Structure Flexibility of the Cu₂ZnSnS₄ Absorber in Low-Cost Photovoltaic Cells: From the Stoichiometric to the Copper-Poor Compounds

Here we present for the very first time a single-crystal investigation of the Cu-poor Zn-rich derivative of Cu2ZnSnS4. Nowadays, this composition is considered as the one that delivers the best photovoltaic performances in the specific domain of Cu2ZnSnS4-based thin-film solar cells. The existence of this nonstoichiometric phase is definitely demonstrated here in an explicit and unequivocal manner on the basis of powder and single-crystal X-ray diffraction analyses coupled with electron microprobe analyses. Crystals are tetragonal, space group I4̅, Z = 2, with a = 5.43440(15) Å and c = 10.8382(6) Å for Cu2ZnSnS4 and a = 5.43006(5) Å and c = 10.8222(2) Å for Cu1.71Zn1.18Sn0.99S4.