Electronic transport in polymers

Abstract

Over the last decade it has become increasingly clear, through broad application of time-of-flight drift mobility techniques, that a diverse assortment of disordered molecular materials in fact share, in remarkable detail, a common pattern of features in their transport properties. In this paper the latter will be clearly illustrated using as the principal examples certain of the polysilylenes and polygermylenes and poly-tetraphenylbenzidine, a polymer incorporating a transport-active aromatic amine on its main chain. The features demonstrated are as follows. Firstly the drift mobility exhibits a convoluted field and temperature dependence which is characterized by In μ proportional to E^1/2 where the slope of this relation typically diminishes with increasing temperature and can even undergo sign reversal and an activated type of temperature dependence which is itself field dependent. In addition, some studies appear to indicate a sensitivity of transport activation to changes in microscopic polarizability and constraints imposed on configurational degrees of freedom. Secondly there is a characteristic composition-specific temperature range over which time-of-flight transients exhibit a well defined plateau, do not exhibit anomalous scaling with field and film thickness and yet display a broad asymmetric leading edge inconsistent with simple diffusive broadening. Thirdly mobilities which can be systematically modified by doping. Finally a characteristic and abrupt alteration of transport behaviour occurs at the glass transition. The extent to which these observations can be accounted for solely on the basis of disorder, the common characteristic of these systems, or whether site relaxation constitutes an additional pervasive requirement is currently undergoing vigorous scrutiny.