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 ZnCd_1-SSe_1- alloys and ZnSO_1-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 ZnCd_1-SSe_1- and ZnSO_1- alloys is investigated by first-principles density functional theory calculations. For quaternary ZnCd_1-SSe_1- 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 ZnCd_1-SSe_1- alloys are established. For ZnSO_1- 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 ZnSO_1- alloys is composition dependent, and is larger for dilute ZnSO_1- 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 ZnCd_1-SSe_1- alloys and ZnSO_1- alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that ZnCd_1-SSe_1- alloys and ZnSO_1- alloys are promising buffer layers for high-efficiency CIGS solar cells.