Amorphous silicon produced from the chemical vapor decomposition of silane at ~600 °C offers a pure silicon network containing no bonded-hydrogen and involving native defects of the order of 1 x 1019 cm-3. Doped phosphorus or boron atoms in the CVD a-Si interact with the defects to reduce the gap states and the spin density as well. The mechanism of the defect compensation has been interpreted in terms of complex-defect formation through the reaction between three-fold dopant atoms and divacancies. Very low concentration of hydrogen atoms less than 0.5 at.% diffused into CVD a-Si eliminates the spin and dramatically improves the doping efficiency. Photovoltaic cells utilizing the weakly hydrogenated CVD a-Si have realized a conversion efficiency of 2.7 % without an antireflection coating