Bonded hydrogen in nanocrystalline silicon photovoltaic materials: Impact on structure and defect density

We have performed a detailed structural and optical investigation of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma-enhanced chemical vapor deposition. The microstructural properties of these thin films are characterized and interpreted physically based on the growth mechanism. Infrared spectroscopy reveals that the bonded hydrogen in a platelet-like configuration, which is believed to be located at grain boundaries, greatly affects oxygen incursions into nc-Si:H thin films, whereas electron spin resonance observations link these incursions to the introduction of dangling bond defects. Consequently, we propose that in nc-Si:H thin films, high bonded-hydrogen content in grain boundaries is of great importance in forming hydrogen-dense amorphous tissues around the small crystalline grains, i.e., compact grain boundary structures with good passivation. Such structures effectively prevent post-deposition oxidation of grain boundary surfaces, which might lead to the formation of dangling bond defects.