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)Se-2 (CIGSe) confirm that the chalcopyrite structure (space group I (4) over bar 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/3)Se-2 series when the copper vacancy ratio (z) increases: the chalcopyrite structure turns to a modified-stannite structure (I (4) over bar 2m) when z >= 0.26. There is a continuous evolution of the structure from Cu-0.74(In0.5Ga0.5)(1.09)Se-2 to Cu-0.25(In0.5Ga0.5)(1.2)sSe(2) ((i.e. Cu(In0.5Ga0.5)(5)Se-8), including Cu-0.4(In0.5Ga0.5)(1.2)Se-2 (i.e. Cu(In0.5Ga0.5)(3)Se-5). 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/3)Se-2 compounds (z not equal 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. (C) 2010 Elsevier Inc. All rights reserved.