In this work, a new type of short water vapor treatment of the interface between the p- and i-layer is presented. This novel treatment is performed under vacuum below 1 mbar for 5 min and considerably reduces the i-layer boron contamination in amorphous silicon (a-Si:H) p-i-n solar cells prepared in single-chamber reactors. A significant advantage is that the substrate with the p-layer can remain loaded in the reactor during this oxidation treatment. The high effectiveness of this treatment in reducing the boron cross-contamination is directly supported by secondary ion mass spectroscopy measurements, by tracing the boron concentration depth profile across the p-i interface and by quantum efficiency measurements of the deposited cells. By applying this water vapor treatment, 0.3-μm-thick a-Si:H p-i-n solar cells of 1 cm2 with high initial conversion efficiencies of 10.1% are deposited in a commercial large-area (35×45 cm2) single-chamber PECVD KAI™ reactor and can clearly compete with cells deposited in multi-chamber systems. Light soaking of these cells for 1200 h at 50 °C leads to stabilized efficiencies of 8.2%. The relative typical efficiency degradation of 20% of such 0.3-μm-thick single-junction cells demonstrates that this treatment does not affect the stability in a negative manner. © 2003 Elsevier B.V. All rights reserved.