Abstract: Electroluminescent films
Authors: Schlam Elliott 
Published: July 2002

Electroluminescent (EL) phenomena offer certain real advances for use in display devices. Among the more significant advantages is high luminous conversion efficiency (in the order of 1 lm/W) at low cost with considerable flexibility in fabrication, size, and display format. While the phenomenon of EL in single-crystal materials (light-emitting diodes) is relatively well understood, this is not the case for polycrystalline powders, or thin films; however, it is these configurations which most exhibit the aforementioned advantages. The defect chemistry giving rise to EL in the II-VI chalcogenides has been widely studied, but the level of knowledge in this area is not sufficient for correlation to predetermined synthesis and fabrication procedures. Furthermore, in general, it has been found in the past that those device configurations and synthesis procedures that lead to particularly desirable properties, such as high brightness or suitability for matrix addressing, result in concomitant undesirable trade-offs. Such trade-offs involve the inability to achieve both of these features and unacceptably short operating life. In the last few years both powdered phosphors and thin films have evolved which exhibit very satisfactory operational characteristics as well as acceptable operating life. The material involved to date is ZnS activated with Mn2+ or Cu+. Generally, the key to achieving long life has been careful materials and processing control, with most significant attention being paid to the co-activator, not the activator.
Thin films of manganese-activated ZnS, which show good efficiency and long life, have been demonstrated both in the USA (Sigmatron, Inc.)1 and Japan (Sharp Corp.).2 The devise configurations are straightforward sandwich-type structures with insulator layers isolating the active material. In both cases, there is an apparent lack of Cu+ which is commonly used as a co-activator in manganese-activated ZnS. The active films may be deposited by thermal evaporation or sputtering, and the activator incorporated by co-evaporation or inclusion in the ZnS source. In any event, the ac-operated devices exhibit good display contrast and appear to be capable of thousands of hours of operation. In the Sigmatron device, advantage is taken of low duty cycle, pulsed operation, which seems to extend life considerably while not seriously degrading output luminosity. This is so because of the long relaxation time (∠1 ms) of the light emission, which results in an enhanced time-averaged luminosity. One great advantage to the thin film is its transparency. A black absorbing layer may be deposited immediately behind the transparent light-emitting film, absorbing most of the ambient illumination and thereby resulting in high display contrast with relatively low light output. Such devices are comfortably legible in direct sunlight, while having an output luminosity of only 20 fL. In powdered phosphors significant advances have been demonstrated by Vecht,3 with onion-skin-type phosphor particles incorporating a CuxS skin and Mn2 activator. A 100-fL luminosity and thousands of hours of life under low duty cycle, dc, pulsed operation have been demonstrated. The question of whether the Cu-rich phase contributes Cu to the active material is not fully resolved, and again the presence of the Cu as a possible co-activator appears critical. Another powdered phosphor configuration has been demonstrated by Lehman4 in his hyper-maintenance phosphor. This material looks like the common Cu-activated ZnS, but instead of using chlorine for a charge-compensating co-activator, bromine is used. This appears to be the main ingredient for long-life operation.
Operated by conventional sinusoidal voltages, Lehman’s phosphor is not suitable for matrix operation since it has a relatively low discrimination ratio. However, very significant results by Brody,4 in the deposition of thin film transistor (TFT) matrix arrays, have shed new light in such phosphors because the TFT array takes the entire burden of the drive and addressing function. Fully operational matrix displays have been demonstrated, incorporating these two technologies.
As briefly outlined here,5 there have been a number of demonstrations of activated ZnS exhibiting good efficiency and acceptably long operating life. Materials processing and deposition procedures and judicious choice of activators and co-activators have been the factors that have resulted in these encouraging results. Electroluminescent films show great potential for meeting all of the significant requirements for flat-matrix displays. In order to be fully exploited for commercial and military applications, a broader industrial and technological base in EL phenomena and devices should be established.