Light‐Emitting Electrochemical Cells Based on Color‐Tunable Inorganic Colloidal Quantum Dots

Large‐area, ultrathin light‐emitting devices currently inspire architects and interior and automotive designers all over the world. Light‐emitting electrochemical cells (LECs) and quantum dot light‐emitting diodes (QD‐LEDs) belong to the most promising next‐generation device concepts for future flexible and large‐area lighting technologies. Both concepts incorporate solution‐based fabrication techniques, which makes them attractive for low cost applications based on, for example, roll‐to‐roll fabrication or inkjet printing. However, both concepts have unique benefits that justify their appeal. LECs comprise ionic species in the active layer, which leads to the omission of additional organic charge injection and transport layers and reactive cathode materials, thus LECs impress with their simple device architecture. QD‐LEDs impress with purity and opulence of available colors: colloidal quantum dots (QDs) are semiconducting nanocrystals that show high yield light emission, which can be easily tuned over the whole visible spectrum by material composition and size. Emerging technologies that unite the potential of both concepts (LEC and QD‐LED) are covered, either by extending a typical LEC architecture with additional QDs, or by replacing the entire organic LEC emitter with QDs or perovskite nanocrystals, still keeping the easy LEC setup featured by the incorporation of mobile ions.     

Efficient Light‐Emitting Electrochemical Cells (LECs) Based on Ionic Iridium(III) Complexes with 1,3,4‐Oxadiazole Ligands

Eight new iridium(III) complexes 1‐8 , with 1,3,4‐oxadiazole (OXD) derivatives as the cyclometalated C^N ligand and/or the ancillary N^N ligands are synthesized and their electrochemical, photophysical, and solid‐state light‐emitting electrochemical cell (LEC) properties are investigated. Complexes 1 , 2 , 7 and 8 are additionally characterized by single crystal X‐ray diffraction. LECs based on complexes 1‐8 are fabricated with a structure indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/cationic iridium complex:ionic liquid/Al. LECs of complexes 1 – 6 with OXD derivatives as the cyclometalated ligands and as the ancillary ligand show yellow luminescence (λ_max = 552–564 nm). LECs of complexes 7 and 8 with cyclometalated C^N phenylpyridine ligands and an ancillary N^N OXD ligand show red emission (λ_max 616–624 nm). Using complex 7 external quantum efficiency (EQE) values of >10% are obtained for devices (210 nm emission layer) at 3.5 V. For thinner devices (70 nm) high brightness is achieved: red emission for 7 (8528 cd m^?2 at 10 V) and yellow emission for 1 (3125 cd m^?2 at 14 V).     

High-performance azure blue quantum dot light-emitting diodes via doping PVK in emitting layer

Highly bright and efficient azure blue quantum dot-based light-emitting diodes (QD-LEDs) have been demonstrated by employing ZnCdSe core/multishell QDs as emitters and the crucial development we report here is the ability to dramatically enhance the efficiency and brightness through doping poly vinyl(N-carbazole) (PVK) in the emissive layer to balance the charge injection. The best device displays remarkable features like maximum luminance of 13,800 cd/m², luminous efficiency of 6.41 cd/A, and external quantum efficiency (EQE) of 8.76%, without detectable red-shift and broadening in electroluminescence (EL) spectra with increasing voltage as well as good spectral matching between photoluminescence (PL) and EL. Such azure blue quantum-dot LEDs show a 140% increase in external quantum efficiency compared with QD-LEDs without PVK. More important, the peak efficiency of the QD-LEDs with PVK dopant is achieved at luminance of about 1000 cd/m², and high efficiency (EQE > 8%) can be maintained with brightness ranging from 200 to 2400 cd/m². There are two main aspects of the role of PVK in the proposed system. Firstly, the lower HOMO of PVK than (poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB) can reduce the potential barrier for 0.4 eV at the interface of QDs and hole transport layer which could result in higher hole injection efficiency along with good EQE as compared to TFB-only HTLs. Secondly, with PVK acting as buffer layer of TFB and QDs, the exciton energy transfer from the organic host to the QDs can be effectively improved.     

Binaphthyl‐Containing Green‐ and Red‐Emitting Molecules for Solution‐Processable Organic Light‐Emitting Diodes

Strong intermolecular interactions usually result in decreases in solubility and fluorescence efficiency of organic molecules. Therefore, amorphous materials are highly pursued when designing solution‐processable, electroluminescent organic molecules. In this paper, a non‐planar binaphthyl moiety is presented as a way of reducing intermolecular interactions and four binaphthyl‐containing molecules ( BNCM s): green‐emitting BBB and TBT as well as red‐emitting BTBTB and TBBBT , are designed and synthesized. The photophysical and electrochemical properties of the molecules are systematically investigated and it is found that TBT , TBBBT , and BTBTB solutions show high photoluminescence (PL) quantum efficiencies of 0.41, 0.54, and 0.48, respectively. Based on the good solubility and amorphous film‐forming ability of the synthesized BNCM s, double‐layer structured organic light‐emitting diodes (OLEDs) with BNCM s as emitting layer and poly(N‐vinylcarbazole) (PVK) or a blend of poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] and PVK as hole‐transporting layer are fabricated by a simple solution spin‐coating procedure. Amongst those, the BTBTB based OLED, for example, reaches a high maximum luminance of 8315 cd · m⁻² and a maximum luminous efficiency of 1.95 cd · A⁻¹ at a low turn‐on voltage of 2.2 V. This is one of the best performances of a spin‐coated OLED reported so far. In addition, by doping the green and red BNCM s into a blue‐emitting host material poly(9,9‐dioctylfluorene‐2,7‐diyl) high performance white light‐emitting diodes with pure white light emission and a maximum luminance of 4000 cd · m⁻² are realized.     

Transparent InP Quantum Dot Light‐Emitting Diodes with ZrO2 Electron Transport Layer and Indium Zinc Oxide Top Electrode

Because of outstanding optical properties and non‐vacuum solution processability of colloidal quantum dot (QD) semiconductors, many researchers have developed various light emitting diodes (LEDs) using QD materials. Until now, the Cd‐based QD‐LEDs have shown excellent properties, but the eco‐friendly QD semiconductors have attracted many attentions due to the environmental regulation. And, since there are many issues about the reliability of conventional QD‐LEDs with organic charge transport layers, a stable charge transport layer in various conditions must be developed for this reason. This study proposes the organic/inorganic hybrid QD‐LEDs with Cd‐free InP QDs as light emitting layer and inorganic ZrO₂ nanoparticles as electron transport layer. The QD‐LED with bottom emission structure shows the luminescence of 530 cd m^?2 and the current efficiency of 1 cd/A. To realize the transparent QD‐LED display, the two‐step sputtering process of indium zinc oxide (IZO) top electrode is applied to the devices and this study could fabricate the transparent QD‐LED device with the transmittance of more than 74% for whole device array. And when the IZO top electrode with high work‐function is applied to top transparent anode, the device could maintain the current efficiency within the driving voltage range without well‐known roll‐off phenomenon in QD‐LED devices.     

Core/Shell PbSe/PbS QDs TiO2 Heterojunction Solar Cell

Quasi type‐II PbSe/PbS quantum dots (QDs) are employed in a solid state high efficiency QD/TiO₂ heterojunction solar cell. The QDs are deposited using layer‐by‐layer deposition on a half‐micrometer‐thick anatase TiO₂ nanosheet film with (001) exposed facets. Theoretical calculations show that the carriers in PbSe/PbS quasi type‐II QDs are delocalized over the entire core/shell structure, which results in better QD film conductivity compared to PbSe QDs. Moreover, PbS shell permits better stability and facile electron injection from the QDs to the TiO₂ nanosheets. To complete the electrical circuit of the solar cell, a Au film is evaporated as a back contact on top of the QDs. This PbSe/PbS QD/TiO₂ heterojunction solar cell produces a light to electric power conversion efficiency (η) of 4% with short circuit photocurrent (J_sc) of 17.3 mA/cm². This report demonstrates highly efficient core/shell near infrared QDs in a QD/TiO₂ heterojunction solar cell.     

Optimizing carrier balance of a red quantum-dot light-emitting electrochemical cell with a carrier injection layer of cationic Ir(III) complex

Solid-state light-emitting electrochemical cells (LECs) with promising features of solution processability, low-voltage operation and compatibility with inert cathode metals have shown great potential in display and lighting applications in recent years. Among the reported emissive materials for LECs, ionic transition metal complexes (iTMCs) have relatively higher electroluminescence (EL) efficiencies due to their phosphorescent property. However, the red iTMCs generally exhibit moderate color saturation and low emission efficiency, limiting their display applications. To improve color saturation and device efficiency of red LECs, efficient quantum dots (QDs) with narrow emission bandwidth are good alternative emissive materials. In this work, efficient and saturated red QD LECs employing iTMC carrier injection layers to provide in situ electrochemical doping are demonstrated. The thicknesses of iTMC and red-QD layers are systematically adjusted to achieve the best carrier balance. In the optimized device, the iTMC carrier injection layer facilitates hole injection into the red-QD layer while electrons are injected from the cathode into the red-QD layer directly since the electron injection barrier is low. The Commission Internationale de I'Eclairage (CIE) coordinates of the EL spectra approach the red standard point of National Television System Committee (NTSC). High external quantum efficiency and current efficiency reaching 9.7% and 16.1 cd A⁻¹, respectively. These results confirm superior carrier balance in such a simple iTMC/QD bilayer device structure. Furthermore, compared with iTMC LECs, less degree of device efficiency roll-off upon increasing device current is observed in QD LECs since a shorter excited-state lifetime of fluorescent QDs reduces the probability of collision exciton quenching. Saturated and efficient red EL with mitigated efficiency roll-off from red-QD LECs employing iTMC carrier injection layers confirms that they are good candidates of saturated light sources for displays.     

Cadmium-free quantum dots based violet light-emitting diodes: High-efficiency and brightness via optimization of organic hole transport layers

Bright and efficient violet quantum dot (QD) based light-emitting diodes (QD-LEDs) with heavy-metal-free ZnSe/ZnS have been demonstrated by choosing different hole transport layers, including poly(4-butyl-phenyl-diphenyl-amine) (poly-TPD), poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB), and poly-N-vinylcarbazole (PVK). Violet QD-LEDs with maximum luminance of about 930 cd/m², the maximum current efficiency of 0.18 cd/A, and the peak EQE of 1.02% when poly-TPD was used as HTL. Higher brightness and low turn-on voltage (3.8 V) violet QD-LEDs could be fabricated when TFB was used as hole transport material. Although the maximum luminance could reach up to 2691 cd/m², the devices exhibited only low current efficiency (∼0.51 cd/A) and EQE (∼2.88%). If PVK is used as hole transport material, highly efficient violet QD-LEDs can be fabricated with lower maximum luminance and higher turn-on voltages compared with counterpart using TFB. Therefore, TFB and PVK mixture in a certain proportion has been used as HTL, turn-on voltage, brightness, and efficiency all have been improved greatly. The QD-LEDs is fabricated with 7.39% of EQE and 2856 cd/m² of maximum brightness with narrow FWHM less than 21 nm. These results represent significant improvements in the performance of heavy-metal-free violet QD-LEDs in terms of efficiency, brightness, and color purity.     

Toward Highly Efficient Solid‐State White Light‐Emitting Electrochemical Cells: Blue‐Green to Red Emitting Cationic Iridium Complexes with Imidazole‐Type Ancillary Ligands

Using imidazole‐type ancillary ligands, a new class of cationic iridium complexes ( 1 – 6 ) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2′‐bipyridine)‐type ancillary ligands, imidazole‐type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue‐green to red emitting cationic iridium complexes are obtained. Single‐layer light‐emitting electrochemical cells (LECs) based on cationic iridium complexes show blue‐green to red electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A^?1 are achieved for the blue‐green‐emitting, yellow‐emitting, and orange‐emitting devices, respectively. By doping the red‐emitting complex into the blue‐green LEC, white LECs are realized, which give warm‐white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color‐rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A^?1, and 10 lm W^?1, respectively, which are the highest for white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.     

Optimization of carrier transport layer: A simple but effective approach toward achieving high efficiency all-solution processed InP quantum dot light emitting diodes

High performance quantum dot light emitting diodes (QD-LED) are being considered as a next-generation technology for energy efficient solid-state lighting and displays. In recent years, cadmium (Cd)-based QLEDs have made great progress in performance, which is close to commercial applications. However, the performance of environmentally friendly Cd-free QD-LED still needs to be improved. In this letter, using InP/ZnS quantum dots (QDs), an environmentally friendly red QDs material, as the light emitting layer, low-cost all-solution processed red InP/ZnS QD-LED are fabricated. The optimized device with a hybrid multilayered structure employing an organic double hole transport layer (HTL) with doping small molecules (TFB/PVK:TAPC) and an inorganic ZnMgO nanoparticles (NPs) electron transport layer (ETL), here TFB, PVK and TAPC represent poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4’-(N-(p-butylphenyl))-diphenylamine)], poly (9-vinlycarbazole) and 1,1-bis [4-[N,N′-di (p-tolyl)amino]phenyl]-cyclohexane, respectively. The best device exhibits a peak current efficiency (CE) of 7.58 cd A⁻¹, which is 2.4 times higher than the control device using PVK (HTL) and ZnO (ETL). At the same time, turn-on voltage dropped from 2.8 V (control devices) to 2.4 V. These superb QD-LED performances originate not only from the improved hole injection by the introduction of a double hole layer and the reduced the quenching of excitons by using ZnMgO NPs ETL but also from increasing the hole mobility with doping of small molecule materials in PVK to balance the carrier transportation. This work provides a simple and feasible idea with optimization the carrier transport for realizing high-efficiency QD-LED devices.     

Fine‐Tuning of Yellow or Red Photo‐ and Electroluminescence of Functional Difluoro‐boradiazaindacene Films

The present work describes the synthesis of difluoro‐boradiazaindacenes (Bodipy) functionalized at the central 8‐position by phenylamino moieties easily transformable into phenyl amide scaffoldings. Molecules carrying three linear or branched chains were prepared and characterized. An X‐ray crystal structure for the pivotal trimethoxyphenyl‐Bodipy derivative was determined, and the packing is discussed in terms of molecular interactions; a key feature for the formation of thin films. All of the dyes are thermally stable up to 170 °C but no liquid‐crystalline phases are observed. Reversible reduction and oxidation processes occur around +0.97 and −1.34 V, respectively, versus saturated calomel electrode in solution and the electroactivity and photoluminescence are maintained in thin films produced by vacuum evaporation. Interestingly, two distinct emissions are observed at 550 and 635 nm by electroluminescence of the trimethoxyphenyl‐Bodipy derivative, corresponding to the luminescence of isolated molecules and dimers, respectively. Doping Alq₃ films with this Bodipy molecule by vacuum evaporation produces organic light‐emitting diodes (OLEDs) in which very efficient energy transfer from the Alq₃ matrix to the Bodipy occurs by a resonance mechanism involving the first Bodipy excited state. Yellow light (550 nm, 344 cd m⁻² at 15 V) is emitted at low doping concentration (7 mol %), whereas red light (635 nm, 125 cd m⁻² at 15 V) is emitted at higher concentration (19 mol %). Dispersion of the Bodipy into a fluorescent poly(N‐vinylcarbazole) polymer (PVK) (≈3 mol % per repeating unit of PVK) by solution processing exclusively produces yellow emission owing to the isolated Bodipyfluorophore (550 nm, 213 cd m⁻² at 15 V). The second excited state of the Bodipy dye is likely involved during energy transfer from the PVK matrix.     

Highly Efficient Quantum Dot Light‐Emitting Diodes by Inserting Multiple Poly(methyl methacrylate) as Electron‐Blocking Layers

This work presents a new device architecture integrating multiple poly(methyl methacrylate) (PMMA) electron‐blocking layers (EBL) in quantum dot light‐emitting diodes (QD‐LEDs). The device utilizes red‐emitting CdSe/ZnS QD with a novel structure where multiple PMMA EBLs are sandwiched between a pair of QD layers. A systematic optimization of QD‐LED structures has shown that a device including two PMMA and three QD layers performs the best, achieving a current efficiency of 17.8 cd A^?1 and a luminance of 194 038 cd m^?2. Numerical simulation of a simplified model of the proposed QD‐LED structure verifies that the structure consisting of two PMMA and three QD layers provides significant improvement in electroluminescent intensity. The simulation provides further insight into the origin of the effect of the PMMA EBL by showing that the addition of PMMA EBL reduces the electron leakage from the active QD region and enhances electron confinement, leading to an increased electron concentration in the QD active layers and a higher radiative recombination rate. The experimental and theoretical studies presented in this work demonstrate that multiple layers of PMMA can act as efficient EBLs in the fabrication of QD‐LEDs of improved performance.     

Fully Printed Light‐Emitting Electrochemical Cells Utilizing Biocompatible Materials

The use of biomaterials and bioinspired concepts in electronics will enable the fabrication of transient and disposable technologies within areas ranging from smart packaging and advertisement to healthcare applications. In this work, the use of a nonhalogenated biodegradable solid polymer electrolyte based on poly(ε‐caprolactone‐co‐trimethylene carbonate) and tetrabutylammonium bis‐oxalato borate in light‐emitting electrochemical cells (LECs) is presented. It is shown that the spin‐cast devices exhibit current efficiencies of ≈2 cd A^?1 with luminance over ≈12 000 cd m^?2, an order of magnitude higher than previous bio‐based LECs. By a combination of industrially relevant techniques (i.e., inkjet printing and blade coating), the fabrication of LEC devices on a cellulose‐based flexible biodegradable substrate showing lifetimes compatible with transient applications is demonstrated. The presented results have direct implications toward the industrial manufacturing of biomaterial‐based light‐emitting devices with potential use in future biodegradable/biocompatible electronics.     

Construction of White‐Light‐Emitting Silk Protein Hybrid Films by Molecular Recognized Assembly among Hierarchical Structures

The fabrication of bio‐hybrid functional films is demonstrated by applying a materials assembly technique. Based on the hierarchical structures of silk fibroin materials, functional molecular/materials, i.e., quantum dots (QDs), can be fixed to amino acid groups in silk fibroin films. It follows that white‐light‐emitting QD silk hybrid films are obtained by hydrogen bond molecular recognition to the –COO^– groups functionalized to blue luminescent ZnSe (5.2 nm) and yellow luminescent CdTe (4.1 nm) QDs in a molar ratio of 30:1 of ZnSe to CdTe QDs. Simultaneously, a systematic blue shift in the emission peak is observed from the QD solution to QDs silk fibroin films. The significant blue shift hints the appearance of the strong interaction between QDs and silk fibroins, which causes strong white‐light‐emitting uniform silk films. The molecular recognized interactions are confirmed by high resolution transmission electron microscopy, field scanning electron microscope, and attenuated total internal reflectance Fourier transform infrared spectroscopy. The QD silk films show unique advantages, including simple preparation, tunable white‐light emission, easy manipulation, and low fabrication costs, which make it a promising candidate for multicomponent optodevices.     

Cover Picture: White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant (Adv. Funct. Mater. 7/2006)

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized by Wang and co‐workers on p. 957. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue and orange emission from the corresponding emitting species. A single‐layer device has been fabricated that has performance characteristics roughly comparable to those of organic white‐light‐emitting diodes with multilayer device structures. New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

Monodisperse Starburst Oligofluorene‐Functionalized 4,4′,4″‐Tris(carbazol‐9‐yl)‐triphenylamines: Their Synthesis and Deep‐Blue Fluorescent Properties for Organic Light‐Emitting Diode Applications

Four monodisperse starburst oligomers bearing a 4,4′,4″‐tris(carbazol‐9‐yl)‐triphenylamine (TCTA) core and six oligofluorene arms are synthesized and characterized. The lengths of oligofluorene arms vary from one to four fluorene units, giving the starburst oligomers molecular weights ranging from 3072 to 10 068 Da (1 Da = 1.66 × 10^–27 kg). All of the starburst oligomers have good film‐forming capabilities, and display bright, deep‐blue fluorescence (λ_max = 395–416 nm) both in solution and in the solid state, with the quantum efficiencies of the films (Φ_PL) varying between 27 and 88 %. Electrochemical studies demonstrate that these materials have large energy gaps, and are stable for both p‐doping and n‐doping processes. Electroluminescent devices are successfully fabricated using these materials as hole‐transporting emitters, and emit deep‐blue light. Devices with luminance values up to 1025 cd m^–2 at 11 V and luminous efficiencies of 0.47 cd A^–1 at 100 cd m^–2 have been produced, which translates to an external quantum efficiency of 1.4 %. In addition, these large‐energy‐gap starburst oligomers are good host materials for red electrophosphorescence. The luminance of the red electrophosphorescent devices is as high as 4452 cd m^–2, with a luminous efficiency of 4.31 cd A^–1 at 15 mA cm^–2: This value is much higher than those obtained from the commonly used hole‐transporting materials, such as poly(vinyl carbazole) (PVK) (1.10 cd A^–1 at 16 mA cm^–2).     

Ion Transport beyond the Polyether Paradigm: Introducing Oligocarbonate Ion Transporters for Efficient Light‐Emitting Electrochemical Cells

The light‐emitting electrochemical cell (LEC) is fundamentally dependent on mobile ions for its operation. In polymer LECs, the mobile ions are commonly provided by dissolving a salt in an ion transporter, with the latter almost invariably being an ether‐based compound. Here, the synthesis, characterization, and application of a new class of carbonate‐based ion transporters are reported. A polymer LEC, comprising a star‐branched oligocarbonate endowed with aliphatic side groups as the ion transporter, features a current efficacy of 13.8 cd A^?1 at a luminance of 1060 cd m^?2, which is a record‐high efficiency/luminance combination for a singlet‐emitting LEC. It is further established that the design principles of a high‐performance carbonate ion transporter constitute the selection of an oligomeric structure over a corresponding polymeric structure and the endowment of the oligomer with functional side chains to render it compatible with the polymeric emitter.     

Water‐Soluble Lacunary Polyoxometalates with Excellent Electron Mobilities and Hole Blocking Capabilities for High Efficiency Fluorescent and Phosphorescent Organic Light Emitting Diodes

High performance solution‐processed fluorescent and phosphorescent organic light emitting diodes (OLEDs) are achieved by water solution processing of lacunary polyoxometalates used as novel electron injection/transport materials with excellent electron mobilities and hole blocking capabilities. Green fluorescent OLEDs using poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐(1,4‐benzo‐{2,1′,3}‐thiadiazole)] (F8BT) as the emissive layer and our polyoxometalates as electron transport/hole blocking layers give a luminous efficiency up to 6.7 lm W^?1 and a current efficiency up to 14.0 cd A^?1 which remained nearly stable for about 500 h of operation. In addition, blue phosphorescent OLEDs (PHOLEDs) using poly(9‐vinylcarbazole) (PVK):1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7) as a host and 10.0 wt% FIrpic as the blue dopant in the emissive layer and a polyoxometalate as electron transport material give 12.5 lm W^?1 and 30.0 cd A^?1 power and luminous efficiency, respectively, which are among the best performance values observed to date for all‐solution processed blue PHOLEDs. The lacunary polyoxometalates exhibit unique properties such as low electron affinity and high ionization energy (of about 3.0 and 7.5 eV, respectively) which render them as efficient electron injection/hole blocking layers and, most importantly, exceptionally high electron mobility of up to 10^?2 cm² V^?1 s^?1.     

Highly Flexible Organic Nanofiber Phototransistors Fabricated on a Textile Composite for Wearable Photosensors

Highly flexible organic nanofiber phototransistors are fabricated on a highly flexible poly(ethylene terephthalate) (PET) textile/poly(dimethylsiloxane) (PDMS) composite substrate. Organic nanofibers are obtained by electrospinning, using a mixture of poly(3,3^″′‐didodecylquarterthiophene) (PQT‐12) and poly(ethylene oxide) (PEO) as the semiconducting polymer and processing aid, respectively. PDMS is used as both a buffer layer for flattening the PET textile and a dielectric layer in the bottom‐gate bottom‐contact device configuration. PQT‐12:PEO nanofibers can be well‐aligned on the textile composite substrate by electrospinning onto a rotating drum collector. The nanofiber phototransistors fabricated on the PET/PDMS textile composite substrate show highly stable device performance (on‐current retention up to 82.3 (±6.7)%) under extreme bending conditions, with a bending radius down to 0.75 mm and repeated tests over 1000 cycles, while those prepared on film‐type PET and PDMS‐only substrates exhibit much poorer performances. The photoresponsive behaviors of PQT‐12:PEO nanofiber phototransistors have been investigated under light irradiation with different wavelengths. The maximum photoresponsivity, photocurrent/dark‐current ratio, and external quantum efficiency under blue light illumination were 930 mA W^?1, 2.76, and 246%, respectively. Furthermore, highly flexible 10 × 10 photosensor arrays have been fabricated which are able to detect incident photonic signals with high resolution. The flexible photosensors described herein have high potential for applications as wearable photosensors.     

High‐Efficiency and Stable Quantum Dot Light‐Emitting Diodes Enabled by a Solution‐Processed Metal‐Doped Nickel Oxide Hole Injection Interfacial Layer

Stabilization is one critical issue that needs to be improved for future application of colloidal quantum dot (QD)‐based light‐emitting diodes (QLEDs). This study reports highly efficient and stable QLEDs based on solution‐processsed, metal‐doped nickel oxide films as hole injection layer (HIL). Several kinds of metal dopants (Li, Mg, and Cu) are introduced to improve the hole injection capability of NiO films. The resulting device with Cu:NiO HIL exhibits superior performance compared to the state‐of‐the‐art poly(3,4‐ethylenedioxythiophene):poly(styrene‐sulfonate) (PEDOT:PSS)‐based QLEDs, with a maximum current efficiency and external quantum efficiency of 45.7 cd A^?1 and 10.5%, respectively. These are the highest values reported so far for QLEDs with PEDOT:PSS‐free normal structure. Meanwhile, the resulting QLED shows a half‐life time of 87 h at an initial luminance of 5000 cd m^?2, almost fourfold longer than that of the PEDOT:PSS‐based device.