2,7‐Bis(diarylamino)‐9,9‐dimethylfluorenes as Hole‐Transport Materials for Organic Light‐Emitting Diodes

2,7‐Bis(p‐methoxyphenyl‐m′‐tolylamino)‐9,9‐dimethylfluorene ( 1′ ), 2,7‐bis(phenyl‐m′‐tolylamino)‐9,9‐dimethylfluorene ( 2′ ) and 2,7‐bis(p‐fluorophenyl‐m′‐tolylamino)‐9,9‐dimethylfluorene ( 3′ ) have been synthesized using the palladium‐catalyzed reaction of the appropriate diarylamines with 2,7‐dibromo‐9,9‐dimethylfluorene. These molecules have glass‐transition temperatures 15–20 °C higher than those for their biphenyl‐bridged analogues, and are 0.11–0.14 V more readily oxidized. Fluorescence spectra and fluorescence quantum yields for dimethylfluorene‐bridged and biphenyl‐bridged species are similar, but the peaks of the absorption spectra of 1′ – 3′ are considerably red‐shifted relative to those of their biphenyl‐bridged analogues. Time‐of‐flight hole mobilities of 1′ – 3′ /polystyrene blends are in a similar range to those of the biphenyl‐bridged analogues. Analysis according to the disorder formalism yields parameters rather similar to those for the biphenyl species, but with somewhat lower zero‐field mobility values. Density functional theory (DFT) calculations suggest that the enforced planarization of the fluorene bridge leads to a slightly larger reorganization energy for the neutral/cation electron‐exchange reaction than in the biphenyl‐bridged system. Organic light‐emitting diodes have been fabricated using 1′ – 3′ /polystyrene blends as the hole‐transport layer and tris(8‐hydroxy quinoline)aluminium as the electron‐transport layer and lumophore. Device performance shows a correlation with the ionization potential of the amine materials paralleling that seen in biphenyl‐based systems, and fluorene species show similar performance to biphenyl species with comparable ionization potential.     

High‐Efficiency Red Phosphorescent Iridium Dendrimers with Charge‐ Transporting Dendrons: Synthesis and Electroluminescent Properties

A series of 1‐phenylisoquinoline derivatives encapsulated with peripheral arylamines as dendrons are synthesized by using the Ullmann reaction and palladium‐catalyzed aromatic carbon–carbon Suzuki‐coupling reactions. Red‐emitting dendritic iridium complexes (called G1‐1 , G1‐2 , and G2 ) are synthesized using the following derivatives: N,N‐diphenyl‐3′‐isoquinolin‐4‐biphenylaniline, N,N‐di(9,9‐dimethylfluorenyl‐3′‐isoquinolin‐4‐biphenylaniline, N,N‐di(4′‐di(2′‐(9′,9′‐dimethylfluorenyl)amine)biphenyl‐3′‐isoquinolin‐4‐biphenylaniline as the first ligands and 5‐methyl‐3‐(pyridin‐2′‐yl)‐1H‐1,2,4‐triazole as an ancillary ligand. The obtained dendrimers are soluble in common organic solvents, and uniform thin films can be spin‐coated from such solutions. Devices fabricated from dendritic iridium complexes G1‐2 and G2 with a small molecule host are fabricated by spin‐coating from chloroform solution in different device configurations. G1‐2 and G2 show similar device performances with maximum external quantum efficiencies (EQEs) of 12.8 % and 11.8 % (photons/electron) and luminous efficiency of 9.2 cd A^–1 and 8.5 cd A^–1 at 0.1 mA cm^–2, respectively. Devices based on polymer host poly(9,9‐dioctylfluorene)(PFO) (30 % PBD (2‐(4‐biphenyl)‐5‐(4‐tert‐butylphenyl‐1,3,4‐oxadiazole)) show a slightly higher efficiency for G1‐2 , with a maximum EQE of 13.9 % at a much higher current density of 6.4 mA cm^–2 and luminance of 601 cd m^–2.     

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.     

Influence of Molecular Dipoles on the Photoluminescence and Electroluminescence of Dipolar Spirobifluorenes

This Full Paper investigates a series of strongly fluorescent donor–acceptor‐substituted spirobifluorene compounds, red 2‐diphenylamino‐7‐(2,2‐dicyanovinyl)‐9,9′‐spirobifluorene (DCV), green 2‐diphenylamino‐9,9′‐spirobifluorene7‐carxoxaldehyde (CHO), and blue 2‐diphenylamino‐7‐(2,2‐diphenylvinyl)‐9,9′‐spirobifluorene (DPV), together with their spiro‐linked “dimeric” analogs, 2DCV, 2,2′‐bis(diphenylamino)‐9,9′‐spirobifluorene‐7,7′‐dicarboxaldehyde (2CHO), and 2,2′‐bis(diphenylamino)‐7,7′‐bis(2,2‐diphenylvinyl)‐9,9′‐spirobifluorene (2DPV), respectively. The emission optical density and, hence, the intensity of photoluminescence (PL) or electroluminescence (EL) of the “dimeric” analogs is presumed to increase, which is beneficial for organic light‐emitting diode (OLED) applications. The physical properties, including the dipole moments obtained from quantum chemistry calculations, emission solvatochromism, fluorescence quantum yield (Φ_f) as well as the EL of these six spirobifluorene compounds have been examined in detail. We found that Φ_f as well as OLED performance (EL efficiency and intensity) of the strongly dipolar DCV decrease significantly in the “dimeric” analog 2DCV, but less so in the moderately dipolar CHO and 2CHO, and only slightly in the weakly dipolar DPV and 2DPV. This is parallel to the intramolecular dipole moment, which is large for 2DCV, medium for 2CHO, and very small for 2DPV. Here, we show for the first time systematically that the luminescence intensity is closely correlated with the local electric field induced by the molecular dipole. A strong electric field may facilitate radiationless decay channels with a charge‐transfer nature, leading to a high quenching rate. Consistent with this conclusion, which is derived from the red DCV/2DCV and green CHO/2CHO, our new blue fluorophore DPV with an essentially zero dipole moment has successfully achieved one of the best electrofluorescent blue OLEDs. At the same time, by doping the highly dipolar DCV into an isolated environment with the low‐polarity Alq₃ as the host matrix, we obtained a very high performance of saturated yellow OLEDs as well, This is possibly due to the reduction of emission‐quenching dipoles from the neighboring molecules. Our results have provided an important insight in designing luminescent materials, as follows: molecular dipole moments should be kept at a low magnitude to avoid quenching induced by a strong local electric field in the chromophore.     

A Hole‐Transporting Material with Controllable Morphology Containing Binaphthyl and Triphenylamine Chromophores

An organic compound with two triphenylamine moieties linked with binaphthyl at the 3,3′‐positions (2,2′‐dimethoxyl‐3,3′‐ di(phenyl‐4‐yl‐diphenyl‐amine)‐[1,1′]‐binaphthyl, TPA–BN–TPA) can be synthesized by Suzuki coupling. Amorphous and homogeneous films are obtained by either vacuum deposition or spin‐coating from solution in good solvents, while single crystals are grown in an appropriate polar solvent. X‐ray crystallography showed that a TPA–BN–TPA crystal is a multichannel structure containing solvent molecules in the channels. The intramolecular charge‐transfer state resulting from amino conjugation effects is observed by solvatochromic experiments. The high glass‐transition temperature (130 °C) and decomposition temperature (439 °C) of this material, in combination with its reversible oxidation property, make it a promising candidate as a hole‐transport material for light‐emitting diodes. With TPA–BN–TPA as the hole‐transporting layer in an indium tin oxide/TPA–BN–TPA/aluminum tris(8‐hydroxyquinoline)/Mg:Ag device, a brightness of about 10 100 cd m^–2 at 15.6 V with a maximum efficiency of 3.85 cd A^–1 is achieved, which is superior to a device with N,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐[1,1′‐biphenyl]‐4,4′‐diamine as the hole‐transporting layer under the same conditions. Other devices with TPA–BN–TPA as the blue‐light‐emitting layer or host for a blue dye emitter are also studied.     

Starburst DCM‐Type Red‐Light‐Emitting Materials for Electroluminescence Applications

Three new starburst DCM (4‐(dicyanomethylene)‐2‐methyl‐6‐[4‐(dimethylaminostyryl)‐4H‐pyran]) derivatives, 4,4′,4′′‐tris[2‐(4‐dicyanomethylene‐6‐t‐butyl‐4H‐pyran‐2‐yl)‐ethylene]triphenylamine (TDCM), 4,4′,′′‐tris[2‐(4‐(1′,3′‐indandione)‐6‐t‐butyl‐4H‐pyran‐2‐yl)‐ethylene]triphenylamine (TIN), and 4‐methoxy‐4′,4′′‐bis[2‐(4‐(1′,3′‐indandione)‐6‐t‐butyl‐4H‐pyran‐2‐yl)‐ethylene]triphenylamine (MBIN), have been designed and synthesized for application as red‐light emitters in organic light‐emitting diodes (OLEDs). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) reveal their extremely high glass‐transition temperatures and decomposition temperatures, as well as their low tendency to crystallize. Photoluminescence and electroluminescence measurements show that they exhibit a greatly restricted concentration‐quenching effect compared to DCM1 (4‐(dicyanomethylene)‐2‐methyl‐6‐[p‐(N,N‐dimethylamino)‐styryl]‐4H‐pyran), a simple but typical DCM‐type dye, as a result of their non‐planar, three‐dimensional structures that result from their unique propeller‐like triphenylamine electron‐donating cores. The peripheral electron‐withdrawing moieties also play a key role in the restriction of concentration quenching. That is, TIN and MBIN, bearing 1,3‐indandione acceptors, emit more efficiently than TDCM and DCM1, which have dicyanomethylene as acceptors at a high doping concentration of 10 wt.‐% in poly(9‐vinylcarbazole) (PVK) film, irrespective of whether they are photoexcited or electroexcited, though their fluorescence quantum yields in dilute solutions are much lower than that of DCM1. By way of the co‐doping approach, the electroluminescence device with the configuration indium tin oxide (ITO)/PVK:MBIN(10 wt.‐%):tris(4‐(2‐phenylethynyl)‐phenyl)amine (TPA; 30 wt.‐%) (70 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 20 nm)/tris(8‐quinolinolato) aluminum (Alq₃;15 nm)/LiF (0.3 nm)/Al (150 nm) exhibits a turn‐on voltage of 5.1 V, a maximum luminance of 6971 cd m^–2, a maximum efficiency of 6.14 cd A^–1 (405 cd m^–2), and chromaticity coordinates of (0.66,0.33). The encouraging electroluminescence performance suggests potential applications of the starburst DCM red‐light emitters in OLEDs.     

Color Tuning in Benzo[1,2,5]thiadiazole‐Based Small Molecules by Amino Conjugation/Deconjugation: Bright Red‐Light‐Emitting Diodes

Bipolar compounds (referred to in general as btza ) containing a benzo[1,2,5]thiadiazole core and peripheral diarylamines and/or 4‐tert‐butylphenyl moieties have been synthesized via palladium‐catalyzed cross‐coupling reactions of 4,7‐dibromobenzo[1,2,5]thiadiazole with appropriate stannyl compounds. These compounds are fluorescent and the emission color ranges from green to red. The fluorescence of the compounds originates from a charge‐transfer process and exhibits solvatochromism. These red‐light‐emitting materials are amorphous and devices of different configurations were fabricated: I) ITO/ btza /TPBI/Mg:Ag; II) ITO/ btza /Alq₃/Mg:Ag; III) ITO/ btza /Mg:Ag (where ITO = indium tin oxide, TPBI = 1,3,5‐tris(N‐phenylbezimidazol‐2‐yl)benzene, and Alq₃ = tris(8‐hydroxyquinoline)aluminum). The performance of some of the red‐light‐emitting devices appears to be very promising.     

Synthesis,Structure, Electronic State,and Luminescent Properties of Novel Blue‐Light‐Emitting Aryl‐Substituted 9,9‐Di(4‐(di‐p‐tolyl)aminophenyl)fluorenes

Novel blue‐light‐emitting fluorene derivatives 5a–c and 7a–c containing bulky and highly emissive groups, namely pyrene, 10‐phenylanthracene‐9‐yl and 10‐(4′‐diphenylaminophenyl)anthracene‐9‐yl groups, as well as hole‐injecting/transporting triarylamines were synthesized. Single crystals of compounds 5a , 5c , 7a , and 7c were grown and their crystal structures were determined by X‐ray diffraction. The four fluorene derivatives have nonplanar molecular structures, which reduce the intermolecular interaction and the likelihood of molecular aggregation or excimer formation. No unwanted long‐wavelength emission was observed in the photoluminescence (PL) spectra of the 5a–c and 7a–c thin films. Their PL spectra reveal excellent thermal stability after annealing treatment under air and ambient light. All of the six compounds show high fluorescence quantum yields and outstanding thermal stabilities. The 2‐aryl and 2,7‐diaryl substituents at the fluorene molecule have a significant effect on the photophysical properties and the thermal characteristics. The six compounds show almost the same energy levels for the highest occupied molecular orbitals (HOMOs) of about ?5.20 eV, which allows effective hole injection. The C2‐ and C7‐aryl substituents play a relatively less‐important role in the HOMO energy levels, which depend mainly on the triphenylamino groups at the C9 position. The molecular orbitals, excitation energy, and emission energy were calculated to explain the real origin of their photophysical characteristics. The HOMOs are mainly localized on the triphenylamino groups at the C9 position, while the lowest unoccupied molecular orbitals (LUMOs) have a significant orbital density at the C2‐ and/or C7‐aryl substituents. Pure‐blue‐light‐emitting diodes based on 2,7‐diaryl‐9,9‐di(triarylamino)fluorenes were fabricated.     

Amorphous Diphenylaminofluorene‐Functionalized Iridium Complexes for High‐Efficiency Electrophosphorescent Light‐Emitting Diodes

Two new phosphorescent iridium(III ) cyclometalated complexes, [Ir(DPA‐Flpy)₃] ( 1 ) and [Ir(DPA‐Flpy)₂(acac)] ( 2 ) ((DPA‐Flpy)H = (9,9‐diethyl‐7‐pyridinylfluoren‐2‐yl)diphenylamine, Hacac = acetylacetone), have been synthesized and characterized. The incorporation of electron‐donating diphenylamino groups to the fluorene skeleton is found to increase the highest occupied molecular orbital (HOMO) levels and add hole‐transporting ability to the phosphorescent center. Both complexes are highly amorphous and morphologically stable solids and undergo glass transitions at 160 and 153 °C, respectively. These iridium phosphors emit bright yellow to orange light at room temperature with relatively short lifetimes (< 1 μs) in both solution and the solid state. Organic light‐emitting diodes (OLEDs) fabricated using 1 and 2 as phosphorescent dopant emitters constructed with a multilayer configuration show very high efficiencies. The homoleptic iridium complex 1 is shown to be a more efficient electrophosphor than the heteroleptic congener 2 . Efficient electrophosphorescence with a maximum external quantum efficiency close to 10 % ph/el (photons per electron), corresponding to a luminance efficiency of ～ 30 cd A^–1 and a power efficiency of ～ 21 lm W^–1, is obtained by using 5 wt.‐% 1 as the guest dopant.     

Highly Efficient Solid‐State Near‐Infrared Emitting Material Based on Triphenylamine and Diphenylfumaronitrile with an EQE of 2.58% in Nondoped Organic Light‐Emitting Diode

The development of efficient near‐infrared (NIR) emitting material is of current focus. Donor–acceptor (D–A) architecture has been proved to be an effective strategy to obtain narrow energy gap. Herein, a D–A‐type NIR fluorescent compound 2,3‐bis(4′‐(diphenylamino)‐[1,1′‐biphenyl]‐4‐yl)fumaronitrile (TPATCN) is synthesized and fully characterized. As revealed by theoretical calculations and photophysical experiments, TPATCN exerts the advantages of the relatively large dipole moment of the charge transfer state and a certain degree of orbital overlap of the local excited state. A highly mixed or hybrid local and charge transfer excited state might occur to simultaneously achieve both a large fraction of singlet formation and a high quantum efficiency in D–A system. TPATCN exhibits strong NIR fluorescence with the corresponding thin film quantum efficiency of 33% and the crystal efficiency of 72%. Remarkably, the external quantum efficiency of nondoped NIR organic light‐emitting diode (OLED) reaches 2.58% and remains fairly constant over a range of 100–300 mA cm^?2, which is among the best results for NIR OLEDs reported so far.     

Fluorene‐Based Copolymers for Red‐Light‐Emitting Diodes

The syntheses of new fluorene‐based π‐conjugated copolymers; namely, poly((5,5″‐(3′,4′‐dihexyl‐2,2′;5′,2″‐terthiophene 1′,1′‐dioxide))‐alt‐2,7‐(9,9‐dihexylfluorene)) (PFTORT), poly((5,5″″‐(3″,4″‐dihexyl‐2,2′:5′,2′:5″,2‴:5‴,2″″‐quinquethiophene 1″,1″‐dioxide))‐alt‐2,7‐(9,9‐dihexylfluorene)) (PFTTORTT), and poly((5,5‐E‐α‐(2‐thienyl)methylene)‐2‐thiopheneacetonitrile)‐alt‐2,7‐(9,9‐dihexylfluorene)) (PFTCNVT), are reported. In the solid state, PFTORT and PFTCNVT present red–orange emission (with a maximum at 610 nm) while PFTTORTT shows a red emission with a maximum at 666 nm. In all cases, electrochemical measurements have revealed p‐ and n‐dopable copolymers. All these copolymers have been successfully tested in simple light‐emitting diodes and show promising results for orange‐ and red‐light‐emitting devices.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene Blue Host and Side‐Chain‐Located Orange Dopant

Four single polymers with two kinds of attachment of orange chromophore to blue polymer host for white electroluminescence (EL) were designed. The effect of the side‐chain attachment and main‐chain attachment on the EL efficiencies of the resulting polymers was compared. The side‐chain‐type single polymers are found to exhibit more efficient white EL than that of the main‐chain‐type single polymers. Based on the side‐chain‐type white single polymer with 4‐(4‐alkyloxy‐phenyl)‐7‐(4‐diphenylamino‐phenyl)‐2,1,3‐benzothiadiazoles as the orange‐dopant unit and polyfluorene as the blue polymer host, white EL with simultaneous orange (λ_max = 545 nm) and blue emission (λ_max = 432 nm/460 nm) is realised. A single‐layer device (indium tin oxide/poly(3,4‐ethylenedioxythiophene)/polymer/Ca/Al) made of these polymers emits white light with the Commission Internationale de l'Éclairage coordinates of (0.30,0.40), possesses a turn‐on voltage of 3.5 V, luminous efficiency of 10.66 cd A^–1, power efficiency of 6.68 lm W^–1, and a maximum brightness of 21 240 cd m^–2.     

Novel Heteroleptic CuI Complexes with Tunable Emission Color for Efficient Phosphorescent Light‐Emitting Diodes

A series of orange‐red to red phosphorescent heteroleptic Cu^I complexes (the first ligand: 2,2′‐biquinoline (bq), 4,4′‐diphenyl‐2,2′‐biquinoline (dpbq) or 3,3′‐methylen‐4,4′‐diphenyl‐2,2′‐biquinoline (mdpbq); the second ligand: triphenylphosphine or bis[2‐(diphenylphosphino)phenyl]ether (DPEphos)) have been synthesized and fully characterized. With highly rigid bulky biquinoline‐type ligands, complexes [Cu(mdpbq)(PPh₃)₂](BF₄) and [Cu(mdpbq)(DPEphos)](BF₄) emit efficiently in 20 wt % PMMA films with photoluminescence quantum yield of 0.56 and 0.43 and emission maximum of 606 nm and 617 nm, respectively. By doping these complexes in poly(vinyl carbazole) (PVK) or N‐(4‐(carbazol‐9‐yl)phenyl)‐3,6‐bis(carbazol‐9‐yl) carbazole (TCCz), phosphorescent organic light‐emitting diodes (OLEDs) were fabricated with various device structures. The complex [Cu(mdpbq)(DPEphos)](BF₄) exhibits the best device performance. With the device structure of ITO/PEDOT/TCCz:[Cu(mdpbq)(DPEphos)](BF₄) (15 wt %)/TPBI/LiF/Al (III), a current efficiency up to 6.4 cd A^–1 with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.61, 0.39) has been realized. To our best knowledge, this is the first report of efficient mononuclear Cu^I complexes with red emission.     

The Fabrication and Characterization of Single‐Component Polymeric White‐Light‐Emitting Diodes

By using Ni⁰‐mediated polymerization, we have systematically synthesized a series of fluorene‐based copolymers composed of blue‐, green‐, and red‐light‐emitting comonomers with a view to producing polymers with white‐light emission. 2,7‐Dibromo‐9,9‐dihexylfluorene, {4‐(2‐[2,5‐dibromo‐4‐{2‐(4‐diphenylamino‐phenyl)‐vinyl}‐phenyl]‐vinyl)‐phenyl}‐diphenylamine (DTPA), and 2‐{2‐(2‐[4‐{bis(4‐bromo‐phenyl)amino}‐phenyl]‐vinyl)‐6‐tert‐butyl‐pyran‐4‐ylidene}‐malononitrile (TPDCM) were used as the blue‐, green‐, and red‐light‐emitting comonomers, respectively. It was found that the emission spectra of the resulting copolymers could easily be tuned by varying their DTPA and TPDCM content. Thus with the appropriate red/green/blue (RGB) unit ratio, we were able to obtain white‐light emission from these copolymers. A white‐light‐emitting diode using the polyfluorene copolymer containing 3 % green‐emitting DTPA and 2 % red‐emitting TPDCM (PG3R2) with a structure of indium tin oxide/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid)/PG3R2/Ca/Al was found to exhibit a maximum brightness of 820 cd m^–2 at 11 V with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33,0.35), which are close to the standard CIE coordinates for white‐light emission (0.33,0.33).     

Full‐Color Delayed Fluorescence Materials Based on Wedge‐Shaped Phthalonitriles and Dicyanopyrazines: Systematic Design,Tunable Photophysical Properties,and OLED Performance

Purely organic light‐emitting materials, which can harvest both singlet and triplet excited states to offer high electron‐to‐photon conversion efficiencies, are essential for the realization of high‐performance organic light‐emitting diodes (OLEDs) without using precious metal elements. Donor–acceptor architectures with an intramolecular charge‐transfer excited state have been proved to be a promising system for achieving these requirements through a mechanism of thermally activated delayed fluorescence (TADF). Here, luminescent wedge‐shaped molecules, which comprise a central phthalonitrile or 2,3‐dicyanopyrazine acceptor core coupled with various donor units, are reported as TADF emitters. This set of materials allows systematic fine‐tuning of the band gap and exhibits TADF emissions that cover the entire visible range from blue to red. Full‐color TADF‐OLEDs with high maximum external electroluminescence quantum efficiencies of up to 18.9% have been demonstrated by using these phthalonitrile and 2,3‐dicyanopyrazine‐based TADF emitters.     

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.     

A DCM‐Type Red‐Fluorescent Dopant for High‐Performance Organic Electroluminescent Devices

2‐(2‐tert‐Butyl‐6‐((E)‐2‐(2,6,6‐trimethyl‐2,4,5,6‐tetrahydro‐1H‐pyrrolo[3,2,1‐ij]quinolin‐8‐yl)vinyl)‐4H‐pyran‐4‐ylidene)malononitrile (DCQTB) is designed and synthesized in high yield for application as the red‐light‐emitting dopant in organic light‐emitting diodes (OLEDs). Compared with 4‐(dicyanomethylene)‐2‐tert‐butyl‐6‐(1,1,7,7,‐tetramethyljulolidyl‐9‐enyl)‐4H‐pyran (DCJTB), one of the most efficient red‐emitting dopants, DCQTB exhibits red‐shifted fluorescence but blue‐shifted absorption. The unique characteristics of DCQTB with respect to DCJTB are utilized to achieve a red OLED with improved color purity and luminous efficiency. As a result, the device that uses DCQTB as dopant, with the configuration: indium tin oxide (ITO)/N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB; 60 nm)/tris(8‐quinolinolato) aluminum (Alq₃):dopant (2.3 wt %) (7 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 12 nm)/Alq₃(45 nm)/LiF(0.3 nm):Al (300 nm), shows a larger maximum luminance (L_max = 6021 cd m^–2 at 17 V), higher maximum efficiency (η_max = 4.41 cd A^–1 at 11.5 V (235.5 cd m^–2)), and better chromaticity coordinates (Commission Internationale de l'Eclairage, CIE, (x,y) = (0.65,0.35)) than a DCJTB‐based device with the same structure (L_max = 3453 cd m^–2 at 15.5 V, η_max = 3.01 cd A^–1 at 10 V (17.69 cd m^–2), and CIE (x,y) = (0.62,0.38)). The possible reasons for the red‐shifted emission but blue‐shifted absorption of DCQTB relative to DCJTB are also discussed.     

Stabilized Blue Emission from Polyfluorene‐Based Light‐Emitting Diodes: Elimination of Fluorenone Defects

Polyfluorene (PF)‐based light‐emitting diodes (LEDs) typically exhibit device degradation under operation with the emergence of a strong low‐energy emission band (at ～ 2.2–2.4 eV). This longer wavelength band converts the desired blue emission to blue–green or even yellow. We have studied both the photoluminescence (PL) and electroluminescence (EL) of PFs with different molecular structures and found that the low‐energy emission band originates from fluorenone defects which are introduced by photo‐oxidization, thermal oxidation, or during device fabrication. X‐ray photo‐emission spectroscopy (XPS) results show that the oxidation of PF is strongly catalyzed by the presence of calcium. The fluorenone defects generate a stronger contribution to the EL than to the PL. By utilization of a novel electron‐transporting material as a buffer layer between the emissive PF and the Ca/Ag (Ba/Ag) cathode, the blue EL emission from the PF was stabilized.     

Crosslinkable TAPC‐Based Hole‐Transport Materials for Solution‐Processed Organic Light‐Emitting Diodes with Reduced Efficiency Roll‐Off

1‐Bis[4‐[N,N‐di(4‐tolyl)amino]phenyl]‐cyclohexane (TAPC) has been widely used in xerography and organic light‐emitting diodes (OLEDs), but derivatives are little known. Here, a new series of solution‐processable, crosslinkable hole conductors based on TAPC with varying highest occupied molecular orbital (HOMO) energies from ?5.23 eV to ?5.69 eV is implemented in blue phosphorescent OLEDs. Their superior perfomance compared with the well‐known N4,N4,N4′,N4′‐tetraphenylbiphenyl‐4,4′‐diamine (TPDs) analogues regarding hole‐injection and mobility, electron and exciton blocking capabilities, efficiency, and efficiency roll‐off is demonstrated. Overall, the TAPC‐based devices feature higher luminous and power efficiency over a broader range of brightness levels and reduced efficiency roll off. A systematic broadening of the emission zone is observed as the hole‐injection barrier between the anode and the hole‐transporting layer increased.     

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.