High‐Transconductance Organic Thin‐Film Electrochemical Transistors for Driving Low‐Voltage Red‐Green‐Blue Active Matrix Organic Light‐Emitting Devices

Switching and control of efficient red, green, and blue active matrix organic light‐emitting devices (AMOLEDs) by printed organic thin‐film electrochemical transistors (OETs) are demonstrated. These all‐organic pixels are characterized by high luminance at low operating voltages and by extremely small transistor dimensions with respect to the OLED active area. A maximum brightness of ≈900 cd m^?2 is achieved at diode supply voltages near 4 V and pixel selector (gate) voltages below 1 V. The ratio of OLED to OET area is greater than 100:1 and the pixels may be switched at rates up to 100 Hz. Essential to this demonstration are the use of a high capacitance electrolyte as the gate dielectric layer in the OETs, which affords extremely large transistor transconductances, and novel graded emissive layer (G‐EML) OLED architectures that exhibit low turn‐on voltages and high luminescence efficiency. Collectively, these results suggest that printed OETs, combined with efficient, low voltage OLEDs, could be employed in the fabrication of flexible full‐color AMOLED displays.     

High‐Mobility ZnO Thin Film Transistors Based on Solution‐processed Hafnium Oxide Gate Dielectrics

The properties of metal oxides with high dielectric constant (k) are being extensively studied for use as gate dielectric alternatives to silicon dioxide (SiO₂). Despite their attractive properties, these high‐k dielectrics are usually manufactured using costly vacuum‐based techniques. In that respect, recent research has been focused on the development of alternative deposition methods based on solution‐processable metal oxides. Here, the application of the spray pyrolysis (SP) technique for processing high‐quality hafnium oxide (HfO₂) gate dielectrics and their implementation in thin film transistors employing spray‐coated zinc oxide (ZnO) semiconducting channels are reported. The films are studied by means of admittance spectroscopy, atomic force microscopy, X‐ray diffraction, UV–Visible absorption spectroscopy, FTIR, spectroscopic ellipsometry, and field‐effect measurements. Analyses reveal polycrystalline HfO₂ layers of monoclinic structure that exhibit wide band gap (≈5.7 eV), low roughness (≈0.8 nm), high dielectric constant (k ≈ 18.8), and high breakdown voltage (≈2.7 MV/cm). Thin film transistors based on HfO₂/ZnO stacks exhibit excellent electron transport characteristics with low operating voltages (≈6 V), high on/off current modulation ratio (～10⁷) and electron mobility in excess of 40 cm² V^?1 s^?1.     

Layered,Nanonetwork Composite Cathodes for Flexible,High‐Efficiency,Organic Light Emitting Devices

In this work, the application of an aluminum (Al)/multiwall carbon nanotube (MWCNT)/Al, multilayered electrode to flexible, high‐efficiency, alternating current driven organic electroluminescent devices (AC‐OEL), is reported. The electrode is fabricated by sandwiching a spray‐cast nanonetwork film of MWCNTs between two evaporated layers of Al. The resulting composite film facilitates a uniform charge distribution across a robust crack‐free electrode under various bending angles. It is demonstrated that these composite electrodes stabilize the power efficiency of flexible devices for bending angles up to 120°, with AC‐OEL device power efficiencies of ≈22 lm W^?1 at luminances of ≈4000 cd m^?2 (using no output coupling). Microscopic examination of the Al/MWCNTs/Al electrode after bending of up to 1300 cycles suggests that the nanotubes significantly enhance the mechanical properties of the thin Al layers while providing a moderate modification to the work function of the metal. While the realization of robust, high‐brightness, and high‐efficiency AC‐OEL devices is potentially important in their future lighting applications, it is anticipated that this to also have significant impact in standard organic light emitting diodes lighting applications.     

Controlled Polarizability of One‐Nanometer‐Thick Oxide Nanosheets for Tailored,High‐κ Nanodielectrics

An important challenge in current microelectronics research is the development of techniques for making smaller, higher‐performance electronic components. In this context, the fabrication and integration of ultrathin high‐κ dielectrics with good insulating properties is an important issue. Here, we report on a rational approach to produce high‐performance nanodielectrics using one‐nanometer‐thick oxide nanosheets as a building block. In titano niobate nanosheets (TiNbO₅, Ti₂NbO₇, Ti₅NbO₁₄), the octahedral distortion inherent to site‐engineering by Nb incorporation results in a giant molecular polarizability, and their multilayer nanofilms exhibit a high dielectric constant (160–320), the largest value seen so far in high‐κ nanofilms with thickness down to 10 nm. Furthermore, these superior high‐κ properties are fairly temperature‐independent with low leakage‐current density (<10^?7 A cm^?2). This work may provide a new recipe for designing nanodielectrics desirable for practical high‐κ devices.     

High‐Speed,Low‐Voltage,and Environmentally Stable Operation of Electrochemically Gated Zinc Oxide Nanowire Field‐Effect Transistors

Single‐crystal, 1D nanostructures are well known for their high mobility electronic transport properties. Oxide‐nanowire field‐effect transistors (FETs) offer both high optical transparency and large mechanical conformability which are essential for flexible and transparent display applications. Whereas the “on‐currents” achieved with nanowire channel transistors are already sufficient to drive active matrix organic light emitting diode (AMOLED) displays; it is shown here that incorporation of electrochemical‐gating (EG) to nanowire electronics reduces the operation voltage to ≤2 V. This opens up new possibilities of realizing flexible, portable, transparent displays that are powered by thin film batteries. A composite solid polymer electrolyte (CSPE) is used to obtain all‐solid‐state FETs with outstanding performance; the field‐effect mobility, on/off current ratio, transconductance, and subthreshold slope of a typical ZnO single‐nanowire transistor are 62 cm²/Vs, 10⁷, 155 μS/μm and 115 mV/dec, respectively. Practical use of such electrochemically‐gated field‐effect transistor (EG FET) devices is supported by their long‐term stability in air. Moreover, due to the good conductivity (≈10^?2 S/cm) of the CSPE, sufficiently high switching speed of such EG FETs is attainable; a cut‐off frequency in excess of 100 kHz is measured for in‐plane FETs with large gate‐channel distance of >10 μm. Consequently, operation speeds above MHz can be envisaged for top‐gate transistor geometries with insulator thicknesses of a few hundreds of nanometers. The solid polymer electrolyte developed in this study has great potential in future device fabrication using all‐solution processed and high throughput techniques.     

Solution‐Processed Highly Efficient Alternating Current‐Driven Field‐Induced Polymer Electroluminescent Devices Employing High‐k Relaxor Ferroelectric Polymer Dielectric

Organic thin‐film electroluminescent (EL) devices, such as organic light‐emitting diodes (OLEDs), typically operate using constant voltage or direct current (DC) power sources. Such approaches require power converters (introducing power losses) and make devices sensitive to dimensional variations that lead to run away currents at imperfections. Devices driven by time‐dependent voltages or alternating current (AC) may offer an alternative to standard OLED technologies. However, very little is known about how this might translate into overall performance of such devices. Here, a solution‐processed route to creating highly efficient AC field‐induced polymer EL (FIPEL) devices is demonstrated. Such solution‐processed FIPEL devices show maximum luminance, current efficiency, and power efficiency of 3000 cd m^?2, 15.8 cd A^?1, and 3.1 lm W^?1 for blue emission, 13 800 cd m^?2, 76.4 cd A^?1, and 17.1 lm W^?1 for green emission, and 1600 cd m^?2, 8.8 cd A^?1, and 1.8 lm W^?1 for orange‐red emission. The high luminance and efficiency, and solution process pave the way to industrial roll‐to‐roll manufacturing of solid state lighting and display.     

Facile Assembly of High‐Quality Organic–Inorganic Hybrid Perovskite Quantum Dot Thin Films for Bright Light‐Emitting Diodes

Organic‐inorganic hybrid perovskite (CH₃NH₃PbX₃, X = Cl, Br or I) quantum dots (QDs) have shown superior optoelectronic properties and have been regarded as a most ideal material for next‐generation optoelectronic devices, particularly for QDs‐based light‐emitting diodes (QLEDs). However, there are only a few reports on CH₃NH₃PbX₃ QLEDs and the reported performance is still very poor, primarily due to the difficulties in the fabrication of high‐quality compact QDs thin films. In this work, an electric‐field‐assisted strategy is developed for efficient fabrication of uniform CH₃NH₃PbBr₃ QDs thin films with high photoluminescence quantum yields (PLQY, 80%–90%) from dilute CH₃NH₃PbBr₃ QDs suspensions (≈0.1 mg mL^‐1) within 5 mins. Benefited from the high‐quality CH₃NH₃PbBr₃ QDs thin films, the corresponding QLEDs deliver a highly bright green emission with maximum luminances of 12450 cd m². Furthermore, a current efficiency of 12.7 cd A^‐1, a power efficiency of 9.7 lm W^‐1, and an external quantum efficiency (EQE) of 3.2% were acheived by enhancing the hole injection. This performance represents the best results for CH₃NH₃PbBr₃ QDs‐based QLEDs reported to date. These results indicate an important progress in the fabrication of high‐performance CH₃NH₃PbX₃ QLEDs and demonstrate their huge potential for next‐generation displays and lighting.     

Low‐Power Nonvolatile Charge Storage Memory Based on MoS2 and an Ultrathin Polymer Tunneling Dielectric

Low‐power, nonvolatile memory is an essential electronic component to store and process the unprecedented data flood arising from the oncoming Internet of Things era. Molybdenum disulfide (MoS₂) is a 2D material that is increasingly regarded as a promising semiconductor material in electronic device applications because of its unique physical characteristics. However, dielectric formation of an ultrathin low‐k tunneling on the dangling bond‐free surface of MoS₂ is a challenging task. Here, MoS₂‐based low‐power nonvolatile charge storage memory devices are reported with a poly(1,3,5‐trimethyl‐1,3,5‐trivinyl cyclotrisiloxane) (pV3D3) tunneling dielectric layer formed via a solvent‐free initiated chemical vapor deposition (iCVD) process. The surface‐growing polymerization and low‐temperature nature of the iCVD process enable the conformal growing of low‐k (≈2.2) pV3D3 insulating films on MoS₂. The fabricated memory devices exhibit a tunable memory window with high on/off ratio (≈10⁶), excellent retention times of 10⁵ s with an extrapolated time of possibly years, and an excellent cycling endurance of more than 10³ cycles, which are much higher than those reported previously for MoS₂‐based memory devices. By leveraging the inherent flexibility of both MoS₂ and polymer dielectric films, this research presents an important milestone in the development of low‐power flexible nonvolatile memory devices.     

High‐Performance Quantum‐Dot Light‐Emitting Diodes Using NiOx Hole‐Injection Layers with a High and Stable Work Function

Solution‐processed oxide thin films are actively pursued as hole‐injection layers (HILs) in quantum‐dot light‐emitting diodes (QLEDs), aiming to improve operational stability. However, device performance is largely limited by inefficient hole injection at the interfaces of the oxide HILs and high‐ionization‐potential organic hole‐transporting layers. Solution‐processed NiO_x films with a high and stable work function of ≈5.7 eV achieved by a simple and facile surface‐modification strategy are presented. QLEDs based on the surface‐modified NiO_x HILs show driving voltages of 2.1 and 3.3 V to reach 1000 and 10 000 cd m^?2, respectively, both of which are the lowest among all solution‐processed LEDs and vacuum‐deposited OLEDs. The device exhibits a T₉₅ operational lifetime of ≈2500 h at an initial brightness of 1000 cd m^?2, meeting the commercialization requirements for display applications. The results highlight the potential of solution‐processed oxide HILs for achieving efficient‐driven and long‐lifetime QLEDs.     

High‐Throughput Combinatorial Optimizations of Perovskite Light‐Emitting Diodes Based on All‐Vacuum Deposition

Light‐emitting diodes (LEDs) based on lead halide perovskites demonstrate outstanding optoelectronic properties and are strong competitors for display and lighting applications. While previous halide perovskite LEDs are mainly produced via solution processing, here an all‐vacuum processing method is employed to construct CsPbBr₃ LEDs because vacuum processing exhibits high reliability and easy integration with existing OLED facilities for mass production. The high‐throughput combinatorial strategies are further adopted to study perovskite composition, annealing temperature, and functional layer thickness, thus significantly speeding up the optimization process. The best rigid device shows a current efficiency (CE) of 4.8 cd A^?1 (EQE of 1.45%) at 2358 cd m^?2, and best flexible device shows a CE of 4.16 cd A^?1 (EQE of 1.37%) at 2012 cd m^?2 with good bending tolerance. Moreover, by choosing NiO_x as the hole‐injection layer, the CE is improved to 10.15 cd A^?1 and EQE is improved to a record of 3.26% for perovskite LEDs produced by vacuum deposition. The time efficient combinatorial approaches can also be applied to optimize other perovskite LEDs.     

Surface‐Modified High‐k Oxide Gate Dielectrics for Low‐Voltage High‐Performance Pentacene Thin‐Film Transistors

In this study, pentacene thin‐film transistors (TFTs) operating at low voltages with high mobilities and low leakage currents are successfully fabricated by the surface modification of the CeO₂–SiO₂ gate dielectrics. The surface of the gate dielectric plays a crucial role in determining the performance and electrical reliability of the pentacene TFTs. Nearly hysteresis‐free transistors are obtained by passivating the devices with appropriate polymeric dielectrics. After coating with poly(4‐vinylphenol) (PVP), the reduced roughness of the surface induces the formation of uniform and large pentacene grains; moreover, –OH groups on CeO₂–SiO₂ are terminated by C₆H₅, resulting in the formation of a more hydrophobic surface. Enhanced pentacene quality and reduced hysteresis is observed in current–voltage (I–V) measurements of the PVP‐coated pentacene TFTs. Since grain boundaries and –OH groups are believed to act as electron traps, an OH‐free and smooth gate dielectric leads to a low trap density at the interface between the pentacene and the gate dielectric. The realization of electrically stable devices that can be operated at low voltages makes the OTFTs excellent candidates for future flexible displays and electronics applications.     

Characteristics of Solution‐Processed Small‐Molecule Organic Films and Light‐Emitting Diodes Compared with their Vacuum‐Deposited Counterparts

Although significant progress has been made in the development of vacuum‐deposited small‐molecule organic light‐emitting diodes (OLEDs), one of the most desired research goals is still to produce flexible displays by low‐cost solution processing. The development of solution‐processed OLEDs based on small molecules could potentially be a good approach but no intensive studies on this topic have been conducted so far. To fabricate high‐performance devices based on solution‐processed small molecules, the underlying nature of the produced films and devices must be elucidated. Here, the distinctive characteristics of solution‐processed small‐molecule films and devices compared to their vacuum‐deposited counterparts are reported. Solution‐processed blue OLEDs show a very high luminous efficiency (of about 8.9 cd A^–1) despite their simplified structure. A better hole‐blocking and electron‐transporting layer is essential for achieving high‐efficiency solution‐processed devices because the solution‐processed emitting layer gives the devices a better hole‐transporting capability and more electron traps than the vacuum‐deposited layer. It is found that the lower density of the solution‐processed films (compared to the vacuum‐deposited films) can be a major cause for the short lifetimes observed for the corresponding devices.     

Over 100 cd A−1 Efficient Quantum Dot Light‐Emitting Diodes with Inverted Tandem Structure

Quantum dot light‐emitting diodes (QLEDs) with tandem structure are promising candidates for future displays because of their advantages of pure emission color, long lifetime, high brightness, and high efficiency. To obtain efficient QLEDs, a solution‐processable interconnecting layer (ICL) based on poly(3, 4‐ethylenedioxythiophene)/polystyrene sulfonate/ZnMgO is developed. With the proposed ICL, all‐solution‐processed, inverted, tandem QLEDs are demonstrated with high current efficiency (CE) of 57.06 cd A^?1 and external quantum efficiency (EQE) of 13.65%. By further optimizing the fabrication processes and using a hybrid deposition technique, the resultant tandem QLEDs exhibit a very high CE over 100 cd A^?1 and an impressive EQE over 23%, which are the highest values ever reported and are comparable with those of the state‐of‐the‐art phosphorescent organic LEDs. Moreover, the efficiency roll‐off, a notorious phenomenon in phosphorescent LEDs, is significantly reduced in the developed QLEDs. For example, even at a very high brightness over 200 000 cd m^?2, the tandem QLEDs can still maintain a high CE of 96.47 cd A^?1 and an EQE of 22.62%. The proposed ICL and the developed fabrication methods allow for realization of very efficient tandem QLEDs for next generation display and lighting applications.     

High‐Performance Organic Light‐Emitting Diodes Based on Intramolecular Charge‐Transfer Emission from Donor–Acceptor Molecules: Significance of Electron‐ Donor Strength and Molecular Geometry

Organic light‐emitting diodes based on intramolecular‐charge‐transfer emission from two related donor–acceptor (D–A) molecules, 3,7‐[bis(4‐phenyl‐2‐quinolyl)]‐10‐methylphenothiazine (BPQ‐MPT) and 3,6‐[bis(4‐phenyl‐2‐quinolyl)]‐9‐methylcarbazole (BPQ‐MCZ), were found to have electroluminescence (EL) efficiencies and device brightnesses that differ by orders of magnitude. High brightness (> 40 000 cd m^–2) and high efficiency (21.9 cd A^–1, 10.8 lm W^–1, 5.78 % external quantum efficiency (EQE) at 1140 cd m^–2) green EL was achieved from the BPQ‐MPT emitter, which has its highest occupied molecular orbital (HOMO) level at 5.09 eV and a nonplanar geometry. In contrast, diodes with much lower brightness (2290 cd m^–2) and efficiency (1.4 cd A^–1, 0.66 lm W^–1, 1.7 % EQE at 405 cd m^–2) were obtained from the BPQ‐MCZ emitter, which has its HOMO level at 5.75 eV and exhibits a planar geometry. Compared to BPQ‐MCZ, the higher‐lying HOMO level of BPQ‐MPT facilitates more efficient hole injection/transport and a higher charge‐recombination rate, while its nonplanar geometry ensures diode color purity. White EL was observed from BPQ‐MCZ diodes owing to a blue intramolecular charge‐transfer emission and a yellow–orange intermolecular excimer emission, enabled by the planar molecular geometry. These results demonstrate that high‐performance light‐emitting devices can be achieved from intramolecular charge‐transfer emission, while highlighting the critical roles of the electron‐donor strength and the molecular geometry of D–A molecules.     

Printed,Flexible, Organic Nano‐Floating‐Gate Memory: Effects of Metal Nanoparticles and Blocking Dielectrics on Memory Characteristics

The effects of using a blocking dielectric layer and metal nanoparticles (NPs) as charge‐trapping sites on the characteristics of organic nano‐floating‐gate memory (NFGM) devices are investigated. High‐performance NFGM devices are fabricated using the n‐type polymer semiconductor, poly{[N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)), and various metal NPs. These NPs are embedded within bilayers of various polymer dielectrics (polystyrene (PS)/poly(4‐vinyl phenol) (PVP) and PS/poly(methyl methacrylate) (PMMA)). The P(NDI2OD‐T2) organic field‐effect transistor (OFET)‐based NFGM devices exhibit high electron mobilities (0.4–0.5 cm² V^?1 s^?1) and reliable non‐volatile memory characteristics, which include a wide memory window (≈52 V), a high on/off‐current ratio (I_on/I_off ≈ 10⁵), and a long extrapolated retention time (>10⁷ s), depending on the choice of the blocking dielectric (PVP or PMMA) and the metal (Au, Ag, Cu, or Al) NPs. The best memory characteristics are achieved in the ones fabricated using PMMA and Au or Ag NPs. The NFGM devices with PMMA and spatially well‐distributed Cu NPs show quasi‐permanent retention characteristics. An inkjet‐printed flexible P(NDI2OD‐T2) 256‐bit transistor memory array (16 × 16 transistors) with Au‐NPs on a polyethylene naphthalate substrate is also fabricated. These memory devices in array exhibit a high I_on/I_off (≈10^{4 ± 0.85}), wide memory window (≈43.5 V ± 8.3 V), and a high degree of reliability.     

Solution‐Processed,Alkali Metal‐Salt‐Doped,Electron‐Transport Layers for High‐Performance Phosphorescent Organic Light‐Emitting Diodes

High‐performance, blue, phosphorescent organic light‐emitting diodes (PhOLEDs) are achieved by orthogonal solution‐processing of small‐molecule electron‐transport material doped with an alkali metal salt, including cesium carbonate (Cs₂CO₃) or lithium carbonate (Li₂CO₃). Blue PhOLEDs with solution‐processed 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) electron‐transport layer (ETL) doped with Cs₂CO₃ show a luminous efficiency (LE) of 35.1 cd A^?1 with an external quantum efficiency (EQE) of 17.9%, which are two‐fold higher efficiency than a BPhen ETL without a dopant. These solution‐processed blue PhOLEDs are much superior compared to devices with vacuum‐deposited BPhen ETL/alkali metal salt cathode interfacial layer. Blue PhOLEDs with solution‐processed 1,3,5‐tris(m‐pyrid‐3‐yl‐phenyl)benzene (TmPyPB) ETL doped with Cs₂CO₃ have a luminous efficiency of 37.7 cd A^?1 with an EQE of 19.0%, which is the best performance observed to date in all‐solution‐processed blue PhOLEDs. The results show that a small‐molecule ETL doped with alkali metal salt can be realized by solution‐processing to enhance overall device performance. The solution‐processed metal salt‐doped ETLs exhibit a unique rough surface morphology that facilitates enhanced charge‐injection and transport in the devices. These results demonstrate that orthogonal solution‐processing of metal salt‐doped electron‐transport materials is a promising strategy for applications in various solution‐processed multilayered organic electronic devices.     

Novel Film‐Casting Method for High‐Performance Flexible Polymer Electrodes

A new film‐casting method for polymer electrodes is reported, in which thickness‐controlled drop‐casting (TCDC), using polyaniline doped with camphorsulfonic acid (PANI:CSA) is used. By combining the advantages of conventional spin‐casting and drop‐casting methods, and by rigorously controlling the film formation parameters, flexible polymer electrodes with high conductivity and excellent transmittance can be produced. The PANI:CSA electrodes cast by the TCDC method exhibited constant thickness‐independent conductivities of ～600 S cm^?1 down to a film thickness of 0.2 μm, and a high optical transmittance of about 85% at 550 nm. Furthermore, the new casting method significantly reduced the sheet resistance (～90 Ω/square) of the PANI:CSA electrodes compared with the conventional spin‐cast films, enhancing the performance of the devices deposited on plastic substrates. The flexible polymer light‐emitting diode produced a brightness of 6000 cd m^?2, and the flexible polymer solar cell exhibited a power conversion efficiency of 2%, both of which were much higher than those of the devices fabricated by the conventional spin‐casting method.     

High‐Performance,Air‐Stable,Top‐Gate,p‐Channel WSe2 Field‐Effect Transistor with Fluoropolymer Buffer Layer

High‐performance, air‐stable, p‐channel WSe₂ top‐gate field‐effect transistors (FETs) using a bilayer gate dielectric composed of high‐ and low‐k dielectrics are reported. Using only a high‐k Al₂O₃ as the top‐gate dielectric generally degrades the electrical properties of p‐channel WSe₂, therefore, a thin fluoropolymer (Cytop) as a buffer layer to protect the 2D channel from high‐k oxide forming is deposited. As a result, a top‐gate‐patterned 2D WSe₂ FET is realized. The top‐gate p‐channel WSe₂ FET demonstrates a high hole mobility of 100 cm²_­ V^?1 s^?1 and a I_ON/I_OFF ratio > 10⁷ at low gate voltages (V_GS ca. ?4 V) and a drain voltage (V_DS) of ?1 V on a glass substrate. Furthermore, the top‐gate FET shows a very good stability in ambient air with a relative humidity of 45% for 7 days after device fabrication. Our approach of creating a high‐k oxide/low‐k organic bilayer dielectric is advantageous over single‐layer high‐k dielectrics for top‐gate p‐channel WSe₂ FETs, which will lead the way toward future electronic nanodevices and their integration.     

Kinked Star‐Shaped Fluorene/ Triazatruxene Co‐oligomer Hybrids with Enhanced Functional Properties for High‐Performance,Solution‐Processed,Blue Organic Light‐Emitting Diodes

A novel series of kinked star‐shaped oligofluorene/triazatruxene hybrids are conveniently prepared via a powerful microwave‐enhanced multiple coupling methodology. Constructing kinked star‐shaped architectures can effectively suppress crystallization and aggregation. The resulting materials are highly amorphous, showing stable amorphous morphology against crystallization. A triazatruxene core endows the materials with elevated highest occupied molecular orbital (HOMO) levels that are well matched to the anode work function, leading to a significantly improved hole‐injection property. They hybrids are highly luminescent in both solution (quantum yield is 0.52–0.80) and the solid‐state (quantum yield is 0.45–0.76) with bright blue emission. Remarkably, solution‐processed devices displaying single‐layer electroluminescence (EL) based on these oligomers exhibit efficient blue EL and demonstrate striking color stability, almost unchanged with increasing driving voltage. The best device performance has a rather low turn‐on voltage (3.3 V) and a high device efficiency (2.16 % @ 2382 cd m^–2) as well as a high brightness (7714 cd m^–2 @ 10 V) with CIE coordinates of (0.16, 0.15); it shows remarkably better EL performance than devices based on linear oligofluorene or polyfluorene counterparts. The results prove that an oligomer with kinked star‐shaped architecture is extremely promising for efficient and stable blue EL. The reasons for the enhanced functional properties and the improved color stability are discussed in relation to the chemical structures and components.     

Thermally Activated Delayed Fluorescence Material as Host with Novel Spiro‐Based Skeleton for High Power Efficiency and Low Roll‐Off Blue and White Phosphorescent Devices

Efficiency roll‐off in blue organic light‐emitting diodes especially at high brightness still remains a vital issue for which the excitons density‐dependent mechanism of host materials takes most responsibility. Additionally, the efficiency roll‐off leads to high power consumption and reduces the operating lifetime because higher driving voltage and current are required. Here, by subtly modifying the triphenylamine to oxygen‐bridged quasi‐planar structure, a novel thermally activated delayed fluorescence type blue host Tri‐o‐2PO is successfully developed. Efficiency roll‐off based on Tri‐o‐2PO is ultralow with external quantum efficiency (EQE) just dropping by around 2% in the high luminance range from 1000 cd m^?2 to 10 000 cd m^?2. As expected, low turn‐on voltage (≈2.9 V) of device is also achieved, which is close to the theory limit value (≈2.62 V). Super‐high power efficiency (≈60 lm W^?1) and EQE (>22%) are also achieved when utilizing Tri‐o‐2PO as host. Furthermore, two‐color warm‐white light with CIE of (0.45, 0.43) and correlated color temperature of 2921 K is also fabricated and a champion EQE of 21% is delivered. These excellent performances prove the strategy of bridging the triphenylamine to reduce ΔE_st is validated and suggest the great potential of this novel skeleton.