Crystal structure re-investigation in wide band gap CIGSe compounds

There is agreement in the literature that Cu(In1 − xGax)Se2 (CIGSe) absorber used in solar cells has an optimum composition (x ≈ 0.3) corresponding to a band gap (1.1–1.2 eV) far below the theoretical value giving the maximum (1.4–1.5 eV). This paper presents a re-investigation of the crystal structure of bulk CIGSe compounds for both stoichiometric and Cu-poor compositions. Regardless of the gallium content, all the stoichiometric compounds are found to adopt the well-known chalcopyrite structure (space group I-42d) while a modification of the structure is evidenced for the high Ga-content Cu-poor compounds. The X-ray diffraction analyses demonstrate that the crystal structure of Cu0.743In0.543 Ga0.543Se2 is derived from that of the stannite structure (space-group I-42m). Ab-initio calculations show a strong dependence of the electronic structure near the Fermi level with the copper content. Such modifications are expected to significantly change the optical properties of Cu-poor CIGSe materials.