Post-transit photocurrent analysis, based on the time-of-flight transient photoconductivity technique, was successfully carried out for a series of amorphous selenium $(a-\mathrm{Se})$ thin films. The method allows a determination of the density of gap states beyond the shallow tail states from the emission-dominated post-transit currents. Prominent hole and electron traps were resolved some 0.4–0.5 eV above the valence-band edge and 0.55–0.65 eV below the conduction-band edge. These two traps represent the thermally accessible levels of the $D^{+}$ and $D^{-}$ intrinsic negative-U defects in $a-\mathrm{Se}.$ The tail end of the transient currents and an apparent temperature dependence below 0 °C of the resolved densities can be interpreted as indications for a further set of hole and electron traps close to the Fermi level, but experimental uncertainties and a simulation of the temperature dependence show that such assignment remains questionable.

• Received 5 October 1998

DOI:https://doi.org/10.1103/PhysRevB.59.10607