Experimental study of the density of states in the band gap of a-Se

The energy levels of the thermally accessible states of the negative-U defects in a-Se have been determined from the activation energy of the steady-state photocurrents in the mono- and bimolecular recombination regimes, and independently from the hole and electron emission currents of the post-transit time-of-flight (TOF) signals. Indications for the distribution of tail states are obtained from standard transient photocurrent measurements in a gap cell geometry, as well as from an analysis of the drift mobility characteristics as measured through TOF with sandwich cells. Rather steep tail state distributions are found, with characteristic energies below 30 meV.