Heterojunction solar cells have been manufactured by depositing n-type a-Si:H on p-type 1-2 Omega cm Cz single-crystalline silicon substrates. An efficiency of 14.2% has been obtained for 1 cm(2) solar cells by using a simple (Al/(p) c-Si/(n) a-Si:Hi/ITO/metal grid) structure. With an additional surface texturing, we have reached an efficiency of 15.3% for 1 cm2 solar cells. We have investigated the dark IV-curves in order to contribute to a better understanding of the basis of solar cells.