Abstract
Thin-films of gold-metalized polypropylene submitted to DC and AC electric fields in conditions where electrical discharges in ambient are avoided emit light in the visible part of the optical domain. The origin of the light is discussed with the help of light intensity vs. voltage characteristics and spectral analysis. Two components of the light with different features have been unraveled being due to an interfacial phenomenon and to the polymer itself. The first one is attributed to the formation of surface plasmons excited at the metal/dielectric interfaces by charge injection/extraction. These coherent collective excitations of the electron gas at the metal surface decay by emitting a typical optical signature in the red part of the visible domain. The second component of the electroluminescence spectrum is attributed to the formation of excitonic states in polypropylene excited upon charge recombination or hot electron effects. A strong evidence is provided by the spectral shape analysis which shows the spectrum exhibits identical features with the cathodo-luminescence spectrum of the material. This main component of the emission spectrum under AC field would also be involved in the emission under DC stress, the latter being so weak preventing a spectral analysis. These excitonic states provide a route for a series of reactions associated with chemical degradation of the polymer during its ageing under high field. Finally, it is shown that similar excited species are generated in polyethylene as well pointing towards a generic origin of these excited states in polyolefins.
