Influence of Ge substitution on the structure and optical properties of Cu2ZnSn1−xGexS4 photovoltaic materials

Abstract

Germanium substitution in the well-known photovoltaic material Cu₂ZnSnS₄ has been previously shown to improve power conversion efficiencies, but detailed information about the local structure within solid solutions Cu₂ZnSn_1−xGe_xS₄ has so far been lacking. Given the many ways the metal cations can be distributed, several structural models for the site distribution are possible. In conjunction with powder X-ray diffraction, multinuclear magnetic resonance spectroscopy (^63/65Cu, ⁶⁷Zn, ⁷³Ge, and ¹¹⁹Sn) was applied to evaluate these models because these nuclei are sensitive to variations in the coordination environments around specific atoms. The ⁶⁷Zn NMR spectra indicate that Zn atoms are in lower symmetry environments in Cu₂ZnGeS₄ than in Cu₂ZnSnS₄, as their different quadrupole coupling constants suggested. The ⁷³Ge and ¹¹⁹Sn NMR spectra show gradual changes in chemical shift, indicating that Ge and Sn atoms are randomly mixed in tetrahedral sites within the entire range of solid solubility. The ^63/65Cu NMR spectra collected at high field (21.1 T) reveal two distinct Cu sites for all members of Cu₂ZnSn_1−xGe_xS₄, consistent with the structure model in space group I, as also supported by DFT calculations of NMR parameters. Ge substitution increases the experimental band gap from 1.5 eV in Cu₂ZnSnS₄ to 2.0 eV in Cu₂ZnGeS₄ because the conduction band minimum is raised to higher energy, as shown by electronic structure calculations.