Anomalous Field Dependence of the Hole Mobility in a Molecular Doped Polymer

Photocurrent transients are determined in polycarbonate doped with 17 wt% of p‐p‐EFTP (see structure below) as a function of the electric field strength and the temperature. The hole mobilities are obtained both by a graphical method and by computational analysis based on an analytical expression for the transient photocurrent which has been recently proposed by Scott et al. The comparison of the results obtained with the two methods shows that, although the absolute values of the mobilities differ by a factor of 1.5, the field and temperature dependencies of the hole mobility are similar. The system studied shows an anomalous field dependence of the mobility which at low fields increases with increasing field to a maximum above which it decreases with increasing field. Such a behaviour is analysed in the framework of two different theoretical models: one based on the Marcus theory of electron‐transfer processes and Bässler's model which takes into account the fluctuations in hopping sites due to the simultancous presence of diagonal and off‐diagonal disorder. While the first model does not provide a satisfactory description of the experimental data, the latter cannot explain qualitatively the observed field dependence over a wide range of applied fields. The predicted saturation of the drift velocity of the holes at high fields does not agree with the observed temperature dependence of the mobility high fields. This disagreement with the conventional models is probably due to the large dimensions of the molecules allowing a change in the free energy for nearest‐neighbour hops which is large compared to the polaron binding energy or the width of the distribution of the density of states.