This paper focuses on the relation between p/i-interface layer properties and the light stability of the corresponding solar cells. In a series of cells containing different p/i-interface structures large differences in relative degradation were found. These differences are explained by a redistribution of the electric field due to the insertion of different p/i-interface layers leading to different collection from the i-layer volume in the course of i-layer degradation. Based on this hypothesis, we developed an optimized design of the p/i-interface region, which increases the initial efficiency without introducing additional degradation. a-Si:H/a-Si:H stacked cells including this new design exhibit only 12% degradation after 300 hours of one sun light-soaking. A stabilized efficiency of 9% was achieved.