Organic light-emitting diodes (OLEDs) are being increasingly applicable in flat-panel displays, such as television and mobile phone, and have a great potential to become the next generation solid-state lighting sources. One of the most attractive strategies is to construct OLEDs with low cost purely organic materials, where high cost phosphorescent materials containing rare or noble metals are not involved in the device fabrication. We have proposed a simple strategy towards high-performance fluorescent OLEDs. The designed devices are merely composed of stacked p-type hole-transport layer and n-type electron-transport layer, resulting in a planar pn heterojunction configuration similar to their inorganic counterparts, namely light emitting diodes (LEDs). In this novel device, the excitons are generated and decay at the pn junction interface due to the synergetic interaction of p-type and n-type materials, finally giving the light emission. In addition, a novel strategy towards simultaneously low-driving voltage and high efficiency fluorescent OLEDs was also proposed using an active planar pn heterojunction as the exciton generation region and conventional fluorescent dye or thermally activated delayed fluorescence (TADF) material as the emitter. The current findings highlight the significance of the role of the organic active planar pn heterojunction in fabrication of high-performance and structure-simplified OLEDs with conventional fluorescent and TADF emitters, and also enlighten that organic active pn heterojunction should play a much more important role in electric-to-light conversion devices as it has done in the light-to-electric photovoltaic devices.
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