| Hydrogenated amorphous silicon (a-Si:H) is a thin-film semiconducting material that has generated much interest in the photovoltaic community due to its device-quality, low-cost production and favorable electronic properties. In the last twenty years, much research has focused on understanding the origin of and eliminating the electronic metastability, unique to a-Si:H, known as the Staebler-Wronski effect (SWE). This has led to efforts to fully characterize the microstructure of a-Si:H, particularly in relation to the incorporated hydrogen, which is thought to play a role in the metastability. Proton nuclear magnetic resonance (1H-NMR) has proven to be one of the most useful tools in achieving this goal starting as early as 1981 with a seminal paper by Reimer et al. in which hydrogen was shown to be incorporated in two distinct environments, or phases. In this dissertation, we extend the proton NMR study of a-Si:H to (a) studies of hydrogen dynamics, (b) the structural order of the a-Si network, and (c) of confinements effects on the NMR properties of hydrogen gas trapped in nanovoids. In (a) we report monotonic, reversible equilibrium changes in the concentration of freely rotating hydrogen molecules as the sample temperature is varied up to 200°C, and irreversible changes at higher temperatures. In (c), we demonstrate that confinement of hydrogen gas to nanoscopic volume leads to a nonzero intermolecular dipolar coupling that is the same for all spins in the gas system; in some special samples we use this effect to show that the nanovoids are elongated and aligned in the film and to calculate their average volume. We suggest that the molecular hydrogen observed in (a) is likely to exist in similar nanovoids, and that a reversible conversion of surface bonded hydrogen to molecular hydrogen could explain the results in (a). Finally, in (b) we demonstrate that an NMR technique can be used to measure bulk magnetic susceptibility precisely in a-Si:H, and that film-to-film variation in the measured susceptibility indicates differences in overall structural order of the backbone a-Si network. These differences are interpreted in terms of a heterogeneity of the a-Si network, wherein films containing nanoscale ordered regions correspond to both ‘higher overall order’ and ‘more heterogeneous’. It is suggested that a larger heterogeneity leads to the nanovoid structure that is associated with the dynamic behavior observed in (a). |
