Development of ZnSe xTe1-x p-type contacts for high efficiency tandem structures

With a band gap of 1.7 eV CdSe is a near-ideal top cell for a tandem solar cell using CuIn xGa1-xSe2 (CIGS) as the bottom cell. We and others have demonstrated that CdSe has excellent electronic properties that should result in efficiencies of 18%. The primary obstacle to meeting that objective is low Voc due to not having an effective p-contact. We have made some progress in this regard with ZnSe and ZnTe, but each has limitations that limit Voc below the needed 1+ V. ZnSe is not easily doped p-type, and ZnTe's valence band is not as low as desired. In our recent work we have been combining the two binaries to try to get around these limitations. Films are deposited using conventional co-evaporation to be consistent with manufacturing constraints for solar cells. In one approach we are forming the ternary ZnSe xTe1-x. While giving up a bit of ZnSe's favorable valence band location, we hope to enhance dopability. One of the difficulties that we encountered was maintaining stoichiometry for our targeted Te/Se ratio of 1.0. Such films are typically Zn-rich and not dopable. We found deposition techniques that allow access to stoichiometric films with the desired ratio and have measured modest conductivity. We are also investigating superlattice structures as another way of combining the properties of the binaries. This approach avoids competition between the group VI elements during deposition allowing more control over stoichiometry. However, an added difficulty is posed by the activated N dopant environment in the chamber in that it enhances loss of Te during deposition. The superlattice approach provides means of compensation and is producing stoichiometric films, but conductivity is not yet evident