Energy transfer in polymer electrophosphorescent light emitting devices with single and multiple doped luminescent layers

Abstract

We study energy transfer in efficient polymer electrophosphorescent organic light emitting diodes (PHOLEDs) using poly(9-vinylcarbazole) (PVK) host doped with one or more phosphorescent cyclometalated Ir(III) complexes. Single dopant double heterostructure PHOLEDs exhibited saturated color luminescence due to emissive triplet metal-to-ligand charge-transfer to ground state transitions of the Ir(III) dopants. Blue PHOLEDs, excited by an endothermic process from the host polymer, exhibited an emission maximum at a wavelength of ${\lambda }_{\max }\text{=474\hspace{0.17em}}\mathrm{nm},$ with an external quantum efficiency of ${\eta }_{\mathrm{ext}}\text{=1.3±0.1\%}$ and luminous power efficiency of ${\eta }_p\text{=0.8±0.1\hspace{0.17em}}\mathrm{lm/W}.$ The green PHOLED exhibited ${\eta }_{\mathrm{ext}}\text{=5.1±0.1\%,}$ with ${\eta }_{\mathrm{ext}}\text{>2\%}$ for both red and yellow emission. Resonant energy transfer between green emitting fac-tris (2-phenylpyridyl)Ir(III) and red emitting $\mathrm{bis}{\text{[2-(2}}^{\prime }$ -benzothienyl)-pyridinato-$\mathrm{N},{\mathrm{C}}^{\text{3′}}$ ](acetylacetonate)Ir(III) was observed to nearly double the efficiency of red emission when both dopants were simultaneously blended in a PVK host. PHOLEDs containing a blend of red, yellow, and blue Ir(III) complex dopants produced white light emission with ${\eta }_{\mathrm{ext}}\text{=2.1±0.1\%.}$ Our results suggest that deep lying energy states in the PVK conductive matrix may limit the energy transfer efficiency in phosphor doped polymer OLEDs.