Phase resolved electroluminescence measurements in thin films of low density polyethylene using a charge coupled device camera

Electroluminescence (EL) produced by a commercially available additive free low density polyethylene film has been investigated under a 50 Hz AC electrical stress. The spatial distribution, spectral characteristics and the phase angle relationship of EL with respect to the 50 Hz applied AC voltage were studied using a sensitive Peltier cooled charge-couple device (CCD) camera. The experimental results from several test samples show that the EL can be divided into two different groups according to the EL spatial distribution: (1) homogeneous EL and (2) inhomogeneous EL. At the same applied voltage, the spectra of the homogenous and inhomogeneous EL were found to be significantly different. Homogeneous EL had maximum emission intensity in the red and near infrared (>700 nm), whereas the inhomogeneous emission had maximum intensity at shorter wavelengths (550-600 nm). Phase resolved measurements showed that homogeneous EL always produced equal emission intensity in the positive and negative half-cycles (symmetric distribution), whereas, inhomogeneous EL produced an asymmetric intensity distribution. To explain these results, it is suggested that homogeneous emission is associated with charge injection and bipolar recombination in the surface states on both sides of the sample, whereas inhomogeneous emission is associated with field enhancement at defects on one of the two sides of the film which allow charge to be injected into states more representative of the bulk material.