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

Abstract In this contribution, two new hole-transport units, 10 H -benzo[4,5]thieno[3,2- b ]indole (BTI) and 10 H -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)-10 H -benzo[4,5]thieno[3,2- b ]indole.( mBTITrz ) and 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10 H -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 ( 3 LE), 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/m 2 and an extremely low efficiency roll-off of 3.2% at 5000 cd/m 2 and 9.0% at 10,000 cd/m 2 .