Control of Voc in CdSe solar cells

Simulations indicate that efficiencies in the 25 - 30% range can be attained with CdSe/CIGS tandem cells. A key to meeting this objective is attaining Voc´s in excess of 1 volt in the CdSe cell. The p window layer for CdSe is a member of the ZnSe_xTe_1-x family of compounds which have the requisite band structure. To be effective, the p window contact must have effective contact energy of order six eV. Progress has been made toward achieving this energy but is stymied by difficulties in effectively doping the layer. In this paper we report new insights into the operable contact mechanism for the p window layer. While an apparent doping level of order 1020/cm³ can be attained for ZnTe:Cu, needed tunneling contact to this layer cannot be attained. Comparisons of experimental results with simulations also indicate that the expected electron barrier due to the CdSe/ZnSe_xTe_1-x conduction band offset is not operative. We use these results to propose a new model for the contact that results in consistency between simulation and experiment. This new insight is expected to expedite the further advancement of Voc in CdSe to enable the attainment of the tandem efficiency indicated above.