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 intens ities 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 t he cathode, the photocurrent remains ohmic. We found conditions? however, u nder which the space charge spreads through a significant fraction of the s ample causing the photocurrent to become subohmic. We have measured the tot al space charge as a function of photocurrent in the ohmic and subohmic reg ime and explored the conditions for a spatial spread of the charge resultin g in subohmicity, as a function of temperature, light intensity and the con centration of dangling bond defects. Available models predict that the phot ocurrent, I. is proportional to the square root of the voltage, U, in the s ubohmic regime. Experimentally, we find I proportional to U-m with m = 0.25 +/- 0.04. We suggest directions for improving the models. (C) 2000 Publish ed by Elsevier Science B.V. All rights reserved.