Modeling a.c. thin-film electroluminescent devices

Abstract

Since the work at Sharp Corporation demonstrated the possibility for long lived thin-film electroluminescent devices, there has been considerable interest in this technology for display applications. As this technology moves out of the research laboratory, there is a need for accurate models that one can use to characterize thin-film EL elements. Such models are necessary for process optimization, for the design of optimum driving circuitry, and also to make projections of expected performance limits in a particular application. In a.c.-coupled thin-film EL devices, the active layer (usually ZnS:Mn) is sandwiched between two insulator layers. This structure is the subject of this paper. The presence of the insulator layer results in certain features that are unique to this particular construction, and we have found certain special techniques for characterizing device operation to be particularly useful. The “ Q-Vfigure,” analogous to the “ H-B” diagram for a magnetic material, provides a graphic representation of device operation that is useful for both qualitative and quantitative descriptions. In this paper, a series of simple-to-complex models for device operation are offered that are useful for analysis in applications. The simpler models, though useful for some purposes, fail to give a complete description. The more complete models require a much more detailed knowledge of the underlying processes and so are more cumbersome to use. They can, however, provide insight into the operation. A detailed exposition of experimental data for typical a.c. thin-film EL devices is given, which includes electrical and optical data along with suggestions of underlying physical mechanisms that may provide a model explaining this data. Special consideration is given to the role of tunneling and space charge in determining the electrical characteristics and to the special features of the Mn ++ center in ZnS. It is suggested that an Auger process involving this center can provide a link between concentration quenching and the occurrence of hysteresis or memory effect. The nature of brightness saturation in ZnS:Mn devices is reviewed. The saturation data strongly suggest an interaction between Mn ++ excited states that provide nonradiative decay channels.