Transparent Back Contacts for Thin CdTe-Based Tandem Cells

CdTe cells have excellent potential for use as top cells in multijunction (tandem) thin-film solar cell structures, but their use is limited by the lack of high performance transparent back contacts to the CdTe. This project will build 1) on recent advances by the Heben group at the University of Toledo in designing carbon nanotube structures for transparent solar cell contacts and 2) on advances by Xunlight 26 Solar in scaling up toward the manufacturing of magnetron sputtered CdS/CdTe devices with very thin and smooth CdTe layers ideal for applying transparent back contacts. This collaborative effort between Xunlight 26 Solar and the University of Toledos Wright Center for PV Innovation and Commercialization will emphasize the development of carbon nanotube (CNT) materials and ultra-thin metal layers for high performance transparent back contacts to cells with very thin (0.5 - 1 m) CdTe layers. High transparency will be required in the spectral range appropriate for CI(G)S bottom cells with bandgaps near ~1.1 eV. Commercial Applications and Other Benefits: High conductivity, transparent back contacts for CdTe and related II-VI devices will be developed for tandem thin-film solar cells devices that open the possibility of cost-effective thin film devices with efficiencies considerably higher than current generation of CdTe or CI(G)S devices. X26 will focus, in the near term, on four-terminal devices that do not require current matching in the two junctions, that allow each device to be processed under optimal conditions, and that use the superstrate carrier (glass or polymer) for the CdTe cell and the substrate carrier (glass or stainless steel or polymer) for the CIGS cell as the encapsulating materials. This project will focus on glass carriers, but successful development would be applicable to CdTe/CIGS tandems on flexible stainless steel and polymer carriers as well. The transparent back contacts will have dual use in single-junction semitransparent CdTe modules