Comparison of transparent back contacts for CdTe top cells in tandem thin-film photovoltaic devices

Two-junction tandem thin-film photovoltaic cells are composed of a wide-bandgap top cell overlying a narrower-bandgap bottom cell. CdTe, with a bandgap of 1.5 eV, can function effectively as a top cell when paired with a bottom cell of appropriate bandgap. Effective two-junction cells require a back contact to the top cell that is transparent to the sub-bandgap radiation absorbed by the bottom cell. In this study, we fabricate CdTe top cells with transparent back contacts. We form the ohmic contact to CdTe by using semi-transparent Cu-doped ZnTe as the back-contact interface layer. We compare Al-doped ZnO, indium tin oxide, and single-wall carbon nanotube networks for use as the final transparent conductor. These transparent cells demonstrate device efficiencies comparable to those of control devices using the standard Ti metallization.