CONTROLLED NUCLEATION OF THIN MICROCRYSTALLINE LAYERS FOR THE RECOMBINATION JUNCTION IN A-SI STACKED CELLS

In high-efficiency a-Si:H based stacked cells, at least one of the two layers that form the internal n/p junction has preferentially to be m icrocrystalline so as to obtain sufficient recombination at the juncti on [1-6]. The crucial point is the nucleation of a very thin mu c-Si:H layer on an amorphous (i-layer) substrate [2,4]. In this study, fast nucleation is induced through the treatment of the amorphous substrate by a CO2 plasma. The resulting n-layers with a high crystalline fract ion were, however, found to reduce the V-oc when incorporated in tande m cells. The reduction of the V-oc could be restored only by a precise control of the crystalline fraction of the n-layer. As a technologica lly more feasible alternative, we propose a new, combined n-layer, con sisting of a first amorphous layer for a high V-oc, and a second micro crystalline layer, induced by CO2 treatment, for a sufficient recombin ation at the n/p junction. Resulting tandem cells show no V,, losses c ompared to two standard single cells, and an efficient recombination o f the carriers at the internal junction as proved by the low series re sistance (15 Omega cm(2)) and the high FF (greater than or equal to 75 %) of the stacked cells.