Host engineering for improving the performance of blue phosphorescent organic light-emitting devices

We report an effective means to greatly improve the performance of blue phosphorescent organic light-emitting devices by doping well-known electron-or hole-transporting triplet host material, specifically, 3-(4-biphenyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ) or 4,4',4 ''-tris(N-carbazolyl) triphenylamine (TCTA), into emissive layer to form mixed host structures. A superior device performance with high current and power efficiencies of 47.4 cd/A and 37.3 lm/W, respectively, and more importantly, a theoretical maximum of external quantum efficiency of 21.9%, can be achieved for an optimized TCTA-doped device. On the other hand, doping of TAZ into hole-transporting host material would degrade device performance. It is found that, in addition to the well-known effects of charge carrier balance, both energy levels and triplet energies of mixed host materials play crucial roles in determining the device performance. As confirmed by the time-resolved phosphorescence decay studies, the lifetime of FIrpic is strongly dependent on the triplet energy of host materials and the shorter lifetime of triplet excitons with high Stern-Volmer quenching constant of 0.1564% (1)in TAZ-doped films is the origin of the low electroluminescence efficiency of the TAZ-doped devices. (C) 2011 Elsevier B.V. All rights reserved.