Efficient Near‐Infrared Electroluminescence at 840 nm with “Metal‐Free” Small‐Molecule:Polymer Blends

Due to the so‐called energy‐gap law and aggregation quenching, the efficiency of organic light‐emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light‐emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn‐on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole‐based emitter and a novel indacenodithiophene‐based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution‐processed active layer, not leveraging triplet‐assisted emission.     

Roll‐to‐Roll Cohesive,Coated, Flexible,High‐Efficiency Polymer Light‐Emitting Diodes Utilizing ITO‐Free Polymer Anodes

This paper reports solution‐processed, high‐efficiency polymer light‐emitting diodes fabricated by a new type of roll‐to‐roll coating method under ambient air conditions. A noble roll‐to‐roll cohesive coating system utilizes only natural gravity and the surface tension of the solution to flow out from the capillary to the surface of the substrate. Because this mechanism uses a minimally cohesive solution, the roll‐to‐roll cohesive coating can effectively realize an ultra‐thin film thickness for the electron injection layer. In addition, the roll‐to‐roll cohesive coating enables the fabrication of a thicker polymer anode film more than 250 nm at one time by modification of the surface energy and without wasting the solution. It is observed that the standard sheet resistance deviation of the polymer anode is only 2.32 Ω/□ over 50 000 bending cycles. The standard sheet resistance deviation of the polymer anode in the different bending angles (0 to 180°) is 0.313 Ω/□, but the case of the ITO‐PET is 104.93 Ω/□. The average surface roughness of the polymer anode measured by atomic force microscopy is only 1.06 nm. Because the surface of the polymer anode has a better quality, the leakage current of the polymer light‐emitting diodes (PLEDs) using the polymer anode is much lower than that using the ITO‐PET substrate. The luminous power efficiency of the two devices is 4.13 lm/W for the polymer anode and 3.21 lm/W for the ITO‐PET. Consequently, the PLEDs made by using the polymer anode exhibited 28% enhanced performance because the polymer anode represents not only a higher transparency than the ITO‐PET in the wavelength of 560 nm but also greatly reduced roughness. The optimized the maximum current efficiency and power efficiency of the device show around 6.1 cd/A and 5.1 lm/W, respectively, which is comparable to the case of using the ITO‐glass.     

Purely Organic Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

The design of thermally activated delayed fluorescence (TADF) materials both as emitters and as hosts is an exploding area of research. The replacement of phosphorescent metal complexes with inexpensive organic compounds in electroluminescent (EL) devices that demonstrate comparable performance metrics is paradigm shifting, as these new materials offer the possibility of developing low‐cost lighting and displays. Here, a comprehensive review of TADF materials is presented, with a focus on linking their optoelectronic behavior with the performance of the organic light‐emitting diode (OLED) and related EL devices. TADF emitters are cross‐compared within specific color ranges, with a focus on blue, green–yellow, orange–red, and white OLEDs. Organic small‐molecule, dendrimer, polymer, and exciplex emitters are all discussed within this review, as is their use as host materials. Correlations are provided between the structure of the TADF materials and their optoelectronic properties. The success of TADF materials has ushered in the next generation of OLEDs.     

High‐Performance,Solution‐Processed,and Insulating‐Layer‐Free Light‐Emitting Diodes Based on Colloidal Quantum Dots

Quantum‐dot light‐emitting diodes (QLEDs) may combine superior properties of colloidal quantum dots (QDs) and advantages of solution‐based fabrication techniques to realize high‐performance, large‐area, and low‐cost electroluminescence devices. In the state‐of‐the‐art red QLED, an ultrathin insulating layer inserted between the QD layer and the oxide electron‐transporting layer (ETL) is crucial for both optimizing charge balance and preserving the QDs' emissive properties. However, this key insulating layer demands very accurate and precise control over thicknesses at sub‐10 nm level, causing substantial difficulties for industrial production. Here, it is reported that interfacial exciton quenching and charge balance can be independently controlled and optimized, leading to devices with efficiency and lifetime comparable to those of state‐of‐the‐art devices. Suppressing exciton quenching at the ETL–QD interface, which is identified as being obligatory for high‐performance devices, is achieved by adopting Zn_0.9Mg_0.1O nanocrystals, instead of ZnO nanocrystals, as ETLs. Optimizing charge balance is readily addressed by other device engineering approaches, such as controlling the oxide ETL/cathode interface and adjusting the thickness of the oxide ETL. These findings are extended to fabrication of high‐efficiency green QLEDs without ultrathin insulating layers. The work may rationalize the design and fabrication of high‐performance QLEDs without ultrathin insulating layers, representing a step forward to large‐scale production and commercialization.     

Degradation Mechanisms in Small‐Molecule and Polymer Organic Light‐Emitting Diodes

Degradation in organic light‐emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small‐molecule and polymer OLEDs is presented. For small‐molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance‐efficiency loss and operating‐voltage rise. For polymer OLEDs, it is shown how the charge‐transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.     

Inkjet‐Printed High‐Efficiency Multilayer QLEDs Based on a Novel Crosslinkable Small‐Molecule Hole Transport Material

Quantum dots light‐emitting diodes (QLEDs) have attracted much interest owing to their compatibility with low‐cost inkjet printing technology and potential for use in large‐area full‐color pixelated display. However, it is challenging to fabricate high efficiency inkjet‐printed QLEDs because of the coffee ring effects and inferior resistance to solvents from the underlying polymer film during the inkjet printing process. In this study, a novel crosslinkable hole transport material, 4,4′‐bis(3‐vinyl‐9H‐carbazol‐9‐yl)‐1,1′‐biphenyl (CBP‐V) which is small‐molecule based, is synthesized and investigated for inkjet printing of QLEDs. The resulting CBP‐V film after thermal curing exhibits excellent solvent resistance properties without any initiators. An added advantage is that the crosslinked CBP‐V film has a sufficiently low highest occupied molecular orbital energy level (≈?6.2 eV), high film compactness, and high hole mobility, which can thus promote the hole injection into quantum dots (QDs) and improve the charge carrier balance within the QD emitting layers. A red QLED is successfully fabricated by inkjet printing a CBP‐V and QDs bilayer. Maximum external quantum efficiency of 11.6% is achieved, which is 92% of a reference spin‐coated QLED (12.6%). This is the first report of such high‐efficiency inkjet‐printed multilayer QLEDs and demonstrates a unique and effective approach to inkjet printing fabrication of high‐performance QLEDs.     

OLEDs: Degradation Mechanisms in Small‐Molecule and Polymer Organic Light‐Emitting Diodes (Adv. Mater. 34/2010)

Degradation in organic light‐emitting diodes (OLEDs) is a complex problem. Depending upon the materials and the device architectures used, the degradation mechanism can be very different. In this Progress Report, using examples in both small molecule and polymer OLEDs, the different degradation mechanisms in two types of devices are examined. Some of the extrinsic and intrinsic degradation mechanisms in OLEDs are reviewed, and recent work on degradation studies of both small‐molecule and polymer OLEDs is presented. For small‐molecule OLEDs, the operational degradation of exemplary fluorescent devices is dominated by chemical transformations in the vicinity of the recombination zone. The accumulation of degradation products results in coupled phenomena of luminance‐efficiency loss and operating‐voltage rise. For polymer OLEDs, it is shown how the charge‐transport and injection properties affect the device lifetime. Further, it is shown how the charge balance is controlled by interlayers at the anode contact, and their effects on the device lifetime are discussed.     

Air‐Stable and High‐Performance Solution‐Processed Organic Light‐Emitting Devices Based on Hydrophobic Polymeric Ionic Liquid Carrier‐Injection Layers

A lot of research, mostly using electron‐injection layers (EILs) composed of alkali‐metal compounds has been reported with a view to increase the efficiency of solution‐processed organic light‐emitting devices (OLEDs). However, these materials have intractable properties, such as a strong affinity for moisture, which cause the degradation of OLEDs. Consequently, optimal EIL materials should exhibit high electron‐injection efficiency as well as be stable in air. In this study, polymer light‐emitting devices (PLEDs) based on the commonly used yellow‐fluorescence‐emitting polymer F8BT, which utilize poly(diallyldimethylammonium)‐based polymeric ionic liquids, are experimentally and analytically investigated. As a result, the optimized PLED employing an EIL comprising poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (poly(DDA)TFSI), which is expected to display good moisture resistance because of water repellency of fluorocarbon groups, exhibits excellent storage stability in air and electroluminescence performance with a low turn‐on voltage of 2.01 V, maximum external quantum efficiency of 9.00%, current efficiency of 30.1 cd A^?1, and power efficiency of 32.4 lm W^?1. The devices with poly(DDA)TFSI show one of the highest efficiencies as compared to the reported standard PLEDs. Moreover, poly(DDA)TFSI is applied as a hole‐injection layer (HIL). The optimized PLED using poly(DDA)TFSI as the HIL exhibits performances comparable to those of a device that uses a conventional poly(3,4‐ethylenedioxy‐thiophene):poly(4‐styrenesulfonate) HIL.     

Semiconductor Nanowire Light‐Emitting Diodes Grown on Metal: A Direction Toward Large‐Scale Fabrication of Nanowire Devices

Bottom‐up nanowires are attractive for realizing semiconductor devices with extreme heterostructures because strain relaxation through the nanowire sidewalls allows the combination of highly lattice mismatched materials without creating dislocations. The resulting nanowires are used to fabricate light‐emitting diodes (LEDs), lasers, solar cells, and sensors. However, expensive single crystalline substrates are commonly used as substrates for nanowire heterostructures as well as for epitaxial devices, which limits the manufacturability of nanowire devices. Here, nanowire LEDs directly grown and electrically integrated on metal are demonstrated. Optical and structural measurements reveal high‐quality, vertically aligned GaN nanowires on molybdenum and titanium films. Transmission electron microscopy confirms the composition variation in the polarization‐graded AlGaN nanowire LEDs. Blue to green electroluminescence is observed from InGaN quantum well active regions, while GaN active regions exhibit ultraviolet emission. These results demonstrate a pathway for large‐scale fabrication of solid state lighting and optoelectronics on metal foils or sheets.     

Perovskite Light‐Emitting Diodes with Improved Outcoupling Using a High‐Index Contrast Nanoarray

Organic–inorganic hybrid perovskite light‐emitting diodes (PeLEDs) are promising for next‐generation optoelectronic devices due to their potential to achieve high color purity, efficiency, and brightness. Although the external quantum efficiency (EQE) of PeLEDs has recently surpassed 20%, various strategies are being pursued to increase EQE further and reduce the EQE gap compared to other LED technologies. A key point to further boost EQE of PeLEDs is linked to the high refractive index of the perovskite emissive layer, leading to optical losses of more than 70% of emitted photons. Here, it is demonstrated that a randomly distributed nanohole array with high‐index contrast can effectively enhance outcoupling efficiency in PeLEDs. Based on a comprehensive optical analysis on the perovskite thin film and outcoupling structure, it is confirmed that the nanohole array effectively distributes light into the substrate for improved outcoupling, allowing for 1.64 times higher light extraction. As a result, highly efficient red/near‐infrared PeLEDs with a peak EQE of 14.6% are demonstrated.     

White Organic Light‐Emitting Devices for Solid‐State Lighting

White organic light‐emitting devices (WOLEDs) have advanced over the last twelve years to the extent that these devices are now being considered as efficient solid‐state lighting sources. Initially, WOLEDs were targeted towards display applications for use primarily as liquid‐crystal display backlights. Now, their power efficiencies have surpassed those of incandescent sources due to improvements in device architectures, synthesis of novel materials, and the incorporation of electrophosphorescent emitters. This review discusses the advantages and disadvantages of several WOLED architectures in terms of efficiency and color quality. Hindrances to their widespread acceptance as solid‐state lighting sources are also noted.     

Efficient flexible phosphorescent polymer light-emitting diodes based on silver nanowire-polymer composite electrode

Blue, green, and red electrophosphorescent polymer light-emitting diodes have been fabricated on silver nanowire-polymer composite electrode. The devices are 20%-50% more efficient than control devices on ITO/glass and exhibit small efficiency roll-off at high luminances. The blue PLEDs were repeatedly bent to 1.5 mm radius concave or convex with calculated strain in the emissive layer approximately 5% (tensile or compressive).     

Highly Deformable and See‐Through Polymer Light‐Emitting Diodes with All‐Conducting‐Polymer Electrodes

Despite the high expectation of deformable and see‐through displays for future ubiquitous society, current light‐emitting diodes (LEDs) fail to meet the desired mechanical and optical properties, mainly because of the fragile transparent conducting oxides and opaque metal electrodes. Here, by introducing a highly conductive nanofibrillated conducting polymer (CP) as both deformable transparent anode and cathode, ultraflexible and see‐through polymer LEDs (PLEDs) are demonstrated. The CP‐based PLEDs exhibit outstanding dual‐side light‐outcoupling performance with a high optical transmittance of 75% at a wavelength of 550 nm and with an excellent mechanical durability of 9% bending strain. Moreover, the CP‐based PLEDs fabricated on 4 µm thick plastic foils with all‐solution processing have extremely deformable and foldable light‐emitting functionality. This approach is expected to open a new avenue for developing wearable and attachable transparent displays.     

Inkjet Printing—Process and Its Applications

In this Progress Report we provide an update on recent developments in inkjet printing technology and its applications, which include organic thin‐film transistors, light‐emitting diodes, solar cells, conductive structures, memory devices, sensors, and biological/pharmaceutical tasks. Various classes of materials and device types are in turn examined and an opinion is offered about the nature of the progress that has been achieved.     

Aggregation‐Induced Dual‐Phosphorescence from Organic Molecules for Nondoped Light‐Emitting Diodes

Aggregation‐induced emission (AIE) is a beneficial strategy for generating highly effective solid‐state molecular luminescence without suffering losses in quantum yield. However, the majority of reported AIE‐active molecules exhibit only strong fluorescence, which is not ideal for electrical excitation in organic light‐emitting diodes (OLEDs). By introducing various substituent groups onto the biscarbazole compound, a series of molecular materials with aggregation‐induced phosphorescence (AIP) is designed, which exhibits two distinctly different phosphorescence bands and an absolute solid‐state room‐temperature phosphorescence quantum yield up to 64%. Taking advantage of the AIE feature, the AIP molecules are fabricated into OLEDs as a homogeneous light‐emitting layer, which allows for relatively small efficiency roll‐off and shows an external electroluminescence quantum yield of up to 5.8%, more than the theoretical limit for purely fluorescent OLED devices. The design showcases a promising strategy for the production of cost‐effective and highly efficient OLED technology.     

Transient Light‐Emitting Diodes Constructed from Semiconductors and Transparent Conductors that Biodegrade Under Physiological Conditions

Transient forms of electronics, systems that disintegrate, dissolve, resorb, or sublime in a controlled manner after a well‐defined operating lifetime, are of interest for applications in hardware secure technologies, temporary biomedical implants, “green” consumer devices and other areas that cannot be addressed with conventional approaches. Broad sets of materials now exist for a range of transient electronic components, including transistors, diodes, antennas, sensors, and even batteries. This work reports the first examples of transient light‐emitting diodes (LEDs) that can completely dissolve in aqueous solutions to biologically and environmentally benign end products. Thin films of highly textured ZnO and polycrystalline Mo serve as semiconductors for light generation and conductors for transparent electrodes, respectively. The emitted light spans a range of visible wavelengths, where nanomembranes of monocrystalline silicon can serve as transient filters to yield red, green, and blue LEDs. Detailed characterization of the material chemistries and morphologies of the constituent layers, assessments of their performance properties, and studies of their dissolution processes define the underlying aspects. These results establish an electroluminescent light source technology for unique classes of optoelectronic systems that vanish into benign forms when exposed to aqueous conditions in the environment or in living organisms.     

Cover Picture: Multilayer Polymer Light‐Emitting Diodes: White‐Light Emission with High Efficiency (Adv. Mater. 17/2005)

White‐light‐emitting polymer diodes can be fabricated by solution processing using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer, and water‐soluble (or ethanol‐soluble) polymers and/or small molecules as the hole‐injection/transport layer (HIL/HTL) and the electron injection/transport layer (EIL/ETL), as reported on p. 2053 by Gong, Bazan, Heeger and co‐workers. Illumination‐quality light is obtained from these multilayer, high‐performance devices, with stable CIE coordinates, color temperatures, and high color‐rendering indices all close to those of “pure” white light. The cover illustration envisages the incorporation of the fabrication technique with low‐cost manufacturing technology in order to produce large areas of high‐quality white light.     

Introduction of Red‐Green‐Blue Fluorescent Dyes into a Metal–Organic Framework for Tunable White Light Emission

The unique features of the metal–organic frameworks (MOFs), including ultrahigh porosities and surface areas, tunable pores, endow the MOFs with special utilizations as host matrices. In this work, various neutral and ionic guest dye molecules, such as fluorescent brighteners, coumarin derivatives, 4‐(dicyanomethylene)‐2‐methyl‐6‐(p‐dimethylaminostyryl)‐4H‐pyran (DCM), and 4‐(p‐dimethylaminostyryl)‐1‐methylpyridinium (DSM), are encapsulated in a neutral MOF, yielding novel blue‐, green‐, and red‐phosphors, respectively. Furthermore, this study introduces the red‐, green‐, and blue‐emitting dyes into a MOF together for the first time, producing white‐light materials with nearly ideal Commission International ed'Eclairage (CIE) coordinates, high color‐rendering index values (up to 92%) and quantum yields (up to 26%), and moderate correlated color temperature values. The white light is tunable by changing the content or type of the three dye guests, or the excitation wavelength. Significantly, the introduction of blue‐emitting guests in the methodology makes the available MOF host more extensive, and the final white‐light output more tunable and high‐quality. Such strategy can be widely adopted to design and prepare white‐light‐emitting materials.     

Stretchable Light‐Emitting Diodes with Organometal‐Halide‐Perovskite–Polymer Composite Emitters

Intrinsically stretchable light‐emitting diodes (LEDs) are demonstrated using organometal‐halide‐perovskite/polymer composite emitters. The polymer matrix serves as a microscale elastic connector for the rigid and brittle perovskite and induces stretchability to the composite emissive layers. The stretchable LEDs consist of poly(ethylene oxide)‐modified poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate as a transparent and stretchable anode, a perovskite/polymer composite emissive layer, and eutectic indium–gallium as the cathode. The devices exhibit a turn‐on voltage of 2.4 V, and a maximum luminance intensity of 15 960 cd m^?2 at 8.5 V. Such performance far exceeds all reported intrinsically stretchable LEDs based on electroluminescent polymers. The stretchable perovskite LEDs are mechanically robust and can be reversibly stretched up to 40% strain for 100 cycles without failure.     

Inkjet Printed Bilayer Light‐Emitting Electrochemical Cells for Display and Lighting Applications

A new bilayer light‐emitting electrochemical cell (LEC) device, which allows well‐defined patterned light emission through an easily adjustable, mask‐free, and additive fabrication process, is reported. The bilayer stack comprises an inkjet‐printed lattice of micrometer‐sized electrolyte droplets, in a “filled” or “patterned” lattice configuration. On top of this, a thin layer of light‐emitting compound is deposited from solution. The light emission is demonstrated to originate from regions proximate to the interfaces between the inkjetted electrolyte, the light‐emitting compound, and one electrode, where bipolar electron/hole injection and electrochemical doping are facilitated by ion motion. By employing KCF₃SO₃ in poly(ethylene glycol) as the electrolyte, Super Yellow as the light‐emitting compound, and two air‐stabile electrodes, it is possible to realize filled lattice devices that feature uniform yellow–green light emission to the naked eye, and patterned lattice devices that deliver well‐defined and high‐contrast static messages with a pixel density of 170 PPI.