We investigated the properties of hydrogenated amorphous silicon oxide (a-Si1-xOx:H) deposited near the phase transition between amorphous and microcrystalline structures. a-Si1-xOx:H films were prepared by plasma-enhanced chemical vapor deposition using a gas mixture of silane, hydrogen, and carbon dioxide. The film structure was changed from amorphous to microcrystalline phase by increasing hydrogen dilution. Optical and electrical characterizations revealed that wide-gap a-Si1-xOx:H films were deposited under phase transition conditions. We also fabricated a-Si1-xOx:H single-junction p-i-n solar cells by varying the hydrogen dilution for the i-layer. The solar cells showed a maximum open circuit voltage of 1.04 V (J(sc) = 7.92 mA/cm(2), FF = 0.64, E-ff = 5.2%) when the i-layer was deposited under phase transition conditions. (C) 2009 The Japan Society of Applied Physics