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 microcrystalline so as to obtain sufficient recombination at the junction [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 fraction were, however, found to reduce the V-oc when incorporated in tandem 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 technologically more feasible alternative, we propose a new, combined n-layer, consisting of a first amorphous layer for a high V-oc, and a second microcrystalline layer, induced by CO2 treatment, for a sufficient recombination at the n/p junction. Resulting tandem cells show no V,, losses compared to two standard single cells, and an efficient recombination of the carriers at the internal junction as proved by the low series resistance (15 Omega cm(2)) and the high FF (greater than or equal to 75%) of the stacked cells.