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

Wavelength-resolved electroluminescence and current measurements have been performed on polyethylene naphthalate thin films submitted to high direct c urrent 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. expe riments when the held is increased. This behaviour provides the basis for a model in which the time evolution of the spectrum shape is attributed to t he progressive charging of the material, through homopolar injection. The n et result is an increase of the internal field in the charge-free region wh ere luminescence occurs.