Zhongbin Wu

Teaching an old acceptor new tricks: rationally employing 2,1,3-benzothiadiazole as input to design a highly efficient red thermally activated delayed fluorescence emitter

The exploitation of blue to orange emissive thermally activated delayed fluorescence (TADF) materials has been conducted comprehensively, while the equally important red TADF materials have been studied at a relatively slow pace. Three D–A–D structured fluorescent molecules, N4,N4,N7,N7-tetraphenylbenzo[c][1,2,5]thiadiazole-4,7-diamine (BTZ–DPA), 4,7-bis(9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazole (BTZ–CZ) and 4,7-bis(9,9-dimethylacridin-10(9H)-yl) benzo[c][1,2,5]thiadiazole (BTZ–DMAC) were designed and synthesized by rationally employing 2,1,3-benzothiadiazole as an acceptor. The introduction of 2,1,3-benzothiadiazole not only presents an efficient input for the design of red TADF emitters, but also provides benefits for the resulting high-efficiency organic light-emitting diodes (OLEDs) which show a maximum external quantum efficiency of 8.8% at a luminance of 1.06 cd m−2 with the emission peak at 636 nm.

Reduced efficiency roll-off in all-phosphorescent white organic light-emitting diodes with an external quantum efficiency of over 20%

Although previously reported all-phosphorescent white organic light-emitting diodes (WOLEDs) exhibit impressive electroluminescence efficiencies, problems remain in terms of the severe efficiency roll-off at high brightness. Here, a smart design of the emissive zone structure is presented to make full use of generated excitons to realize high efficiency all-phosphorescent WOLEDs with reduced efficiency roll-off. The fabricated WOLED shows a maximum power efficiency (PE) of 46.6 lm W−1, a current efficiency (CE) of 46.4 cd A−1 and an external quantum efficiency (EQE) of 22.4%. These values remain as high as 41.3 lm W−1, 46.2 cd A−1 and 22.0%, respectively, at a brightness of 1000 cd m−2, exhibiting less pronounced efficiency roll-off. The critical current density, where the EQE declines by half from its peak, reaches 220 mA cm−2, which should be a high value for all-phosphorescent WOLEDs with an external quantum efficiency of over 20%.

A 1 kW-class multi-stage heat-driven thermoacoustic cryocooler system operating at liquefied natural gas temperature range

This article introduces a multi-stage heat-driven thermoacoustic cryocooler capable of reaching cooling capacity about 1 kW at liquefied natural gas temperature range without any moving mechanical parts. The cooling system consists of an acoustically resonant double-acing traveling wave thermoacoustic heat engine and three identical pulse tube coolers. Unlike other traditional traveling wave thermoacoustic heat engines, the acoustically resonant double-acting thermoacoustic heat engine is a closed-loop configuration consists of three identical thermoacoustic conversion units. Each pulse tube cooler is bypass driven by one thermoacoustic heat engine unit. The device is acoustically completely symmetric and therefore “self-matching” for efficient traveling-wave thermoacoustic conversion. In the experiments, with 7 MPa helium gas as working gas, when the heating temperature reaches 918 K, total cooling capacity of 0.88 kW at 110 K is obtained with a resonant frequency of about 55 Hz. When the heating tempera...

Achieving Extreme Utilization of Excitons by an Efficient Sandwich-Type Emissive Layer Architecture for Reduced Efficiency Roll-Off and Improved Operational Stability in Organic Light-Emitting Diodes.

It has been demonstrated that the efficiency roll-off is generally caused by the accumulation of excitons or charge carriers, which is intimately related to the emissive layer (EML) architecture in organic light-emitting diodes (OLEDs). In this article, an efficient sandwich-type EML structure with a mixed-host EML sandwiched between two single-host EMLs was designed to eliminate this accumulation, thus simultaneously achieving high efficiency, low efficiency roll-off and good operational stability in the resulting OLEDs. The devices show excellent electroluminescence performances, realizing a maximum external quantum efficiency (EQE) of 24.6% with a maximum power efficiency of 105.6 lm W(-1) and a maximum current efficiency of 93.5 cd A(-1). At the high brightness of 5,000 cd m(-2), they still remain as high as 23.3%, 71.1 lm W(-1), and 88.3 cd A(-1), respectively. And, the device lifetime is up to 2000 h at initial luminance of 1000 cd m(-2), which is significantly higher than that of compared devices with conventional EML structures. The improvement mechanism is systematically studied by the dependence of the exciton distribution in EML and the exciton quenching processes. It can be seen that the utilization of the efficient sandwich-type EML broadens the recombination zone width, thus greatly reducing the exciton quenching and increasing the probability of the exciton recombination. It is believed that the design concept provides a new avenue for us to achieve high-performance OLEDs.

Managing Excitons and Charges for High-Performance Fluorescent White Organic Light-Emitting Diodes

The simultaneous realization of high efficiency, stable spectra, high color rendering index (CRI), and low-efficiency roll-off in a fluorescent white organic light-emitting diode (WOLED) still remains a big challenge. Here, we demonstrate high-performance conventional fluorescent-dopant-based WOLEDs by strategic management of singlet and triplet excitons within an efficient emissive zone. This design consists of two separated red/green sub-EMLs with ultralow doping concentration and a sandwiched sub-EML doped with red and green fluorescent dyes at a relatively high concentration, which can harness all electrogenerated excitons and reduce the energy loss to the utmost extent. Accordingly, the resulting WOLED realizes an external quantum efficiency (EQE) of 18.2% with a maximum power efficiency of 44.6 lm W-1. At the practical luminance of 1000 cd m-2 for the lighting source, the EQE still remains as high as 16.2% with a CRI of 82 and stable color spectra. A comprehensive understanding of the device working mechanism is performed to guide design of efficient and stable fluorescent WOLEDs.

New AIEgens containing dibenzothiophene-S,S-dioxide and tetraphenylethene moieties: similar structures but very different hole/electron transport properties

Three AIEgens were designed and constructed using tetraphenylethene and dibenzothiophene-S,S-dioxide, which show good thermal stability as verified by thermogravimetric and differential scanning calorimetry analyses. Single-carrier devices indicate that the control of the transport properties of these AIEgens can be realized by conveniently modifying the linkage mode of their two construction blocks. The obtained experimental results might open up a new option for the design of efficient blue AIEgens. Furthermore, when they are utilized as sky-blue emitting layers in OLEDs, the devices exhibit the highest efficiencies with the maximum external quantum efficiency of 3.62%, current efficiency of 8.66 cd A−1 and power efficiency of 5.28 lm W−1.

NUMERICAL SIMULATION OF A THREE-STAGE STIRLING-TYPE PULSE TUBE CRYOCOOLER FOR 4K OPERATION

The thermoacoustically driven two-stage pulse tube cooler has recently reached a lowest temperature of about 18.1K in our lab, and we are now developing a three-stage Stirling-type pulse tube cooler working at about 4K to match the thermoacoustic engine. In this paper, thermoacoustic theory is employed to simulate the three-stage pulse tube cooler. In order to decrease the cooling power losses of the second and third stage cold tips, two thermal bridges are introduced on the first and second stage cold tips to cool down the second and the third stage pulse tubes. Thus, the temperature gradients in the pulse tubes near the cold tips decrease, so do the cooling power losses through the thermal conductivity of the pulse tubes. After numerical optimization, the pulse tube cooler reaches a cooling temperature of 3.87K with 207 Watts of input acoustic power.

Highly efficient inverted organic light-emitting diodes using composite organic heterojunctions as electrode-independent injectors

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode have attracted increasing attention for display applications because of their easy integration with the n-type transistors based on low-cost and highly-uniform amorphous silicon (a-Si), and transparent amorphous oxide semiconductors (TAOSs). Up to date, indium tin oxide (ITO) has been widely used as the transparent electrode, but the dogged issue of using ITO as the cathode in IOLEDs is the large energy barrier for electron injection due to its high work function. In this work, a kind of organic charge generation layer (CGL), comprising of a p-type semiconductor/bulk heterojunction (BHJ)/n-type semiconductor (p/BHJ/n), is introduced on the ITO cathode to fabricate high performance red, green and blue IOLEDs. It is found that the utilization of the composite organic heterojunction CGL as an electron injector greatly enhanced the electron injection, thus significantly improving the electroluminescence efficiency of the resulting IOLEDs. More importantly, their performances are independent of the work function of the used cathode. It is experimentally demonstrated that the electrons injected into the emitting layer are from the generated charges in composite organic heterojunction CGLs, which are completely different in the injection manner from electrodes in conventional OLEDs. It is believed that our studies provide a promising method to fabricate high performance IOLEDs regardless of the choice of electrodes, which will benefit to integrate IOLEDs on the n-type TFTs.

Managing excitons for high performance hybrid white organic light-emitting diodes by using a simple planar heterojunction interlayer

High performance hybrid white organic light-emitting diodes (WOLEDs) were fabricated by inserting a planar heterojunction interlayer between the fluorescent and phosphorescent emitting layers (EMLs ...

High performance hybrid tandem white organic light-emitting diodes by using a novel intermediate connector

We propose a novel intermediate connector for fluorescence/phosphorescence hybrid tandem white organic light emitting diodes (WOLEDs). The core of our concept is to insert a thin layer of calcium (Ca) between lithium 8-hydroxyquinolinolate (Liq) and 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) to construct the intermediate connector. The resultant hybrid tandem WOLEDs exhibit very high device performance. For the color-complementary devices, the maximum current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) reach 106.3 cd A−1, 51.4 lm W−1 and 39.6%, respectively, without any out-coupling techniques. At the luminance intensity of 1000 cd m−2, the efficiencies can still be maintained at 102.8 cd A−1, 46.9 lm W−1 and 38.8%. We also obtain a color rendering index (CRI) as high as 93 in a three-primary-color device. Moreover, both the complementary and three-primary-color devices possess excellent stability for the luminance increased from 1000 cd m−2 to 10 000 cd m−2.