Effect of Impurity Ions and Permanent Dipoles for Device Performance of Thin-Film Electroluminescent Diodes
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ABSTRACT Double-layer EL diodes composed of a spin-coated Polyvinylcarbazole (PVCz) layer and a vacuum-sublimed tris-(8-hydroxyquinoline)aluminum (Alq) layer were prepared. The diodes with the same device structure but with PVCz layer with added ionic impurities were also prepared. The diodes were driven at constant voltage and allowed to stand under short-circuit or reverse bias conditions. Observations of luminance-current density-voltage relations at constant voltage driving were repeated. The decrease of both luminance and current density during constant voltage driving were observed. Both spontaneous and reverse-bias assisted recovery of device performances were observed and these degradation and recovery phenomena were discussed in terms of the movement of ionic impurities in organic layers. INTRODUCTION Today, basic working mechanism for charge-injection type electroluminescent (EL) diodes, which consists of double-injection of positive and negative charges, their transport, recombination and production of neutral excited states, and emissive transition to ground states, has been well understood. Molecular materials useful for EL diodes, whether small molecules or polymers, are classified into insulators with high resistivity, even though high charge-carrier mobility and high quantum efficiency of fluorescence are important requisites[ 1,2]. In other words, very little amount of electronic charges are present in molecular films without charge injection from outer electrodes. Once electric field is applied, large amount of positive and negative charges pass through molecular solid films. One should note that fundamental electric properties of molecular materials for EL diodes are governed by insulating characteristics and dielectric properties. Little attention has been paid for this view points, because all the attentions have been focused on the design of specific small molecules and polymers with high hole and electron transporting capabilities. It is well known that purities of molecular materials are not so high enough compared with the cases of inorganic semiconductors. Thus it is always assumed that organic thin films contain small amounts of impurities, even when they are fabricated from high purity materials under careful processing technique. Impurities, especially ionic species in insulating films move slowly under applied electric field and form an internal electric field to the opposite direction to the applied field, even though their mobilities are extremely low. Very recently, we found that ionic impurities give significant effects on device performances of EL diodes. Possible origins of long-term irreversible degradation processes in EL diodes, such as degradation of organic layers due to spontaneous crystallization of dye molecules, corrosion of metal electrodes and exfoliation of organic layers from electrodes, have been extensively studied [ 3-6]. Thus the origins of irreversible degradation have been reasonably ascribed to chemical or physical changes 611 Mat. Res. Soc. Symp. Proc. Vol. 4
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