The transport and trapping of electrons in polymers

Abstract

The conductivity of a polymer is determined by the availability of charge carriers and their mobility, and both terms are dominated by the trapping states in the polymer. The distribution in energy of these states may be studied by populating the states and observing the current that results from the relaxation of the trapped charges. In our experiments the states are populated by a pulse of electrons from an injection beam, and the surface potential of the polymer film is monitored by a second electron beam that passes above the charged surface; we have used this technique to study both charge trapping and the isothermal discharge process that follows a charging pulse. The general case of charge decay from an arbitrary distribution of states with retrapping can be analyzed by using a multiple trapping model, and this analysis yields the distribution of trapping states in the polymer. The analysis takes on a particularly simple form in the limit of negligible retrapping. Experimentally we have concentrated on PS-2 polystyrene and on several mono-dispersed polystyrenes. We find a change in the distribution of trapping states with molecular weight; there is also a change in trap distribution near the glass transition temperature.