Cong Fan

Simple Bipolar Molecules Constructed from Biphenyl Moieties as Host Materials for Deep‐Blue Phosphorescent Organic Light‐Emitting Diodes

Simple is good! Based on biphenyl molecules, two bipolar host materials with high triplet energies have been rationally designed, synthesized, and fully characterized. Deep blue phosphorescent organic light-emitting diodes, which employ the new hosts and an iridium(III) complex as triplet emitter, show a maximum current efficiency of 40 cd A(-1), a maximum power efficiency of 36 lm W(-1), and a maximum external quantum efficiency of 19.5 %.

Tetraphenylsilane derivatives spiro-annulated by triphenylamine/carbazole with enhanced HOMO energy levels and glass transition temperatures without lowering triplet energy: host materials for efficient blue phosphorescent OLEDs

Two new host materials, SiBSTPA and SiBSCz, were designed and synthesized based on 4,4′-bis(triphenylsilyl)-biphenyl (BSB). Their thermal, electrochemical, electronic absorption and photoluminescent properties were fully investigated. The introduction of spiro-annulated triphenylamine/carbazole moieties on 4,4′-bis(triphenylsilyl)-biphenyl (BSB) increases the HOMO energy levels from −6.49 eV (BSB) to −5.30 eV for SiBSTPA and −5.56 eV for SiBSCz, and accordingly facilitates hole injection from the nearby hole-transporting layer. Compared to 4,4′-bis(triphenylsilyl)-biphenyl (BSB), higher glass transition temperatures (Tg) were observed at 133 °C for SiBSTPA and 129 °C for SiBSCz, owing to the rigid spiro-annulated structures. Meanwhile, the perpendicular conformation between the triphenylamine or carbazole plane and the biphenyl plane effectively prevents the extension of the π-conjugation and consequently causes no depreciation of their triplet energies (ca. 2.75 eV). Phosphorescent organic light-emitting devices (PhOLEDs) with the following configuration: ITO/NPB/TCTA/EML/TAZ/LiF/Al were fabricated by using the two host materials and the blue emitter bis[2-(4′,6′-difluorophenyl)pyridinato-N,C2′]iridium(III) picolate (FIrpic) as the guest. These devices exhibited good performance with the maximum current efficiency of 21.4 cd A−1 and the maximum power efficiency of 15.6 lm W−1.

Extremely Accessible Potassium Nitrate (KNO3) as the Highly Efficient Electrolyte Additive in Lithium Battery.

The systematic investigation of RNO3 salts (R = Li, Na, K, and Cs) as electrolyte additives was carried out for lithium-battery systems. For the first time, the abundant and extremely available KNO3 was proved to be an excellent alternative of LiNO3 for suppression of the lithium dendrites. The reason was ascribed to the possible synergetic effect of K(+) and NO3(-) ions: The positively charged K(+) ion could surround the lithium dendrites by electrostatic attraction and then delay their further growth, while simultaneously the oxidative NO3(-) ion could be reduced and subsequently profitable to the reinforcement of the solid-electrolyte interphase (SEI). By adding KNO3 into the practical Li-S battery, the discharging capacity was enhanced to average 687 mAh g(-1) from the case without KNO3 (528 mAh g(-1)) during 100 cycles, which was comparable to the one with the well-known LiNO3 additive (637 mAh g(-1)) under the same conditions.

Random terpolymer with a cost-effective monomer and comparable efficiency to PTB7-Th for bulk-heterojunction polymer solar cells

A new random terpolymer PTB7-Th-T2 has been designed and synthesized by incorporating a significantly lower cost monomer, 2,2′-bithiophen, for application as a donor material in polymer solar cells (PSCs). By replacing 25 mol% of extremely expensive 3-fluorothieno[3,4-b]thiophene-2-carboxylate monomer in the famous PTB7-Th by a >30-times lower cost bithiophene, the new terpolymer PTB7-Th-T2 shows a comparable HOMO energy level and increased absorption intensity in the wide wavelength range of 400–700 nm. In addition, in a polymer/PC71BM blended film, the hole mobility has improved from 1.67 × 10−4 for PTB7-Th to 2.49 × 10−4 cm2 V−1 s−1 for PTB7-Th-T2. A power conversion efficiency (PCE) of 7.05% has been achieved in a polymer bulk heterojunction photovoltaic device with the structure of ITO/PEDOT:PSS/PTB7-Th-T2:PC71BM (1:1.5, w/w)/Ca/Al by using 3% of 1,8-diiodooctane (DIO) as a solvent additive. Furthermore, through introducing an amino-substituted perylene diimide (PDIN) as the cathode interlayer, a high fill factor (FF) of 67.38% and PCE of 8.19% have been obtained; these values are higher than those of the control polymer PTB7-Th of 63.26% and 7.93%, respectively at the same device conditions.

Pretreatment of Lithium Surface by Using Iodic Acid (HIO3) To Improve Its Anode Performance in Lithium Batteries

Iodic acid (HIO3) was exploited as the effective source to build an artificial solid-electrolyte interphase (SEI) on the surface of Li anode. On one hand, HIO3 is a weak solid-state acid and can be easily handled to remove most ion-insulating residues like Li2CO3 and/or LiOH from the pristine Li surface; on the other hand, both the products of LiI and LiIO3 resulted from the chemical reactions between Li metal and HIO3 are reported to be the ion-conductive components. As a result, the lower voltage polarization and impedance, longer cycling lifetime and higher Coulombic efficiency have been successfully achieved in the HIO3-treated Li-Li and Li-Cu cells. By further using the HIO3-treated Li anode into practical Li-S batteries, the impressive results also have been obtained, with average discharge capacities of 719 mAh g-1 for 200 cycles (0.2 C) and 506 mAh g-1 for 500 cycles (0.5 C), which were better than the Li-S batteries using the pristine Li anode (552 and 401 mAh g-1, respectively) under the same conditions.

Efficient Pt(II) emitters assembled from neutral bipyridine and dianionic bipyrazolate: designs, photophysical characterization and the fabrication of non-doped OLEDs

Potential dianionic chelates, 5,5′-bis(trifluoromethyl)-2H,2′H-3,3′-bipyrazole (bipzH2) and 5,5′-(1-methylethylidene)-bis(3-trifluoromethyl-1H-pyrazole) (mepzH2), were synthesized from Claisen condensation employing ethyl trifluoroacetate with 2,3-butanedione and with 3,3-dimethyl-2,4-pentanedione, followed by hydrazine cyclization. These chelates were then utilized in the preparation of four emissive Pt(II) metal complexes [Pt(tbbpy)(bipz)] (1), [Pt(msbpy)(bipz)] (2), [Pt(tbbpy)(mepz)] (3) and [Pt(msbpy)(mepz)] (4), where tbbpy and msbpy represent 4,4′-di-t-butyl-2,2′-bipyridine and 4,4′-dimesityl-2,2′-bipyridine, respectively. Single crystal X-ray structural analyses of 2 and 3 were executed to unveil the basic coordination geometry around the Pt(II) center as well as the π–π stacking interaction in the solid state. These complexes are essentially non-emissive in solution (Q. Y. = 0.2–0.4%), but are highly luminescent in the solid state with QY of 52% and 83% and τobs of 368 ns and 8.37 μs for 1 and 3, respectively. Their photophysical properties were measured and discussed on the basis of computational approaches. For applications, non-doped organic light emitting diodes (OLEDs) were fabricated using 1 and 3 as emitters, exhibiting red-orange emission with a maximum luminance of 43 000 cd m−2, an EQE of 19.0%, a CE of 21.0 cd A−1 and a PE of 15.5 lm W−1, and yellow emission with a maximum luminance of 5100 cd m−2, an EQE of 7.1%, a CE of 21.0 cd A−1 and a PE of 11.3 lm W−1, respectively. The particularly higher OLED efficiencies of 1versus3 highlight the design principle of Pt(II) based phosphors, particularly for the fabrication of a non-doped OLED architecture.

Ternary Organic Solar Cells with Coumarin7 as the Donor Exhibiting Greater Than 10% Power Conversion Efficiency and a High Fill Factor of 75%

Ternary bulk heterojunction (BHJ) is a brilliant photovoltaic technology for improving the performance of organic solar cells (OSCs), because the light absorption range can be significantly extended by using multiple donors or acceptor materials. In this paper, coumarin7 (C7), a small organic molecule typical led used in organic light-emitting diodes, was initially exploited as second electron-donor component in ternary bulk heterojunction OSCs along with conventional blend system spolythieno[3,4-b]-thiophene/benzodithiophene(PTB7) and [6,6]-phenyl-C71 -butyric acid methyl(PC71 BM). A champion PCE value of 10.28% was realized in the ternary OSCs when incorporated with 10 wt % C7 doping ratio in the donors, corresponding to about 35% enhancement compared with the PTB7:PC71BM-based OSCs, a high fill factor (FF) of 75.03%, a short-circuit currentdensity (Jsc) of 18.72 mA cm-2 and an open-circuit voltage (Voc) of 0.73 V. The enhanced performance of the ternary OSCs can be attributed to the simultaneous improvement of the FF and the Jsc. In addition to extended light absorption, a perfect nanofiber filament active layer morphology is obtained due to the good compatibility between C7 and PTB7, which facilitates the balance of charge transportation and the suppression of charge recombination. This investigation suggests that coumarin derivatives, which have completely different structure with polymer donors, can also be used to fabricate ternary solar cells and have the potential applications to obtain amazing performance after further device engineering and optimization.

Cost-effective synthesis of α-carboline/pyridine hybrid bipolar host materials with improved electron-transport ability for efficient blue phosphorescent OLEDs

Two α-carboline/pyridine hybrid bipolar host materials, 2,6-bis(9-H-pyrido[2,3-b]indol-9-yl)pyridine (2,6-CbPy) and 9,9′,9′′-(pyridine-2,4,6-triyl)tris(9-H-pyrido[2,3-b]indole) (2,4,6-CbPy) are designed and synthesized through an efficient one-step catalyst-free C–N coupling reaction. The thermal, electrochemical and photoluminescence properties of the two host materials have been fully investigated. The introduction of the α-carboline instead of carbazole in 2,6-CbPy and 2,4,6-CbPy significantly improves the electron transporting abilities, and simultaneously maintains the good hole transporting properties with 2,6-CzPy and 2,4,6-CzPy. By using them as hosts for FIrpic-doped blue phosphorescent OLEDs, a maximum current efficiency of 44.1 cd A−1 and power efficiency of 38.5 lm W−1 has been achieved for the 2,4,6-CbPy-based device. The maximum current efficiency is higher than 42.1 cd A−1 of the carbazole-containing 2,4,6-CzPy under the same device conditions.

Enhanced reversibility and electrochemical performances of mechanically alloyed Cu3P achieved by Fe addition

Cu3P is a potential anode material for lithium-ion batteries with its comparable gravimetric capacity, but several times higher volumetric capacity (4732 mA h cm−3) than graphite. However, the cycling stability of Cu3P is poor at low discharge potentials and high current densities. In this work, Fe addition is employed as a simple strategy to modulate the composition and phase constitution of Cu3P nanopowders synthesized by wet mechanical alloying, and thereby to tune the electrochemical performance of the anode. The addition of Fe results in a composite constitute containing Cu3P as the major phase and some other minor phases including Cu, α-Fe and FeP, which are combinationally determined by X-ray diffraction, energy dispersive X-ray spectroscopy and Mossbauer spectroscopy. Electrochemical tests reveal that both the cycling stability and the rate capability of the electrodes are improved by Fe addition. The Cu3P electrode with 10% Fe addition shows the best cell performance, with the capacity being remarkably improved by over 100%, from 82 mA h g−1 to 178 mA h g−1 after 50 cycles at 0.75C between 2.0 V and 0.5 V vs. Li/Li+. The improvement of the electrochemical performance is engendered by a synergetic effect of the microstructure change of the powders and the presence of Fe-related minor phases, leading to increased electronic conductivity as well as enhanced electrochemical reversibility of the electrode.

Preparation and characterization of flexible lithium iron phosphate/graphene/cellulose electrode for lithium ion batteries

In this work, a free-standing flexible composite electrode was prepared by vacuum filtration method with LiFePO4, graphene and nanofibrillated cellulose (NFC). Compared with the pure LiFePO4 electrode, the resulting flexible composite (LiFePO4/graphene/NFC) electrode showed excellent mechanical flexibility, and possessed an enhanced initial discharge capacity of 151 mA h/g (0.1 C) and a good capacity retention rate with only 5% loss after 60 cycles due to suitable electrolyte wettability at the interface. Furthermore, the NFC and graphene formed a three-dimensional conductive framework, which provided high-speed electron conduction in the composite and reduced electrode polarization during charging-discharging processes. Moreover, the composite electrode could endure bending tests up to 1000 times, highlighting preferable mechanical strength and durability. These results demonstrated that the as-fabricated electrodes could be applied as flexible electrodes with an embedded power supply.