Hydrogenated amorphous silicon (a-Si:H) p-i-n solar cells were grown in a single, non-load-locked chamber by using 13.56 MHz plasma. Residual air impurities were controlled with high vacuum pumping, or not controlled, and the concentrations were measured at the onset, before p-layer deposition. The low pressures (approximately 10(-7)-10(-8) Torr) of N-2 and O-2 near the lowest vacuum pressure of the chamber, incorporated base contamination levels of nitrogen and oxygen (approximately 10(18)-10(19) atoms/cm(3)). High pressures of H2O (10(-5)-10(-6) Torr) resulted in extremely high oxygen contamination levels (approximately 10(21) atoms/cm(3)), which was the dominant residual impurity. High concentration un-stabilized H2O showed a rapidly decreasing rate which induced non-uniform oxygen doping and resulted in a non-uniform distribution of the internal electric field in the i-layer. The net loss (Delta QE(0,V)) of quantum efficiency (QE) of the cells between zero and a forward bias showed the increase of QE loss in the long-wavelength region for increasing forward bias resulted in poor performance of the low fill factor and energy conversion efficiency, and a high series resistance of the cells. The removal of residual water vapor from the chamber is a key factor in improving the performance of a-Si:H solar cells. (C) 2013 Elsevier B.V. All rights reserved.