New Ambipolar Hosts Based on Carbazole and 4,5-Diazafluorene Units for Highly Efficient Blue Phosphorescent OLEDs with Low Efficiency Roll-Off

Two ambipolar host materials, 9,9-bis(9-methylcarbazol-3-yl)-4,5-diazafluorene (MCAF) and 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene (PCAF), comprising two electron-donating carbazole units and an electron-accepting 4,5-diazafluorene group, have been designed, synthesized, and characterized....

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Bibliographic Details
Published in:Chemistry of materials 2012-02, Vol.24 (4), p.643-650
Main Authors: Zheng, Cai-Jun, Ye, Jun, Lo, Ming-Fai, Fung, Man-Keung, Ou, Xue-Mei, Zhang, Xiao-Hong, Lee, Chun-Sing
Format: Article
Language:English
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Summary:Two ambipolar host materials, 9,9-bis(9-methylcarbazol-3-yl)-4,5-diazafluorene (MCAF) and 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene (PCAF), comprising two electron-donating carbazole units and an electron-accepting 4,5-diazafluorene group, have been designed, synthesized, and characterized. Given the nonplanar structure of the sp3-hybridized C9 atom of the 4,5-diazafluorene unit, MCAF and PCAF exhibit high triplet energy levels of 2.82 and 2.83 eV, as well as high glass-transition temperatures of 187 and 188 °C, respectively. Equipped with ambipolar transport properties as well as suitable highest occupied and lowest unoccupied molecular orbital energy levels, the two compounds excellently perform in blue phosphorescent organic light-emitting devices (PHOLEDs). The MCAF-based blue PHOLED has a very low turn-on voltage of 2.6 V, a high current efficiency of 32.2 cd A–1, a high external quantum efficiency of 17.9%, a high power efficiency of 31.3 lm W–1, and a low efficiency roll-off with a high efficiency of 27.6 cd A–1 even at 10 000 cd m–2. These values are among the highest ever reported for devices doped with iridium(III) bis[2-(4′,6′-difluorophenyl)pyridinato-N,C(2′)]-picolinate.
ISSN:0897-4756
1520-5002
DOI:10.1021/cm2036647