Influence of Ge substitution on the structure and optical properties of Cu₂ZnSn_1-xGe_xS₄ photovoltaic materials

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 Cu2ZnSn1-xGexS4 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 (Cu-63/65, Zn-67, Ge-73, and Sn-119) was applied to evaluate these models because these nuclei are sensitive to variations in the coordination environments around specific atoms. The Zn-67 NMR spectra indicate that Zn atoms are in lower symmetry environments in Cu2ZnGeS4 than in Cu2ZnSnS4, as their different quadrupole coupling constants suggested. The Ge-73 and Sn-119 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 Cu-63/65 NMR spectra collected at high field (21.1 T) reveal two distinct Cu sites for all members of Cu2ZnSn1-xGexS4, 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.