Optimizing the conjugation between N,N′-dicarbazolyl-3,5-benzene and triphenylphosphine oxide as bipolar hybrids for highly efficient blue and single emissive layer white phosphorescent OLEDs

• Two bipolar host materials mCPpPO and mCPmPO have been disigned and synthesized.
• High efficiency and low roll off blue device were fabricated.
• The high efficiency single emission layer white PhOLED basd on mCPmPO was obtained.

Two bipolar host materials, mCPpPO and mCPmPO have been synthesized by Ni(II)/Zn-catalyzed cross-coupling of diphenylphosphine oxide and corresponding aryl bromide. The photophysical properties, HOMO/LUMO orbital distribution and triplet levels of these host materials are investigated and optimized by tuning the linking modes between electron acceptor triphenylphosphine oxide and electron donor N,N′-dicarbazolyl-3,5-benzene (mCP). When mCP is linked to the meta-position of benzene of triphenylphosphine oxide, the hybrid (mCPmPO) shows much higher steric hinderance than the para-position linked analogue (mCPpPO) so that it possesses a higher triplet energy. Equipped with the bipolar transport properties, mCPmPO-based blue PhOLED doped FIrpic shows a maximum current efficiency (ηc,max) of 40.0 cd/A, a maximum power efficiency (ηp,max) of 39.7 lm/W, corresponding the maximum external quantum efficiency (ηEQE,max) of 20.3%, and the current efficiency still maintain to 34.8 cd/A even at 1000 cd/m2. Based on the optimized triplet energy level, the single emission layer white PhOLED hosted by mCPmPO shows ηc,max, ηp,max and ηEQE,max of 46.9 cd/A, 39.7 lm/W and 17.6%, respectively.

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