Spectral features of the luminescence of PE subjected to various excitation sources

Electroluminescence (EL) of insulating polymers is now recognized as a warning sign for electrical aging. In order to further understand the emission process, we have compared the luminescence spectra of the same material when the emitting states are created by different excitation sources. Additive free and carbonyl containing low density polyethylene films were used. Light emission was stimulated by photon irradiation in the ultraviolet range (photoluminescence), by the interaction between a cold plasma and the polymer surface leading to a post-discharge emission (plasma induced luminescence), and by the electric field in the absence of any gaseous discharge (EL). Plasma induced luminescence and electroluminescence exhibit similar spectral features in the wavelength range between 500 and 650 nm. It is concluded that the same kind of chromophores is involved in both processes. They could be conjugated groups acting as recombination centers because charges are selectively trapped on these groups. This explains why photoluminescence does not show similar emission features since all the material chromophores are excited upon UV irradiation. Moreover, it is shown that the EL-features are sensitive to structural relaxation of the polymer probably through the interaction between emitting species and the surrounding molecules.