VOC enhancement in CdSe solar cells using ZnSexTe1-x:N window layers

Performance of the CdSe top cell is currently limiting performance of CdSe/CIGS tandems due to low Voc. This limitation is primarily due to the p- layer contact. In this paper we report efforts to use ZnSexTe1-x to produce a contact with good band alignment with CdSe and dopability. Both co-evaporated homogeneous films and superlattice(SL) structures of the ternary have been incorporated in devices as the p contact. In devices with the ZnSexTe1-x p contact doped with activated N the Fermi level is estimated to have moved 75 mV closer to the CdSe valence band relative to devices with a ZnSe contact. A corresponding increase in Voc of 50 mV is observed in the SL structures, but there is a decrease for the co-evaporated structures. The results suggest that Voc is controlled by the location of the p-contact Fermi level, and that Voc can be further improved by the use of properly configured and doped ZnSexTe1-x ternary layers.