Density of states in the mobility gap of stabilized a-Se from electron time-of-flight photocurrent analysis

The distribution of localized states in a-Se in the upper part of the mobility gap has been investigated by comparing the measured and calculated electron transient time-of-flight (TOF) photocurrents. The theoretical analysis of multiple-trapping transport has been done by the discretization of a possible continuous distribution of localized density of states (DOS) and the use of the Laplace transform formalism. The continuous distribution of localized states has been approximated in the interval [Ec, Ec−0.6 eV] by a spline function passing through nodes, whose positions have been subject to optimization. Another parameter of optimization was the integrated concentration of states lying deeper than Ec−0.6 eV. A possible effective DOS has the following features: a peak at ∼0.30 eV below Ec with a peak at ∼1017 eV−1 cm−3 and a shoulder at 0.45–0.50 eV below Ec with an amplitude 1014–1015 eV−1 cm−3. The concentration of deep states varies from 1011 to 1014 cm−3. The effects of alloying with As and doping with Cl are also investigated.