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.     

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.     

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.     

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.     

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.     

Optical performance of efficient blue/near UV nitropyridine-conjugated anthracene (NAMA) based light emitting diode

9-[(5-nitropyridin-2-aminoethyl) iminiomethyl]-anthracene (NAMA) was synthesized for the first time from the reaction between 9-antracene carboxaldehyde and 2-(2-aminoethylamino)-5-nitropyridine under mild reaction conditions. The structure of the nitropyridine-conjugated anthracene (NAMA) was characterized using ¹H-NMR, ¹³C-NMR and elemental analysis techniques. Furthermore, electroluminescent behavior of the NAMA was investigated by using a material based on polycyclic aromatic hydrocarbons (PAHs) in organic light-emitting diodes (OLEDs). It was observed that the NAMA exhibits blue/near UV emission at 410 nm with a high density signal. The optimized device structure, ITO/CuPc/α−NPD/NAMA/Alq₃/LiF/Al is characterized by blue/near UV electroluminescence (EL) and a high current density with 7735 cd m⁻² maximum brightness at approximately 10.4 V. The emitting color of the device showed the blue/near UV emission (x, y) = (0.14, 0.10) at 277.1 mA cm⁻² in CIE (Commission Internationale de l’Eclairage) chromaticity coordinates with a long operational lifetime (180 h).     

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.     

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.     

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.     

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.     

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.     

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.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

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.     

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.     

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.     

Efficient Bipolar Blue AIEgens for High‐Performance Nondoped Blue OLEDs and Hybrid White OLEDs

Blue organic luminescent materials play a crucial role in full‐color display and white lighting but efficient ones meeting commercial demands are very rare. Herein, the design and synthesis of tailor‐made bipolar blue luminogens with an anthracene core and various functional groups are reported. The thermal stabilities, photophysical properties, electronic structures, electrochemical behaviors, carrier transport abilities, and electroluminescence performances are systematically investigated. The luminogen TPE‐TAPBI containing a tetraphenylethene moiety shows aggregation‐induced emission, while another luminogen TriPE‐TAPBI bearing a triphenylethene unit exhibits light aggregation‐caused quenching. In comparison with TriPE‐TAPBI, TPE‐TAPBI has stronger blue emission in neat film and functions more efficiently in nondoped organic light‐emitting diodes (OLEDs). High maxima current, power, and external quantum efficiencies of 7.21 cd A^?1, 6.78 lm W^?1, and 5.73%, respectively, are attained by the nondoped blue OLED of TPE‐TAPBI (CIE_x,y = 0.15, 0.16). Moreover, efficient two‐color hybrid warm white OLEDs (CIE_x,y = 0.457, 0.470) are achieved using TPE‐TAPBI neat film as the blue‐emitting component, which provide total current, power, external quantum efficiencies of up to 70.5 lm W^?1, 76.0 cd A^?1, and 28% at 1000 cd m^?2, respectively. These blue and white OLEDs are among the most efficient devices with similar colors in the literature.     

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.     

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.     

Dual enhancing properties of LiF with varying positions inside organic light-emitting devices

A multilayer organic light-emitting device (OLED) has been fabricated with a thin (0.3 nm) lithium fluoride (LiF) layer inserted inside an electron transport layer (ETL), aluminum tris(8-hydroxyquinoline) (Alq₃). The LiF electron injection layer (EIL) has not been used at an Al/Alq₃ interface in the device on purpose to observe properties of LiF. The electron injection-limited OLED with the LiF layer inside 50 nm Alq₃ at a one forth, a half or a three forth position assures two different enhancing properties of LiF. When the LiF layer is positioned closer to the Al cathode, the injection-limited OLED shows enhanced injection by Al interdiffusion. The Al interdiffusion at least up to 12.5 nm inside Alq₃ rules out the possible insulating buffer model in a small molecule bottom-emission (BE) OLED with a thin, less than one nanometer, electron injection layer (EIL). If the position is further away from the Al cathode, the Al diffusion reaches the LiF layer no longer and the device shows the electroluminescence (EL) enhancement without an enhanced injection. The suggested mechanism of LiF EL efficiency enhancer is that the thin LiF layer induces carrier trap sites and the trapped charges alters the distribution of the field inside the OLED and, consequently, gives a better recombination of the device. By substituting the Alq₃ ETL region with copper phthalocyanine (CuPc), all of the electron injection from the cathode of Al/CuPc interface, the induced recombination at the Alq₃ emitting layer (EML) by the LiF EL efficiency enhancer, and the operating voltage reduction from high conductive CuPc can be achieved. The enhanced property reaches 100 mA/cm² of current density and 1000 cd/m² of luminance at 5 V with its turn-on slightly larger than 2 V. The enhanced device is as good as our previously reported non-injection limited LiF EIL device [Yeonjin Yi, Seong Jun Kang, Kwanghee Cho, Jong Mo Koo, Kyul Han, Kyongjin Park, Myungkeun Noh, Chung Nam Whang, Kwangho Jeong, Appl. Phys. Lett. 86 (2005) 213502].