p-i-n Heterostructures deposited by ultra high vacuum plasma enhanced chemical vapour deposition (UHV-PECVD) using silane and methane gases with and without hydrogen dilution of the i-layer have been studied. Amorphous silicon carbon alloy (a-SiC:H) has been used in the p(+) and i-layers and amorphous silicon (a-Si:H) in the n layer. The effect of hydrogen dilution on the defect properties of i-layer films has been studied by photothermal deflection spectroscopy and constant photocurrent method. Hydrogen dilution of the i-layer changes the J (V) characteristics and notably under illumination: the fill factor decreases, the short-circuit current density J,, increases and, moreover, a cross over was observed between J (V) characteristics in dark and under illumination of the diluted cells. A two diodes model has been used to explain this phenomenon. Measurements of the diode junction capacity show higher capacitance in the diluted i-layer p-i-n structure than in the undiluted ones at low frequencies. This effect is due to the decrease of the defects density induced mainly by the increase of the capture reemission time (or delay time) of charges. (c) 2006 Elsevier Ltd. All rights reserved.