Alternative Window Schemes for CuInSe2-Based Solar Cells: Final Report, 3 November 1995--31 December 1997

This work demonstrated high-efficiency CIGS cells based on highly resistive ZnO buffer layers grown by MOCVD. One cell based on NREL CIGS and a ZnO buffer layer exhibited an active-area efficiency of nearly 14%. This result is one of the best efficiencies reported for a 'direct' ZnO/CIGS cell made with a vacuum process. This work has established that the buffer layer is truly ZnO and not an alloyresulting from interdiffusion of ZnO and CIGS. Also established is that a two-step growth procedure like those described as 'baseline' and 'modified' lead to efficient cells. To achieve high efficiencies, ZnO buffer layers must have resistivities greater than 10/sup 4/ ohm-cm. Efficient cells are obtained whether the high resistivity is achieved as-grown, or after exposure to air. Because cellefficiencies are lower than predicted by simulation studies when the buffer resistivity is lower than 10/sup 4/ ohm-cm, we conclude that the resistivity is simply related to processing. In particular, the resistivity correlates with the excess zinc in the MOCVD ZnO film, which can, in turn, diffuse into the CIS absorber and possibly cause enhanced current loss mechanisms. Future studies willfocus on identifying process pathways that will lead to resistive ZnO buffer layers, and thus, large cell efficiencies. A limited effort was devoted to ZnSe buffer layers. A completed cell with a ZnSe buffer layer and RF-sputtered ZnO TCO was fabricated and produced power with a total-area efficiency of 9.5%. Future studies will concentrate on developing approaches to deposit conductive ZnO ontop of the ZnSe buffer layer without degrading properties of the ZnSe/CIS structure.