Optimized trade-off between electroluminescent stability and efficiency in solution-processed WOLEDs adopting functional iridium(III) complexes with 9-phenyl-9-phosphafluorene oxide (PhFlPO) moiety

Abstract

Employing complimentary color phosphorescent emitters of the well-known FIrpic and new unsymmetrical orange emitter Ir-POB with 9-phenyl-9-phosphafluorene oxide (PhFlPO) moiety, solution-processed white organic lighting-emitting diodes (WOLEDs) had been fabricated to cope with the variation of EL spectra at different working voltage through excitation of both FIrpic and Ir-POB with the same mechanism in the device. Through constructing a thin exciton formation zone by the energy level layout between host 4,4′,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and 1,3,5-tris-(N-(phenyl)-benzimidazole)-benzene (TPBi), both FIrpic and Ir-POB in the single emission layer can be excited by energy-transfer mechanism to bring stable white EL spectra in wide range of driving voltages with small CIE coordinate variation of Δx = 0.0044 and Δy = 0.0197. Furthermore, in order to overcome the disadvantage associated with energy-transfer mechanism, both FIrpic and Ir-POB have been excited by direct charge-trapping mechanism in WOLEDs by judiciously controlling the energy level layout between phosphorescent emitters and co-hosts polyvinyl carbazole (PVK) and 1,3-bis(5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7). The concerned WOLEDs not only maintain stable white EL in even wider driving voltage rang with CIE coordinate variation of Δx = 0.0078 and Δy = 0.0053, but also furnish much higher EL efficiencies of maximum external quantum efficiency (ηext) of 22.4%, a maximum current efficiency (ηL) of 58.4 cd A−1 and a maximum power efficiency (ηP) of 41.3 lm W−1. Hence, by unifying excitation mechanism of the phosphorescent emitters, optimized trade-off between white EL stability and high EL efficiencies have been successfully achieved. Definitely, these results can provide crucial information for developing WOLEDs with high performances.