陈时友 教授
华东师范大学极化材料与器件教育部重点实验室

时 间： 9月25日（星期四）下午四点
地 点： 唐仲英楼A313

摘要：The I2-II-IV-VI4 (I=Cu, Ag, II=Zn, Cd, IV=Si, Ge, Sn, VI=S, Se, Te) quaternary semiconductors, such as kesterite Cu2ZnSnS4 and Cu2ZnSnSe4, are drawing considerable attention recently as the active layers in thin-film solar cells.The additional number of elements in these quaternary compounds, relative to binaryII-VI or III-V semiconductors, results in increased freedom for tuning the material properties. However, the understanding of theirphysical properties is still poor.In this talk I will show that these semiconductors can be derived from binary II-VI semiconductors through element-mutation, i.e., from II-VI (ZnS) to I-III-VI2 (CuGaS2) and then to I2-II-IV-VI4 (Cu2ZnSnS4).Following the mutation, we can determine the crystal structures of the I2-II-IV-VI4 semiconductors and reveal the chemical trends in their electronic band structure, which makes the band structure engineering possible. The band gaps of I2-II-IV-VI4 compounds can be tuned from negative (metal or topological insulator)to more than 4 eV (wide-gap semiconductor).Furthermore, the increased structural freedom enhances the miscibility (component-uniformity) of the Cu2ZnSn(S,Se)4 and Cu2Zn(Sn,Ge)Se4 alloys, so their band gaps can be tuned continuously and also linearly as a function of the alloy composition.
  Although the band gap is more flexible, the defect control becomes very challenging in quaternary semiconductors. A large variety of intrinsic lattice defects and defect complexes can be formed, which have important influence on the carrier concentration, electron-hole recombinationand hence the solar cell performance. Using first-principles calculations, we identified the dominant defects and predicted their ionization energy levels, which are confirmed by the recent experimental measurements and used for the optimization of Cu2ZnSn(S,Se)4 solar cell performance through the synthesis-condition control.
  Through similar element-mutation in GaN or InN, ternary ZnSnN2 and ZnGeN2 semiconductors can also be designed. Their mixed-cation alloys Zn(Sn,Ge)N2, which have better miscibility than (In, Ga)N alloys and span a band gap range from 2.0 eV to 3.5 eV, are recently synthesized.