Passivation of nanocrystalline silicon photovoltaic materials employing a negative substrate bias.

Hydrogenated nanocrystalline silicon (nc-Si:H) shows great promise in the application of third-generation thin film photovoltaic cells. However, the mixed-phase structure of nc-Si:H leads to many defects existing in this important solar energy material. Here we present a new way to passivate nc-Si:H films by tuning the negative substrate bias in plasma-enhanced chemical vapor deposition. Microstructures of the nc-Si:H films prepared under a negative bias from 0 to -300 V have been characterized using Raman, x-ray diffraction, transmission electron microscope, and optical transmission techniques. A novel passivation effect on nc-Si:H films has been identified by the volume fraction of voids in nc-Si:H, together with the electrical properties obtained by electron spin resonance and effective minority lifetime measurements. The mechanism of the passivation effect has been demonstrated by infrared spectroscopy, which illustrates that the high-energy H atoms and ions accelerated by an appropriate bias of -180 V can form more hydrides along the grain boundaries and effectively prevent oxygen incursions forming further Si-O/Si interface dangling bonds in the nc-Si:H films. The detrimental influence of a bias over -180 V on the film quality due to the strong ion bombardment of species with excessively high energy has also been observed directly from the surface morphology by atomic force microscopy.