Kexiang Wang

Highly Efficient Red and White Organic Light-Emitting Diodes with External Quantum Efficiency beyond 20% by Employing Pyridylimidazole-Based Metallophosphors

Two highly efficient red neutral iridium(III) complexes, Ir1 and Ir2, were rationally designed and synthesized by selecting two pyridylimidazole derivatives as the ancillary ligands. Both Ir1 and Ir2 show nearly the same photoluminescence emission with the maximum peak at 595 nm (shoulder band at about 638 nm) and achieve high solution quantum yields of up to 0.47 for Ir1 and 0.57 for Ir2. Employing Ir1 and Ir2 as emitters, the fabricated red organic light-emitting diodes (OLEDs) show outstanding performance with the maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 20.98%, 33.04 cd/A, and 33.08 lm/W for the Ir1-based device and 22.15%, 36.89 cd/A, and 35.85 lm/W for the Ir2-based device, respectively. Furthermore, using Ir2 as red emitter, a trichromatic hybrid white OLED, showing good warm white emission with low correlated color temperature of <2200 K under the voltage of 4-6 V, was fabricated successfully. The white device also realizes excellent device efficiencies with the maximum EQE, CE, and PE reaching 22.74%, 44.77 cd/A, and 46.89 lm/W, respectively. Such high electroluminescence performance for red and white OLEDs indicates that Ir1 and Ir2 as efficient red phosphors have great potential for future OLED displays and lightings applications.

Bipolar hosts and non-doped deep-blue emitters (CIEy = 0.04) based on phenylcarbazole and 2-(2-phenyl-2H-1,2,4-triazol-3-yl)pyridine groups

We have designed and synthesized four bipolar materials, TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz, using 2-(2-phenyl-2H-1,2,4-triazol-3-yl)pyridine (TAZ) as an electron-withdrawing group and phenylcarbazole as an electron-donating group. Their rigid molecular structures improve their thermal stability with high glass transition temperatures: 99 °C for TAZ-2Cz, 100 °C for TAZ-3Cz and 103 °C for TAZ-4Cz. Employing TAZ-1Cz, TAZ-2Cz, TAZ-3Cz and TAZ-4Cz as hosts, green phosphorescent organic light-emitting devices (PhOLEDs) with fac-tris(2-phenylpyridine)iridium as an emitter display maximum external quantum efficiencies (EQEs) of 15.8, 15.5, 16.0 and 13.0%, respectively. And blue PhOLEDs hosted by TAZ-1Cz, TAZ-2Cz and TAZ-3Cz achieved maximum current efficiencies of 8.6, 11.0 and 10.6 cd A−1, respectively. Furthermore, non-doped OLEDs using TAZ-1Cz and TAZ-4Cz as emitters exhibit electroluminescence (EL) peaks at 400 and 412 nm with CIE coordinates of (0.16, 0.04) and (0.15, 0.04), which are located in the deep-blue region.

Manipulation and exploitation of singlet and triplet excitons for hybrid white organic light-emitting diodes with superior efficiency/CRI/color stability

A high performing tetra-chromatic hybrid white organic light-emitting diode (WOLED), with a fluorescent blue emitting layer (EML) of bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2) sandwiched between a pair of a phosphor-doped hole transporting layer (HTL) and an electron transporting layer (ETL), was developed. This was achieved by controlling the location of the green phosphor doped in the HTL and yellow phosphor doped in the ETL at ∼1 nm away from the HTL/EML and EML/ETL interfaces, and incorporating an ultrathin red phosphorescence layer (<0.1 nm) in the center of the Bepp2 EML. The resulting hybrid WOLED exhibits good warm white emission, showing stable electroluminescence spectra with a maximum color rendering index (CRI) of 94 and a low correlated color temperature of 2440–2468 K over a wide voltage range of 5–9 V. Meanwhile, this WOLED also achieves a high device efficiency, a maximum current efficiency and power efficiency, 29.51 l m W−1 and 17.71%, respectively, and an external quantum efficiency of up to 34.15 cd A−1. Such a high performance is realized through the precise manipulation and effective exploitation of singlet and triplet excitons via a novel device design. Moreover, the proposed WOLED also removes the additional interlayers between the fluorescent and phosphorescent emitting regions that are commonly employed in the conventional hybrid WOLEDs, inducing a simplified device structure with reduced heterojunction interfaces, which is very beneficial for promoting the commercial development of WOLEDs.

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.

Ultra-simple white organic light-emitting diodes employing only two complementary colors with color-rendering index beyond 90

White organic light-emitting diodes (WOLEDs) with an ultra-high color rendering index (CRI) (≥90) are considered to be crucial for special lighting applications, such as in hospitals, art galleries, and museums. However, most reported WOLEDs with a CRI of ≥90 almost all use three or more emitters, and usually suffer from a complicated device structure. In this work, an exciplex formed between a 4,4′,4′′-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (m-MTDATA) donor and a bis[2-(2-hydroxyphenyl)-pyridine]beryllium (Bepp2) acceptor, exhibiting a broad-spectrum emission, was employed as a yellow emitter. And a thin 4,4′,4′′-tri(9-carbazoyl)triphenylamine (TCTA) layer (2–6 nm) as a carrier adjustment layer was inserted into the Bepp2 layer of the exciplex to block some of the electrons at the TCTA/Bepp2 interface, inducing a blue light emission from Bepp2. A series of ultra-simple di-chromatic WOLEDs, using only three organic materials, were demonstrated. By changing the thickness of TCTA, the proposed WOLEDs achieve an ultra-high CRI of 92, which, to our knowledge, is by far the simplest structure for a complementary WOLED with a CRI over 90. Besides, the optimized WOLED, at a practical luminance of 3000 cd m−2, shows an ultra-high CRI of 90, and also realizes a high maximum current efficiency and power efficiency of 8.7 cd A−1 and 10.1 lmW−1, respectively. This novel design concept provides a new avenue for achieving simple-structured, but ultra-high-CRI WOLEDs.

Precise manipulation of the carrier recombination zone: a universal novel device structure for highly efficient monochrome and white phosphorescent organic light-emitting diodes with extremely small efficiency roll-off

Achieving superior device efficiency and very small efficiency roll-off simultaneously for all phosphorescent OLEDs (PHOLEDs) is still an open challenge. In this work, a universal novel device structure, having mixed hosts sandwiched between hole- and electron-transporting hosts, was proposed, and a series of monochrome and white PHOLEDs based on the proposed novel device structure were developed. All the resulting PHOLEDs achieve a maximum external quantum efficiency (EQE) exceeding the theoretical limit, reaching 21.71%, 23.85%, 23.99%, 21.79%, and 23.15% for green, yellow, red, blue, and white PHOLEDs, respectively. Moreover, apart from blue PHOLEDs using inefficient blue phosphor, other monochrome and white PHOLEDs show extremely small efficiency roll-off. At a practical luminance of 5000 cd m−2, the EQE is still up to 20.34%, 20.95%, 20.07%, and 16.06% for green, yellow, red, and white PHOLEDs, respectively. Such high device performance is testified from the precise manipulation of the carrier recombination zone via a novel device structure, which contributes to a strictly limited and broadened carrier recombination zone, a balanced distribution of electrons and holes, as well as consequentially reduced triplet exciton aggregation and polaron formation, thus effectively boosting the device efficiency and suppressing the notorious triplet–triplet annihilation and triplet–polaron quenching.

High efficiency and low roll-off green OLEDs with simple structure by utilizing thermally activated delayed fluorescence material as the universal host

Abstract We achieved high-efficiency and low-roll-off green fluorescent and phosphorescent organic light-emitting diodes (OLEDs) simultaneously by adopting the thermally activated delayed fluorescence material of bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone as the universal host. At a luminance of 1000 cd/m2, fluorescent OLEDs based on C545T get a current efficiency, power efficiency, and external quantum efficiency (EQE) of 31.8 cd/A, 25.0 lm/W, and 9.26%, respectively. This is almost the highest efficiency based on C545T at the luminance of 1000 cd/m2 to date. On the other hand, phosphorescent OLEDs with Ir(ppy)3 as the emitter realize a maximum current efficiency, power efficiency, and EQE of 64.3 cd/A, 62.4 lm/W, and 18.5%, respectively. More important, the EQE remains 17.8% at the representative luminance of 1000 cd/m2 and the roll-off ratio is just 3.78%. The transient photoluminescence decay measurement demonstrates that the up-conversion of host triplet excitons plays a key role in the high efficiency and low roll-off. More detailed discussions are also given.

Non-phosphor-doped fluorescent/phosphorescent hybrid white organic light-emitting diodes with a sandwiched blue emitting layer for simultaneously achieving superior device efficiency and color quality

In this study, an efficient deep-blue fluorescent OLED with the maximum EQE of 5.92% was achieved by employing a sandwiched blue emitting layer (EML) of 4P-NPD/4P-NPD : Bepp2 (1 : 1)/Bepp2. It was demonstrated that the blue light emission simultaneously originated from 4P-NPD and Bepp2, and the main carrier recombination zone was located at the co-doped 4P-NPD : Bepp2 (1 : 1) layer. Based on the above facts, by incorporating complementary phosphorescent ultra-thin EMLs (UEMLs) into different positions of EML of the above blue device, a series of non-phosphor-doped two- and three-color hybrid white OLEDs (WOLEDs) were fabricated, of which the utilization and the energy transfer of singlet and triplet excitons were investigated in detail. Finally, several proposed non-phosphor-doped four-color hybrid WOLEDs with complementary phosphorescent UEMLs simultaneously incorporated into the center and the two sides of the sandwiched blue EML were developed. The resulting WOLEDs showed high external quantum efficiencies (EQEs) ranging from 18.88% to 21.46%, demonstrating complete exciton utilization. Also, the optimized WOLED simultaneously achieved a high maximum EQE and a color rendering index of 19.35% and 93, respectively. Such a superior device performance can be due to (i) the highly efficient blue fluorescence emission realized by the sandwiched blue EML and (ii) effective manipulation and exploitation of singlet and triplet excitons by the novel device structure.