Combining emissions of hole- and electron-transporting layers simultaneously for simple blue and white organic light-emitting diodes with superior device performance

An efficient deep-blue fluorescent organic light-emitting diode (OLED), showing an emission peak at 432 nm, a high luminance of 14 140 cd m(-2), and an external quantum efficiency (EQE) of 5.07% (which exceeds the theoretical limit), was developed by employing a simple bilayer structure. Such high device performance was achieved by combining emissions of the hole-transporting layer 4P-NPD and the electron-transporting layer Bepp2, simultaneously. Furthermore, based on the blue emission of the above-mentioned 4P-NPD/Bepp2, by handily arranging complementary phosphors doped in both 4P-NPD and Bepp2 sides, a series of simple two-, three-, and four-color hybrid white OLEDs (WOLEDs) without the interlayer between fluorescent and phosphorescent emitting layers were demonstrated. These hybrid WOLEDs realize superior device efficiency, the maximum EQE reaching 18.87%, 15.49%, and 18.44% for optimized two-, three-, and four-color WOLEDs, respectively. Moreover, the four-color hybrid WOLED also exhibits a very high color rendering index (CRI) of 93-94 over a wide luminance range of 83.68-17 050 cd m(-2). To our knowledge, this is among the best efficiencies for very high CRI WOLEDs using such simple structures. The realization of high device performance was explored in detail, and was ascribed to the precise management and effective utilization of singlet and triplet excitons via the proposed novel device structure.