Non-Passive Behavior of Equivalent Circuit Components in AC Powder Electroluminescence (ACPEL) Lamps

Abstract

For the first time, the voltage and frequency characteristics of a single layer AC powder electroluminescent lamp have been examined in detail to reveal the individual contributions of the material components involved. Statistical modelling has been employed to refine the equivalent circuit description of the lamp. DC-blocked resistance-capacitance networks can be reduced to a single effective resistance and capacitance in series. The frequency dependence of these two quantities in the range 4-1600 Hz has been used to unravel the behavior of the different underlying resistance and capacitance components at different voltage amplitudes in the range 25-150 V. The resistive contribution, R, of the activated ZnS phosphor is shown to be non-passive, and obeys the form: R(V,f) = R 0 (V).f −1/3 .e −T.f , where V is the applied voltage, f is the frequency and T is a time constant, at all voltages. For both ZnS and BaTiO 3 , other characteristics indicate the presence of a thinner, non-polarized region within each semiconducting particle located within the particle's crust. Thick film AC electroluminescence, also referred to as AC powder electroluminescence (ACPEL), can be achieved with a simplyconstructed layer structure to produce a surface-distributed light source. The first examples were reported by Destriau in 1936.1 Today, the use of modern materials 2 allows the structure of the AC electroluminescent device to be reduced to just one active layer between two electrodes; this makes the manufacture of this light source particularly economical. The mechanism of operation of ACPEL devices has been the subject of some debate. [3][4][5] As the voltage across the sandwich increases, initially no light is emitted until a voltage threshold is exceeded, when it is thought that carriers (holes and electrons) are generated in the phosphor matrix by the applied field. These carriers are swept by the field in opposite directions, before eventually recombination occurs with the emission of light. This may occur both before and after the voltage across the device is reversed. After exceeding the voltage threshold for light emission, further increases in the voltage produce a near linear increase in luminescence. 2The economic advantage offered by ACPEL light sources is offset by two main weaknesses: efficacy and longevity, both of which are at least an order of magnitude worse than for the OLED alternative as a surface-distributed light source. In order to tackle efficacy improvement, we believe that a thorough characterization of the ACPEL device is a strategic prerequisite. The purpose of this work, then, has been to derive and characterize a reliable equivalent circuit, and identify as far as possible the contributions which the individual material components make to elements of that circuit. Over the last several decades, ACPEL equivalent circuits ranging from a simple capacitance to more complex passive resistance -capacitance networks 6,7 have been proposed, but have lacked detailed experimental confirmation. In the related ...