1, Colorado State University, Fort Collins, Colorado, United States
3, National Renewable Energy Laboratory, Golden, Colorado, United States
As single junction silicon solar cells approach there theoretical limits, tandems show a clear path to higher efficiencies. CdTe alloys can be tuned with magnesium (CdMgTe) or zinc (CdZnTe) for ideal tandem pairing with silicon. A II-VI/Si tandem holds the greatest promise for inexpensive, high-efficiency top cells that can be quickly deployed in the market using existing polycrystalline CdTe manufacturing lines combined with mature silicon production lines. Currently all high efficiency polycrystalline CdTe cells use a chloride based passivation process to passivate grain boundaries and bulk defects, making it an essential step for any CdTe alloy manufacturing process. CdMgTe and CdZnTe absorbers have exhibited historically limited performance associated with absorber degradation during passivation. Due to the reactive nature of the material during CdCl2 treatment, CdMgTe and CdZnTe absorbers have exhibited poorly passivated absorber behavior. Capping layers deposited prior to CdCl2 treatments have been reported to reduce the reactive nature of the absorbers and help passivate II-VI front and back interfaces. This research aims to study and compare the material and electrical effects of CdTe, CdS, Al2O3, and MZO capping layers on the CdMgTe and CdZnTe absorbers. These various capping layers have been shown to reduce the degradation during CdCl2 passivation and improve bulk absorber quality identified and discussed using transmittance, QE, PL, TEM, EDX, PL, and TRPL. Bulk minority carrier lifetimes over 5 ns and world record efficiencies have been achieved using these capping layers.