ZnTe:Cu Contact Optimization Strategies for Single-Junction and Multijunction CdS/CdTe PV Device Designs

The ability to produce high-performance CdS/CdTe photovoltaic (PV) devices that incorporate high-transparency back contacts for multijunction thin-film PV applications will require an even greater level of understanding than has been required for single-junction devices. This study reports some of our initial investigations at NREL to modify the ZnTe:Cu contact process previously developed forsingle-junction applications for optimal use as a transparent back contact. We have succeeded in producing devices incorporating a transparent ZnTe:Cu/ITO/metal-grid contact that demonstrates nominally identical light I-V (LIV) performance to the ZnTe:Cu/Ti contact used in single-junction devices. However, we have determined that the transparent conducting oxide (TCO), CdS, CdTe, and ZnTe:Culayers are all factors in the optical absorption within the device. Finally, we have concluded that optimizing the transparent ZnTe:Cu contact for use with NREL-produced device material will require a more detailed understanding of the evolution of the junction region during the contact process.