A new benzothienoindole-based bipolar host material for efficient green phosphorescent organic light-emitting diodes with extremely small efficiency roll-off

In this contribution, two new hole-transport units, 10H-benzo[4,5]thieno[3,2-b]indole (BTI) and 10H-benzofuro[3,2-b]indole (BFI), were developed to design and synthesize bipolar host materials namely 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-benzo[4,5]thieno[3,2-b]indole.(mBTITrz) and 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-benzofuro[3,2-b]indole.(mBFITrz), for phosphorescent organic light-emitting diodes (PHOLEDs). The effect of heteroatoms in the hole-transport unit on the physicochemical and electroluminescence properties of the hosts were investigated in detail. Interestingly, the phosphorescence of the compounds was highly dependent on the hole-transport unit, because it originated from their local excited state (³LE), which was attributed to the interrupted strong electronic communication between donor and acceptor units by meta-mode of conjugation on the phenyl linker. Consequently, the mBTITrz showed high triplet energy (E_T) of ∼2.88 eV compared to its counterpart mBFITrz (E_T ∼ 2.65 eV). Both compounds displayed marked thermal stability with high thermal decomposition temperatures of above 410 °C and glass-transition temperatures of above 103 °C. In addition, the single carrier device studies revealed a bipolar charge transporting character for the compounds. Furthermore, the compounds were evaluated as bipolar hosts for green PHOLEDs by employing Ir(ppy)₃ dopant. The mBTITrz-hosted device demonstrated much better performance than did the mBFITrz-hosted device, with a maximum external quantum efficiency of 21.3% and maximum current efficiency of 76.0 cd/A. Notably, the mBTITrz-based device exhibited excellent efficacy stability with uncompromised efficiency roll-off at 1000 cd/m² and an extremely low efficiency roll-off of 3.2% at 5000 cd/m² and 9.0% at 10,000 cd/m².