First Principles Study Of The High-pressure Properties Of Cu₂ZnSnS₄

Cu₂ZnSnS₄?CZTS?with many advantages is used in the field of solar cell,and belongs to the thermoelectric materials,which has been attracted much attentions for many advantages.Moreover,materials based on the doped CZTS have also been attracted much attentions.In this study,we have investigated the structural stability of the kesterite?KS?,stannite?ST?,primitive mixed Cu-Au?PMCA?,wurtzite-kesterite?WKS?,and wurtzite-stannite?WST?structures for CZTS.Based on the calculations above,the structural properties,band structural and optical properties,phonon spectrum modulation patterns of the KS,ST,PMCA,WKS,and WST structures for CZTS were systematically studied under pressure.At the same time,we have explored the structural parameters,band structural and optical properties of the two new atomic configurations?space group P21 and CMC21?CZTS.The major work is as follows:1.We have systematically investigated the structural properties of the KS,ST,PMCA,WST,and WKS phases for CZTS under the different pressure,and gained the following conclusion.?1?Under 0 GPa,it is found that the order of stability is determined as KS>ST>PMCA>WKS>WST.Moreover,it is found that the KS,ST,PMCA,WST,and WKS structures of CZTS can't satisfy the mechanical stability criteria under 14.93?13.29?14.1?18.55,and 22.1 GPa.The pressure induces phase transition,which was studied.The phase transition between the KS and ST structures occurs at about 32 GPa.At about 52,57.7,58.3 GPa,the phase transitions and mixed crystal states between KS and PMCA occur.When the pressure is about 52 GPa,there exist transition or mixed crystal state between WKS and WST structures.?2?The elastic constants,bulk modulus?B?,shear modulus?G?,young's modulus?E?,GH/BH,poisson's ratio???of five phases of CZTS under pressure were calculated.For these structures,because the bond strength among atoms responds to stress in different degree,the elastic constants have different tendency.As the pressure increases,the ductile behavior becomes better and better,and the stiffnesses and plasticity of the KS and ST structures for CZTS decrease and increases,respectively.However,the stiffnesses of the KS and ST CZTS increase.For the PMCA structure of CZTS,with the increase of the pressure,plasticity is improved,and toughness behavior is getting better and better.For the WST structure,when the pressure increases,the plasticity of the material is better at 32 GPa;GH/BH changes with the increase of the pressure,the structure of the WST CZTS shows ductile behavior.Meanwhile,for the WKS CZTS,as the pressure increases,and the structure of the WKS CZTS shows ductile and plastic behavior under pressure.2.The electronic and optical properties of the KS,ST,PMCA,WST,and WKS structures for CZTS under varied pressures are further studied.?1?To study the phase transitions and structural properties under pressure,the further study of structural properties for the KS,ST,PMCA,WST,and WKS under pressure were calculated.The order of the band gaps is WKS_Eg>KS_Eg>WST_Eg>ST_Eg.?e.g.,KSEg represents the band gap of the KS structure?which has not been changed by the pressure-dependence.It is found that in the KS and ST structures,as the pressure increases,the band gaps first increase linely and then decrease with increasing pressure.As the pressure increases,the KS,ST,WST,and WKS structures for CZTS are still the direct-band-gap semiconductors.However,when the pressure increases to more than 32 GPa,the WKS and WST have a metallic property.The PMCA CZTS has first metallic property and then becomes the indirect-band-gap semiconductor with the pressure increasing.At the same time,we have researched the populations of CZTS under varied pressure.?2?The optical properties of the KS,ST,PMCA,WST,and WKS structures for CZTS were investigated.The results show that for the KS,ST,PMCA,WST,and WKS structures,as the pressure increases,the reflectivities decrease,and absorption coefficients and refractivities increase.In addition,when the pressure increases,the peaks of all optical properties for the five phases CZTS shift up the high energies,and the peaks gradually widen.3.We have investigated the structural,electronic and optical properties of two structures of CZTS under pressure,which has barely been reported.?1?We constructed two models of CZTS?the space group,CMC21 and P21?.The calculated results show that the CMC21 and P21 phases can't satisfy the mechanical stability criteria under 20.4 and 25.1 GPa,respectivly.The elastic properties of the two phases for CZTS were calculated under pressure.It is found that as the pressure increases,the stiffness of the CMC21 phase is the largest under 51 GPa,however,it is the least under 32 GPa.Moreover,the plasticity is improved.CMC21 CZTS possesses ductile behavior under different pressure.In addition,the P21 phase has good toughness and plastic properties,and toughness is getting better and better with the increase of the pressure.The bulk modulus of P21 CZTS increases with increasing pressure.?2?The band structures show that the CMC21 and P21 phases of CZTS belong to the direct-band-gap semiconductors.Nevertheness,the CMC21 and P21 CZTS have the metallic properties under 51 GPa and 32 GPa,respectively.On the basis of the calculated band structures,we have explored the optical properties of the two phases,and it is found that the dielectric spectrums,absorption spectrums,and reflection spectrums of the two phases move up the high energies with the pressure increasing.