Band-Structure Engineering of ZnxCd1-xSySe1-y Quaternary Alloys as the Window Layer for CdTe Solar Cells

With the optimization of the back contact, improvement of carrier lifetime, and the use of Se-alloyed CdTe as the absorber, the power conversion efficiency (PCE) of CdTe solar cells has now risen beyond 22%. However, further improvement of the PCE of CdTe-based solar cells by optimization of the window layer has not been fully discussed. In this work, we propose that ZnxCd1-xSySe1-y alloys can be tuned as promising window layers of both CdTe and CdSexTe1-x solar cells when band alignment and lattice match are taken into account. Using first-principles calculations and the special quasirandom structure approach, we investigate the compositional dependence of formation energies, lattice parameters, bandgap energies, and band alignments of ZnxCd1-xSySe1-y alloys. We find that the interaction parameters and thus the miscibility temperatures of ZnxCd1-xSySe1-y alloys are directly proportional to the lattice mismatch between the mixing elements, and are larger for mixed-cation alloys than for mixed-anion alloys. The lattice parameter of ZnxCd1-xSySe1-y exhibits a linear dependence to its composition. The bowing of band-gap energies of ZnxCd1-xSySe1-y alloys is quite large, due to the symmetry-reductioninduced intraband coupling in quaternary alloys. Finally, we illustrate the optimal composition range of ZnxCd1-xSySe1-y alloys as the window layer for both CdTe solar cells and CdSexTe1-x solar cells. Our work provides a guideline for further improving the performance of CdTe and CdSexTe1-x solar cells by adopting ZnxCd1-xSySe1-y alloys as promising window layers.