Optical-bias effects in electron-drift measurements and defect relaxation in a-Si:H

We report measurements of the effects of optical bias upon photocharge transients in a-Si:H. Transients were recorded from 100 ns to 10 s following a laser pulse. Without optical bias these transients exhibit a form consistent with trapping of electrons by a deep level; this result is consistent with extensive prior research. However, even low levels of optical bias (generation rate 1017 cm-3 s-1) suppress deep trapping, leading to an enhancement of electron drift by as much as one decade. We propose that charge state transitions associated with optical bias leave defects in metastable configurations. The corresponding transition energies lie closer to the conduction band than for the defects relaxed, equilibrium configuration.