Organic light-emitting devices based on a highly robust osmium(II) complex

An osmium(II) complex, $Os(fptz)2(PPh2Me)2$ [$fptz=3$-trifluoromethyl-5-(2-pyridyl)-1,2,4-triazole, $PPh2Me=methyl$-diphenyl-phosphan], when doped in a $4,4′-N,N′$-dicarbazolebiphenyl host as a phosphorescent red emitter in organic light-emitting devices, has the unusual property of resisting concentration quenching and sustaining its external quantum efficiency (EQE) (8%–9%) over a wide range of doping concentration (20%–40%). Single-crystal x-ray diffraction evidence is presented that the difference in doping behaviors can be linked to the octahedral configuration of $Os(fptz)2(PPh2Me)2$⁠, which could prevent the undesirable molecular aggregation from occurring at high concentration, thus delaying the onset of quenching. Accordingly, the high doping concentration of $Os(fptz)2(PPh2Me)2$ leads to a significant suppression of luminescence quenching at high brightness/current and achieves a “flat” EQE versus brightness response owing to the increase in the emission sites in the emission layer.