We recently demonstrated how the short circuit current of an a-Si:H/c-Si hetero junction solar cell (SHJ) can be significantly improved to above 40 mA/cm2 by replacing the standard a-Si:H(p) emitter by a silicon oxide emitter containing p-doped silicon nano-crystallites. While we could obtain a conversion efficiency of 20.3%, the cell suffered from a lower fill factor of 72.9% compared to 77.0% for our standard process. In this paper, we address this issue both theoretically and experimentally. We found that a thin (∼3 nm) highly doped nanocrystalline silicon layer on top of the emitter can greatly improve the fill factor. Using 1D device simulation, we explain the prevalent loss mechanism, which is originating mostly from poor tunnel recombination at the TCO/emitter interface rather than in the bulk of the emitter. Both have their origin in the lower effective dopant concentration of the nano-crystalline silicon oxide emitter, we suspect. From the model, implications for further development can be derived.