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

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.