Dependence of electroluminescence intensity and spectral distribution on ageing time in polyethylene naphthalate as modelled by space charge modified internal field

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.