Structural and Electronic Properties of Polycrystalline Cu(In,Ga)(S,Se)2 Alloys

Abstract

ABSTRACTWe present a systematic study on the polycrystalline Cu(In,Ga)(S,Se)2 alloys with a gallium to indium ratio of Ga/(Ga+In)<0.3 and a sulfur to selenium ratio varying in the range between 0<S/(S+Se)<1. All samples were grown by coevaporation of the elements at constant rates under high vacuum conditions. The formation of island-like (Cu,S,Se) segregations correlate with the sulfur to selenium ratio in the layer and are found in the growth region near the copper rich phase boundary. These segregations are related to a preferred incorporation of sulfur in the copper rich growth mode. We obtain solar cell grade material from an indium rich growth mode up to a sulfur to selenium ratio of S/(S+Se)=0.9. A detailed analysis of the electronic and optical properties of Mo/CIGSSe/CdS/ZnO:Al heterojunctions allows us to determine the energetic position of the bands within the Cu(In,Ga)(S,Se)2 alloy system. We find that contrary to the Cu(In,Ga)(Se)2 alloy system the valence band position is significantly lowered with increasing bandgap.