Optical properties and electronic structures of Cu₂ZnSnS₄, Cu₂ZnGeS₄, and Cu₂Zn(Ge,Sn)S₄ and Cu₂Zn(Ge,Sn)Se₄ solid solutions

Abstract

Cu2Zn(GexSn1−x)S4 (CZGTS) samples were synthesized by a mechanochemical process and sequential heating. The phases in the obtained powders were analyzed by X-ray diffraction. The band-gap energies of the CZGTS samples were determined by the diffuse reflectance spectra of UV–vis–NIR spectroscopy. The band gap energy of the CZGTS system linearly increased from 1.49 eV for Cu2ZnSnS4 (x = 0.0) to 2.25 eV for Cu2ZnGeS4 (x = 1.0). Their energy levels of valence band maximum (VBM) from the vacuum level were estimated from the ionization energies measured by photoemission yield spectroscopy (PYS). The energy levels of conduction band minimum (CBM) were determined by addition of the band-gap energies to the VBM levels. The energy level of VBM of the CZGTS solid solution was almost constant. On the other hand, the CBM level of the CZGTS solid solution linearly increased from −3.96 eV for Cu2ZnSnS4 (x = 0.0) to −3.28 eV for Cu2ZnGeS4 (x = 1.0) with the increasing Ge content. For CZGTS solar cells with CdS buffer layer, unfavorable cliff-type conduction band offset was expected. We also synthesized Cu2ZnSnSe4, Cu2ZnGeSe4, and Cu2Zn(Ge,Sn)Se4 (CZGTSe) solid solution samples and determined their energy levels of VBM and CBM. For Cu2Zn(GexSn1−x)Se4 system with 0.3 ≦ x ≦ 1.0, similar cliff-type conduction band offset was is expected. However, desirable positive spike-type conduction band offset was expected for the Cu2Zn(GexSn1−x)Se4 solar cells with 0.0 ≦ x ≦ 0.2 and CdS buffer layer.