Long Gao

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.

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 14u2006140 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 83.68–17u2006050 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.

Improved color stability of complementary WOLED with symmetrical doped phosphors in single host: experimental verification and mechanism analysis

In this work, by symmetrically doping blue and orange phosphors in a single host, a complementary all-phosphor white organic light-emitting diode (WOLED) was demonstrated. Compared with the reference white device with blue-orange sequential cascade dopants in a single host, the proposed WOLED with (blue-orange-blue) symmetrical arranged dopants shows an improved spectra stability with a change in Commission Internationale de LEclairage (CIE) coordinates of less than (±0.020, ±0.010) as the driving voltage increases from 4 V to 9 V, corresponding to the luminance increasing from 100 cd m−2 to about 15u2006000 cd m−2. The improvement of color stability should be ascribed to the symmetric dopants in the multi-doped single host being able to effectively offset the spectral change caused by the shift of carrier recombination zone with the increase of driving voltage. This is confirmed by systematic probe devices, where the shift of carrier recombination zone was observed to give a more detailed analysis.

High color stability and CRI (>80) fluorescent white organic light-emitting diode based pure emission of exciplexes by employing merely complementary colors

High color stability and CRI (>80) pure exciplex WOLEDs with merely complementary colors of orange- and blue-exciplexes are realized with the application of spacers. The WOLEDs with a three spacer structure achieve a maximum current efficiency, power efficiency and external quantum efficiency (EQE) of 16.2 cd A−1, 11.3 lm W−1 and 7.92%, respectively. Besides, a standard white light point close to Commission Internationale de l’Eclairage (CIE) coordinates of (0.31 ± 0.00, 0.37 ± 0.02) and a very high color rendering index (CRI) of ∼83 with two colors emitting are obtained simultaneously. The balanced emission and natural broad emission band of the exciplex are responsible for the stable white light spectra and high CRI. We also find that the location of the spacers, and the amount of them, play a key role in the electroluminescence performance of the WOLEDs and more detailed discussions are given below.

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.

Low efficiency roll-off and high color stability pure fluorescent white organic light-emitting diode based exciplex host

Pure fluorescent WOLEDs with low efficiency roll-off and high color stability were realized by employing an exciplex host. Due to incomplete energy transfer from the blue exciplex host of mCPu2006:u2006PO-T2T to the orange fluorescent dopant of rubrene, the WOLEDs showed a maximum current efficiency, power efficiency and EQE of 14.2 cd A−1, 12.8 lm W−1 and 4.90%, respectively. To our surprise, a rather small roll-off ratio of 8.2% from the maximum EQE to the EQE at 1000 cd m−2 and stable white light-emitting spectra with CIE coordinates of (0.384 ± 0.001, 0.439 ± 0.002) from 4 V to 8 V were obtained simultaneously. The bipolarity and triplet exciton up-conversion of the exciplex host played key roles in delivering excellent performance. More detailed discussions are also provided.

Polyfluorene-based white light conjugated polymers incorporating orange iridium(III) complexes: the effect of steric configuration on their photophysical and electroluminescent properties

Different kinds of polyfluorene-based white light conjugated polymers with phosphorescent iridium(III) complexes as orange emission groups and polyfluorene as blue emission groups were designed and synthesized. On the basis of adjusting substituent positions on iridium(III) complexes, the conjugated polymers exhibited different steric configurations, i.e. hyperbranched and linear structures, and the PL emission peaks of iridium(III) complexes had a significant change. Compared to linear conjugated polymers, hyperbranched white light conjugated polymers showed the best thermal stability and film forming properties. The white light single-emissive-layer devices with simplified configuration were also prepared in a wet process. All these devices realized good electroluminescence, especially the hyperbranched conjugated polymers in which the roll off phenomenon at high current density was effectively suppressed. Furthermore, EL spectra of hyperbranched polymers exhibited good stability at different driving voltages. A maximum luminance of 2469 cd m−2, a maximum current efficiency of 1.73 cd A−1 and the commission internationale de lEclairage (CIE) coordinates of (0.25, 0.23) showed white light was achieved from the HPF-Ir10 devices.

Highly efficient chlorine functionalized blue iridium(III) phosphors for blue and white phosphorescent organic light-emitting diodes with the external quantum efficiency exceeding 20%

The absence of highly efficient and stable blue phosphors is still a bottleneck for the development of high quality monochrome and white phosphorescent organic light-emitting diodes (OLEDs). Here, we propose a chlorine-functionalization strategy for blue phosphors, and a series of aromatic chlorine-based high-efficiency blue iridium(III) phosphors (Ir1–Ir4) containing various ancillary ligands was designed and prepared. These phosphors show intensive blue emissions withPL peaks of 468–476 nm and a high ΦPL of 0.70–0.85 in dichloromethane. Using these blue phosphors as emitters, the fabricated OLEDs also realize similar blue emissions with the peaks located at 472, 484, 476, and 476 nm for the Ir1–Ir4-based OLEDs, respectively. Moreover, the Ir2- and Ir3-based OLEDs exhibit extremely high EQEs exceeding the theoretical limit, reaching 20.43% and 21.41%, respectively. In addition, the two-color white OLED with Ir3 as the blue emitter also achieves an extremely high device efficiency, having a maximum EQE of up to 20.17%. The excellent blue emission and high device efficiency indicates that the four phosphors have huge potential applications for high performance blue and white phosphorescent OLEDs.