The introduction of a nc-SiOx:H material as window layer in single junction a-Si:H n-i-p solar cell leads to a Voc enhancement of 80 mV compared to a μc-Si:H p-layer. According to numerical modeling of the Voc, both the higher work function p-layer and the conduction band offset (CBO) at the i/p interface match well with the experimental Voc increase with the oxygen content. Using the differential temperature method, the builtin voltage (Vbi) of the cells with the two different p-layers is measured to be similar, agreeing well with the CBO model. Thus we attribute the improvement of the Voc to the reduction of recombination at the i/p interface, as a consequence of the CBO in this region.