In order to elucidate the transport mechanism in a-Si:H/c-Si heterojunction solar cells under high forward bias (U>0.5 V), we conducted temperature-dependent measurements of current-voltage (I-V) curves in the dark and under illumination. ZnO:Al/(p)a-Si:H/(n)c-Si/(n +)a-Si:H cells are compared with inversely doped structures and the impact of thin undoped a-Si:H buffer layers on charge carrier transport is explored. The solar cell I-V curves are analyzed employing a generalized twodiode model which allows fitting I-V data for a broad range of samples. The fitting results are complemented with numerical simulations using AFORS-HET under consideration of microscopic a-Si:H parameters as determined by constant-final-stateyield photoelectron spectroscopy (CFSYS) to identify possible origins for a systematic increase of the high-forward-bias ideality factor along with the open-circuit voltage (V oc). It is further shown that also for a-Si:H/c-Si heterojunctions, dark I-V curve fit parameters can unequivocally be linked to V oc under illumination, which may prove helpful for device assessment. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.