Achieving an excellent device performance with slow roll-off efficiency is still a crucial requirement for organic light-emitting diodes (OLED) in displays and white lighting applications. Therefore, we selected carbazole as the hole transporting group (p-type unit), phenylene as the linkage unit and pyridine as the electron transporting unit (n-type unit), to construct two bipolar host materials o-2CzPy and m-2CzPy, by varying the ortho- and meta-linking modes.Their chemical structures were confirmed by mass spectrometry (TOF-MS-EI), 1H and 13C NMR spectroscopy, and elemental analysis. These two materials have good thermal stability with the thermal decomposition temperatures of than 360 ℃ and efficient hole- and electron-transporting properties. FIrpic and Ir(ppy)3 based blue and green PhOLEDs incorporating o-2CzPy and m-2CzPy as hosts show pretty color purity, good high efficiencies and slow efficiency roll-off. Among them, the maximum current efficiency of 45.16 cd A-1 and external quantum efficiency of 24.44% were achieved for o-2CzPy based blue device, which are close to the best values for FIrpic-based blue PhOLEDs with single bipolar host reported in recent literatures. Furthermore, the o-2CzPy based green device achieved the high efficiencies of 28.58% and 96.91 cd A-1, and stable emission color of ΔCIEy≤0.0001. Even at the practical brightness of 1000 cd m-2 and 6000 cd m-2, the external quantum efficiencies of o-2CzPy hosted green device still keep as high as 28.03% and 24.70%, with the slight efficiency reduction of 1.9% and 13.6%. These excellent results indicate that these bipolar host materials may find practical applications in highly efficient and stable OLED displays.
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