Top-Emitting OLED Using Praseodymium Oxide Coated Platinum as Hole Injectors

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Abstract
Praseodymium oxide (Pr/sub 2/O/sub 3/) coated platinum (Pt) was investigated as a composite hole-injection layer for "top-emitting" organic light-emitting diodes (OLEDs) based on copper (II) phthalocyanine-N, N'-diphenyl-N, N' bis(3-methylphenyl-1, 1'-biphenyl-4, 4'-diamine-tris-8-hydroxyquinoline aluminum. Aluminum was used as the current-carrying and reflecting anode electrode underneath the composite hole-injection layer. The resulting radiation pattern was found to be highly non-Lambertian. With 1-nm Pr/sub 2/O/sub 3/ on 2-nm Pt, a luminance of /spl sim/1400 cd/m/sup 2/ in the normal direction was obtained. When the intensity was integrated over all angles, it was determined that these OLEDs emitted 30% more radiation than their conventional "bottom-emitting" counterparts. An external quantum efficiency of 1.32% and a power efficiency of 1.1 lm/W were obtained at 100 cd/m/sup 2/. The difference between top- and bottom-emitting diodes is explained in terms of microcavity effects.