First-principles Study On Surface Passivation Of Cu₂ZnSnS₄ And Cu_0.714GaTe₂ Doping

Cu₂Zn Sn S₄?CZTS?belongs to P-type semiconductor,and has a kesterite structure in nature.Because of its low cost and excellent photoelectric properties,it attracts more and more attention of researchers as the absorbing layer of solar cells.The stucture of CZTS-based thin film solar cell mainly originates fromthe structure of more mature Cu In Ga Se₂?CIGS?solar cells:front electrode|Zn O window layer|Cd S buffer layer|CZTS absorber layer|Mo back electrode|galss.The performance of CZTS depends not only on the structure and properties of the materials in each layer,but also on the changes in structure and properties that result from the surface treatment of material of each layer.The surface of the semiconductor material often leads to the appearance of surface electronic states that have a negative effect on the transmission of photogenerated carriers in solar cells.In this thesis,the first-principles calculation method was used to systematically investigate the electronic properties of the bulk Cu2Zn Sn S4,surface reconstruction,and mechanism of adsorption and passivation of F,Cl and H atoms on Cu2Zn Sn S4?112?surface.Our results show that a surface reconstruction occurs on the Cu-Zn-Sn-terminated Cu₂Zn Sn S₄?1^-1^-2^-?surface.When a single F,Cl or H atom is adsorbed on the S-terminated Cu₂Zn Sn S₄?112?surface,all three kinds of atoms exhibit an optimal stability at a specific top adsorption site in comparison with the bridge,hcp and fcc sites.When coverage is 0.5 ML,H,Cl or F atoms occupy two top sites on the Cu2Zn Sn S4?112?surface,which have the lowest adsorption energy.The surface states of S-terminated Cu₂Zn Sn S₄?112?slab near the Fermi level are mainly contributed by the Cu-3d orbital and the S-3p orbital electrons.For 0.5 ML H,Cl and F sorption,the surface states near the Fermi level are partially reduced.It has also been found that the passivation effect of H atom for surface states is the most significant and the effect of Cl atom is better than that of F atom.The main reason for the decrease of the surface states is that adsorption atoms obtain electrons from the S atoms,and the state density peaks of the Cu and S atoms at the Fermi level almost disappear completely.In addition,the chalcopyrite semiconductor Cu_0.72Ga Te₂ is a new type of thermoelectric material with Cu vacancy defects and which is regarded as preferred thermoelectric materials.In this thesis,the first-principles calculation was also used to study the changes of the band and density of states of Sb-doped Cu_0.714Ga Te₂.It is found that the band gap of Cu_0.714Ga Sb_x Te₂ disappears at the doping concentration of x=0.1 and 0.4,which shows the metal properties.