Investigating electroluminescence excitation by space charge measurements under transient stress in cross linked polyethylene

Carrier transport in 150 μm thick XLPE films under ca. 130 kV/mm was characterized by both transient space charge and EL measurements. Both positive and negative charges were injected from the electrodes. For relatively short stressing time (order of minutes), the field distribution was not significantly changed due to the evolution of these charges. Injected positive and negative charges propagated with similar speed; however, the negative charges were more dispersive in their velocity. It was about 800 seconds after the application of the step voltage when the injected positive and negative charges met in the bulk. A strong EL was observed soon after the step voltage application and decayed quickly within several tens of seconds. Subsequently, the EL intensity showed a gradual increase. The first intense emission was interpreted as an interfacial phenomenon, possibly associated with minute protrusions in the interface. The gradual increase of the signal was correlated with the recombination of two kinds of carriers injected from both electrodes. The EL intensity with periodical application of voltage led to lower intensity. The memory effect was explained as the result of cumulative relaxation of the field around the protrusions.