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The existing interpretation and models of electroluminescence (EL) in poly(ethylene-2, 6-naphthalate) (PEN) suggest a mechanism triggered by the impact excitation and/or ionization of the luminescent centres by hot electrons. Excitation by bipolar charge recombination has not been considered extensively because of the lack of knowledge of the behaviour of electrons and holes in insulating films subjected to a dc stress. An investigation of the internal space charge distribution versus field is believed to constitute a relevant approach to discussing the EL excitation process. Transient and steady-state EL of PEN films are analysed with the support of space charge profiles obtained by the laser induced pressure pulse (LIPP) method for dc fields in the range of 25-225 kV mm-1. For field values up to 75 kV mm-1, the internal charge is dominated by dipolar polarization without any excitation of EL. Space charge polarization is evidenced from 75 kV mm-1 upwards, the LIPP response being a superposition of space charge and dipolar polarization effects. Space charge polarization dominates in the field range where EL is continuously emitted (E>150 kV mm-1). The EL excitation mechanism is discussed on the basis of the recorded charge profiles.

Wavelength-resolved electroluminescence and current measurements have been performed on polyethylene naphthalate thin films submitted to high direct current fields (3.4 MV cm-1) for several days. The change in the shape of the emission spectrum over time resembles that observed in short-term experiments when the field is increased. This behaviour provides the basis for a model in which the time evolution of the spectrum shape is attributed to the progressive charging of the material, through homopolar injection. The net result is an increase of the internal field in the charge-free region where luminescence occurs.