Electron time-of-flight measurements in a-Si_1-xC_x:H

We have measured the temperature-dependent electron drift mobility in a series of hydrogenated amorphous silicon-carbon alloys using time-of-flight. The specimens were prepared at Solarex using the gas mixture procedures which have recently yielded improvement in the solar conversion efficiency of wide bandgap solar cells. As the bandgap increased due to carbon alloying the electron drift mobility decreased by as much as a factor 30 at some temperatures. The cells with 1.75 eV, 1.81 eV, and 1.87 eV bandgaps had thermally activated drift mobilities over the temperature range 120 K - 200 K; this is associated with simple multiple-trapping behavior. Specimens with bandgaps near 1.90 eV did not have simply activated drift mobilities; we have not accounted for this behavior, but it suggests that the bandtail broadening description used to account for the effects of germanium alloying on the electron drift mobility may not be simply applicable to carbon alloying.