Silicon Degradation in Monolithic II-VI/Si Tandem Solar Cells

II-VI/Si tandem solar cells have strong potential for high efficiency at low cost by combining the two most widely used solar cell materials: silicon and cadmium telluride (CdTe). However, there are challenges with this merger, as loss of minority-carrier lifetime in the silicon bottom cell can be caused by growth of a II-VI cell on top. Silicon lifetime degradation in monolithic II-VI/Si structures is measured here on experimental samples for CdTe deposition temperatures between 400 and 500 °C, with variable In₂O₃:ZnO (IZO) thickness between the CdTe and silicon, and with and without CdCl₂ postdeposition treatment. Results indicate that the CdCl₂ treatment has the strongest effect on silicon lifetime reduction, followed by temperature and IZO thickness. Potential causes are discussed, and the effect on monolithic II-VI/Si two-junction solar cells is modeled. Remarkably, many silicon samples in the study were able to maintain >400 μs lifetimes, with some exceeding 1 ms, consistent with >30% projected efficiency in fully integrated II-VI/Si tandem solar cells.