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.