Sign Inversion of Magnetoconductance in Organic Semiconductors by Different Spin-Mixing Channels at Charge-Transfer Interfaces

Organic semiconductors have shown obvious spin-related magnetoconductance (MC) even without the involvement of any magnetic elements, bringing an emerging research field called as organic spintronics. Tuning the MC sign is crucial to realize practical application based on these spin effects. Herein, we report the manipulation of MC signs in organic photodiode based on the ground-state and excited-state charge–transfer (CT) interfaces at room temperature. Different to the traditional CT interfaces that normally show positive MC, the ground-state CT (GSCT) interface presents negative MC due to the dominant spin-mixing channel from weakly bound polaron-pairs states, in which a main reverse intersystem crossing from the triplet to singlet states will lead to the opposite signs of magnetic field dependence. By adjusting the ground-state or excited-state CT process in one tandem device under electric and optical excitations, the device could reveal a low-field controlled current inverter. Our work shows the important role of organic CT states in the manipulation of the spin-related magneto-optoelectronic properties in organic semiconductors

Room-Temperature Sol–Gel Derived Molybdenum Oxide Thin Films for Efficient and Stable Solution-Processed Organic Light-Emitting Diodes

Molybdenum oxide (MoO3) thin films were prepared by sol–gel methods at room temperature from the precursors of MoO3 powder mixing into NH3 or H2O2 solution and then directly treated by UV-ozone instead of widely used high-temperature annealing. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) characteristics demonstrated that the room-temperature sol–gel derived MoO3 thin films exhibited excellent uniformity, unchanged chemical structure, and high work function. For the first time, the novel solution-processed MoO3 thin films were successfully applied as the hole injection layers (HILs) for solution-processed organic light-emitting diodes (OLEDs). The efficiencies of the resulting OLEDs were comparable or even higher than that of the device using PEDOT:PSS as the HIL. More importantly, the lifetimes of the solution-processed OLEDs are improved by nearly 2 orders of magnitude. This study should provide a potential approach to develop low-cost, high-performance, and long-lifetime OLEDs for practical applications

Efficient Non-doped Near Infrared Organic Light-Emitting Devices Based on Fluorophores with Aggregation-Induced Emission Enhancement

A family of donor–acceptor–donor (D–A–D) type near-infrared (NIR) fluorophores containing rigid nonplanar conjugated tetraphenylethene (TPE) moieties was designed and synthesized through Stille coupling reactions with electron-deficient [1,2,5]­thiadiazolo­[3,4-g]­quinoxaline (QTD) or benzo­[1,2-c;4,5-c′]­bis­[1,2,5]­thiadiazole (BBTD) as acceptors. The absorption, fluorescence, and electrochemical properties were studied. These compounds exhibited good aggregation-induced emission enhancement (AIEE) property, as a result of the twisted TPE units, which restrict the intramolecular rotation and reduce the π–π stacking. Photoluminescence of these chromophores ranges from 600 to 1100 nm, and their HOMO–LUMO gaps are between 1.85 and 1.50 eV. Non-doped organic light-emitting diodes (OLEDs) based on these fluorophores were made and exhibited EL emission spectra peaking from 706 to 864 nm. The external quantum efficiency (EQE) of these devices ranged from 0.89% to 0.20% and remained fairly constant over a range of current density of 100–300 mA cm^–2. The device with the highest solid fluorescence efficiency emitter <b>1a</b> shows the best performance with a maximum radiance of 2917 mW Sr^–1 m^–2 and EQE of 0.89%. A contrast between nondoped and doped OLEDs with these materials confirms that AIEE compounds are suitable for fabricate efficient nondoped NIR OLEDs

Photocontrolled All-Organic Magnetoelectric Switch Based on Two Types of Charge Transfer Complexes at Room Temperature

Integrating all optical, electrical, and magnetic properties into a single device is still difficult in the construction of multifunctional spintronic devices. Herein, by combining two kinds of charge transfer (CT) complexes in a single organic device, a polarity-adjustable room-temperature magnetoconductance (MC) has been achieved at a fixed bias voltage. Specifically, the interfacial ground-state charge transfer (GSCT) complex and the excited-state charge transfer (ESCT) complex exhibit different MC profiles and hence together generate an optical-incentive MC without any ferromagnetic electrodes. The simulation of the MC profiles suggests that the GSCT-induced hyperfine interaction (HFI) dominates in the negative MC in the dark, while the ESCT-induced Δg mechanism dominates in the positive MC with illumination. Furthermore, an ultrahigh-amplitude MC (over 10 000%) is obtained near the turn-on voltage at room temperature. More importantly, the MC responses can be controlled and manipulated by magnetic field, applied electric field, and variable intensity and wavelength of light excitation. Finally, the universality of the design principle in devices is demonstrated by the change of the series of materials, in which the electron and hole transport layers are replaced by other corresponding materials of the same type, respectively. This work offers some clues for realizing pure organic multifunctional devices coupling light-electricity-magnetism in the future

An Ultrafast Organic Photodetector with Low Dark Current for Optical Communication Systems

Photodetectors are widely applied in imaging, communication, medical detection, and many other fields. Organic photodetectors (OPDs) have attracted extensive attention due to their inherent mechanical flexibility, low cost, good light absorption, and ease of processing. However, the low response speed of OPDs has always been a major problem, which hinders their application in the field of fast optical communication. In addition, fabricating high-speed OPDs again has the problems of large dark current and complex structure. Here, a high-speed OPD based on a simple planar heterojunction with a bandwidth of 107 MHz is achieved. More significantly, the dark current of the device is effectively suppressed by electrode injection control. Moreover, this nanosecond OPD is successfully applied to optical communication with a transmission rate up to 40 Mbit/s

Tuning the Photophysical Properties and Energy Levels by Linking Spacer and Topology between the Benzimidazole and Carbazole Units: Bipolar Host for Highly Efficient Phosphorescent OLEDs

A new series of benzimidazole/carbazole hybrids with different linking spacers or linking topologies between the benzimidazole and carbazole moieties were facilely prepared, and their thermal, photophysical, and electrochemical properties were investigated. With the incorporation of rigid benzimidazole moiety, these compounds possess excellent thermal stability with high glass-transition temperatures (Tg) of 137−186 °C and the thermal-decomposition temperatures (Td) of 479−544 °C. 2 and 3 with the m-terphenyl unit as the linking spacer between the carbazole and the benzimidazole moieties exhibit significant blue shifts as compared to 1 and 4 with the phenyl unit because the longer linking spacer alleviate intramolecular charge transfer. Their HOMO and LUMO energy levels vary in the range of 5.50−5.63 and 2.02−2.35 eV, respectively. Devices employing the new compounds as the host for the green emitter of Ir(ppy)3 were fabricated with the configurations of ITO/MoO3 (10 nm)/NPB (80 nm)/Host: 9 wt % Ir(ppy)3 (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al(100 nm). Their EL efficiencies follow the order of 3 > 2 > 1 ≈ 4, which correlates with their triplet energy and the separation of HOMO and LUMO distributions at hole- and electron-transporting moieties. A maximum current efficiency of 70.2 cd A−1 and a maximum power efficiency of 73.4 lm W1− were achieved when 3 was used as the host. A facile strategy to manipulate the spatial distribution of energy levels and triplet energy of hosts by changing linking spacers or linking topologies is demonstrated

Highly Efficient Deep Blue Aggregation-Induced Emission Organic Molecule: A Promising Multifunctional Electroluminescence Material for Blue/Green/Orange/Red/White OLEDs with Superior Efficiency and Low Roll-Off

For the constant demand of organic light-emitting diodes (OLEDs) with high efficiency, long lifetime, and low cost for display and lighting applications, the development of high-performance organic electroluminescence materials is key. Aggregation-induced emission (AIE) luminogens (AIEgens) provide a promising choice for their excellent performance in nondoped devices. Here we report a multifunctional deep blue AIE material, which can be used not only as an excellent blue emitter but also as a good host of green/orange/red phosphors. A deep blue nondoped OLED with a CIEy of 0.08 and high external quantum efficiency (EQE) of 7.0% is achieved. Furthermore, a series of green/orange/red phosphorescent OLEDs with high efficiency and low roll-off are obtained. Impressively, hybrid white OLEDs (WOLEDs) based on the deep blue AIEgen exhibit simultaneously high CRI (>90), excellent efficiency (EQEmax> 25%, PEmax = 99.9 lm W–1 for two-color WOLEDs, PEmax = 60.7 lm W–1 for four-color WOLEDs), low roll-off (PE1000nit = 72.1 lm W–1 for two-color WOLEDs, PE1000nit = 43.5 lm W–1 for four-color WOLEDs), and superior stable color, indicative of the multifunction of AIEgens. Accordingly, this work opens a new direction for achieving high-performance OLEDs, particularly offering a smart but simple way to depress the efficiency roll-off and reduce the cost of OLEDs for practical applications

Novel Thermally Stable Blue-Light-Emitting Polymer Containing N,N,N‘,N‘-Tetraphenyl−Phenylenediamine Units and Its Intramolecular Energy Transfer

Novel Thermally Stable Blue-Light-Emitting Polymer Containing N,N,N‘,N‘-Tetraphenyl−Phenylenediamine Units and Its Intramolecular Energy Transfe

Upper Excited Triplet State-Mediated Intersystem Crossing for Anti-Kasha’s Fluorescence: Potential Application in Deep-Ultraviolet Sensing

Owing to the Kasha rule, only the lowest excited state S1 contributes to the photoemission or other photoinduced processes in general, causing a waste of photoenergy and the limitation of application scenarios. The anti-Kasha effect offers the possibility of utilizing high-energy excited states Sn to develop novel functions and applications. Here, an anti-Kasha fluorescence has been experimentally found in a pure organic molecule and investigated in detail to reveal the underlying mechanism. The experimental lines of evidence of the excitation mapping spectrum and time-resolved photoluminescence spectrum suggest that the anti-Kasha emission is governed by an upper excited triplet state Tn. Intersystem crossing (ISC) from S5 to Tn can successfully compete with internal conversion, followed by reverse ISC from Tn to S2 to form upper-state emission. Further, utilizing this effect for the deep-ultraviolet light sensor is discussed. These findings provide keen insights into manipulating the excited-state evolution, while offering the possibility for utilizing the high-energy excited states to explore novel functions and applications of organic molecules

Supramolecular Architectures, Photophysics, and Electroluminescence of 1,3,4-Oxadiazole-Based Iridium(III) Complexes: From μ-Dichloro Bridged Dimer to Mononuclear Complexes

One μ-dichloro bridged diiridium complex and three mononuclear iridium(III) complexes based on the 1,3,4-oxadiazole derivatives as cyclometalated ligands and acetylacetonate (acac) or dithiolates O,O‘-diethyldithiophosphate (Et2dtp) or N,N‘-diethyldithiocarbamate (Et2dtc) as ancillary ligands have been synthesized and systematically studied by X-ray diffraction analysis. The results reveal that three mononuclear complexes all adopt distorted octahedral coordination geometry around the iridium center by two chelating ligands with cis-C−C and trans-N−N dispositions, which have the same coordination mode as the diiridium dimer. The dinuclear complex crystallizes in the monoclinic system and space group C2/c, whereas three mononuclear iridium complexes are all triclinic system and space group P1̄. In the stacking structure of the dimer, one-dimensional tape-like chains along the b-axis are formed by hydrogen bondings, which are strengthened by π stacking interactions between phenyl rings of 1,3,4-oxadiazole ligands. Then these chains assemble a three-dimensional alternating peak and valley fused wave-shape structure. In each stacking structure of three mononuclear complexes, two molecules form a dimer by the C−H···O hydrogen bondings, and these dimers are connected by π stacking interactions along the b-axis, constructing a zigzag chain. Then these zigzag chains are interacted by π stacking along the a-axis, building up a two-dimensional structure. All complexes emit green with emission wavelengths in the range of 501−535 nm, depending on the structures of cyclometalated ligands and ancillary ligands. Electroluminescent devices using complexes 2−4 as phosphorescent dopants have been fabricated. A high-efficiency green emission device with a maximum luminous efficiency of 5.26 cd/A at a current density of 1.38 mA/cm2 and a maximum brightness of 2594 cd/m2 at 15.5 V has been achieved using 2 as the emitter