Two Novel Neutral Cyclometalated Iridium(III) Complexes Based on 10,11,12,13-Tetrahydrodibenzo[a,c]phenazine for Efficient Red Electroluminescence.

Two novel neutral phosphorescent iridium(III) complexes (Ir1 and Ir2) were rationally designed and synthesized with high yields using 10,11,12,13-tetrahydrodibenzo[a,c]phenazine as the main ligand. The two complexes showed bright-red phosphorescence (625 nm for Ir1, and 620 nm for Ir2, in CH2Cl2), high-luminescence quantum efficiency (0.32 for Ir1, and 0.35 for Ir2), obvious solvatochromism and good thermostability. Then, they were used to fabricate high-efficiency red OLEDs via vacuum evaporation; the maximum current efficiency, power efficiency, and external quantum efficiency of the red devices based on Ir1 and Ir2 are 13.47/15.22 cd/A, 10.35/12.26 lm/W, and 10.08/7.48%, respectively.

Carbon nanotube-embedded hollow carbon nanofibers as efficient hosts for advanced lithium-sulfur batteries.

Lithium-sulfur (Li-S) batteries have attracted great research attention because of their high energy density and low cost. However, shuttling effects of polysulfides and insulation from elemental sulfur hinder their practical application. Herein, we report hollow carbon nanofibers filled with carbon nanotubes (denoted as HCNF/CNT) as host materials for sulfur to mitigate the shuttling behavior and improve the kinetics of insulative sulfur. The as-prepared HCNF/CNT with nano-conductive domains in the hollow carbon nanofibers enables high loading and efficient utilization of sulfur. Owing to their unique structural superiority, the sulfur-encapsulated HCNF/CNT cathode materials for Li-S batteries deliver excellent electrochemical performance, including high specific capacity of 1156 mA h g-1 at 0.2 C, good rate performance and cycling stability with a capacity retention of 77.2% after 200 cycles at 2 C. Such a unique structure can provide inspiration for the rational structural design of carbon materials as hosts for high performance Li-S batteries.

Robust singlet fission process in strong absorption π-expanded diketopyrrolopyrroles.

Singlet fission (SF) has drawn tremendous attention as a multiexciton generation process that could mitigate the thermal loss and boost the efficiency of solar energy conversion. Although a SF-based solar cell with an EQE above 100% has already been fabricated successfully, the practical efficiency of the corresponding devices is plagued by the limited scope of SF materials. Therefore, it is of great importance to design and develop new SF-capable compounds aiming at practical device application. In the current contribution, via a π-expanded strategy, we presented a new series of robust SF chromophores based on polycyclic DPP derivatives, Ex-DPPs. Compared to conventional DPP molecules, Ex-DPPs feature strong absorption with a fivefold extinction coefficient, good molecular rigidity to effectively restrain non-radiative deactivation, and an expanded π-skeleton which endow them with well-suited intermolecular packing geometries for achieving efficient SF process. These results not only provide a new type of high-efficiency SF chromophore but also address some basic guidelines for the design of potential SF materials targeting practical light harvesting applications.

Solution-Processed Yellow Organic Light-Emitting Diodes Based on Two New Ionic Ir (III) Complexes.

Two new and efficient cationic yellow-emissive Ir (III) complexes (Ir1 and Ir2) are rationally designed by using 2-(4-chloro-3-(trifluoromethyl)phenyl)-4-methylquinoline as the main ligand, and, respectively, 4,4'-dimethyl-2,2'-bipyridyl and 4,4'-dimethoxy-2,2'-bipyridyl as the ancillary ligands. Both complexes show enhanced phosphorescence (546 nm with 572 nm as shoulder and high phosphorescent quantum efficiency in solution, which is in favor of efficient solution-processed phosphorescent organic light-emitting diodes. Compared with Ir2, the Ir1-based device displays excellent device performance, with maximum external quantum efficiency, current efficiency, and power efficiency of up to 7.92%, 26.32 cd/A and 15.31 lm/W, respectively, thus proving that the two new ionic Ir (III) complexes exhibit great potential for future solution-processed electroluminescence.

Recent progress on organic light-emitting diodes with phosphorescent ultrathin (<1nm) light-emitting layers.

In recent years, phosphorescent dyes forming ultrathin light-emitting layers (<1 nm, UEMLs) have been widely applied to fabricate monochromatic and white organic light-emitting diodes (OLEDs) owing to its merits of simplified device structure and preparation process, more flexible design, lower material consumption, and complete exciton utilization. In addition, it was demonstrated that the OLEDs with UEMLs achieved high electroluminescence performance comparable to the conventional doping-based devices. Structurally, OLEDs were structured with phosphorescent UEMLs inserted into nonluminous materials, heterojunction interface as well as into luminescent materials including phosphorescent, conventional fluorescent, thermally activated delayed fluorescence, and exciplex emitters. We carefully reviewed the successful applications of UEMLs in OLEDs and underlying working mechanism of corresponding devices, and also emphasized the representative achievements about OLEDs with UEMLs, aimed at forming a comprehensive summary of the present research for UEMLs-based OLEDs. In the end, we also gave an outlook for the future development of UEMLs-based OLEDs.

Improved electrical ideality and photoresponse in near-infrared phototransistors realized by bulk heterojunction channels.

The factors that affect the electrical ideality and photoresponse in near-infrared (NIR) organic phototransistors (OPTs) are still nebulous. Here, simultaneous increase in electrical ideality and NIR response in the OPTs is realized by applying a bulk heterojunction (BHJ) channel. The acceptor in the channel helps to trap the undesirable injected electrons, avoiding the accumulation of the electrons at the active channel/dielectric interface, and thereby improving the hole transporting. Use of a BHJ channel also helps reducing the contact resistance in the OPTs. The electrical stability is then improved with mitigated dependence of charge mobility on gate voltage in the saturation region. The BHJ channel also offers an improved photoresponse through enhanced exciton dissociation, leading to more than one order of magnitude increase in responsivity than that in a control OPT. The results are encouraging, which pave the way for the development of high-performing NIR OPTs.

A Low-Temperature Solution-Processed CuSCN/Polymer Hole Transporting Layer Enables High Efficiency for Organic Solar Cells.

The hole transporting layers (HTLs) between the electrode and light absorber play a vital role in charge extraction and transport processes in organic solar cells (OSCs). Herein, a bilayer structure HTL of CuSCN/TFB is formed by soluble copper(I) thiocyanate (CuSCN) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl)))] (TFB). The excellent charge extraction capability is proved in nonfullerene PM6:Y6 and fullerene PTB7-Th:PC71BM blend system-based cells. The introduction of TFB tunes the work function and polishes the interfacial contact between the HTL and light absorber, which favors the hole extraction process in cells. Meanwhile, lower recombination loss, higher exciton dissociation probability, and larger domain size are observed in CuSCN/TFB HTL-based cells compared to those of the reference cell with the pristine CuSCN HTL, which significantly improve the photovoltaic performance. As a result, a champion efficiency of 15.10% is obtained, which is >14% higher than the efficiency of 13.15% obtained in the reference cell. This study suggests that CuSCN/TFB is a promising HTL to achieve high efficiency for OSCs.

Low-temperature direct synthesis of perovskite nanocrystals in water and their application in light-emitting diodes.

Cesium lead halide perovskite nanocrystals (PNCs) have aroused tremendous research attention because of their excellent optoelectronic properties. Herein, we developed a facile and green low-temperature strategy free of organic solvents, in which only pure water was adopted as the solvent, to synthesize CsPbBr3 NCs. Intriguingly, although formed with the assistance of water, the obtained CsPbBr3 NCs present a cubic crystal structure, photoluminescence quantum yield (PLQY) of 75%, and narrow emission line width for bright green emission. Furthermore, both electroluminescence (EL) and photoluminescence (PL)-based light-emitting diodes (LEDs) present intrinsic green emission originating from the as-prepared CsPbBr3 NCs. Hence, this work offered a novel eco-friendly avenue for the preparation of perovskite NCs for their practical applications in LEDs.

Synthesis and properties of hyperbranched polymers for white polymer light-emitting diodes.

In this work, a series of hyperbranched copolymers with fluorene-alt-carbazole as the branches, three-dimensional-structured spiro[3.3]heptane-2,6-dispirofluorene (SDF) as the core, and iridium 1-(4-bromophenyl)-isoquinoline (acetylacetone) (Ir(Brpiq)2acac) as the dimming group were synthesized by one-pot Suzuki polycondensation for white emission. All copolymers show great thermal stabilities and high hole-transporting ability due to the introduction of the carbazole unit. The hyperbranched structures for copolymers can suppress the interchain interactions efficiently, and help to form amorphous films. The fabricated polymer light-emitting devices (PLEDs) based on the above synthesized copolymers realize good white light emission, and achieve high electroluminescence (EL) performance. For example, for the optimized PLED, the maximum luminance and current efficiency reach 6210 cd m-2 and 6.30 cd A-1, respectively, indicating the synthesized hyperbranched copolymers have potential application in solution-processable white polymer light-emitting diodes.

High-Performance Organic Electroluminescence: Design from Organic Light-Emitting Materials to Devices.

Organic electroluminescence is considered as the most competitive alternative for the future solid-state displays and lighting techniques owing to many advantages such as self-luminescence, high efficiency, high contrast, high color rendering index, ultra-thin thickness, transparency, flat and flexibility, etc. The development of high-performance organic electroluminescence has become the continuing focus of research. In this personal account, a brief overview of representative achievements in our study on the design of highly efficient novel organic light-emitting materials (including fluorescent materials, phosphorescent iridium(III) complexes and conjugated polymers bearing phosphorescent iridium(III) complex) and high-performance device structures together with working principles are given. At last, we will give some perspectives on this fascinating field, and also try to provide some potential directions of research on the basis of the current stage of organic electroluminescence.

Carbon dot-based white and yellow electroluminescent light emitting diodes with a record-breaking brightness.

First, oleophilic carbon dots (CDs) with a fluorescence quantum yield (QY) of 41% were synthesized by a one-pot microwave-assisted carbonization method. Then, CD-based electroluminescent light emitting diodes (CD-LEDs) were prepared. The impact of CD aggregation on the brightness of CD-LEDs was studied. The results show that, to some extent, with the decrease of the aggregation of the CD film, the luminescence quenching of the CD-LEDs gradually decreased, and the luminance of the CD-LEDs gradually increased. Hence, in order to improve the dispersion of CDs and reduce the aggregation of CDs and the luminescence quenching of the devices, host-guest doping was adopted to effectively improve the brightness of CD-LEDs. In this work, the yellow emission of the doped devices is mainly derived from the direct carrier trapping on CDs. Moreover, white and yellow CD-LEDs were obtained from the same oleophilic CDs by tuning the structure of the devices. The white CD-LEDs exhibit a high color rendering index (CRI) of 83 with a luminance of 455.2 cd m-2. The yellow CD-LEDs show the maximum brightness of 339.5 cd m-2 and excellent color stability. The results show that the luminescence quenching of CD-LEDs was resisted and the brightness of CD-LEDs was improved by using host-guest doping.

Solution-Processable ZnO/Carbon Quantum Dots Electron Extraction Layer for Highly Efficient Polymer Solar Cells.

In this work, we report the effort to develop high-efficiency inverted polymer solar cells (PSCs) by applying a solution-processable bilayer ZnO/carbon quantum dots (C-QDs) electron extraction layer (EEL). It is shown that the use of the bilayer EEL helps to suppress the exciton quenching by passivating the ZnO surface defects in the EEL, leading to an enhanced exciton dissociation, reduced charge recombination and more efficient charge extraction probability, and thereby achieving high power conversion efficiency (PCE). The inverted PSCs, based on the blend of poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} and [6,6]-phenyl C71-butyric acid methyl ester, possess a significant improvement in PCE of ∼9.64%, which is >27% higher than that of a control cell (∼7.59%). The use of a bilayer ZnO/C-QD EEL offers a promising approach for attaining high-efficiency inverted PSCs.

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 l'Eclairage (CIE) coordinates of (0.25, 0.23) showed white light was achieved from the HPF-Ir10 devices.

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.