Yanqin Miao

Two novel indolo[3,2-b]carbazole derivatives containing dimesitylboron moieties: synthesis, photoluminescent and electroluminescent properties

Two novel indolo[3,2-b]carbazole derivatives, 5,11-di(4′-dimesitylboronphenyl)indolo[3,2-b]carbazole (DDBICZ) and 2,8-dimesitylboron-5,11-di(4′-dimesitylboronphenyl)indolo[3,2-b]carbazole (DDDBICZ), were synthesized by introducing two dimesitylboron groups and/or four dimesitylboron groups (as electron-acceptors) into the indolo[3,2-b]carbazole moiety (as an electron-donor). The structures of these two compounds were fully characterized by elemental analysis, mass spectrometry and proton nuclear magnetic resonance spectroscopy methods. Their thermal properties were studied by thermogravimetric analysis and differential scanning calorimetry. Their electrochemical and photophysical properties were studied by electrochemical methods, UV-vis absorption spectroscopy and fluorescence spectroscopy. The charge-transporting properties of DDBICZ and DDDBICZ were studied by fabricating single carrier devices using them as charge-transporting layers. The results reveal that DDBICZ and DDDBICZ have high thermal stability (the decomposition temperature of DDBICZ = 201 °C and the decomposition temperature of DDDBICZ = 210 °C) and good electrochemical and electron-transporting properties. Moreover, in order to examine the electroluminescent properties of DDBICZ and DDDBICZ, Device A and Device B were fabricated by using them as light-emitting layers, respectively. The turn-on voltage, maximum luminance and maximum luminance efficiency of Device A are 6.1 V, 5634 cd m−2 and 2.96 cd A−1, whereas those of Device B are 3.6 V, 1363 cd m−2 and 2.88 cd A−1.

Modelling of piezoresistive response of carbon nanotube network based films under in-plane straining by percolation theory

Carbon nanotube (CNT) networks used for strain detection are one of the encouraging findings in the application of sensors. Recently, our group has discovered that in-plane strain sensors built from CNT films without surfactants exhibit a high piezoresistive response in CNT networks. In this paper, a study on modeling of piezoresistive response of CNT network based films without surfactants by percolation theory for in-plane strain detection is presented. In particular, within a conductive region, a model was built to predict piezoresistive response of CNT networks and the model predicted result shows an agreement with the experimental result.

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.

Efficient tandem organic light-emitting device based on photovoltaic-type connector with positive cycle

We designed a tandem organic light-emitting device based on an organic photovoltaic-type charge generation connector (CGC) of fullerene carbon 60/copper(II) phthalocyanine. The CGC can absorb a portion of photons radiated from emission zone and form excitons which disassociated into free charges at PN junction interface without energy barrier, leading to low driving voltage and better charge balance. The efficiency increases remarkably with increasing current density, even beyond two folds compared with single unit device under higher current density, meaning slower roll-off. The whole process is a positive cycle, and actually enhances the utilization of internal radiation and the overall performance of tandem device.

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.

Optimal nitrogen and phosphorus codoping carbon dots towards white light-emitting device

Through a one-step fast microwave-assisted approach, nitrogen and phosphorus co-doped carbon dots (N,P-CDs) were synthesized using ammonium citrate (AC) as a carbon source and phosphates as additive reagent. Under the condition of an optimal reaction time of 140 s, the influence of additive with different N and P content on fluorescent performance of N,P-CDs was further explored. It was concluded that high nitrogen content and moderate phosphorus content are necessary for obtaining high quantum yield (QY) N,P-CDs, among which the TAP-CDs (CDs synthesized using ammonium phosphate as additive reagent) show high quantum yield (QY) of 62% and red-green-blue (RGB) spectral composition of 51.67%. Besides, the TAP-CDs exhibit satisfying thermal stability within 180 °C. By virtue of good optical and thermal properties of TAP-CDs, a white light-emitting device (LED) was fabricated by combining ultraviolet chip with TAP-CDs as phosphor. The white LED emits bright warm-white light with the CIE chromaticity coordinat...

Influence of aligned carbon nanotube networks on piezoresistive response in carbon nanotube films under in-plane straining

The use of carbon nanotube (CNT) films for strain detection is one of the most encouraging findings in the field of sensors. Our previous studies have shown that the density of the CNT networks (randomly distributed) plays an important role in governing the piezoresistive response in CNT films under the in-plane straining. The influence of aligned CNT networks on the piezoresistive response is presented. In particular, the CNTs are first decorated with iron oxide nanoparticles (to enhance magnetism), and then the decorated CNTs are used to form aligned CNT networks in the films with the aid of a magnetic field. The films are loaded in the three points bending tester, and their piezoresistive response is investigated under a cycled in-plane straining. The results show that the CNT networks aligned at 0° to electrodes and 90° to electrodes exhibit the highest and the lowest piezoresistive sensitivity, respectively, but the poor repeatability (i.e., severe resistant reduction −6% to −27%) is also found in th...

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.

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.