White organic light-emitting diodes (WOLEDs) are currently under intensive research and development worldwide as a new generation light source to replace problematic incandescent bulbs and fluorescent tubes. One of the major challenges facing WOLEDs has been to achieve high energy efficiency and high color rendering index simultaneously to make the technology competitive against other alternative technologies such as inorganic LEDs. Here, an all-phosphor, four-color WOLEDs is presented, employing a novel device design principle utilizing molecular energy transfer or, specifically, triplet exciton conversion within common organic layers in a cascaded emissive zone configuration to achieve exceptional performance: an 24.5% external quantum efficiency (EQE) at 1000 cd/m2 with a color rendering index (CRI) of 81, and an EQE at 5000 cd/m2 of 20.4% with a CRI of 85, using standard phosphors. The EQEs achieved are the highest reported to date among WOLEDs of single or multiple emitters possessing such high CRI, which represents a significant step towards the realization of WOLEDs in solid-state lighting.
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