Enabling high efficiency and long lifetime of pure blue phosphorescent organic light emitting diodes by simple cyano modified carbazole-based host

Abstract

Herein, we report two simple and efficient high triplet energy bipolar host materials named 2'-(9H-carbazol-9-yl)-[1,1′-biphenyl]-2-carbonitrile (CzBPCN) and 9-(2′-cyano-[1,1′-biphenyl]-2-yl)-9H-carbazole-3-carbonitrile (CNCzBPCN) by connecting carbazole type donor and cyano acceptor on 2 and 2′ positions of a biphenyl linker, respectively. The effect of cyano substitution on carbazole building block on photophysical and electroluminescence properties was unveiled. Both the compounds revealed high triplet energy (ET) of above 3.0 eV attributed to the interrupted interchromophoric electronic interactions between the donor and acceptor units. The computational studies and carrier transport analysis proved that the compounds secured good bipolar charge transporting feature. The applicability of these materials as hosts for blue phosphorescent organic light emitting diodes (PhOLEDs) was tested. Interestingly, the CNCzBPCN hosted device exhibited superior performance with three fold improved external quantum efficiency (EQEmax) of 20.2% over the CzBPCN hosted device which showed EQEmax of 7.1%. Importantly, the CNCzBPCN hosted device demonstrated 11 fold extended operational lifetime of 121 h up to 50% of its intimal luminance (L0 = 200 cd/m2) compared to CzBPCN. We believe that this work demonstrates the cost-effective and efficient high triplet energy bipolar host materials for simultaneous achievement of high EQE over 20% and long operational lifetime of blue PhOLEDs.