Space charges resulting from photocurrents exceeding the thermionic emission currents in a-Si:H

When the photocurrent exceeds the thermionic emission current of electrons and holes, a photoconductor cannot remain neutral. A positive space charge appears near the cathode when the contribution of holes to the photocurrent is less than that of electrons. That happens in a-Si:H at all light intensities at low temperature and, at room temperature and above, at high light intensity. As long as the space charge is limited to a narrow region near the cathode, the photocurrent remains ohmic. We found conditions, however, under which the space charge spreads through a significant fraction of the sample causing the photocurrent to become subohmic. We have measured the total space charge as a function of photocurrent in the ohmic and subohmic regime and explored the conditions for a spatial spread of the charge resulting in subohmicity, as a function of temperature, light intensity and the concentration of dangling bond defects. Available models predict that the photocurrent, I, is proportional to the square root of the voltage, U, in the subohmic regime. Experimentally, we find I∝Um with m=0.25±0.04. We suggest directions for improving the models.