Carrier mobility and density of states in microcrystalline silicon film compositions, probed using time-of-flight photocurrent spectroscopy

The technique of time-of-flight photocurrent spectroscopy, and its application in the study of carrier transport in microcrystalline silicon films over a range of material compositions, is described Measurements reveal that the hole mobility is greatly increased in comparison to amorphous films, by up to a factor of 300 at room temperature, at a Raman crystalline volume fraction of 30% or more Significant enhancements are evident at concentrations below 10% A more modest increase in the electron mobility, by up to a factor of five, is found in highly crystalline films, while in low-crystallinity films the electron mobility appears to fall substantially below that measured in amorphous silicon This is consistent with the poor blue response observed in the corresponding thick p-i-n solar cells A comparison of results from time-of-flight and transient photocurrent techniques applied to films prepared at higher crystallinities reveals differences in the density of states distributions, which suggest that charge transport in these films may be anisotropic