High‐Performance Hybrid White Organic Light‐Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll‐Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter

Abstract

wileyonlinelibrary.compast few years, hybrid WOLEDs, combining the blue fl uorophors and longwavelength phosphors, have attracted substantial attention owing to the unique merits of high effi ciency and excellent stability. [ 2 ] In principle, to achieve a theoretical maximum internal quantum efficiency for hybrid WOLEDs, a prerequisite key is that all electrically generated singlet and triplet excitons must be effectively utilized for the white emission. [ 2,3 ] Enormous efforts have been devoted to simultaneously harvest both the singlet and triplet excitons in single-emissive-layer (single-EML), [ 4 ] and multi-emissive-layer (multi-EML) hybrid WOLEDs. [ 5 ] In the single-EML hybrid WOLEDs, the precise manipulation of phosphorescent emitter concentration in blue fl uorophore host is very necessary to suppress the singlet exciton transfer from the blue fl uorophore to the phosphors via Förster energy transfer. In this case, the phosphorescent dopant concentration and the property of used blue fl uorophore host have obvious effects on the device performance and there exist the problems of notorious spectrum shift with the increased operational voltages. Alternatively, the multi-EML counterparts provide a reliable strategy Thermally activated delayed fl uorescence (TADF)-based white organic lightemitting diodes (WOLEDs) are highly attractive because the TADF emitters provide a promising alternative route to harvest triplet excitons. One of the major challenges is to achieve superior effi ciency/color rendering index/ color stability and low effi ciency roll-off simultaneously. In this paper, highperformance hybrid WOLEDs are demonstrated by employing an effi cient blue TADF emitter combined with red and green phosphorescent emitters. The resulting WOLED shows the maximum external quantum effi ciency, current effi ciency, and power effi ciency of 23.0%, 51.0 cd A −1 , and 51.7 lm W −1 , respectively. Moreover, the device exhibits extremely stable electroluminescence spectra with a high color rendering index of 89 and Commission Internationale de L'Eclairage coordinates of (0.438, 0.438) at the practical brightness of 1000 cd m −2 . The achievement of these excellent performances is systematically investigated by versatile experimental and theoretical evidences, from which it is concluded that the utilization of a blue-green-red cascade energy transfer structure and the precise manipulation of charges and excitons are the key points. It can be anticipated that this work might be a starting point for further research towards high-performance hybrid WOLEDs.

De novo design of D–σ–A molecules as universal hosts for monochrome and white phosphorescent organic light-emitting diodes

“…Since light is only detected from the forward-viewing direction, it is believed that the total energy emitted from a device is ∼1.7 times greater than that from the forward-viewing one 45. It is expected that the PE of device WS can ideally reach over 100 lm W –1 at 1000 cd m –2 , which is comparable to that of commercial fluorescent tubes, representing one of the most efficient white OLEDs so far 16,46–50. To the best of our knowledge, this is the first single host based white PhOLED that can simultaneously realize EQE ≥25% at high brightness (≥1000 cd m –2 ) and extremely stable EL emission with ΔCIE < (0.01, 0.01) from 1000 to 10 000 cd m –2 .…”

High-efficiency/CRI/color stability warm white organic light-emitting diodes by incorporating ultrathin phosphorescence layers in a blue fluorescence layer

“…In order to achieve high device efficiency, phosphorescent materials have been widely used to fabricate such devices because they allow manufacturers to harvest 25% singlet excitons and 75% triplet excitons, thus achieving internal quantum efficiency of 100%, compared with the 25% theoretical value for fluorescent materials [3][4][5][6][7][8]. However, the all-phosphorescent WOLEDs are limited by i) the absence of efficient blue phosphors with long operational lifetimes that are suitable for commercial applications [9,10], and ii) the difficulty of conducting carrier injection from an adjacent carrier transporting layer to the large-bandgap host material with higher triplet state energy [11]. Thus, one promising approach to achieve WOLEDs with high efficiency and long lifetime is to combine emissions from blue fluorophores and long wavelength phosphors, also called fluorescence/ phosphorescence (F/P)-based hybrid WOLEDs.…”

“…Towards this end, a series of novel device structures for hybrid WOLEDs have been reported. For hybrid WOLEDs, two types of dominant device architectures have been proposed: the single-emissive layer (single-EML) structure [11,13,[15][16][17][18] and the multi-emissive layer (multi-EML) structure [1,10,11,14,19,20]. In order to obtain ideal white emission in single-EML hybrid WOLEDs, the doping concentration of phosphors in the blue fluorophore host must be precisely controlled.…”

“…In order to obtain ideal white emission in single-EML hybrid WOLEDs, the doping concentration of phosphors in the blue fluorophore host must be precisely controlled. This must be done in order to suppress the transfer of singlet excitons from the blue fluorophore to the phosphors via Förster energy transfer [10][11][12]15], thus inducing a complicated device preparation process. In the multi-EML hybrid WOLEDs, for harvesting complete electrically generated excitons, a commonly used configuration is to incorporate an interlayer between the fluorescent and the phosphorescent dopant layers, which separates the singlet and triplet excitons to their individual emitting zones [1,11,15,21,22].…”

“…In fact, the excellent device stability and high CRI for WOLEDs are also key factors that are as important as other parameters, such as high efficiency, high brightness, and low cost [3,8,12,18,25,26]. Although some reported hybrid WOLEDs have realized high efficiency, problems still exist, including poor color stability and low CRI [10,15]. Especially, for a simplified hybrid WOLED, simultaneously realizing high efficiency, ultra-high CRI, and good color stability remains a big challenge.…”