Poly-Si thin film which has an ESR spin density of 6.9 E16/cm(3) was obtained by Cat-CVD with a deposition rate of 5.4 Angstrom /s at a relatively high deposition pressure (approx. 30 Pa) with a high hydrogen dilution (SiH4/H-2 = 0.05). The low ESR spin density films were applied to solar cells as a photo-active layer. Although the dark V-1 curves of the cell showed diode characteristics, the photo V-1 curves showed very small photo-current densities, lower than 0.5 mA/cm(2). A large amount of higher order silane molecules at the present high deposition pressure might be one of the causes of this small photo-current density, as well as grain boundary recombination and an incubation layer at the first stage of film deposition.