In this work we present a study of the structural, optoelectronic and transport properties of a series of Si films deposited in a parameter region (namely hydrogen dilution) corresponding to a transition from amorphous-to-nanocrystalline silicon by hot-wire (HW) and radio-frequency plasma enhanced chemical vapor deposition (RF) on plastic substrates at 150 °C. To achieve a higher deposition rate of Si films by RF we used a relatively high power density (350 mW/cm2) and deposition pressure (1.5 Torr). For certain hydrogen dilution values, these deposition conditions can lead to the formation of Si crystals in the silane plasma and to a growth of polymorphous silicon film. This material has improved carrier transport properties (ambipolar diffusion length = 220 nm) and very high photosensitivity (>5 × 106). The best HW amorphous silicon films exhibited lower photosensitivity (7 × 104) and an ambipolar diffusion length of only 100 nm. For solar cell fabrication, we optimized the RF deposition conditions to produce very thin amorphous and nanocrystalline phosphorous and boron doped silicon layers. Our best n–i–p solar cell, with a polymorphous Si intrinsic layer deposited on plastic, has an efficiency of 5.5%, FF = 52.5%, VOC = 920 mV, JSC = 11.6 mA/cm2. For solar cells with a nanocrystalline Si active layer deposited on glass the following results were achieved: efficiency = 3.4%, FF = 43.5%, VOC = 460 mV, JSC = 17.2 mA/cm2; and on plastic substrate: efficiency = 2.2%, FF = 32.7%, VOC = 397 mV, JSC = 17.2 mA/cm2.