98_laurent_spectral_analysis

98_laurent_spectral_analysis - I EEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 5 No. 4, August 1998 527 Spectral Analysis of Optical Emission Due to Isothermal Charge Recombination in Poly ole fins G. Teyssedre, L. Cisse, C. Laurent, F. Massines Laboratoire de Genie Electrique, Universite Paul Sabatier, Toulouse, France and P. Tiemblo Instituto de Ciencia y Tecnologia de Polimeros, Madrid, Spain ABSTRACT Light emission from electrically stressed polymers, so called electroluminescence, is a subject of great interest because it is associated with electrical aging andl dielectric breakdown of highly insulating materials. Radiative recombination of charge carriers on luminescent centers gener- ally is evoked as the main contribution to the emission, but this is difficult to assess in poly- olefins since the spectral features associated with this mechanism are not known. The rea- son is twofold. First, there is a lack of knowledge on the nature of the luminescent centers in these materials, and second, it is difficult to get wavelengt h-resolved electroluminescence spectra due to the low light level. By using an alternative activation method, we have isolated the emission spectrum associated with isothermal charge recombination in polyethylene and polypropylene. Electrical charges of both polarities are brought to the surface of the samples by contact with a cold plasma powered at a frequency of =5 kHz. The kinetic and spectral features of the recombination-induced emission are recorded after discharge switch off. The recombi- nation mechanism is considered on the basis of the light decay kinetics whereas the chemical nature of the luminescent centers is discussed by a comparison with the photoluminescence spectrum of the polymer. It is shown that charges recombine by tunneling from traps to the luminescent centers which are unsaturated species. The chromophores involved in photo- and recombination-induced luminescence appear to be the same, but not the electronic transitions. Light emitted upon charge recombination has been assigned to transitions from the lowest ly- ing triplet states of poly-enone sequences. 1 INTRODUCTION OLYMERS electroluminescence (EL) is an exciting field of research P because it is associated with electrical aging of the materials and could provide a basis for monitoring the degradation rate of electrically stressed samples [l]. This would only be achieved through a full inter- pretation of the emission spectrum. A first step towards this goal has been reached recently in aromatic polyesters [2,3], but the case of poly- olefins is much more difficult to handle. They are a-bonded polymers which can emit light only in the visible range through isolated chro- mophores, i.e., unsaturated species, situated as in chain, side-chain, or end chain groups, or else, through the presence of additives (e.g. antiox- idant) or contaminants. In order to propose an interpretation of the EL emission in these materials, one needs to know the spectral features of the emission due to the recombination of electrical charges which is a likely part, if not all, of the EL spectrum.
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98_laurent_spectral_analysis - I EEE Transactions on...

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