Extremely low driving voltage electrophosphorescent green organic light-emitting diodes based on a host material with small singlet–triplet exchange energy without p- or n-doping layer

Abstract

In order to achieve low driving voltage, electrophosphorescent green organic light-emitting diodes (OLEDs) based on a host material with small energy gap between the lowest excited singlet state and the lowest excited triplet state (ΔEST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC–TRZ) with ΔEST of only 0.11eV has been found to be bipolar and used as the host for green OLEDs based on tris(2-phenylpyridinato) iridium(III) (Ir(ppy)3). A very low onset voltage of 2.19V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68cd/A and 60lm/W, respectively, and no significant roll-off of current efficiency (58cd/A at 1000cd/m2 and 62cd/A at 10,000cd/m2) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer.