We deposited silicon thin films by plasma-enhanced chemical vapor deposition (PECVD) at very low substrate temperatures of 75 and 40 degreesC. Even at these low deposition temperatures, the protocrystalline Si (pc-Si:H) exhibits a high photosensitivity and remarkably enhanced stability against light saturation. This material grows at the borderline between amorphous (a-Si:H) and nanocrystalline (nc-Si:H) phases in the deposition parameter space. Structural and optical characterization revealed a small fraction of crystallites embedded in an amorphous matrix. Thickness-dependent morphology of silicon films was revealed by absolute constant photocurrent method (CPM). We demonstrated the effect of the amorphous-to-nanocrystalline transition on the solar cell performance. The cells with a protocrystalline absorber layer showed an improved fill factor.