CuInSe2, CuIn3Se5, and CuIn5Se8 phases in Cu-poor Cu2Se-In2Se3 pseudo-binary system - Their crystal structures, optical properties and electronic structures

CuInSe2 and Cu-poor Cu-In-Se (CIS) phases such as CuIn3Se5 and CuIn5Se8 in the composition of (1-x) Cu2Se-(x) In2Se3 with 0.5 <= x <= 1.0 were prepared. The crystal structure changed from chalcopyrite-type CuInSe2 to hexagonal CuIn5Se8 through stannite-type CuIn3Se5 with increasing x (decreasing Cu/In ratio). The band-gap energies of stannite-type CuIn3Se5 (1.17 eV) and CuIn5Se8 (1.22-1.24 eV) are larger than that of chalcopyrite-type CuInSe2 (0.99 eV). The energy levels of the valence band maxima (VBMs) from the vacuum level were estimated from the ionization energy measured by photoemission yield spectroscopy (PYS). The energy levels of the VBMs of the Cu-poor Cu-In-Se samples decrease rapidly with decreasing Cu/In ratio. The energy levels of the VBM of CuIn3Se5 (-5.65 eV) and CuIn5Se8, (-5.75-5.95 eV) are deeper than that of CuInSe2 (-5.25 eV).