Crystallographic and optical properties of (Cu, Ag)In₅Se₈ and Ag(Ga, In)₅Se₈ in (Cu, Ag)₂Se-(Ga, In)₂Se₃ pseudo-binary system

Abstract We synthesized stannite-type (Cu 1− x Ag x )In 5 Se 8 with 0.0 ≤ x ≤ 1.0. The lattice constants increased almost linearly with increasing Ag/(Cu + Ag) (ACA) ratio. The band-gap energy, E g , increased monotonically from 1.21 eV for CuIn 5 Se 8 to 1.40 eV for AgIn 5 Se 8 . The energy levels of the valence band maximum (VBM) and conduction band minimum (CBM) from the vacuum level were lower than those of a chalcopyrite-type (Cu,Ag)InSe 2 system. The VBM energy level decreased monotonically from −5.62 eV for CuIn 5 Se 8 to −5.82 eV for AgIn 5 Se 8 with increasing ACA ratio. In contrast, the CBM energy level remained almost constant at about −4.4 eV. We then synthesized stannite-type Ag(Ga x In 1− x ) 5 Se 8 samples with 0.0 ≤ x ≤ 0.8. The E g increased from 1.40 eV for AgIn 5 Se 8 to 1.78 eV for Ag(Ga 0.8 In 0.2 )In 5 Se 8 with increasing Ga/(Ga + In) ratio. The VBM energy level remained almost constant at about −5.8 eV, but the CBM energy level increased from −4.38 eV for AgIn 5 Se 8 to −3.90 eV for Ag(Ga 0.8 In 0.2 )In 5 Se 8 .