Control of electronic structure in Cu(In, Ga)(S, Se)2 for high-efficiency solar cells

Abstract

We have made a three-dimensional (3D) map of the band-gap energy of the Cu(In1–xGax)(SySe1–y)2 (CIGSSe) quinary system. The band-gap energy of the CIGSSe system monotonically increased from CuInSe2 (x = 0.0 and y = 0.0) with simultaneously increasing Ga/(Ga+In) (GGI, x) and S/(S + Se) (SSSe, y) ratios. We also made a 3D map of the energy levels of the valence band maxima (VBMs) and conduction band minima (CBMs) of the CIGSSe system from the ionization energies measured by photoemission yield spectroscopy and the band-gap energies. The substitution effect of S for Se in CuInSe2 was different from that of Ga for In. The VBM level of the CIGSSe system did not change significantly and the CBM level increased with an increasing GGI ratio, x. On the other hand, the VBM level decreased and the CBM level increased with an increasing SSSe ratio, y.