RGB Phosphorescent Organic Light-Emitting Diodes by Using Host Materials with Heterocyclic Cores: Effect of Nitrogen Atom Orientations

A series of host materials 1−7 containing various heterocyclic cores, like pyridine, pyrimidine, and pyrazine, were developed for RGB phosphorescent organic light-emitting diodes (OLEDs). Their energy levels can be tuned by the change of heterocyclic cores and their nitrogen atom orientations, and decrease of singlet−triplet exchange energy (ΔEST) was achieved with introducing one or two nitrogen atoms into the central arylene; this is also consistent with density functional theory calculations. Their carrier mobilities can also be tuned by the choice of heterocyclic cores, giving improved bipolarity compared with that without any heterocyclic cores. Due to the high triplet energy level of the developed host materials, well confinement of triplet excitons of blue emitter iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,C2′) picolinate (FIrpic) was achieved except for 7 due to its low ET. In contrast, triplet energy can be well confined on green emitter fac-tris(2-phenylpyridine) iridium (Ir(PPy)3) and red emitter tris(1-phenylisoquinolinolato-C2,N)iridium(III) (Ir(piq)3) for all the hosts, giving comparable lifetime (τ), photoluminescent quantum efficiency (ηPL), and radiative and nonradiative rate constants (kr and knr). Highly efficient blue and green phosphorescent OLEDs were achieved for 2, exhibiting one of the highest ever efficiencies to date, especially at much brighter luminance for lighting applications. In comparison, the highest efficiencies hitherto were achieved for the red phosphorescent OLED based on 6, which can be attributed to its lower-lying LUMO level and the smallest ΔEST, giving improved electron injection and carrier balance. Different from the blue and green phosphorescent OLEDs based on FIrpic and Ir(PPy)3, the host materials with lower-lying LUMO levels seem to be better hosts for a red emitter Ir(piq)3, achieving improved efficiency and reduced efficiency roll-off at high current density.