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.

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► An effective means to greatly improve the performance of blue phosphorescent OLEDs.
► Doping of either TAZ or TCTA triplet host material to form mixed host structures.
► Superior device performance with EQE of 21.9% is achieved for an optimized TCTA-doped device.
► Energy levels and ET of mixed host materials play crucial roles in determining device performance.
► Low EL efficiency is due to a shorter lifetime of triplet excitons with high KSV.