Highly Efficient Non‐Doped Blue Organic Light‐Emitting Diodes Based on Fluorene Derivatives with High Thermal Stability

A new series of blue‐light‐emitting fluorene derivatives have been synthesized and characterized. The fluorene derivatives have high fluorescence yields, good thermal stability, and high glass‐transition temperatures in the range 145–193 °C. Organic light‐emitting diodes (OLEDs) fabricated using the fluorene derivatives as the host emitter show high efficiency (up to 5.3 cd A^–1 and 3.0 lm W^–1) and bright blue‐light emission (Commission Internationale de L'Eclairage (CIE) coordinates of x = 0.16, y = 0.22). The performance of the non‐doped fluorene‐based devices is among the best fluorescent blue‐light‐emitting OLEDs. The good performance of the present blue OLEDs is considered to derive from: 1) appropriate energy levels of the fluorene derivatives for good carrier injection; 2) good carrier‐transporting properties; and 3) high fluorescence efficiency of the fluorene derivatives. These merits are discussed in terms of the molecular structures.     

A Multifunctional Bipolar Luminogen with Delayed Fluorescence for High‐Performance Monochromatic and Color‐Stable Warm‐White OLEDs

Increasing exciton utilization and reducing exciton annihilation are crucial to achieve high performance of organic light‐emitting diodes (OLEDs), which greatly depend on molecular engineering of emitters and hosts. A novel luminogen (SBF‐BP‐DMAC) is synthesized and characterized. Its crystal and electronic structures, thermal stability, electrochemical behavior, carrier transport, photoluminescence, and electroluminescence are investigated. SBF‐BP‐DMAC exhibits enhanced photoluminescence and promotes delayed fluorescence in solid state and bipolar carrier transport ability, and thus holds multifunctionality of emitter and host for OLEDs. Using SBF‐BP‐DMAC as an emitter, the nondoped OLEDs exhibit maximum electroluminescence (EL) efficiencies of 67.2 cd A^?1, 65.9 lm W^?1, and 20.1%, and the doped OLEDs provide maximum EL efficiencies of 79.1 cd A^?1, 70.7 lm W^?1, and 24.5%. A representative orange phosphor, Ir(tptpy)₂acac, is doped into SBF‐BP‐DMAC for OLED fabrication, giving rise to superior EL efficiencies of 88.0 cd A^?1, 108.0 lm W^?1, and 26.8% for orange phosphorescent OLEDs, and forward‐viewing EL efficiencies of 69.3 cd A^?1, 45.8 lm W^?1, and 21.0% for two‐color hybrid warm‐white OLEDs. All of these OLEDs can retain high EL efficiencies at high luminance, with very small efficiency roll‐offs. The outstanding EL performance demonstrates the great potentials of SBF‐BP‐DMAC in practical display and lighting devices.     

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.     

Multifunctional Fluorene‐Based Oligomers with Novel Spiro‐Annulated Triarylamine: Efficient,Stable Deep‐Blue Electroluminescence,Good Hole Injection,and Transporting Materials with Very High Tg

A series of fluorene‐based oligomers with novel spiro‐annulated triarylamine structures, namely DFSTPA, TFSTPA, and TFSDTC, are synthesized by a Suzuki cross‐coupling reaction. The spiro‐configuration molecular structures lead to very high glass transition temperatures (197–253 °C) and weak intermolecular interactions, and consequently the structures retain good morphological stability and high fluorescence quantum efficiencies(0.69–0.98). This molecular design simultaneously solves the spectral stability problems and hole‐injection and transport issues for fluorene‐based blue‐light‐emitting materials. Simple double‐layer electroluminescence (EL) devices with a configuration of ITO/TFSTPA (device A) or TFSDTC (device B)/ TPBI/LiF/Al, where TFSTPA and TFSDTC serve as hole‐transporting blue‐light‐emitting materials, show a deep‐blue emission with a peak around 432 nm, and CIE coordinates of (0.17, 0.12) for TFSTPA and (0.16, 0.07) for TFSDTC, respectively, which are very close to the National Television System Committee (NTSC) standard for blue (0.15, 0.07). The maximum current efficiency/external quantum efficiencies are 1.63 cd A^?1/1.6% for device A and 1.91 cd A^?1/2.7% for device B, respectively. In addition, a device with the structure ITO/DFSTPA/Alq₃/LiF/Al, where DFSTPA acts as both the hole‐injection and ‐transporting material, is shown to achieve a good performance, with a maximum luminance of 14 047 cd m^?2, and a maximum current efficiency of 5.56 cd A^?1. These values are significantly higher than those of devices based on commonly usedN,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐[1,1′‐biphenyl]‐4,4′‐diamine (NPB) as the hole‐transporting layer (11 738 cd m^?2 and 3.97 cd A^?1) under identical device conditions.     

Unexpected Sole Enol‐Form Emission of 2‐(2′‐Hydroxyphenyl)oxazoles for Highly Efficient Deep‐Blue‐Emitting Organic Electroluminescent Devices

Considerable efforts have been devoted to the development of highly efficient blue light‐emitting materials. However, deep‐blue fluorescence materials that can satisfy the Commission Internationale de l'Eclairage (CIE) coordinates of (0.14, 0.08) of the National Television System Committee (NTSC) standard blue and, moreover, possess a high external quantum efficiency (EQE) over 5%, remain scarce. Here, the unusual luminescence properties of triphenylamine‐bearing 2‐(2′‐hydroxyphenyl)oxazoles ( 3a–3c ) and their applications in organic light‐emitting diodes (OLEDs) are reported as highly efficient deep‐blue emitters. The 3a ‐based device exhibits a high spectral stability and an excellent color purity with a narrow full‐width at half‐maximum of 53 nm and the CIE coordinates of (0.15, 0.08), which is very close to the NTSC standard blue. The exciton utilization of the device closes to 100%, exceeding the theoretical limit of 25% in conventional fluorescent OLEDs. Experimental data and theoretical calculations demonstrate that 3a possesses a highly hybridized local and charge‐transfer excited state character. In OLEDs, 3a exhibits a maximum luminance of 9054 cd m^?2 and an EQE up to 7.1%, which is the first example of highly efficient blue OLEDs based on the sole enol‐form emission of 2‐(2′‐hydroxyphenyl)azoles.     

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.     

Highly Efficient Non‐Doped Blue‐Light‐Emitting Diodes Based on an Anthrancene Derivative End‐Capped with Tetraphenylethylene Groups

A novel blue‐emitting material, 2‐tert‐butyl‐9,10‐bis[4‐(1,2,2‐triphenylvinyl)phenyl]anthracene ( TPVAn ), which contains an anthracene core and two tetraphenylethylene end‐capped groups, has been synthesized and characterized. Owing to the presence of its sterically congested terminal groups, TPVAn possesses a high glass transition temperature (155 °C) and is morphologically stable. Organic light‐emitting diodes (OLEDs) utilizing TPVAn as the emitter exhibit bright saturated‐blue emissions (Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of x = 0.14 and y = 0.12) with efficiencies as high as 5.3 % (5.3 cd A^–1)—the best performance of non‐doped deep blue‐emitting OLEDs reported to date. In addition, TPVAn doped with an orange fluorophore served as an authentic host for the construction of a white‐light‐emitting device that displayed promising electroluminescent characteristics: the maximum external quantum efficiency reached 4.9 % (13.1 cd A^–1) with CIE coordinates located at (0.33, 0.39).     

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.     

Efficient Nondoped Blue Fluorescent Organic Light‐Emitting Diodes (OLEDs) with a High External Quantum Efficiency of 9.4% @ 1000 cd m−2 Based on Phenanthroimidazole−Anthracene Derivative

Organic light‐emitting diodes (OLEDs) can promise flexible, light weight, energy conservation, and many other advantages for next‐generation display and lighting applications. However, achieving efficient blue electroluminescence still remains a challenge. Though both phosphorescent and thermally activated delayed fluorescence materials can realize high‐efficiency via effective triplet utilization, they need to be doped into appropriate host materials and often suffer from certain degree of efficiency roll‐off. Therefore, developing efficient blue‐emitting materials suitable for nondoped device with little efficiency roll‐off is of great significance in terms of practical applications. Herein, a phenanthroimidazole?anthracene blue‐emitting material is reported that can attain high efficiency at high luminescence in nondoped OLEDs. The maximum external quantum efficiency (EQE) of nondoped device is 9.44% which is acquired at the luminescence of 1000 cd m^?2. The EQE is still as high as 8.09% even the luminescence reaches 10 000 cd m^?2. The maximum luminescence is ≈57 000 cd m^?2. The electroluminescence (EL) spectrum shows an emission peak of 470 nm and the Commission International de L'Eclairage (CIE) coordinates is (0.14, 0.19) at the voltage of 7 V. To the best of the knowledge, this is among the best results of nondoped blue EL devices.     

Solution‐Processable Hole‐Generation Layer and Electron‐Transporting Layer: Towards High‐Performance,Alternating‐Current‐Driven,Field‐Induced Polymer Electroluminescent Devices

The effect of solution‐processed p‐type doping of hole‐generation layers (HGLs) and electron‐transporting layer (ETLs) are systematically investigated on the performance of solution‐processable alternating current (AC) field‐induced polymer EL (FIPEL) devices in terms of hole‐generation capability of HGLs and electron‐transporting characteristics of ETLs. A variety of p‐type doping conjugated polymers and a series of solution‐processed electron‐transporting small molecules are employed. It is found that the free hole density in p‐type doping HGLs and electron mobility of solution‐processed ETLs are directly related to the device performance, and that the hole‐transporting characteristics of ETLs also play an important role since holes need to be injected from electrode through ETLs to refill the depleted HGLs in the positive half of the AC cycle. As a result, the best FIPEL device exhibits exceptional performance: a low turn‐on voltage of 12 V, a maximum luminance of 20 500 cd m^?2, a maximum current and power efficiency of 110.7 cd A^?1 and 29.3 lm W^?1. To the best of the authors' knowledge, this is the highest report to date among FIPEL devices driven by AC voltage.     

Manipulation of Charge and Exciton Distribution Based on Blue Aggregation‐Induced Emission Fluorophors: A Novel Concept to Achieve High‐Performance Hybrid White Organic Light‐Emitting Diodes

The aggregation‐induced emission (AIE) phenomenon is important in organic light‐emitting diodes (OLEDs), for it can potentially solve the aggregation‐caused quenching problem. However, the performance of AIE fluorophor‐based OLEDs (AIE OLEDs) is unsatisfactory, particularly for deep‐blue devices (CIEy < 0.15). Here, by enhancing the device engineering, a deep‐blue AIE OLED exhibits low voltage (i.e., 2.75 V at 1 cd m^?2), high luminance (17 721 cd m^?2), high efficiency (4.3 lm W^?1), and low efficiency roll‐off (3.6 lm W^?1 at 1000 cd m^?2), which is the best deep‐blue AIE OLED. Then, blue AIE fluorophors, for the first time, have been demonstrated to achieve high‐performance hybrid white OLEDs (WOLEDs). The two‐color WOLEDs exhibit i) stable colors and the highest efficiency among pure‐white hybrid WOLEDs (32.0 lm W^?1); ii) stable colors, high efficiency, and very low efficiency roll‐off; or iii) unprecedented efficiencies at high luminances (i.e., 70.2 cd A^?1, 43.4 lm W^?1 at 10 000 cd m^?2). Moreover, a three‐color WOLED exhibits wide correlated color temperatures (10 690–2328 K), which is the first hybrid WOLED showing sunlight‐style emission. These findings will open a novel concept that blue AIE fluorophors are promising candidates to develop high‐performance hybrid WOLEDs, which have a bright prospect for the future displays and lightings.     

Mechanistic Study on High Efficiency Deep Blue AIE‐Based Organic Light‐Emitting Diodes by Magneto‐Electroluminescence

Aggregation‐induced emission (AIE) materials are highly attractive because of their excellent properties of high efficiency emission in nondoped organic light‐emitting diodes (OLEDs). Therefore, a deep understanding of the working mechanisms, further improving the electroluminescence (EL) efficiency of the resulting AIE‐based OLEDs, is necessary. Herein, the conversion process from higher energy triplet state (T₂) to the lowest singlet state (SS₁) is found in OLEDs based on a blue AIE material, 4′‐(4‐(diphenylamino)phenyl)‐5′‐phenyl‐[1,1′:2′,1′′‐terphenyl]‐4‐carbonitrile (TPB‐AC), obviously relating to the device efficiency, by magneto‐EL (MEL) measurements. A special line shape with rise at low field and reduction at high field is observed. The phenomenon is further clarified by theoretical calculations, temperature‐dependent MELs, and transient photoluminescence emission properties. On the basis of the T₂‐S₁ conversion process, the EL performances of the blue OLEDs based on TPB‐AC are further enhanced by introducing a phosphorescence doping emitter in the emitting layer, which effectively regulates the excitons on TPB‐AC molecules. The maximum external quantum efficiency (EQE) reaches 7.93% and the EQE keeps 7.57% at the luminance of 1000 cd m^?2. This work establishes a physical insight for designing high‐performance AIE materials and devices in the future.     

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.     

Efficient Deep‐Blue Organic Light‐Emitting Diodes: Arylamine‐Substituted Oligofluorenes

Novel deep‐blue‐light‐emitting diphenylamino and triphenylamino end‐capped oligofluorenes were synthesized by double palladium‐catalyzed Suzuki cross‐coupling of dibromo‐oligofluorene with the corresponding boronic acid as a key step. These oligofluorenes exhibit deep‐blue emission (λ^em_max = 429–432 nm), low and reversible electrochemical oxidation (highest occupied molecular orbital = 5.15–5.20 eV), high fluorescence quantum yield (Φ_FL = 0.61–0.93), and good thermal properties (glass‐transition temperature, T_g = 99–195 °C and decomposition temperature, T_dec > 450 °C). Remarkably, saturated deep‐blue organic light‐emitting diodes, made from these oligofluorenes as dopant emitters, have been achieved with excellent performance and maximum efficiencies up to 2.9 cd A^–1 at 2 mA cm^–2 (external quantum efficiency of 4.1 %) and with Commission Internationale de l'Éclairage (x,y) coordinates of (0.152,0.08), which is very close to the National Television System Committee standard blue.     

Blue‐Light‐Emitting Oligoquinolines: Synthesis,Properties, and High‐Efficiency Blue‐Light‐Emitting Diodes

The synthesis, photophysics, cyclic voltammetry, and highly efficient blue electroluminescence of a series of four new n‐type conjugated oligomers, 6,6′‐bis(2,4‐diphenylquinoline) (B1PPQ), 6,6′‐bis(2‐(4‐tert‐butylphenyl)‐4‐phenylquinoline) (BtBPQ), 6,6′‐bis(2‐p‐biphenyl)‐4‐phenylquinoline) (B2PPQ), and 6,6′‐bis((3,5‐diphenylbenzene)‐4‐phenylquinoline) (BDBPQ) is reported. The oligoquinolines have high glass‐transition temperatures (T_g ≥ 133 °C), reversible electrochemical reduction, and high electron affinities (2.68–2.81 eV). They emit blue photoluminescence with 0.73–0.94 quantum yields and 1.06–1.42 ns lifetimes in chloroform solutions. High‐performance organic light‐emitting diodes (OLEDs) with excellent blue chromaticity coordinates are achieved from all the oligoquinolines. OLEDs based on B2PPQ as the blue emitter give the best performance with a high brightness (19 740 cd m^–2 at 8.0 V), high efficiency (7.12 cd A^–1 and 6.56 % external quantum efficiency at 1175 cd m^–2), and excellent blue color purity as judged by the Commission Internationale de L'Eclairage (CIE) coordinates (x = 0.15,y = 0.16). These results represent the best efficiency of blue OLEDs from neat fluorescent organic emitters reported to date. These results demonstrate the potential of oligoquinolines as emitters and electron‐transport materials for developing high‐performance blue OLEDs.     

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).     

Novel Heterolayer Organic Light‐Emitting Diodes Based on a Conjugated Dendrimer

We demonstrate a novel organic light‐emitting diode (LED) heterolayer structure that contains a conjugated dendrimer as the light‐emitting molecule. The LED was prepared by spin‐coating two dendrimer layers from the same solvent. The device consists of a graded bilayer structure formed from a neat dendrimer film covered with a film consisting of the same dendrimer but doped with the electron‐transporting material 2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole (PBD). In this device, the heterojunction interface present in conventional bilayer organic light‐emitting diodes is eliminated, and is replaced by a graded interlayer. By optimizing the concentration of PBD in the dendrimer, a peak electroluminescence (EL) external quantum efficiency of 0.16 % at 600 cd m^–2 was obtained. The EL quantum efficiency is significantly enhanced in comparison with devices based on a single layer, a conventional bilayer, and a single‐layer doped with PBD. The EL quantum efficiency is a factor of eight larger than that of a conventional bilayer LED made with the conjugated dendrimer as the emissive layer and poly(methylmethacrylate) (PMMA) doped with PBD as the electron‐transporting layer. The best blended device exhibited only one third of the efficiency of the graded device. The improvement in the operating characteristics of the graded device is attributed to the efficient device structure, in which exciton formation is improved by a graded doping profile of electron‐ and hole‐transporting components.     

An Alternative Approach to Constructing Solution Processable Multifunctional Materials: Their Structure,Properties, and Application in High‐Performance Organic Light‐Emitting Diodes

A new series of full hydrocarbons, namely 4,4′‐(9,9′‐(1,3‐phenylene)bis(9H‐fluorene‐9,9‐diyl))bis(N,N‐diphenylaniline) (DTPAFB), N,N′‐(4,4′‐(9,9′‐(1,3‐phenylene)bis(9H‐fluorene‐9,9‐diyl))bis(4,1‐phenylene))bis(N‐phenylnaphthalen‐1‐amine) (DNPAFB), 1,3‐bis(9‐(4‐(9H‐carbazol‐9‐yl)phenyl)‐9H‐fluoren‐9‐yl)benzene, and 1,3‐bis(9‐(4‐(3,6‐di‐tert‐butyl‐9H‐carbazol‐9‐yl)phenyl)‐9H‐fluoren‐9‐yl)benzene, featuring a highly twisted tetrahedral conformation, are designed and synthesized. Organic light‐emitting diodes (OLEDs) comprising DNPAFB and DTPAFB as hole transporting layers and tris(quinolin‐8‐yloxy)aluminum as an emitter are made either by vacuum deposition or by solution processing, and show much higher maximum efficiencies than the commonly used N,N′‐di(naphthalen‐1‐yl)‐N,N′‐diphenylbiphenyl‐4,4′‐diamine device (3.6 cd A^?1) of 7.0 cd A^?1 and 6.9 cd A^?1, respectively. In addition, the solution processed blue phosphorescent OLEDs employing the synthesized materials as hosts and iridium (III) bis[(4,6‐di‐fluorophenyl)‐pyridinato‐N, C²] picolinate (FIrpic) phosphor as an emitter present exciting results. For example, the DTPAFB device exhibits a brightness of 47 902 cd m^?2, a maximum luminescent efficiency of 24.3 cd A^?1, and a power efficiency of 13.0 lm W^?1. These results show that the devices are among the best solution processable blue phosphorescent OLEDs based on small molecules. Moreover, a new approach to constructing solution processable small molecules is proposed based on rigid and bulky fluorene and carbazole moieties combined in a highly twisted configuration, resulting in excellent solubility as well as chemical miscibility, without the need to introduce any solubilizing group such as an alkyl or alkoxy chain.     

Monothiatruxene‐Based,Solution‐Processed Green,Sky‐Blue,and Deep‐Blue Organic Light‐Emitting Diodes with Efficiencies Beyond 5% Limit

The development of blue materials with good efficiency, even at high brightness, with excellent color purity, simple processing, and high thermal stability assuring adequate device lifetime is an important remaining challenge for organic light‐emitting didoes (OLEDs) in displays and lightning applications. Furthermore, these various features are typically mutually exclusive in practice. Herein, four novel green and blue light‐emitting materials based on a monothiatruxene core are reported together with their photophysical and thermal properties, and performance in solution‐processed OLEDs. The materials show excellent thermal properties with high glass transition temperatures ranging from 171 to 336 °C and decomposition temperatures from 352 to 442 °C. High external quantum efficiencies of 3.7% for a deep‐blue emitter with CIE color co‐ordinates (0.16, 0.09) and 7% for green emitter with color co‐ordinates (0.22, 0.40) are achieved at 100 cd m^?2. The efficiencies observed are exceptionally high for fluorescent materials with photoluminescence quantum yields of 24% and 62%, respectively. The performance at higher brightness is very good with only 38% and 17% efficiency roll‐offs at 1000 cd m^?2. The results indicate that utilization of this unique molecular design is promising for efficient deep‐blue highly stable and soluble light‐emitting materials.     

Highly Efficient Green‐Emitting Phosphorescent Iridium Dendrimers Based on Carbazole Dendrons

Green‐emitting iridium dendrimers with rigid hole‐transporting carbazole dendrons are designed, synthesized, and investigated. With second‐generation dendrons, the photoluminescence quantum yield of the dendrimers is up to 87 % in solution and 45 % in a film. High‐quality films of the dendrimers are fabricated by spin‐coating, producing highly efficient, non‐doped electrophosphorescent organic light‐emitting diodes (OLEDs). With a device structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/neat dendrimer/1,3,5‐tris(2‐N‐phenylbenzimidazolyl)benzene/LiF/Al, a maximum external quantum efficiency (EQE) of 10.3 % and a maximum luminous efficiency of 34.7 cd A^–1 are realized. By doping the dendrimers into a carbazole‐based host, the maximum EQE can be further increased to 16.6 %. The integration of rigid hole‐transporting dendrons and phosphorescent complexes provides a new route to design highly efficient solution‐processable dendrimers for OLED applications.