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.     

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.     

Doping‐Induced Charge Trapping in Organic Light‐Emitting Devices

By using pyran‐containing donor–acceptor dyes as doping molecules in organic light‐emitting devices (OLEDs), we scrutinize the effects of charge trapping and polarization induced by the guest molecules in the electro‐active host material. Laser dyes 4‐(dicyanomethylene)‐2‐methyl‐6‐[2‐(julolidin‐9‐yl)phenyl]ethenyl]‐4H‐pyran (DCM2) and the novel 4‐(dicyanomethylene)‐2‐methyl‐6‐{2‐[(4‐diphenylamino)phenyl]ethenyl}‐4H‐pyran (DCM‐TPA) are used as model compounds. The emission color of these polar dyes depends strongly on doping concentration, which we have attributed to polarization effects induced by the doping molecules themselves. Their frontier orbital energy levels are situated within the bandgap of the tris(8‐hydroxyquinoline)aluminum (Alq₃) host matrix and allow the investigation of either electron trapping or both electron and hole trapping. In the case of DCM‐TPA doping, we were able to show that electron trapping leads to a partial shift of the recombination zone out of the doped Alq₃ region. To impede charge‐recombination processes taking place in the undoped host matrix, a charge‐blocking layer efficiently confines the recombination zone inside the doped zone and gives rise to increased luminous efficiency. For a doping concentration of 1 wt.‐% we obtain a maximum luminous efficiency of 10.4 cd A^–1. At this doping concentration, the yellow emission spectrum shows excellent color saturation with CIE chromaticity coordinates x, y of 0.49 and 0.50, respectively. In the case of DCM2 the recombination zone is much less affected for the same doping concentrations, which is ascribed to the fact that both electrons and holes are being trapped. The experimental findings are corroborated with a numerical simulation of the doped multilayer devices.     

High‐Tg Carbazole Derivatives as Blue‐Emitting Hole‐Transporting Materials for Electroluminescent Devices

A series of dicarbazolyl derivatives bridged by various aromatic spacers and decorated with peripheral diarylamines were synthesized using Ullmann and Pd‐catalyzed C–N coupling procedures. These derivatives emit blue light in solution. In general, they possess high glass‐transition temperatures (T_g > 125 °C) which vary with the bridging segment and methyl substitution on the peripheral amine. Double‐layer organic light‐emitting devices were successfully fabricated using these molecules as hole‐transporting and emitting materials. Devices of the configuration ITO/HTL/TPBI/Mg:Ag (ITO: indium tin oxide; HTL: hole‐transporting layer; TPBI: 1,3,5‐tris(N‐phenylbenzimidazol‐2‐yl)benzene) display blue emission from the HTL layer. The EL spectra of these devices appear slightly distorted due to the exciplex formation at the interfaces. However, for the devices of the configuration ITO/HTL/Alq₃/Mg:Ag (Alq₃ = tris(8‐hydroxyquinoline)aluminum) a bright green light from the Alq₃ layer was observed. This clearly demonstrates the facile hole‐transporting property of the materials described here.     

Biocompatible Nanoparticles with Aggregation‐Induced Emission Characteristics as Far‐Red/Near‐Infrared Fluorescent Bioprobes for In Vitro and In Vivo Imaging Applications

Light emission of 2‐(2,6‐bis((E)‐4‐(diphenylamino)styryl)‐4H‐pyran‐4‐ylidene)malononitrile (TPA‐DCM) is weakened by aggregate formation. Attaching tetraphenylethene (TPE) units as terminals to TPA‐DCM dramatically changes its emission behavior: the resulting fluorogen, 2‐(2,6‐bis((E)‐4‐(phenyl(4′‐(1,2,2‐triphenylvinyl)‐[1,1′‐biphenyl]‐4‐yl)amino)styryl)‐4H‐pyran‐4‐ylidene)malononitrile (TPE‐TPA‐DCM), is more emissive in the aggregate state, showing the novel phenomenon of aggregation‐induced emission (AIE). Formulation of TPE‐TPA‐DCM using bovine serum albumin (BSA) as the polymer matrix yields uniformly sized protein nanoparticles (NPs) with high brightness and low cytotoxicity. Applications of the fluorogen‐loaded BSA NPs for in vitro and in vivo far‐red/near‐infrared (FR/NIR) bioimaging are successfully demonstrated using MCF‐7 breast‐cancer cells and a murine hepatoma‐22 (H₂₂)‐tumor‐bearing mouse model, respectively. The AIE‐active fluorogen‐loaded BSA NPs show an excellent cancer cell uptake and a prominent tumor‐targeting ability in vivo due to the enhanced permeability and retention effect.     

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.     

Direct Threat of a UV‐Ozone Treated Indium‐Tin‐Oxide Substrate to the Stabilities of Common Organic Semiconductors

Ultraviolet‐ozone treated indium‐tin‐oxide (UV‐ITO) glass substrates have been widely and unquestioningly used in the field of organic electronics to improve both device performance and stability. Evidence is presented here for rapid decay of common organic films such as N,N′‐bis(naphthalen‐1‐ yl)‐N,N′‐bis(phenyl)‐benzidine (NPB), tris(8‐hydroxy‐quinolinato)aluminum (Alq₃), and rubrene when they are in contact with an UV‐ITO substrate. While the photoluminescence (PL) of these organic films deposited on an UV‐ITO substrate decay rapidly under illumination; those on quartz substrates are comparatively much more stable. Results from X‐ray and UV photoemission spectroscopies (XPS and UPS) further suggest that degradations of the rubrene films on UV‐ITO substrate are mainly attributed to active oxygen species generated upon UV‐ozone treatment. These reactive oxygen species on the UV‐ITO surface behave as a reservoir of oxygen that interacts with rubrene and shifts its highest occupied molecular orbital (HOMO) level away from the Fermi level. This interaction induces a gap‐state in the energy gap of rubrene, which acts as a charge recombination center. More importantly, enhanced stabilities of rubrene‐based organic photovoltaic (OPV) devices are demonstrated when they are fabricated on gold‐coated or trifluoromethane (CHF₃) plasma‐treated ITO. The presented works shows that the commonly used UV‐ITO substrate is a threat to the stability of addlayer organic semiconducting films.     

Binaphthyl‐Containing Green‐ and Red‐Emitting Molecules for Solution‐Processable Organic Light‐Emitting Diodes

Strong intermolecular interactions usually result in decreases in solubility and fluorescence efficiency of organic molecules. Therefore, amorphous materials are highly pursued when designing solution‐processable, electroluminescent organic molecules. In this paper, a non‐planar binaphthyl moiety is presented as a way of reducing intermolecular interactions and four binaphthyl‐containing molecules ( BNCM s): green‐emitting BBB and TBT as well as red‐emitting BTBTB and TBBBT , are designed and synthesized. The photophysical and electrochemical properties of the molecules are systematically investigated and it is found that TBT , TBBBT , and BTBTB solutions show high photoluminescence (PL) quantum efficiencies of 0.41, 0.54, and 0.48, respectively. Based on the good solubility and amorphous film‐forming ability of the synthesized BNCM s, double‐layer structured organic light‐emitting diodes (OLEDs) with BNCM s as emitting layer and poly(N‐vinylcarbazole) (PVK) or a blend of poly[N,N′‐bis(4‐butylphenyl)‐N,N′‐bis(phenyl)benzidine] and PVK as hole‐transporting layer are fabricated by a simple solution spin‐coating procedure. Amongst those, the BTBTB based OLED, for example, reaches a high maximum luminance of 8315 cd · m⁻² and a maximum luminous efficiency of 1.95 cd · A⁻¹ at a low turn‐on voltage of 2.2 V. This is one of the best performances of a spin‐coated OLED reported so far. In addition, by doping the green and red BNCM s into a blue‐emitting host material poly(9,9‐dioctylfluorene‐2,7‐diyl) high performance white light‐emitting diodes with pure white light emission and a maximum luminance of 4000 cd · m⁻² are realized.     

High‐Purity‐Blue and High‐Efficiency Electroluminescent Devices Based on Anthracene

Novel blue‐light‐emitting materials, 9,10‐bis(1,2‐diphenyl styryl)anthracene (BDSA) and 9,10‐bis(4′‐triphenylsilylphenyl)anthracene (BTSA), which are composed of an anthracene molecule as the main unit and a rigid and bulky 1,2‐diphenylstyryl or triphenylsilylphenyl side unit, have been designed and synthesized. Theoretical calculations on the three‐dimensional structures of BDSA and BTSA show that they have a non‐coplanar structure and inhibited intermolecular interactions, resulting in a high luminescence efficiency and good color purity. By incorporating these new, non‐doped, blue‐light‐emitting materials into a multilayer device structure, it is possible to achieve luminance efficiencies of 1.43 lm W^–1 (3.0 cd A^–1 at 6.6 V) for BDSA and 0.61 lm W^–1 (1.3 cd A^–1 at 6.7 V) for BTSA at 10 mA cm^–2. The electroluminescence spectrum of the indium tin oxide (ITO)/copper phthalocyanine (CuPc)/1,4‐bis[(1‐naphthylphenyl)‐amino]biphenyl (α‐NPD)/BDSA/tris(9‐hydroxyquinolinato)aluminum (Alq₃)/LiF/Al device shows a narrow emission band with a full width at half maximum (FWHM) of 55 nm and a λ_max = 453 nm. The FWHM of the ITO/CuPc/α‐NPD/BTSA/Alq₃/LiF/Al device is 53 nm, with a λ_max = 436 nm. Regarding color, the devices showed highly pure blue emission ((x,y) = (0.15,0.09) for BTSA, (x,y) = (0.14,0.10) for BDSA) at 10 mA cm^–2 in Commission Internationale de l'Eclairage (CIE) chromaticity coordinates.     

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).     

Improved Efficiency of Single‐Layer Polymer Light‐Emitting Devices with Poly(vinylcarbazole) Doubly Doped with Phosphorescent and Fluorescent Dyes as the Emitting Layer

We demonstrate novel organic light‐emitting diode (LED) materials that contain a green phosphorescent dye (dmbpy)Re(CO)₃Cl (dmbpy = 4,4′‐dimethyl‐2,2′‐bipyridine), and a red fluorescent dye 4‐dicyanomethylene‐6‐(p‐dimethylaminostyryl)‐2‐methyl‐4H‐pyran (DCM) as dopants and polyvinylcarbazole (PVK) as the host. The photoluminescence (PL) and electroluminescence (EL) properties of these complex materials were studied. The energy transfer efficiency from PVK host to DCM is increased by the (dmbpy)Re(CO)₃Cl co‐dopant, which has an emission energy between that of PVK and DCM. The (dmbpy)Re(CO)₃Cl, which emits a long‐lived phosphorescence, is used as an energy coupler, providing the possibility to harvest both singlet and triplet energy in the devices. The pure red emission from DCM was observed from PL and EL spectra of (dmbpy)Re(CO)₃‐Cl(> 2.0 wt.‐%):DCM(> 0.5 wt. %) doped PVK films, demonstrating an efficient energy transfer from PVK and (dmbpy)Re(CO)₃‐Cl to DCM. By optimizing the concentration of DCM and (dmbpy)Re(CO)₃Cl in PVK, a maximum EL quantum efficiency of 0.42 cd A^–1 at a current density of 9.5 mA cm^–2 was obtained. The EL quantum efficiency of the doubly doped device is significantly enhanced in comparison with both a DCM‐only doped PVK device and a DCM‐doped PVK device with the green fluorescent dye Alq₃ as co‐dopant. The improvement in the operating characteristics of the phosphorescent and fluorescent dye doubly doped device is attributed to efficient energy transfer in the system, in which both triplet and singlet excitons are used for resultant emission in the polymer device.     

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.     

Red Phosphorescent Iridium Complex Containing Carbazole‐Functionalized β‐Diketonate for Highly Efficient Nondoped Organic Light‐Emitting Diodes

A novel red phosphorescent iridium complex containing a carbazole‐functionalized β‐diketonate, Ir(DBQ)₂(CBDK) (bis(dibenzo[f,h]quinoxalinato‐N,C²) iridium (1‐(carbazol‐9‐yl)‐5,5‐dimethylhexane‐2,4‐diketonate)) is designed, synthesized, and characterized. The electrophosphorescence properties of a nondoped device using the title complex as an emitter with a device configuration of indium tin oxide (ITO)/N,N′‐diphenyl‐N,N′‐bis(1‐naphthyl)‐1,1′‐diphenyl‐4,4′‐diamine (NPB; 20 nm)/iridium complex (20 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 5 nm)/tris(8‐hydroxyquinoline) (AlQ; 30 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) are examined. The results show that the nondoped device achieves a maximum lumen efficiency as high as 3.49 lm W^–1. To understand this excellent result observed, two reference complexes Ir(DBQ)₂(acac), where acac is the acetyl acetonate anion, and Ir(DBQ)₂(FBDK), [bis(dibenzo[f,h]quinoxalinato‐N,C²) iridium (1‐(9‐methyl‐fluoren‐9‐yl)‐6,6‐dimethylheptane‐3,5‐diketonate)], have also been synthesized, and as emitters they were examined under the same device configuration. The maximum lumen efficiency of the former compound is found to be 0.26 lm W^–1 while that for the latter is 0.37 lm W^–1, suggesting that the excellent performance of Ir(DBQ)₂(CBDK) can be attributed mainly to an improved hole‐transporting property that benefits the exciton transport. In addition, a bulky diketonate group separates the emitter centers from each other, which is also important for organic light‐emitting diodes.     

Cover Picture: An Organic Light‐Emitting Diode with Field‐Effect Electron Transport (Adv. Funct. Mater. 1/2008)

The cover shows an organic light‐emitting diode with remote metallic cathode, reported by Sarah Schols and co‐workers on p. 136. The metallic cathode is displaced from the light‐emission zone by one to several micrometers. The injected electrons accumulate at an organic heterojunction and are transported to the light‐emission zone by field‐effect. The achieved charge‐carrier mobility and in combination with reduced optical absorption losses because of the remoteness of the cathode may lead to applications as waveguide OLEDs and possibly a laser structure. (The result was obtained in the EU‐funded project “OLAS” IST‐ FP6‐015034.) We describe an organic light‐emitting diode (OLED) using field‐effect to transport electrons. The device is a hybrid between a diode and a field‐effect transistor. Compared to conventional OLEDs, the metallic cathode is displaced by one to several micrometers from the light‐emitting zone. This micrometer‐sized distance can be bridged by electrons with enhanced field‐effect mobility. The device is fabricated using poly(triarylamine) (PTAA) as the hole‐transport material, tris(8‐hydroxyquinoline) aluminum (Alq₃) doped with 4‐(dicyanomethylene)‐2‐methyl‐6‐(julolindin‐4‐yl‐vinyl)‐4H‐pyran (DCM₂) as the active light‐emitting layer, and N,N′‐ditridecylperylene‐3,4,9,10‐tetracarboxylic diimide (PTCDI‐C₁₃H₂₇), as the electron‐transport material. The obtained external quantum efficiencies are as high as for conventional OLEDs comprising the same materials. The quantum efficiencies of the new devices are remarkably independent of the current, up to current densities of more than 10 A cm^–2. In addition, the absence of a metallic cathode covering the light‐emission zone permits top‐emission and could reduce optical absorption losses in waveguide structures. These properties may be useful in the future for the fabrication of solid‐state high‐brightness organic light sources.     

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.     

Ambipolar Conductive 2,7‐Carbazole Derivatives for Electroluminescent Devices

A series of 2,7‐disubstituted carbazole (2,7‐carb) derivatives incorporating arylamines at the 2 and 7 positions are synthesized via palladium‐catalyzed C–N or C–C bond formation. These compounds possess glass transition temperatures ranging from 87 to 217 °C and exhibit good thermal stabilities, with thermal decomposition temperatures ranging from 388 to 480 °C. They are fluorescent and emit in the purple‐blue to orange region. Two types of organic light emitting diodes (OLEDs) were constructed from these compounds: (I) indium tin oxide (ITO)/2,7‐carb (40 nm)/1,3,5‐tris(N‐phenylbenzimidazol‐2‐yl)benzene (TPBI, 40 nm)/Mg:Ag; and (II) ITO/2,7‐carb (40 nm)/tris(8‐hydroxyquinoline) aluminum (Alq₃, 40 nm)/Mg:Ag. In type I devices, the 2,7‐disubstituted carbazoles function as both hole‐transporting and emitting material. In type II devices, light is emitted from either the 2,7‐disubstituted carbazole layer or Alq₃. The devices appear to have a better performance compared to devices fabricated with their 3,6‐disubstituted carbazole congeners. Some of the new compounds exhibit ambipolar conductive behavior, with hole and electron mobilities up to 10^–4 cm² V^–1 s^–1.     

Nanodot‐Enhanced High‐Efficiency Pure‐White Organic Light‐Emitting Diodes with Mixed‐Host Structures

A relatively high‐efficiency, fluorescent pure‐white organic light‐emitting diode was fabricated using a polysilicic acid (PSA) nanodot‐embedded polymeric hole‐transporting layer (HTL). The diode employed a mixed host in the single emissive layer, which comprised 0.5 wt % yellow 5,6,11,12‐tetra‐phenylnaphthacene doped in the mixed host of 50 % 2‐(N,N‐diphenyl‐amino)‐6‐[4‐(N,N‐diphenylamino)styryl]naphthalene and 50 % N,N′‐bis‐(1‐naphthyl)‐N,N′‐diphenyl‐1,10‐biphenyl‐4‐4′‐diamine. By incorporating 7 wt % 3 nm PSA nanodot into the HTL of poly(3,4‐ethylene‐dioxythiophene)‐poly‐(styrenesulfonate), the efficiency at 100 cd m^–2 was increased from 13.5 lm W^–1 (14.7 cd A^–1; EQE: 7.2 %) to 17.1 lm W^–1 (17.6 cd A^–1; EQE: 8.3 %). The marked efficiency improvement may be attributed to the introduction of the PSA nanodot, leading to a better carrier‐injection‐balance.     

High‐Efficiency and Color Stable Blue‐Light‐Emitting Polymers and Devices

Novel fluorene‐based blue‐light‐emitting copolymers with an ultraviolet‐blue‐light (UV‐blue‐light) emitting host and a blue‐light emitting component, 4‐N,N‐diphenylaminostilbene (DPS) have been designed and synthesized by using the palladium‐ catalyzed Suzuki coupling reaction. It was found that both copolymers poly [2,7‐(9,9‐dioctylfluorene)‐alt‐1,3‐(5‐carbazolphenylene)] (PFCz) DPS1 and PFCz‐DPS1‐OXD show pure blue‐light emission even with only 1 % DPS units because of the efficient energy transfer from the UV‐blue‐light emitting PFCz segments to the blue‐light‐emitting DPS units. Moreover, because of the efficient energy transfer/charge trapping in these copolymers, PFCz‐DPS1 and PFCz‐DPS1‐OXD show excellent device performance with a very stable pure blue‐light emission. By using a neutral surfactant poly[9,9‐bis(6'‐(diethanolamino)hexyl)‐fluorene] (PFN‐OH) as the electron injection layer, the device based on PFCz‐DPS1‐OXD5 with the configuration of ITO/PEDOT:PSS/PVK/polymer/PFN‐OH/Al showed a maximum quantum efficiency of 2.83 % and a maximum luminous efficiency of 2.50 cd A^–1. Its CIE 1931 chromaticity coordinates of (0.156, 0.080) match very well with the NTSC standard blue pixel coordinates of (0.14, 0.08). These results indicate that this kind of dopant/host copolymer could be a promising candidate for blue‐light‐emitting polymers with high efficiency, good color purity, and excellent color stability.     

Molecular Engineering of Blue Fluorescent Molecules Based on Silicon End‐Capped Diphenylaminofluorene Derivatives for Efficient Organic Light‐Emitting Materials

Blue fluorescent materials based on silicone end‐capped 2‐diphenylaminofluorene derivatives are synthesized and characterized. These materials are doped into a 2‐methyl‐9,10‐di‐[2‐naphthyl]anthracene host as blue dopant materials in the emitting layer of organic light‐emitting diode devices bearing a structure of ITO/DNTPD (60 nm)/NPB (30 nm)/emitting layer (30 nm)/Alq₃ (20 nm)/LiF (1.0 nm)/Al (200 nm). All devices exhibit highly efficient blue electroluminescence with high external quantum efficiencies (3.47%–7.34% at 20 mA cm^?2). The best luminous efficiency of 11.2 cd A^?1 and highest quantum efficiency of 7.34% at 20 mA cm^?2 are obtained in a device with CIE coordinates (0.15, 0.25). A deep‐blue OLED with CIE coordinates (0.15, 0.14) exhibits a luminous efficiency of 3.70 cd A^?1 and quantum efficiency of 3.47% at 20 mA cm^?2.