Evidence of exciton formation in thin polypropylene films under AC and DC fields and relationship to electrical degradation

Thin-films of gold-metalized polypropylene submitted to AC electric field in conditions where electrical discharges are avoided emit light in the optical domain. The origin of this light is discussed on the basis of spectral analysis performed at different electric field and frequency. Two components of the light with different features have been unraveled being due to the polymer itself and to an interfacial phenomenon. The first one is tentatively attributed to the formation of excitonic states of the polypropylene that could be linked with chemical degradation of the polymer explaining its ageing under high field. The second one has a relationship with the surface roughness of the films and is tentatively attributed to the radiative decay of surface plasmons excited at the dielectric/electrode interface by the injection/extraction of electrical charges.