Solution‐Processible Red Iridium Dendrimers based on Oligocarbazole Host Dendrons: Synthesis,Properties, and their Applications in Organic Light‐Emitting Diodes

A series of novel red‐emitting iridium dendrimers functionalized with oligocarbazole host dendrons up to the third generation ( red‐G3 ) have been synthesized by a convergent method, and their photophysical, electrochemical, and electroluminescent properties have been investigated. In addition to controlling the intermolecular interactions, oligocarbazole‐based dendrons could also participate in the electrochemical and charge‐transporting process. As a result, highly efficient electrophosphorescent devices can be fabricated by spin‐coating from chlorobenzene solution in different device configurations. The maximum external quantum efficiency (EQE) based on the non‐doped device configuration increases monotonically with increasing dendron generation. An EQE as high as 6.3% was obtained as for the third generation dendrimer red‐G3 , which is about 30 times higher than that of the prototype red‐G0 . Further optimization of the device configuration gave an EQE of 11.8% (13.0 cd A^?1, 7.2 lm W^?1) at 100 cd m^?2 with CIE coordinates of (0.65, 0.35). The state‐of‐the‐art performance indicated the potential of these oligocarbazole‐based red iridium dendrimers as solution processible emissive materials for organic light‐emitting diode applications.     

Near-infrared thermally activated delayed fluorescent dendrimers for the efficient non-doped solution-processed organic light-emitting diodes

Two near-infrared (NIR) dendrimers with TADF characteristics are reported to develop the non-doped solution-processed OLED for the first time. The rigid ring end-capped aliphatic chain dendrons are introduced to improve the dissolvability and film-forming ability. The dendrimers possess excellent thermal and morphological stabilities. Simultaneously, the dendrimers exhibit self-host feature that the peripheral carbazole/tricarbazole dendrons can encapsulate the core to prevent concentration quenching. Employing MPPA-3Cz as the emitter, the non-doped solution-processed device exhibits a maximum external quantum efficiency (EQE) of 0.254% with a peak wavelength at 715 nm, which is comparable to the most-efficient solution-processed NIR FOLEDs with similar electroluminescent spectra. Moreover, the device shows negligible efficiency roll-off at high current density. Our results indicate that the design of long-wavelength TADF dendrimers will be a promising strategy for the efficient non-doped solution-processed NIR OLEDs.     

Facile Photo‐Crosslinking of Azide‐Containing Hole‐Transporting Polymers for Highly Efficient,Solution‐Processed,Multilayer Organic Light Emitting Devices

A novel framework of azide containing photo‐crosslinkable, conducting copolymer, that is, poly(azido‐styrene)‐random‐poly(triphenylamine) (X‐PTPA), is reported as a hole‐transporting material for efficient solution‐processed, multi‐layer, organic light emitting diodes (OLEDs). A facile and energy‐efficient crosslinking process is demonstrated with UV irradiation (254 nm, 2 mW/cm²) at a short exposure time (5 min). By careful design of X‐PTPA, in which 5 mol% of the photo‐crosslinkable poly(azido‐styrene) is copolymerized with hole‐transporting poly(triphenylamine) (X‐PTPA‐5), the adverse effect of the crosslinking of azide moieties is prevented to maximize the performances of X‐PTPA‐5. Since the photo‐crosslinking chemistry of azide molecules does not involve any photo‐initiators, superior hole‐transporting ability is achieved, producing efficient devices. To evaluate the performances of X‐PTPA‐5 as a hole‐transporting/electron‐blocking layer, Ir(ppy)₃‐based, solution‐processable OLEDs are fabricated. The results show high EQE (11.8%), luminous efficiency (43.7 cd/A), and power efficiency (10.4 lm/W), which represent about twofold enhancement over the control device without X‐PTPA‐5 film. Furthermore, micro‐patterned OLEDs with the photo‐crosslinkable X‐PTPA‐5 can be fabricated through standard photolithography. The versatility of this approach is also demonstrated by introducing the same azide moiety into other hole‐transporting materials such as poly(carbazole) (X‐PBC).     

Control of Charge Transport in Iridium(III) Complex‐Cored Carbazole Dendrimers by Generation and Structural Modification

Here, the charge transporting properties of a family of highly phosphorescent iridium(III) complex‐cored carbazole dendrimers designed to have improved charge transport by incorporating carbazole units into the dendrons are studied. Firstly, the effect of the dendrimer generation and the role of dendron for materials with one dendron per ligand of the core are considered. It is shown, in contrast to previously reported light‐emitting dendrimers, that in this case the carbazolyl‐based dendrons have an active role in charge transport. Next, the effect on the charge transport of attaching two dendrons per ligand to the dendrimer core is explored. In this latter case, for the so called “double dendron” material a highly non‐dispersive charge transport behavior is observed, together with a time‐of‐flight mobility of the order of 10^?3 cm² V^?1 s^?1. Furthermore the lowest energetic disorder parameter (σ) ever reported for a solution‐processed conjugated organic material is found, σ < 20 meV.     

Charge‐Carrier Balance and Color Purity in Polyfluorene Polymer Blends for Blue Light‐Emitting Diodes

A study of an efficient blue light‐emitting diode based on a fluorescent aryl polyfluorene (aryl‐F8) homopolymer in an inverted device architecture is presented, with ZnO and MoO₃ as electron‐ and hole‐injecting electrodes, respectively. Charge‐carrier balance and color purity in these structures are achieved by incorporating poly(9,9‐dioctylfluorene‐co‐N‐(4‐butylphenyl)‐diphenylamine (TFB) into aryl‐F8. TFB is known to be a hole‐transporting material but it is found to act as a hole trap on mixing with aryl‐F8. Luminance efficiency of ≈6 cd A^?1 and external quantum efficiency (EQE) of 3.1% are obtained by adding a small amount (0.5% by weight) of TFB into aryl‐F8. Study of charge injection and transport in the single‐carrier devices shows that the addition of a small fraction of hole traps is necessary for charge‐carrier balance. Optical studies using UV–vis and fluorescence spectroscopic measurements, photoluminescence quantum yield, and fluorescence decay time measurements indicate that TFB does not affect the optical properties of the aryl‐F8, which is the emitting material in these devices. Luminance efficiency of up to ≈11 cd A^?1 and EQE values of 5.7% are achieved in these structures with the aid of improved out‐coupling using index‐matched hemispheres.     

Solution‐Processible Multi‐component Cyclometalated Iridium Phosphors for High‐Efficiency Orange‐Emitting OLEDs and Their Potential Use as White Light Sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole‐transporting carbazole and fluorene‐based 2‐phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the π‐conjugation through incorporation of electron‐pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest occupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium‐based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo‐ and electroluminescence properties of these phosphorescent carbazolylfluorene‐functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light‐emitting diodes (OLEDs) using these complexes as the solution‐processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole‐transporting layer. These orange‐emitting devices can produce a maximum current efficiency of ～ 30 cd A^–1 corresponding to an external quantum efficiency of ～ 10 % ph/el (photons per electron) and a power efficiency of ～ 14 lm W^–1. The homoleptic iridium phosphors generally outperform the heteroleptic counterparts in device performance. The potential of exploiting these orange phosphor dyes in the realization of white OLEDs is also discussed.     

Thermally Activated Delayed Fluorescence Conjugated Polymers with Backbone‐Donor/Pendant‐Acceptor Architecture for Nondoped OLEDs with High External Quantum Efficiency and Low Roll‐Off

Most thermally activated delayed fluorescence (TADF) emitters have to be doped in the host for fabricating efficient organic light‐emitting diodes (OLEDs) and always suffer from quick efficiency roll‐off at high brightness, which severely affect their commercial application in display and lighting fields. In the work, a series of the polymers are synthesized by copolymerization of two carbazole monomers and one acridine derivative monomer containing benzophenone acceptor group. The obtained polymers therefore possess a conjugated backbone with carbazole/acridine moieties and benzophenone pendant to form the twisted donor/acceptor structure. Consequently, the TADF features inherited from the acridine derivative are maintained and improved by managing the content of acridine derivative monomer in the polymers. Solution‐processed OLEDs obtained from using neat polymer films exhibit comparable performance with organic TADF small molecules, achieving a maximum external quantum efficiency (EQE) of 18.1% and a very slow roll‐off with EQE of 17.8% at the luminance of 1000 cd m⁻².     

Solution‐Processible Carbazole Dendrimers as Host Materials for Highly Efficient Phosphorescent Organic Light‐Emitting Diodes

A group of dendrimers with oligo‐carbazole dendrons appended at 4,4′‐ positions of biphenyl core are synthesized for use as host materials for solution‐processible phosphorescent organic light‐emitting diodes (PHOLEDs). In comparison with the traditional small molecular host 4,4′‐N,N′‐dicarbazolebiphenyl (CBP), the dendritic conformation affords these materials extra merits including amorphous nature with extremely high glass transition temperatures (ca. 376 °C) and solution‐processibility, but inherent the identical triplet energies (2.60–2.62 eV). In comparison with the widely‐used polymeric host polyvinylcarbazole (PVK), these dendrimers possess much higher HOMO levels (–5.61 to –5.42 eV) that facilitate efficient hole injection and are favorable for high power efficiency in OLEDs. The agreeable properties and the solution‐processibility of these dendrimers makes it possible to fabricate highly efficient PHOLEDs by spin coating with the dendimers as phosphorescent hosts. The green PHOLED containing Ir(ppy)₃ (Hppy = 2‐phenyl‐pyridine) dopant exhibits high peak efficiencies of 38.71 cd A^?1 and 15.69 lm W^?1, which far exceed those of the control device with the PVK host (27.70 cd A^?1 and 9.6 lm W^?1) and are among the best results for solution‐processed green PHOLEDs ever reported. The versatility of these dendrimer hosts can be spread to orange PHOLEDs and high efficiencies of 32.22 cd A^?1 and 20.23 lm W^?1 are obtained, among the best ever reported for solution‐processed orange PHOLEDs.     

Design strategy of yellow thermally activated delayed fluorescent dendrimers and their highly efficient non-doped solution-processed OLEDs with low driving voltage

We designed and synthesized two dendrimers TA-Cz and TA-3Cz with TADF characteristics by using non-conjugated aliphatic chains carbazole/tricarbazole as peripheral dendrons. Both dendrimers possess excellent thermal and morphological stabilities. Introduced the phenyl bridge to increase the distance of the emission core TA between donor (D) and acceptor (A) is a promising route to simultaneously achieve small singlet–triplet energy splitting (ΔE_ST) and enhanced PL quantum yields (PLQYs). Furthermore, non-conjugated aliphatic chains carbazole/tricarbazole dendrons were conveniently introduced to the TADF core, which can effective encapsulate the emission core to restrain the concentration quenching effect and make the fluorescence of the core independent. By utilizing TA-3Cz emitter as the non-doped solution-processed emissive layers, the resulting yellow OLED achieved low driving voltage of 2.4 V and superior external quantum efficiency of 11.8%. Thus, our results here provide a facile strategy to obtain highly efficient non-doped solution-processed OLEDs by employing the reasonable molecular design of the TADF core and the utilization of flexible alkyl chain.     

Red‐Light‐Emitting Iridium Complexes with Hole‐Transporting 9‐Arylcarbazole Moieties for Electrophosphorescence Efficiency/Color Purity Trade‐off Optimization

The synthesis, structures, photophysics, electrochemistry and electrophosphorescent properties of new red phosphorescent cyclometalated iridium(III) isoquinoline complexes, bearing 9‐arylcarbazolyl chromophores, are reported. The functional properties of these red phosphors correlate well with the results of density functional theory calculations. The highest occupied molecular orbital levels of these complexes are raised by the integration of a carbazole unit to the iridium isoquinoline core so that the hole‐transporting ability is improved in the resulting complexes relative to those with 1‐phenylisoquinoline ligands. All of the complexes are highly thermally stable and emit an intense red light at room temperature with relatively short lifetimes that are beneficial for highly efficient organic light‐emitting diodes (OLEDs). Saturated red OLEDs, fabricated using these dyes as the phosphorescent dopants both as vacuum‐evaporated and spin‐coated emissive layers, have been achieved in a multilayer configuration with outstanding red color purity at Commission International de L'Éclairage (CIE) coordinates of (0.67,0.33) to (0.68,0.32). Some of the devices can show very high efficiencies with a maximum external quantum efficiency of up to 12 % photons per electron. The excellent performance of these red emitters indicates the advantage of the carbazole module in the ligand framework; demonstrated by an improved hole‐transporting ability that facilitates exciton transport. These materials could thus provide a new avenue for the rational design of heavy‐metal electrophosphors that reveal a superior device efficiency/color purity trade‐off necessary for pure red‐light generation.     

Solution‐Processed Thermally Activated Delayed Fluorescent OLED with High EQE as 31% Using High Triplet Energy Crosslinkable Hole Transport Materials

Solution‐processed organic light‐emitting diodes (OLEDs) with thermally activated delayed fluorescent (TADF) material as emitter have attracted much attention because of their low cost and high performance. However, exciton quench at the interface between the hole injection layer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and emitting layer (EML) in devices can lead to low device performance. Here, a novel high triplet energy (2.89 eV) and crosslinkable hole‐transporting material grafted with oxetane groups, N,N‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy)hexyloxy)phenyl)‐3,5‐di(9H‐carbazol‐9‐yl)benzenamine (Oxe‐DCDPA)), as crosslinked hole transport layer (HTL) into the interface of PEDOT:PSS layer and EML is proposed for prevention of exciton quenching, and among the reported devices with single HTL in solution‐processed TADF‐OLED, the highest external quantum efficiency (EQE)/luminous efficiency (η_L) of 26.1%/94.8 cd A^?1 and 24.0%/74.0 cd A^?1 are achieved for green emission (DACT‐II as emitter) and bluish‐green emission (DMAC‐TRZ as emitter), respectively. Further improvement, using double HTLs, composed of N,N′‐bis(4‐(6‐((3‐ethyloxetan‐3‐yl)methoxy))‐hexylphenyl)‐N,N′‐diphenyl‐4,4′‐diamine with high hole mobility and Oxe‐DCDPA with high triplet energy, leads to the highest EQE/η_L of 30.8%/111.9 cd A^?1 and 27.2%/83.8 cd A^?1 for green emission and bluish‐green emission, respectively. These two devices show the high maximum brightness of 81 100 and 70 000 cd m^?2, respectively.     

Highly Efficient Electron‐Transporting/Injecting and Thermally Stable Naphthyridines for Organic Electrophosphorescent Devices

A series of 1,8‐naphthyridine derivatives is synthesized and their electron‐transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light‐emitting device (OLED) using fac‐tris(2‐phenylpyridine)iridium [Ir(ppy)₃] as a green phosphorescent emitter doped into a 4,4′‐N,N′‐dicarbazolebiphenyl (CBP) host with 4,4′‐bis[N‐(1‐naphthyl)‐N‐phenylamino]biphenyl (a‐NPD) as the hole‐transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4‐bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole‐injecting layer. The turn‐on voltage of the device is 2.5 V using 2,7‐bis[3‐(2‐phenyl)‐1,8‐naphthyridinyl]‐9,9‐dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole‐blocking (HB) and exciton‐blocking layer of bathocuproine (BCP), 13.2–13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2–54.5 lm W^?1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass‐transition temperatures much higher than that of aluminum(III) bis(2‐methyl‐8‐quinolinato)‐4‐phenylphenolate (BAlq), and decomposition temperatures of 510–518 °C, comparable to or even higher than those of Alq₃.     

Highly efficient and stable blue quantum-dot light-emitting diodes based on polyfluorenes with carbazole pendent groups as hole-transporting materials

Highly efficient and stable blue quantum-dot light-emitting diodes (QD-LEDs) have been realized by using poly (9,9-bis(N-(2′-ethylhexyl)-carbazole-3-yl)-2,7-fluorene) (PFCz) as hole-transporting layers (HTLs). Due to the carbazole units as substituents at the 9-position of polyfluorene, PFCz shows higher hole mobility and better electrochemical stability than poly (N-vinlycarbazole) (PVK). As a result, the maximum current efficiency (CE) and external quantum efficiency (EQE) of the blue QD-LEDs increased from 4.32 cd A⁻¹ to 7.9% for PVK HTL to 7.38 cd A⁻¹ and 12.61% for PFCz HTL, respectively. Furthermore, the PFCz-based blue QD-LED exhibited lower turn-on voltage and longer device lifetime than the PVK-based device. The improvement performance of blue QD-LED should be attributed to the conjugated fluorene backbone and the substituents of the carbazole active sites, thus enhancing hole mobility and electrochemical stability. This result demonstrates that polyfluorenes with pendent carbazole groups is a promising hole-transporting materials for improving performance of blue QD-LEDs.     

Highly efficient all-solution processed blue quantum dot light-emitting diodes based on balanced charge injection achieved by double hole transport layers

Solution-processed blue quantum dot light-emitting diodes (QLEDs) suffer from low device efficiency, whereas the balance of electron and hole injection is critical for obtaining high efficiency. Herein, synergistical double hole transport layers (D-HTLs) are employed, which use poly(9-vinylcarbazole) (PVK) stacked on poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(4,4'-(N-(4-butylphenyl) (TFB). The fabrication of D-HTLs is achieved by using dimethyl formamide (DMF) as the solvent for PVK, with which the underlying TFB layer almost remains unwashed and undamaged during the spin-coating process of PVK layer. TFB/PVK D-HTLs form the stepwise energy level for hole injection, which reduces the hole injection barrier and favors the carrier balance in the emission layer (EML). The optimized blue QLED with TFB/PVK D-HTLs shows a maximum external quantum efficiency (EQE) of 13.7%, which is 3-fold enhancement compared to that of the control device with single TFB HTL. The enhancement of the QLED performance can be attributed to the improvement of surface morphology and charge injection balance for the stepwise D-HTLs based QLEDs. This work manifests the positive effect on performance boost by selecting appropriate solvents towards stepwise D-HTLs formation and paves the way to fabricate highly efficient all-solution processed light emitting diodes.     

Solution‐Processible 2,2′‐Dimethyl‐biphenyl Cored Carbazole Dendrimers as Universal Hosts for Efficient Blue,Green, and Red Phosphorescent OLEDs

A series of solution‐processible 2,2′‐dimethyl‐biphenyl cored dendrimers, namely G1MP, G2MP, and G3MP, is designed and synthesized by tuning the generation of periphery carbazole dendron. The resulting dendrimers all show excellent solubility in common organic solvents, and their high‐quality thin films can be formed via spin‐coating with a root‐mean‐square roughness in the range of 0.38–0.54 nm. G3MP, which contains the third‐generation carbazole dendron, has the greatest potential among those made here as an ideal universal host for multicolored triplet emitters. G3MP exhibits good thermal stability, with a glass transition temperature of 368 °C, a triplet energy as high as 2.85 eV enough to prevent the loss of triplet excitons, and suitable HOMO/LUMO levels of –5.30/–2.11 eV to facilitate both hole and electron injection and transport. When using G3MP as the host, highly efficient deep‐blue, blue, green, and red phosphorescent organic light‐emitting diodes (PhOLEDs) are successfully demonstrated, revealing a maximum luminous efficiency up to 18.2, 28.2, 54.0, and 12.7 cd A^–1 with the corresponding Commission Internationale de L'Eclairage (CIE) coordinates of (0.15, 0.23), (0.15, 0.35), (0.38, 0.59), and (0.64, 0.34), respectively. The state‐of‐art performance indicates that dendritic hosts have a favorable prospect of applications in solution‐processed white PhOLEDs and full‐color displays.     

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.     

Fluorene‐Carbazole Dendrimers: Synthesis,Thermal, Photophysical and Electroluminescent Device Properties

Novel hole‐transporting dendrimeric molecules containing dioctylfluorene, spirobi(fluorene) and spiro(cylododecane‐fluorene) as the core unit and different numbers of carbazole and thiophene moieties as the peripheral groups are synthesized. All the dendrimers are characterized by ¹H NMR, ¹³C NMR, FTIR, UV–vis, PL spectroscopy, and MALDI‐TOF. They are thermally stable with high glass transition and decomposition temperatures and exhibit chemically reversible redox processes. They are used as the hole‐transporting layer (HTL) material for multilayer organic light emitting diodes (OLEDs) with a low turn‐on voltage of around 2.5 V and a bright green emission with a maximum luminance of around 25400 cd m^?2.     

Unexpected Propeller‐Like Hexakis(fluoren‐2‐yl)benzene Cores for Six‐Arm Star‐Shaped Oligofluorenes: Highly Efficient Deep‐Blue Fluorescent Emitters and Good Hole‐Transporting Materials

Grafting six fluorene units to a benzene ring generates a new highly twisted core of hexakis(fluoren‐2‐yl)benzene. Based on the new core, six‐arm star‐shaped oligofluorenes from the first generation T1 to third generation T3 are constructed. Their thermal, photophysical, and electrochemical properties are studied, and the relationship between the structures and properties is discussed. Simple double‐layer electroluminescence (EL) devices using T1–T3 as non‐doped solution‐processed emitters display deep‐blue emissions with Commission Internationale de l'Eclairage (CIE) coordinates of (0.17, 0.08) for T1 , (0.16, 0.08) for T2 , and (0.16, 0.07) for T3 . These devices exhibit excellent performance, with maximum current efficiency of up to 5.4 cd A^?1, and maximum external quantum efficiency of up to 6.8%, which is the highest efficiency for non‐doped solution‐processed deep‐blue organic light‐emitting diodes (OLEDs) based on starburst oligofluorenes, and is even comparable with other solution‐processed deep‐blue fluorescent OLEDs. Furthermore, T2‐ and T3‐ based devices show striking blue EL color stability independent of driving voltage. In addition, using T0–T3 as hole‐transporting materials, the devices of indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS)/ T0–T3 /tris(8‐hydroxyquinolinato)aluminium (Alq₃)/LiF/Al achieve maximum current efficiencies of 5.51–6.62 cd A^?1, which are among the highest for hole‐transporting materials in identical device structure.     

Achieving Highly Efficient Fluorescent Blue Organic Light‐Emitting Diodes Through Optimizing Molecular Structures and Device Configuration

Based on the results of first‐principles calculations of the electronic properties of blue light‐emitting materials, the molecular structures of oligofluorenes are optimized by incorporating electron‐withdrawing groups into the molecules to balance hole and electron injection and transport for organic light‐emitting diodes (OLEDs). The result is a remarkable improvement in the maximum external quantum efficiency (EQE) of the undoped device from 2.0% to 4.99%. Further optimization of the device configurations and processing procedures, e.g., by changing the thickness of the emitting layer and through thermal annealing treatments, leads to a very high maximum EQE of 7.40% for the undoped sky‐blue device. Finally, by doping the emitter in a suitable host material, 4,4’‐bis(carbazol‐9‐yl)biphenyl (CBP), at the optimal concentration of 6%, pure blue emission with extremely high maximum EQE of 9.40% and Commission Internationale de l’Eclairage (CIE) coordinates of (0.147, 0.139) is achieved.     

m‐Indolocarbazole Derivative as a Universal Host Material for RGB and White Phosphorescent OLEDs

The host materials designed for highly efficient white phosphorescent organic light‐emitting diodes (PhOLEDs) with power efficiency (PE) >50 lm W^‐1 and low efficiency roll‐off are very rare. In this work, three new indolocarbazole‐based materials (ICDP, 4ICPPy, and 4ICDPy) are presented composed of 6,7‐dimethylindolo[3,2‐a]carbazole and phenyl or 4‐pyridyl group for hosting blue, green, and red phosphors. Among this three host materials, 4ICDPy‐based devices reveal the best electroluminescent performance with maximum external quantum efficiencies (EQEs) of 22.1%, 27.0%, and 25.3% for blue (FIrpic), green (fac‐Ir(ppy)₃), and red ((piq)₂Ir(acac)) PhOLEDs. A two‐color and single‐emitting‐layer white organic light‐emitting diode hosted by 4ICDPy with FIrpic and Ir(pq)₃ as dopants achieves high EQE of 20.3% and PE of 50.9 lm W^?1 with good color stability; this performance is among the best for a single‐emitting‐layer white PhOLEDs. All 4ICDPy‐based devices show low efficiency roll‐off probably due to the excellent balanced carrier transport arisen from the bipolar character of 4ICDPy.