Chemical crystallographic investigation on Cu2S-In2S3-Ga2S3 ternary system

Sulfide chalcopyrites Cu(In,Ga)S2 (CIGS) have been proposed as top cell for tandem solar devices with silicon as bottom cell. In that context, the knowledge of the Cu2S-In2S3-Ga2S3 pseudo-ternary system is of prime importance. This study presents an overall investigation of this system on the basis of a chemical crystallographic approach on bulk materials. Particular attention was paid on the copper-poor side of the so-called stoichiometric CuInS2-CuGaS2 line. Based on our investigations, it turns out that the sulfide CIGS system is much more structurally complex than the selenide Cu(In,Ga)Se2 one (CIGSe). Especially, the adaptability of the chalcopyrite structure toward Cu deficiency is significantly lower for the sulfides than for the selenide counterparts. Correlatively, a large 2-phase domain exists between the CuInS2-CuGaS2 line and the very copper-poor region (Cu(In,Ga)3S5 and Cu(In,Ga)5S8). Additionally, the crystal structures of these very copper-poor compounds contain [InS6] octahedra and then are totally different than the chalcopyrite structure. These features could explain why CIGS-based-solar cells are, up to now, less efficient than the CIGSe ones.