Charge injection and electroluminescence as a prelude to dielectric breakdown

To understand the ac voltage aging of low-mobility materials, the amount of energy involved in the degradation process must be known. Electroluminescence experiments are a choice environment for solving this problem since luminescence spectra give direct information on the energy released during aging. Carrier injection is needed for electroluminescence to take place, but it is not clear whether luminescence occurs at the injection threshold or later on. We investigate this problem using two very sensitive arrangements: a charge detector and a photon-counting system. The experimental threshold fields for charge injection and electroluminescence are compared, as are their ac time variations. In polyethylene, charge injection is detected prior to electroluminescence. But it is found that the two phenomena begin at the same electric field if the space-charge effect is taken into account in the estimation of the luminescence threshold. A discussion of the experimental results yields the picture of a recombination-controlled luminescence. The energy released during the radiative as well as the nonradiative recombination processes induce chemical damage of the dielectric.