Introduction of Cu in CdS and its effect on CdTe solar cells

The use of copper during the fabrication of CdTe solar cells is a common feature for nearly all processing schemes developed for these devices. Copper is typically introduced in CdTe during the application of the back electrode, to enhance device performance by facilitating the formation of an ohmic back contact. However, Cu has also been associated with observed instability in CdTe. For this work, although Cu was utilized during the cell fabrication process, it was eliminated from the back contact formation step, and instead introduced in the CdS film prior to the deposition of the CdTe. The only fabrication step, where Cu was intentionally introduced, was subsequent to the CdS deposition. Both plain graphite and Sb₂Te₃/Mo were used as back contacts. Solar cell results suggested that ohmic contacts to CdTe can be attained with undoped graphite as the back electrode. For devices contacted with plain graphite V_OC´s and FF´s in the range of 800-830 mV and 63-67% have been obtained. For Sb₂Te₃-contacted cells, the incorporation of Cu in CdS has lead to a significant increase in performance even though a back barrier was present in these cells.