C1,C8-modified carbazole-based bipolar host materials for blue phosphorescent electroluminescent devices

Abstract

Herein, we report a new molecular design based on carbazole for the development of high triplet energy bipolar host materials for blue phosphorescent organic light emitting diodes (PhOLEDs). Two new isomeric host materials named, 11CzCzCN and 14CzCzCN were designed and synthesized by modifying the carbazole core with carbazole donor at C1 and cyano acceptor at C8-position. The effective interchromophoric electronic communications between the attached donors and central carbazole were interrupted by virtue of large dihedral angle at C1 and short-axis linkage. Also, the strong charge transfer interactions between the donor at C1 and acceptor at C8 was hampered due to the broken conjugation. As a result, the compounds showed high triplet energy (ET) over 2.75 eV without compromising the bipolar charge transporting property. Interestingly, computations and single carrier device analysis evidenced good bipolar charge transporting ability for 11CzCzCN over its structural isomer 14CzCzCN. The applicability of these compounds as hosts for blue PhOLEDs was explored by using a well-known Flrpic dopant. The 11CzCzCN hosted device showed higher performance with maximum EQE of 20.6%, current efficiency of 39.6 cd/A and efficiency roll off of 8.2% at 1000 cd/m2 over its analogue 14CzCzCN hosted device attributed to its high ET and balanced charge transporting property. We believe that this study would provide valuable insights for executing new molecular designs on carbazole for the development of efficient host materials for blue PhOLEDs.