The transport of low energy electrons in polystyrene

This is a continuation of earlier work (1, 2) in which an electron injection technique was used to study the transport of charge carriers in thin films of polystyrene. In these studies an electron beam is used to inject charge into the free surface of the polymer film, and a higher energy beam monitors the surface potential of the film. From the time dependence of the surface potential and its variation with temperature one should be able to distinguish between the various possible transport mechanisms in the polymer. In our work using a 2 keV beam for charge infection we consistently observe currents which decay as t⁻¹ and this leaves an ambiguity of interpretation, as both trap-modified space-charge-limited tranpsort and beam-induced conductivity with bimolecular recombination can give rise to this same form of current decay. Repeating these charge decay measurements with low energy electrons (which should eliminate the contribution due to beam-induced conductivity) we observe charge decay characteristics which are similar to those using the 2 keV electrons. Thus we are able to distinguish between these two charge decay mechanisms: we conclude that beam induced conductivity effects are second order, and we are indeed observing electron transport in polystyrene, with electron mobility and range determined by the distribution of electron traps.