Electroluminescence excitation mechanisms in an epoxy resin under divergent field

Abstract

Electroluminescence (EL) excitation mechanism has been reported in divergent field situation produced around a set of wires moulded into an epoxy resin. Square voltage pulses not less than 100 ms in duration were used. The electroluminescence was evidenced during the positive and negative fronts of the applied voltage. The EL was not detected during the plateau of the pulse, and it was of equal intensity for different polarity of the applied voltage. The interpretation of the EL excitation involves fast charge injection (dynamic << minimum pulse duration) and charge trapping leading to a field reduction at the wire surface below the minimum field needed to excite the electroluminescence. The evidence of charge accumulation was obtained in multiple pulse experiments where small voltage pulses, of a polarity opposed to the polarity of the main pulse, and of amplitude less than the one needed to excite the EL, were used as a probe. It was shown that the EL disappeared after a time of the order of minutes, giving evidence of space charge dissipation around the wire. A space charge density in the range between 10 and 100 C.m /sup -3/ is needed to reduce the field at the wire surface below the field which excites the EL. The space charge field by itself is also able to extract charges on the negative front of the pulse voltage. This study underlined the importance of transient voltages in charge injection and electroluminescence excitation processes.