Optical properties and electronic structures of Cu2SnS3, Cu2GeS3, and their solid solution Cu2(Ge,Sn)S3

Abstract

Cu2(GexSn1−x)S3 (CGTS) (0 ≤ x ≤ 1.0) powders were synthesized by mixing elemental powders and sequential heating in 5% H2S gas atmosphere. The crystal structures of CGTS solid solution samples were refined by Rietveld refinement of the powder X-ray diffraction data. CGTS has a monoclinic crystal structure with a space group of Cc (No. 9), and the refined lattice parameters decreased linearly from a = 6.653(7) Å, b = 11.532(5) Å, and c = 6.656(2) Å of Cu2SnS3 (x = 0.0) to a = 6.416(3) Å, b = 11.303(2) Å, and c = 6.434(9) Å of Cu2GeS3 (x = 1.0) with increasing Ge, x. The band-gap energy (Eg) of the Cu2(GexSn1−x)S3 solid solution monotonically increased from 0.87 eV for Cu2SnS3 (x = 0.0) to 1.53 eV for Cu2GeS3 (x = 1.0) with increasing Ge, x. The energy level of the valence band maximum (VBM) from the vacuum level was determined from the ionization energy measured by photoelectron yield spectroscopy (PYS). The determined energy levels of VBM of Cu2SnS3 and Cu2GeS3, and their solid solution samples were shallower than those of Cu2ZnSnS4 and Cu2ZnGeS4, and their solid solution. The energy level of conduction band minimum (CBM) was calculated by adding Eg to VBM level. The energy levels of CBM of Cu2SnS3 and Cu2GeS3, and their solid solution samples were deeper than those of Cu2ZnSnS4 and Cu2ZnGeS4, and their solid solution. Even when the Ge content in Cu2(GexSn1−x)S3 solid solution samples increased, the VBM level did not change significantly but the CBM level increased markedly. We also determined the VBM levels of Cu2SnSe3, Cu2GeSe3, and Cu2(Ge,Sn)Se3 solid solution samples. The energy levels of the VBM of Cu2SnSe3 and Cu2GeSe3, and their solid solution samples were also shallower than those of Cu2ZnSnSe4 and Cu2ZnGeSe4, and their solid solution. On the basis of these results, we discuss the band diagrams of Cu2(Ge,Sn)S3 and Cu2GeSe3 solar cells with the following device structure: absorber layer/CdS buffer layer/ZnO layer.