Modeling of charge injection and extraction in a metal/polymer interface through an exponential distribution of surface states

From a brief review of the literature, it is shown that surface states within dielectrics represent a critical step in charge injection whereas only scarce physical descriptions of these phenomena are available. It is confirmed from space charge measurements on low density polyethylene (LDPE) that the work function of metals used as electrodes does not correlate to injected charge quantities, thereby supporting the need for alternative descriptions of injection to the Schottky law. Hence, the influence of surface states on space charge formation in LDPE is investigated using a one-dimensional model of electronic charge injection and transport under dc voltage. Transport and trapping in the bulk of the insulation are accounted for using an exponential distribution of traps with a maximum limit in trap depth. We have modeled interface states considering the same form of distribution, hypothesizing an increasing density of states and trap depth when approaching the surface. We suppose that the interface region with modulated properties extends over 1μm from both surfaces of the 200 μm thick insulation. We also considered a reservoir of charge for providing carriers into the dielectric, instead of a classical injection law. After a sensitivity analysis of model parameters on space charge results, we present simulation results on charge injection for a unipolar and a bipolar model. It is shown that the presence of surface states and the reservoir of charges could explain the particular behavior of charges observed experimentally, as regards heterocharge accumulation, and field enhancement.