The Thermalization and Trapping of Electrons in Polystyrene

An electron injection technique has been used to study the transport of electrons in polystyrene. A 2 kV electron beam is used to inject a short pulse of charge into the free surface of the polymer film, and a second electron beam monitors the surface potential of the film. From the time dependence of the surface potential following the injected pulse one may infer the position of the injected carriers as a function of time; moreover, for the small signal case the time-derivative of surface potential is a measure of the current flow in the polymer. The interaction of the injected electrons with localized states in the polymer can be analyzed most conveniently in terms of a thermalization, or demarcation energy, Em. This energy is so defined that the release of the excess electrons from traps of depth Em occurs in a time equal to the elapsed time from the injection event. Thermalization energy is related to elapsed time through the equation Em= kTln(vot). The time dependence of current is thus related to the energy distribution of traps in the polymer, and by plotting the function tdV/dt versus log(t) we obtain a plot of the trap distribution versus energy. This trap distribution characterizes the electron transport properties of the polymer.