Model for electroluminescence in polymers during the early stage of electrical tree initiation

During tree initiation in ac fields electrons are injected into a conduction state on the negative half-cycle. Shallow and deep acceptor states trap the electrons to form negative space charge. On the positive half-cycle electrons are promoted from donor states and extracted leaving positive space charge behind. The hoping of donated electrons between ionised shallow donor states corresponds to ´hole´ transport. Electroluminescence (EL) is produced by recombination, mainly between mobile electrons and deep-trapped holes. This three-level model is able to explain the long-time increase of the EL in terms of an increase in deep state concentrations, without the density of trapped charge becoming unreasonably high. The calculated spatial distribution of EL is also consistent with experiment.