THE TRAPPING OF ELECTRONS IN POLYSTYRENE

The trapping of electrons in localized states in polystyrene has been studied by means of an electron beam technique. A 2.2 kV beam is used to inject a short pulse of charge into the free surface of a thin film of the polymer, and a second electron beam monitors the surface potential of the film. The surface potential is related to trapped charge density and to the depth of charge penetration. Defining a trapping parameter alpha = 1/mu-tau-E, we can relate the incremental buildup of surface potential to the injected charge density and carrier range. This schubweg model does not take account of field distortion due to trapped space charge in the polymer, but it does provide a useful approximate solution to the trapping process. Charge decay from the polymer involves detrapping from the localized states and the transport of these detrapped electrons through the polymer. In a general solution to this problem the subsequent retrapping of these detrapped carriers must be included, but in the case of thin polymer films at high fields this retrapping can sometimes be neglected. The release of electrons from traps is analyzed in terms of a time and temperature dependent demarcation energy E(m). This energy is related to elapsed time since charge injection via the equation E(m) = kTln(nu-t). Time dependence of charge decay is thus related to the energy distribution of traps in the polymer.