Cu2ZnGe(S1-xSex)4 – The challenge to synthesize single phase material

Highlights: • Multiple quaternary phases in polycrystalline powders of Cu₂ZnGe(S_1-xSe_x)_4. • Huge variation in chemical compositions of quaternary phases per sample. • Broad range of secondary phases, especially in samples with S-rich composition. • Volatility of Ge and S complicates the synthesis. -- Abstract: The variation of the band gap energy in Cu₂ZnGeSe₄ and Cu₂ZnGeS₄ from 1.4 eV to 1.7 eV, which is controlled by different S/(S + Se) ratios renders the Cu₂ZnGe(S_1-xSe_x)₄ solid solution an interesting material for the application in multi-junction solar cells. Nevertheless, this system has a certain complexity due to the existence of different polymorphs. Cu₂ZnGeSe₄ crystallizes in the tetragonal kesterite type structure, whereas Cu₂ZnGeS₄ may crystallize in the tetragonal stannite or the orthorhombic wurtz-stannite type structure, respectively. To gain deeper insights into this complex system a systematic study of the solid solution series Cu₂ZnGe(S_1-xSe_x)₄ was performed using polycrystalline material prepared by solid state reaction. The chemical analysis performed by wavelength dispersive X-ray spectroscopy showed remarkable inhomogeneities with different quaternary phases co-existing within one sample. Additionally, a wide variety of binary and ternary secondary phases as well as elemental Ge was observed. The variety of secondary phases is higher in S-rich samples than in Se-rich samples of the solid solution. Thus, synthesis of Cu₂ZnGe(S_1-xSe_x)₄ mixed crystals with off-stoichiometric composition is readily accompanied by the formation of various secondary phases making it a difficult task to obtain single phase material.