Figure 5: Conduction mechanisms in NC films in dark and under illumination.

(a) 'Dark' charge transport occurs through a manifold of weakly conductive MGB states. In our films, the MGB is completely full, and hence, insulating under the flat-band condition. Application of a negative gate bias leads to injection of holes into the MGB, which lowers the Fermi level and opens a conducting pathway through the MGB states. (b) The mechanism for charge transport changes under illumination. In this case, the photogenerated electrons get rapidly trapped from the conduction-band (CB) levels into the mid-gap states and are transported through the MGB. On the other hand, photogenerated holes, that are much longer lived, are transported via more-overlapping, highly conductive valence-band (VB) states. Hole transport via the VB levels dominates photoconduction. Because of the 'mixed' character of photoconduction, which involves both the VB and MGB states, the photovoltage is determined not by the intrinsic band-gap energy, E_g, but the VB–MGB separation.