We report on the basic properties of amorphous/crystalline hetero-junctions (a-Si:H/c-Si), their effects on the recombination of excess carriers and its influence on the a-Si:H/c-Si hetero-junction solar cells. For that purpose we measured the gap state density distribution of thin a-Si:H layers and determined its dependence on deposition temperature and doping by an improved version of near-UV-photoelectron spectroscopy. Furthermore, the Fermi level position in the a-Si:H and the valence band offset were directly measured. In combination with interface sensitive methods such as surface photovoltage analysis and our numerical simulation program AFORS-HET, we found an optimum in wafer pretreatment, doping and deposition temperature for efficient a-Si:H/c-Si solar cells without an i-type a-Si:H buffer layer. We reached at maximum 19.8% certified efficiency by a deposition at 2 10 degrees C with an emitter doping of 2000 ppm of B2H6 on a well cleaned pyramidally structured c-Si(n) wafer. (C) 2006 Elsevier B.V. All rights reserved.