Transparent back contacts and interconnect junctions for CdTe top cells

The development of a good, low resistance transparent back contact (TBC) to the superstrate type CdS/CdTe or Cd_1-xMg_xTe, Cd_1-xZn_xTe, or Cd_1-xMn_xTe alloy solar cell is crucial for achieving the best possible four-terminal or two-terminal polycrystalline thin-film tandem solar cell. We find that ZnTe:N/ITO serves as a good TBC to CdS/CdTe solar cell yielding 9.1% power conversion efficiency even without the use of intentional copper at the back contact. A transparent back contact permits device measurements with light incident from the contact side. Such contact-side J-V measurements show that the short-circuit current of ~2 mum thick cells with this TBC is reduced to almost half when measured from the contact side. This difference in current matches quite well with the integrated current calculated from QE done under weak DC white light bias. Similar experiments performed on 0.68 mum cells showed little difference in current between glass-side and contact-side illumination consistent with a stronger collection field in the thin cells