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.     

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.     

Pure‐Red Dye for Organic Electroluminescent Devices: Bis‐Condensed DCM Derivatives

In this paper, the bis‐condensed 4‐(dicyanomethylene)‐2‐methyl‐6‐[p‐(dimethylamino)styryl]‐4H‐pyran ( DCM) derivatives are introduced as a new class of red dye for organic light‐emitting devices (OLEDs). They showed more red‐shifted emission than the mono‐substituted DCM derivatives and the emission maxima increased as the electron‐donating ability of the aromatic donor group increased. On the basis of these results, red light‐emitting devices were fabricated with bis‐condensed DCM derivatives as red dopants. For a device of configuration ITO/TPD/Alq₃ + DADB (5.2 wt.‐%)/Alq₃/Al (where ITO is indium tin oxide, TPD is N,N′‐diphenyl‐N,N′‐bis(3‐methylphenyl)‐1,1′‐biphenyl‐4,4′‐diamine, Alq₃ is tris(8‐hydroxyquinoline) aluminum, and DADB is [2,6‐bis[2‐[5‐(dibutylamino)phenyl]vinyl]‐4H‐pyran‐4‐ylidene]propanedinitrile), pure red emission was observed with Commission Internationale de l’Eclairage (CIE 1931) coordinates of (0.658, 0.337) at 25 mA/cm².     

Charge‐Transporting Polymers based on Phenylbenzoimidazole Moieties

A series of novel styrene functionalized monomers with phenylbenzo[d]imidazole units and the corresponding homopolymers are prepared. These side‐chain polymers show high glass‐transition temperatures that even exceed the corresponding value for the common electron‐transporting material 1,3,5‐tris(1‐phenyl‐1H‐benzo[d]imidazol‐2‐yl)benzene (TPBI). Similar electronic behavior between the polymers and TPBI is shown. The polymers are used as matrices for phosphorescent dopants. The fabricated devices exhibit current efficiencies up to 38.5 cd A^?1 at 100 cd m^?2 and maximum luminances of 7400 cd m^?2 at 10 V with a minimum turn‐on voltage as low as 2.70 V in single‐layer devices with an ITO/PEDOT:PSS anode (ITO = indium tin oxide, PEDOT:PSS = poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonate)) and a CsF/Ca/Ag cathode.     

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.     

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.     

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.     

Top‐Emission Organic Light‐Emitting Diode with a Novel Copper/Graphene Composite Anode

The relatively high sheet resistance of graphene compared with indium tin oxide (ITO) blocks the applications of graphene as transparent electrodes in organic light‐emitting diodes. A novel copper (Cu)/graphene composite electrode is presented and employed as the anode of a top‐emission organic light‐emitting diode with the structure of Cu/graphene/V₂O₅/NPB/Alq₃/Alq₃: C545T/Bphen: Cs₂CO₃/Sm/Au. The Cu/graphene composite electrodes are fabricated by growing graphene directly on Cu substrates via the chemical vapor deposition method without any transfer process. The maxima of current efficiency and power efficiency of a typical Cu/graphene composite anode device reach 6.1 cd/A and 7.6 lm/W, respectively, which are markedly higher than those of the control devices with a graphene anode, a Cu anode or an ITO anode. The low sheet resistance of the composite electrode, the high quality of graphene without any transfer process and the avoidance of wave guiding loss in glass or polyethylene terephthalate substrates result in the improvements of light emission efficiencies.     

A Pyridine‐Containing Anthracene Derivative with High Electron and Hole Mobilities for Highly Efficient and Stable Fluorescent Organic Light‐Emitting Diodes

A pyridine‐containing anthracene derivative, 9,10‐bis(3‐(pyridin‐3‐yl)phenyl)anthracene (DPyPA), which comprehensively outperforms the widely used electron‐transport material (ETM), tris(8‐quinolinolato) aluminum (Alq₃), is synthesized. DPyPA exhibits ambipolar transport properties, with both electron and hole mobilities of around 10^?3 cm^?2 V^?1 s^?1; about two orders of magnitude higher than that of Alq₃. The nitrogen atom in the pyridine ring of DPyPA coordinates to lithium cations, which leads to efficient electron injection when LiF/Al is used as the cathode. Electrochemical measurements demonstrate that both the cations and anions of DPyPA are stable, which may improve the stability of devices based on DPyPA. Red‐emitting, green‐emitting, and blue‐emitting fluorescent organic light emitting diodes with DPyPA as the ETM display lower turn‐on voltages, higher efficiencies, and stronger luminance than the devices with Alq₃ as the ETM. The power efficiencies of the devices based on DPyPA are greater by 80–140% relative to those of the Alq₃‐based devices. The improved performance of these devices is attributed to the increased carrier balance. In addition, the device employing DPyPA as the ETM possesses excellent stability: the half‐life of the DPyPA‐based device is 67 000 h—seven times longer than that of the Alq₃‐based device—for an initial luminance of 5000 cd m^?2.     

Preparation and Characterization of Blue‐Luminescent Tris(8‐hydroxyquinoline)‐aluminum (Alq3)

Using differential scanning calorimetry (DSC) measurements in combination with structural and optical characterization we have investigated the formation conditions of different phases of tris(8‐hydroxyquinoline)aluminum (Alq₃). We have identified the δ‐phase as a high‐temperature phase of Alq₃ being composed of the facial stereoisomer, and report an efficient method to obtain blue luminescent Alq₃ by a simple annealing process. This allows the preparation of large amounts of pure δ‐Alq₃ by choosing appropriate annealing conditions, which is necessary for further characterization of this blue‐luminescent phase, and offers the possibility of fabricating blue organic light‐emitting devices (OLEDs) from this material.     

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.     

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.     

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.     

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.     

Silaindacenodithiophene‐Based Low Band Gap Polymers – The Effect of Fluorine Substitution on Device Performances and Film Morphologies

Silaindacenodithiophene is copolymerized with benzo[c][1,2,5]thiadiazole ( BT ) and 4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole ( DTBT ), respectively their fluorinated counter parts 5,6‐difluorobenzo[c][1,2,5]thiadiazole ( 2FBT ) and 5,6‐difluoro‐4,7‐di(thiophen‐2‐yl) benzo[c][1,2,5]thiadiazole ( 2FDTBT ). The influence of the thienyl spacers and fluorine atoms on molecular packing and active layer morphology is investigated with regard to device performances. bulk heterojunction (BHJ) solar cells based on silaindacenodithiophene donor‐acceptor polymers achieved PCE's of 4.5% and hole mobilities of as high as 0.28 cm²/(V s) are achieved in an organic field‐effect transistor (OFET).     

1‐Methyl‐2‐(anthryl)‐imidazo[4,5‐f][1,10]‐phenanthroline: A Highly Efficient Electron‐Transport Compound and a Bright Blue‐Light Emitter for Electroluminescent Devices

A new organic blue‐light emitter 1‐methyl‐2‐(anthryl)‐imidazo[4,5‐f][1,10]‐phenanthroline ( 1 ) has been synthesized and fully characterized. The utility of compound 1 as a blue‐light emitter in electroluminescent (EL) devices has been evaluated by fabricating a series of EL devices A where compound 1 functions as an emitter. The EL spectrum of device series A has the emission maximum at 481 nm with the CIE (Commission Internationale de l'Eclairage) color coordinates 0.198 and 0.284. The maximum luminance of devices in series A is 4000 cd m^–2 and the best external quantum efficiency of device series A is 1.82 %. The utility of compound 1 as an electron injection–electron transport material has been evaluated by constructing a set of EL devices B where 1 is used as either the electron‐injection layer or the electron injection–electron transport layer. The performance of device series B is compared to the standard device in which Alq₃ (tris(8‐hydroxyquinoline) aluminum) is used as the electron injection–electron transport layer. The experimental results show that the performance of 1 as an electron injection–electron transport material is considerably better than Alq₃. The stability of device series B is comparable to that of the standard Alq₃ device. The excellent performance of 1 as an electron injection/transport material may be attributed to the strong intermolecular interactions of 1 in the solid state as revealed by single‐crystal X‐ray diffraction analysis. In addition, compound 1 is a colorless material with a much larger highest occupied molecular orbital–lowest unoccupied molecular (HOMO–LUMO) gap than Alq₃, which renders it potentially useful for a wide range of applications in EL devices.     

Blade coating of Tris(8-hydroxyquinolinato)aluminum as the electron-transport layer for all-solution blue fluorescent organic light-emitting diodes

Organic light-emitting diodes (OLEDs) with a low driving voltage and efficient blue fluorescence were fabricated through blade coating. Tris(8-hydroxyquinolinato)aluminum (Alq₃) is a relatively stable electron-transporting material commonly used in evaporation. However, depositing Alq₃ through a solution process is difficult because of its extremely low solubility organic solvents, a result of its symmetrical molecular structure. In this study, Alq₃ was successfully deposited through blade coating at a very low concentration below 0.1wt%. The OLEDs contained co-dopants BUBD-1 and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph), and a high-band-gap host 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as the emission layer with the following structure: ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:2% BUBD-1:1% DSA-Ph (50 nm)/TPBI (30 nm)/LiF (0.8 nm)/Al (100 nm)or ITO/PEDOT:PSS (40 nm)/VB-FNPD (30 nm)/MADN:3% BUBD-1 (50 nm)tris(8-hydroxyquinolinato)aluminum (Alq₃; 10 nm)/LiF (0.8 nm)/Al (100 nm). 2,7-disubstituted fluorene-based triaryldiamine(VB-FNPD)is the cross-linking transporting material. The device exhibited a peak current efficiency of 5.67 cd/A for Alq₃ and 5.76 cd/A for TPBI. The device with Alq₃ has operated lifetime seven times higher than the device with TPBI.     

Architecture of Conjugated Donor–Acceptor (D–A)‐Type Polymer Films with Cross‐Linked Structures

A series of new donor–acceptor (D–A)‐type semiconducting conjugated polymers (SCPs), which can form cross‐linked structural and supramolecular assembly films by hydrogen‐bonding, is successfully synthesized. The microstructures of supramolecular assembly films are further investigated by X‐ray diffraction (XRD), high‐ resolution transmission electron microscopy (HRTEM), and variable‐temperature Fourier transform infrared (FT‐IR) absorption spectra. As electronic transmission (ET) materials, the SCPs demonstrate superior properties by means of fabricating electron‐only devices with the configuration of ITO/ET (SCPs)/Ca/Al. According to space‐charge‐limited current (SCLC) measurements, fluorine‐containing SCPs exhibit much smaller threshold voltages and much higher electron mobilities than Alq₃. Meanwhile, a significant enhancement for their luminescence properties is verified by the photoluminescence (PL) and electroluminescent (EL) spectra of cross‐linked‐type SCPs, compared to non‐cross‐linked‐type SCPs. The fabricated polymer light‐emitting diodes (PLEDs) with the configuration of ITO/PEDOT:PSS/EML (SCPs)/BCP/LiF/Al are able to emit the color from green to red with moderately low turn‐on voltages. These results suggested that cross‐linked D–A‐type SCP can become a potential candidate as a kind of multifunctional materials applied in the field of optoelectronic devices.     

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.     

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.