Very High Efficiency Orange-Red Light-Emitting Devices with Low Roll-Off at High Luminance Based on an Ideal Host-Guest System Consisting of Two Novel Phosphorescent Iridium Complexes with Bipolar Transport

Two phosphorescent iridium complexes with bipolar transporting ability, namely FPPCA (500 nm) and BZQPG (600 nm), are synthesized and employed as an ideal host-guest system for phosphorescent organic light emitting diodes (PHOLEDs). The devices give very high-efficiency orange-red emission from BZQPG with maximum external quantum efficiency (EQE or eta(ext)) of >27% and maximum power efficiency (PE or eta(p)) of > 75 lm/W, and maintain high levels of 26% and 55 lm/W, 25% and 40 lm/W at high luminance of 1000 and 5000 cd m(-2), respectively, within a range of 8-15 wt% of BZQPG. The realization of such high and stable EL performance results from the coexistence of two parallel paths: i) effective energy transfer from host (FPPCA) to guest (BZQPG) and ii) direct exciton formation on the BZQPG emitter, which can alternately dominate the electrophosphorescent emission. This all-phosphor doping system removes the charge-injection barrier from the charge-transport process to the emissive layer (EML) due to the inherent narrow E-g of both phosphors. Therefore, this ideal host-guest system represents a new design to produce PHOLEDs with high efficiency and low efficiency roll-off using a simple device configuration.