PHOTO-ELECTROLUMINESCENCE IN COMMERCIAL THIN-FILM ELECTROLUMINESCENT PANELS

Pulsed, combined voltage/UV excitation of commercial thin-film electroluminescent (TFEL) display panels produces an increase in their EL output. The UV radiation employed can be either broadband (from a pulsed flash-lamp), or monochromatic (from a nitrogen laser). The UV-induced EL gain is particularly pronounced for EL operation near threshold, and tends to saturate as the panel is operated well above threshold. The magnitude of the EL gain is also dependent on the time relationship of the external, pulsed UV radiation and the voltage pulses used to drive the panel into EL operation. Specifically, the EL gain peaks when the pulsed bias and pulsed UV overlap in time. The EL gain is mainly produced by long-UV radiation, and is absent when the external irradiation consists of visible or near-infrared radiation. The latter observations suggest that the UV irradiation generates photo-carriers in the phosphor layer of the device. The long persistence (up to −300 ms) in the EL gain after UV irradiation led to an investigation of the inter-pulse polarization present in TFEL panels operating above EL threshold. When the UV irradiation and the voltage drive-pulses overlap in time, a strong buildup in polarization is observed. The EL gain will persist as long as this excess polarization is present. The peak intensity of the EL pulse was observed to correlate linearly with the changes in polarization before and after the EL emission. The inter-pulse polarization is variously affected when the UV irradiation is not synchronous with the drive pulses, and such changes are accordingly reflected in the EL output. The relevant mechanism in the case of inter-pulse irradiation appears to be the production of photo-carriers, and their drift in the existing polarization field. The detailed changes in the UV-induced polarization depend on the relative intensity of the negative and positive drive voltages. The combined photoelectric (PE) excitation techniques are proposed as valuable diagnostic adjuncts in characterizing the performance of actual TFEL display devices. © 1990, The Electrochemical Society, Inc. All rights reserved.