Top-emitting OLED using praseodymium oxide coated platinum as hole injectors

Praseodymium oxide (Pr₂O₃) 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 Pr₂O₃ on 2-nm Pt, a luminance of ∼1400 cd/m² 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/m². The difference between top- and bottom-emitting diodes is explained in terms of microcavity effects.