Interface Engineering of Cu(In,Ga)Se₂ Solar Cells by Optimizing Cd- and Zn-Chalcogenide Alloys as the Buffer Layer

The optimization of band alignment at the buffer/absorber interface is realized by tuning compositions of Cd and Zn chalcogenides as the buffer layer toward high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells. Using the special quasi-random structure (SQS) approach, we construct randomly disordered ZnxCd1–xSySe1–y alloys and ZnSxO1–x alloys as alternatives to the traditional CdS buffer layer. The compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of ZnxCd1–xSySe1–y and ZnSxO1–x alloys is investigated by first-principles density functional theory calculations. For quaternary ZnxCd1–xSySe1–y alloys, we find that the miscibility temperatures and the bandgap bowing coefficients are proportional to the lattice mismatch between the mixing elements. The linear dependence of lattice parameters, trinomial dependence of band-gap energies and band-edge positions on the alloy-composition of ZnxCd1–xSySe1–y alloys are established. For ZnSxO1–x alloys, we find the lattice parameters also exhibit a linear dependence on its composition. Because of the large lattice mismatch and the chemical disparity between ZnO and ZnS, the bowing coefficient for the bandgap energies of ZnSxO1–x alloys is composition dependent, and is larger for dilute ZnSxO1–x alloys. With the optimization criteria of moderate spike-like conduction band offset, large valance band offset, sufficiently wide bandgap, and lattice match with respect to the CIGS absorber, we illustrate the optimal composition range of both ZnxCd1–xSySe1–y alloys and ZnSxO1–x alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that ZnxCd1–xSySe1–y alloys and ZnSxO1–x alloys are promising buffer layers for high-efficiency CIGS solar cells.