Contact limitation of secondary photoconductivity in intrinsic a-Si:H

A requirement for secondary photoconductivity is that the supply of current by the contacts is sufficient to keep the photoconductor neutral. This requirement is not fulfilled under most experimental conditions in intrinsic a-Si:H at and below 300 K. We find a positive space charge and a large voltage drop near the cathode which yield a subohmic photoconductivity in the range of thermal quenching at applied fields in excess of 100 V/cm. The onset of the subohmic region moves to larger fields with light soaking.