In this paper we present an analysis of the contact formation between an n-type TCO and a p-type a- Si:H as used for emitter structures in high efficiency a-Si:H/c-Si heterojunction solar cells. By means of transient photoconductance decay measurements it is shown that the deposition of ZnO:Al on (p)a-Si:H/(n)c-Si heterojunctions leads to a reduction of charge carrier lifetime mainly at low injection levels, which is correlated to a reduced implied fill factor. This observation is explained by Schottky contact formation and is reproduced by numerical simulations. An analytical description based on two antiparallel diodes is presented to explain the loss in fill factor of a solar cell and the detrimental effect of the work function difference between ZnO:Al and p-type a-Si:H.