIS from tetragonal I 4 2d structure to a Na Cl-like face-centered cubic Fm 3 m phase,which is quite thermodynamically probable.It occurs at 7.9 GPa with a volume reduction of 15.6%.Moreover,a new possible phase transition of CIS from the cubic Fm 3 m structure to the orthorhombic Cmcm structure under higher pressure is predicted occurring at 49.6GPa for the first time,the corresponding volume reduction is about 0.64%.For instance,used the more accurate HSE06 functional we calculated the electronic properties consisting of density of states and band structure for I 4 2d,Fm 3 m and Cmcm phases,it can be found that the I 4 2d phase CIS is a semiconductor material with a direct band gap,and the band gap becomes wider as the pressure increases.Through analysis the Fm 3 m and Cmcm phases,I show that CIS is a metallic material under high pressure,rather than a semiconductor.Through analysis the amount of charge transfer of I 4 2d CIS indicates that the ionicity becomes greater under pressure.The optical absorption spectra indicate that the absorption edge of I 4 2d CIS becomes larger while the optical energy gap becomes larger with the increase of pressure.Some researchers had interests on the effects of Cu/In local sites in WZ-CIS recently.Cu and In atoms in WZ-CIS unit cell randomly share the same lattice site with the same half possibility,then WZ-CIS shows good electronic and optical performance for solar cells.We calculated the electronic and optical properties of WZ-CIS with different local atomic configurations.Through our research we found that Cu-In configuration has a significant effect on the stabilities and properties of WZ-CIS.WZ-CIS is metallic for local aggregative indium and copper atomic configurations,or is a semiconductor for local even-distributed configurations.Metallic configurations have higher lattice energies while semiconductiveconfigurations have lower lattice energies.This indicates that the material will be unstable with the increase of the degree of aggregation.As the degree of the local aggregation of Cu and In atoms increases,the band gap of the WZ-CIS decreases.The optical properties of WZ-CIS were also calculated.We found that the value of optical band gap also decreases as local aggregation of Cu and In atoms increases.The metallic configurations have a higher absorption coefficient.