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

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 (Delta E-ST) have been fabricated. 2-biphenyl-4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine (PIC-TRZ) with Delta E-ST of only 0.11 eV 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 2.19 V is achieved in devices without p- or n-doping. Maximum current and power efficiencies are 68 cd/A and 60 lm/W, respectively, and no significant roll-off of current efficiency (58 cd/A at 1000 cd/m(2) and 62 cd/A at 10,000 cd/m(2)) have been observed. The small roll-off is due to the improved charge balance and the wide charge recombination zone in the emissive layer. (c) 2012 Elsevier B.V. All rights reserved.