Phosphorescent organic light emitting diodes (PHOLEDs) have undergone tremendous growth over the past two decades. Indeed, they are already prevalent in the form of mobile displays, and are expected to be used in large-area flat panels recently. To become a viable technology for next generation solid-state light source however, PHOLEDs face the challenge of achieving concurrently a high color rendering index (CRI) and a high efficiency at high luminance. To improve the CRI of a standard three color white PHOLED, one can use a greenish-yellow emitter to replace the green emitter such that the gap in emission wavelength between standard green and red emitters is eliminated. However, there are relatively few studies on greenish-yellow emitters for PHOLEDs, and as a result, the performance of greenish-yellow PHOLEDs is significantly inferior to those emitting in the three primary colors, which are driven strongly by the display industry. Herein, a newly synthesized greenish-yellow emitter is synthesized and a novel device concept is introduced featuring interzone exciton transfer to considerably enhance the device efficiency. In particular, high external quantum efficiencies (current efficiencies) of 21.5% (77.4 cd/A) and 20.2% (72.8 cd/A) at a luminance of 1000 cd/m2 and 5000 cd/m2, respectively, have been achieved. These efficiencies are the highest reported to date for greenish-yellow emitting PHOLEDs. A model for this unique design is also proposed. This design could potentially be applied to enhance the efficiency of even longer wavelength yellow and red emitters, thereby paving the way for a new avenue of tandem white PHOLEDs for solid-state lighting.
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