1. Modelling of electroluminescence in polymers under ac stress Junwei Zhao, David H. Mills, George Chen and Paul L. Lewin 19 th January 2011

2. Electroluminescence (EL) in polymers Origin: light emission from the recombination of opposite polarity charge carriers What are charge carriers? Where do the charge carriers come from? What mechanism is behind the existence of these charges? Indication: storage, transport and interaction of charge carriers within insulation materials Implication: the effects of degradation or ageing on polymeric materials

3. Experiment

4. Experimental setup CCD camera and triggering system allows EL emission synchronised with applied field Uniform electrode arrangement with semitransparent gold coated 100 µm LDPE EL experimental setup

5. Measured EL emission Measured EL under 50 Hz, 6 kV pk, ac voltage of various waveforms (a) Applied voltage (b) EL intensity Positive half cycleNegative half cycle

6. Modelling

7. EL can be described by the total recombination rate (TRR) S i is the recombination coefficients Bipolar charge transport model Model description: - Injection and extraction of charge carriers (electron and hole) at boundaries - Charge transport by a field dependent mobility - Deep trapping for electrons and holes - Recombination of electrons and holes x=0x=d Polymeric film Discretization of polymeric film Trapping and recombination of bipolar charges

8. Modelled EL under sinusoidal voltage Electroluminescence per cycle (6 kV 50 Hz) Comparison of simulation and experiment

9. Contribution of charge carriers Density of charge carriers per cycle (6 kV)

10. Distorted injection flux at boundaries (a) Distorted injection field (b) Injection current density (c) Conduction current density

11. Modelled EL under sinusoidal voltage Electroluminescence at increased applied fieldElectroluminescence at increased frequency

12. Modelled EL under triangular & square voltage (a) simulation(b) experiment Comparison between simulation and experimental results

13. Conclusions Satisfying EL simulation results have been achieved using a bipolar charge transport model; Two typical peaks which occur prior to the voltage peak in each cycle are reproduced Charge carriers from the injection at the boundaries contribute more than that from the conduction process to the resultant EL Injection current and conduction current are both distorted from the sinusoidal form due to the presence of space charge