The structure of undoped Si:H films deposited at a high rate of 6-9 Å/s in an RF (13.56 MHz) plasma from hydrogen-silane gas mixtures at various substrate temperatures was studied using TEM (with in-situ annealing), XRD, Raman spectroscopy, optical absorption and hydrogen effusion. It is found that under our conditions the amorphous to crystalline transition occurs in a relatively narrow range of parameters, influenced mainly by hydrogen dilution and to a lesser degree by the substrate temperature. In the crystalline range the material is found to be nanocrystalline (average grain size 20 nm) and the crystals are essentially stable up to 800°C annealing. The crystal structure of a mixed amorphousnanocrystalline phase of samples deposited near the edge of crystallinity is also found to be rather stable. Nanocrystalline Si films deposited under these latter deposition conditions reveal in hydrogen effusion a relatively compact material and show high solar cell efficiencies (6-8%) when incorporated as i-layers in pin solar cells.