The impact of Ga and S concentration and gradient in Cu(In,Ga)(Se,S)2 solar cells

As one of the leading photovoltaic materials, Cu(In,Ga)(Se,S)2 has two distinct advantages over other candidates. First, its conduction band and valence band could be independently tuned by adjusting the Ga/(Ga + In) and S/(S + Se), respectively. Next, its composition distribution could be feasibly tuned by controlling the thin film deposition process. These merits have been utilized in fabricating the Cu(In,Ga)(Se,S)2 solar cells with world record efficiency. However, these features have not yet been satisfactorily simulated. In order to understand the mechanism of high-performance Cu(In,Ga)(Se,S)2 solar cells, we obtained the tunable bandgap of absorber layer and suitable conduction band offset between absorber layer and buffer layer via Ga and S concentration and gradient, by combining both first-principle calculations and numerical simulations with Poisson equation and the continuity equation. In particular, compared with other photovoltaic materials, the bandgap of Cu(In,Ga)(Se,S)2 could be tuned flexibly across the layer thickness to realize spatial energy band optimization with double-concentration-grading absorber. The efficiency of 23.29% was obtained based on theoretical calculations and numerical simulations, which is almost the same as the reported result, in the considerations of (i) optimum bandgap of absorber, (ii) band energy offset, (iii) double (Ga- and S-) concentration grading, and (iv) low defect density in the absorber layer. In addition, we obtained a Cu(In,Ga)(Se,S)2 solar cell with a high S content uniform band gap efficiency of 23.53%. Therefore, we provided an understanding for tunable engineered band energy research in high-efficient Cu(In,Ga)(Se,S)2 solar cells.