Stacked solar cells of amorphous silicon

Abstract

Multiple-junction (stacked) solar cells of hydrogenated amorphous silicon (a-Si:H) have been made, consisting of two and three p+in+ cells. Three kinds of tunnel junction cell interconnections have been tested including n+p+ (a-Si:H), n+/cermet/p+, and n+/metal/cermet/p+ layers. The quality of the tunnel junctions improved in that order. The open circuit voltage (Voc) for a single cell was 0.76 V, for a two-cell stack 1.57 V, and for a three-cell stack 1.92 V. The stacked cell has the potential of increasing the efficiency of a-Si:H solar cells in at least two ways. One is by means of bandgap matching to the solar spectrum. The other is by means of a more complete utilization of a given range of wavelengths which cannot be fully absorbed within the limited depletion width of a single cell. In the present device the a-Si:H materials had the same optical bandgap throughout, thereby precluding bandgap matching, but allowing greater light absorption. Thus far, a modest 2% efficiency has been realized in the two-cell stack and 2.5% in a three-cell stack.