Analysis of the luminescence decay following excitation of polyethylene naphthalate films by an electric field

Cissé

et al. 1998

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 generally is evoked as the main contribution to the emission, but this is difficult to assess in polyolefins since the spectral features associated with this mechanism are not known. The reason 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 recombination 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 lying triplet states of poly-enone sequences.

Section: Discussionsupporting

confidence: 83%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Spectral analysis of optical emission due to isothermal charge recombination in polyolefins

Cissé

et al. 1998

“…The kinetics of the two first components can be described by exponential functions with appropriate time constants. We already discussed the problem of the actual time dependence of the recombination-induced luminescence component in such experiments [13]. Considering literature data, the most general form of luminescence decay controlled by charge recombination takes the form:…”

Section: Charge-recombination Induced Luminescence Analysismentioning

confidence: 99%

Charge Recombination Induced Luminescence of Chemically Modified Cross-Linked Polyethylene Materials

Perego

et al. 2009

Very little is known on the chemical nature of the deep trapping centres that play a role in charge storage in insulating polymeric materials. The analysis of the luminescence emitted during a process of charge recombination can be helpful to unravel the nature of such centres. The spectral features of the luminescence induced by charge recombination are analyzed for three different cross-linked polyethylene (XLPE) materials, chemically-modified or not. In two XLPE formulations, a controlled amount of chemical groups containing an aromatic ring has been grafted to the polymer chains. The other formulation is the same XLPE without chemical modification. Photoluminescence is investigated to settle a background to identify the species involved in charge recombination. It is excited on films at room and liquid nitrogen temperature providing fluorescence and phosphorescence spectra. The luminescence excited by the recombination of trapped electrical charges is subsequently analyzed at low temperature and compared to the photoluminescence emission. The results obtained on chemically-modified XLPE provide a basis to identify the species involved in charge recombination in the non modified material. It turns out that aromatic compounds are involved in photo-and charge recombination-induced luminescence in all the tested XLPE, and that the luminescence has the same spectral features for the two stimulations, contrary to what is observed for low density polyethylene.

“…When the electron is trapped in the neighborhood of the ionized center (typically less than 100 A) tunneling recombination occurs with a typical time-dependence. This process is unlikely to be dominant in the transport problem although tunneling recombination has been evidenced between trapped carriers after the polymer was submitted to a high field [55]. Recombination is a very energetic process when compared to trapping/de-trapping because the potential energy between the two charges reaches 2/3 of the band gap; see Figure 15.…”

Section: Charge Recombinationmentioning

confidence: 99%

Charge dynamics and its energetic features in polymeric materials

Roy

et al. 2013

144

Modeling charge transport and linking charge dynamics with dissipative processes responsible for electrical ageing are a challenging objective for developing a mature approach in insulation design. Such an approach is exemplified here for polyethylenebased materials by introducing two models describing bipolar Space Charge Limited Current in transient and steady states. They rely on two classical descriptions of the distribution function of the energy levels of trap states -single trapping level or exponential distribution. Their predictions are discussed as regards the experimental behavior. They notably highlight the importance of recombination processes in explaining the sigmoidal shape of the steady-state current-voltage characteristic. Also, bipolar charge transport seems to be a necessary factor for the observed features of oscillatory charge packets. The energetic features of charge dynamics is reviewed with particular emphasis on recombination phenomena because the latter promote electronically excited states that are chemically reactive and could be involved in ageing reaction. The relationship between charge recombination and electroluminescence is highlighted through experiments and simulation. The spectral analysis of the emitted light advocates the existence of massive chemical/physical degradation in the electrical regime where recombination is a major factor of the Space Charge Limited Current (SCLC).