High Power Efficiency Yellow Phosphorescent OLEDs by Using New Iridium Complexes with Halogen-Substituted 2-Phenylbenzo[d]thiazole Ligands

On the basis of the yellow iridium phosphor, bis(2-phenylbenzothiozolato-N,C2′)iridium(acetylacetonate) [(bt)2Ir(acac)], the three halogen-substituted analogues were designed and synthesized by introducing the F, Cl, and Br atoms to the 4-position of phenyl ring in the ligand of 2-phenylbenzo[d]thiazole. The optoelectronic properties of all the four iridium complexes were fully investigated. Compared to the 559 nm peak emission of (bt)2Ir(acac) in CH2Cl2 solution, adding F atom caused the peak emission of (4-F-bt)2Ir(acac) blue shift to 540 nm, while adding Cl and Br atoms made the peak emissions of (4-Cl-bt)2Ir(acac) and (4-Br-bt)2Ir(acac) slightly blue shift to 554 and 555 nm, respectively. The PhOLEDs using the four iridium complexes as dopants were initially fabricated in the conventional device structure (device I): ITO/MoO3/NPB/CBP/CBP:dopants/TPBi/LiF/Al. The three halogen-substituted analogues exhibited turn-on voltages of 3.5–3.9 V, maximum current efficiencies of 35.5–52.4 cd A–1, maximum power efficiencies of 18.3–29.4 lm W–1 and maximum external quantum efficiencies (EQE) of 12.1–17.3%, which were superior than the (bt)2Ir(acac)-based device (28.4 cd A–1, 19.9 lm W–1, 9.8%). After reducing the hole-injecting barrier and using better carrier-transporting materials in the optimized device II, ITO/MoO3/TAPC/TCTA/CBP:dopants/TmPyPB/LiF/Al, all the four devices exhibited lower turn-on voltages of 2.9–3.1 V and excellent performance with maximum EQE over 20%. As a result, they showed high power efficiencies in the range of 55.9–83.2 lm W–1. Among the four optimized devices, the (4-F-bt)2Ir(acac)-based device achieved the highest power efficiency of 83.2 lm W–1. Remarkably, the (bt)2Ir(acac)-based device still possessed high current efficiency of 53.5 cd A–1, power efficiency of 23 lm W–1, and EQE of 19.6% at extremely high luminance of 10 000 cd m–2.