Structural investigation of the Cu2Se–In2Se3–Ga2Se3 phase diagram, X-ray photoemission and optical properties of the Cu1−z(In0.5Ga0.5)1+z/3Se2 compounds

Structures of compounds in the Cu2Se–In2Se3–Ga2Se3 system have been investigated through X-ray diffraction. Single crystal structure studies for the so-called stoichiometric compounds Cu(In,Ga)Se2 (CIGSe) confirm that the chalcopyrite structure (space group I4¯2d) is very flexible and can adapt itself to the substitution of Ga for In. On the other hand a structure modification is evidenced in the Cu1−z(In0.5Ga0.5)1+z/3Se2 series when the copper vacancy ratio (z) increases; the chalcopyrite structure turns to a modified-stannite structure (I4¯2m) when z≥0.26. There is a continuous evolution of the structure from Cu0.74(In0.5Ga0.5)1.09Se2 to Cu0.25(In0.5Ga0.5)1.25Se2 ((i.e. Cu(In0.5Ga0.5)5Se8), including Cu0.4(In0.5Ga0.5)1.2Se2 (i.e. Cu(In0.5Ga0.5)3Se5). From this single crystal structural investigation, it is definitively clear that no ordered vacancy compound exists in that series. X-ray photoemission spectroscopy study shows for the first time that the surface of powdered Cu1−z(In0.5Ga0.5)1+z/3Se2 compounds (z≠0) is more copper-poor than the bulk. The same result has often been observed on CIGSe thin films material for photovoltaic applications. In addition, optical band gaps of these non-stoichiometric compounds increase from 1.2 to 1.4 eV when z varies from 0 to 0.75.