Hole Injection in a Model Fluorene–Triarylamine Copolymer

Recent developments in synthesis and purification have yielded conjugated polymers with hole mobilities exceeding 0.01 cm² V^?1 s^?1. Essential to harvesting the potential of these materials in organic light emitting diodes (OLEDs) is the identification of suitable ohmic contacts. Using a model fluorene copolymer that shows high‐mobility, non‐dispersive hole transport, it is demonstrated that electrodes commonly used as anodes in OLEDs are very poor hole injectors. Injection from Au and indium tin oxide anodes is limited by energy barriers of 0.75 and 0.65 eV, respectively, and the injected current is found to be temperature independent—a prediction that was not reproduced by the leading injection model for disordered organic semiconductors. Injection from a poly(3,4‐ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) anode, on the other hand, is found to become less efficient with electric field, a behavior which is currently not understood. In thinner poly[(9,9′‐dioctylfluorenyl‐2,7‐diyl)‐co‐(4,4′‐(N‐(4‐sec‐butyl))diphenylamine)] films, which are of relevance to OLEDs, ohmic losses on the PEDOT:PSS layer are found to limit the flow of current. These results illustrate the opportunity to further improve the performance of OLEDs as well as the challenge posed by high mobility conjugated polymers for the design of hole injection layers.     

The Role of Transition Metal Oxides in Charge‐Generation Layers for Stacked Organic Light‐Emitting Diodes

The mechanism of charge generation in transition metal oxide (TMO)‐based charge‐generation layers (CGL) used in stacked organic light‐emitting diodes (OLEDs) is reported upon. An interconnecting unit between two vertically stacked OLEDs, consisting of an abrupt heterointerface between a Cs₂CO₃‐doped 4,7‐diphenyl‐1,10‐phenanthroline layer and a WO₃ film is investigated. Minimum thicknesses are determined for these layers to allow for simultaneous operation of both sub‐OLEDs in the stacked device. Luminance–current density–voltage measurements, angular dependent spectral emission characteristics, and optical device simulations lead to minimum thicknesses of the n‐type doped layer and the TMO layer of 5 and 2.5 nm, respectively. Using data on interface energetic determined by ultraviolet photoelectron and inverse photoemission spectroscopy, it is shown that the actual charge generation occurs between the WO₃ layer and its neighboring hole‐transport material, 4,4',4”‐tris(N‐carbazolyl)‐triphenyl amine. The role of the adjacent n‐type doped electron transport layer is only to facilitate electron injection from the TMO into the adjacent sub‐OLED.     

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.     

Highly Efficient p‐i‐n and Tandem Organic Light‐Emitting Devices Using an Air‐Stable and Low‐Temperature‐Evaporable Metal Azide as an n‐Dopant

Cesium azide (CsN₃) is employed as a novel n‐dopant because of its air stability and low deposition temperature. CsN₃ is easily co‐deposited with the electron transporting materials in an organic molecular beam deposition chamber so that it works well as an n‐dopant in the electron transport layer because its evaporation temperature is similar to that of common organic materials. The driving voltage of the p‐i‐n device with the CsN₃‐doped n‐type layer and a MoO₃‐doped p‐type layer is greatly reduced, and this device exhibits a very high power efficiency (57 lm W^?1). Additionally, an n‐doping mechanism study reveals that CsN₃ was decomposed into Cs and N₂ during the evaporation. The charge injection mechanism was investigated using transient electroluminescence and capacitance–voltage measurements. A very highly efficient tandem organic light‐emitting diodes (OLED; 84 cd A^?1) is also created using an n–p junction that is composed of the CsN₃‐doped n‐type organic layer/MoO₃ p‐type inorganic layer as the interconnecting unit. This work demonstrates that an air‐stable and low‐temperature‐evaporable inorganic n‐dopant can very effectively enhance the device performance in p‐i‐n and tandem OLEDs, as well as simplify the material handling for the vacuum deposition process.     

n‐Doping of a Low‐Electron‐Affinity Polymer Used as an Electron‐Transport Layer in Organic Light‐Emitting Diodes

n‐Doping electron‐transport layers (ETLs) increases their conductivity and improves electron injection into organic light‐emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n‐doping has been notoriously more difficult for these materials. In this work, n‐doping of the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) is demonstrated via solution processing, using the air‐stable n‐dopant (pentamethylcyclopentadienyl)(1,3,5‐trimethylbenzene)ruthenium dimer [RuCp*Mes]₂. Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes]₂, the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n‐doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. The [RuCp*Mes]₂‐doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.     

Platinum Binuclear Complexes as Phosphorescent Dopants for Monochromatic and White Organic Light‐Emitting Diodes

Efficient blue‐, green‐, and red‐light‐emitting organic diodes are fabricated using binuclear platinum complexes as phosphorescent dopants. The series of complexes used here have pyrazolate bridging ligands and the general formula C^∧NPt(μ‐pz)₂PtC^∧N (where C^∧N = 2‐(4′,6′‐difluorophenyl)pyridinato‐N,C^2′, pz = pyrazole ( 1 ), 3‐methyl‐5‐tert‐butylpyrazole ( 2 ), and 3,5‐bis(tert‐butyl)pyrazole ( 3 )). The Pt–Pt distance in the complexes, which decreases in the order 1 > 2 > 3 , solely determines the electroluminescence color of the organic light‐emitting diodes (OLEDs). Blue OLEDs fabricated using 8 % 1 doped into a 3,5‐bis(N‐carbazolyl)benzene (mCP) host have a quantum efficiency of 4.3 % at 120 Cd m^–2, a brightness of 3900 Cd m^–2 at 12 V, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.11, 0.24). Green and red OLEDs fabricated with 2 and 3 , respectively, also give high quantum efficiencies (～ 6.7 %), with CIE coordinates of (0.31, 0.63) and (0.59, 0.46), respectively. The current‐density–voltage characteristics of devices made using dopants 2 and 3 indicate that hole trapping is enhanced by short Pt–Pt distances (< 3.1 Å). Blue electrophosphorescence is achieved by taking advantage of the binuclear molecular geometry in order to suppress dopant intermolecular interactions. No evidence of low‐energy emission from aggregate states is observed in OLEDs made with 50 % 1 doped into mCP. OLEDs made using 100 % 1 as an emissive layer display red luminescence, which is believed to originate from distorted complexes with compressed Pt–Pt separations located in defect sites within the neat film. White OLEDs are fabricated using 1 and 3 in three different device architectures, either with one or two dopants in dual emissive layers or both dopants in a single emissive layer. All the white OLEDs have high quantum efficiency (～ 5 %) and brightness (～ 600 Cd m^–2 at 10 V).     

Thermally Stable Hole‐Transporting Materials Based upon a Fluorene Core

We report a new class of diamine hole‐transporting materials (HTMs) based upon a fluorene core. Using a fluorene core, rather than a biphenyl group, leads to enhanced thermal stability, as evidenced by glass‐transition (T_g) temperatures as high as 161 °C for N,N′‐iminostilbenyl‐4,4′‐fluorene (ISF). The fluorene‐based HTMs have lower ionization potentials (I_p) than their biphenyl analogs, which leads to more efficient injection of holes from the indium tin oxide (ITO) anode, and higher quantum efficiencies. Devices prepared with fluorene‐based HTMs were operated under thermal stress. The failure of an organic light‐emitting diode (OLED) under thermal stress has a direct correlation with the thermal stability of the HTM that is in contact with the ITO anode. OLEDs based on ISF are stable to over 140 °C.     

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.     

High‐Performance Quantum‐Dot Light‐Emitting Diodes Using NiOx Hole‐Injection Layers with a High and Stable Work Function

Solution‐processed oxide thin films are actively pursued as hole‐injection layers (HILs) in quantum‐dot light‐emitting diodes (QLEDs), aiming to improve operational stability. However, device performance is largely limited by inefficient hole injection at the interfaces of the oxide HILs and high‐ionization‐potential organic hole‐transporting layers. Solution‐processed NiO_x films with a high and stable work function of ≈5.7 eV achieved by a simple and facile surface‐modification strategy are presented. QLEDs based on the surface‐modified NiO_x HILs show driving voltages of 2.1 and 3.3 V to reach 1000 and 10 000 cd m^?2, respectively, both of which are the lowest among all solution‐processed LEDs and vacuum‐deposited OLEDs. The device exhibits a T₉₅ operational lifetime of ≈2500 h at an initial brightness of 1000 cd m^?2, meeting the commercialization requirements for display applications. The results highlight the potential of solution‐processed oxide HILs for achieving efficient‐driven and long‐lifetime QLEDs.     

Interface Modification to Improve Hole‐Injection Properties in Organic Electronic Devices

The performance of organic electronic devices is often limited by injection. In this paper, improvement of hole injection in organic electronic devices by conditioning of the interface between the hole‐conducting layer (buffer layer) and the active organic semiconductor layer is demonstrated. The conditioning is performed by spin‐coating poly(9,9‐dioctyl‐fluorene‐co‐N‐ (4‐butylphenyl)‐diphenylamine) (TFB) on top of the poly(3,4‐ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) buffer layer, followed by an organic solvent wash, which results in a TFB residue on the surface of the PEDOT:PSS. Changes in the hole‐injection energy barriers, bulk charge‐transport properties, and current–voltage characteristics observed in a representative PFO‐based (PFO: poly(9,9‐dioctylfluorene)) diode suggest that conditioning of PEDOT:PSS surface with TFB creates a stepped electronic profile that dramatically improves the hole‐injection properties of organic electronic devices.     

Conjugated Small Molecule for Efficient Hole Transport in High‐Performance p‐i‐n Type Perovskite Solar Cells

The π‐conjugated organic small molecule 4,4′‐cyclohexylidenebis[N,N‐bis(4‐methylphenyl) benzenamine] (TAPC) has been explored as an efficient hole transport material to replace poly(3,4‐ethylenedio‐xythiophene):poly(styrenesulfonate) (PEDOT:PSS) in the preparation of p‐i‐n type CH₃NH₃PbI₃ perovskite solar cells. Smooth, uniform, and hydrophobic TAPC hole transport layers can be facilely deposited through solution casting without the need for any dopants. The power conversion efficiency of perovskite solar cells shows very weak TAPC layer thickness dependence across the range from 5 to 90 nm. Thermal annealing enables improved hole conductivity and efficient charge transport through an increase in TAPC crystallinity. The perovskite photoactive layer cast onto thermally annealed TAPC displays large grains and low residual PbI₂, leading to a high charge recombination resistance. After optimization, a stabilized power conversion efficiency of 18.80% is achieved with marginal hysteresis, much higher than the value of 12.90% achieved using PEDOT:PSS. The TAPC‐based devices also demonstrate superior stability compared with the PEDOT:PSS‐based devices when stored in ambient circumstances, with a relatively high humidity ranging from 50 to 85%.     

The Mechanism of Charge Generation in Charge‐Generation Units Composed of p‐Doped Hole‐Transporting Layer/HATCN/n‐Doped Electron‐Transporting Layers

The rate‐limiting step of charge generation in charge‐generation units (CGUs) composed of a p‐doped hole‐transporting layer (p‐HTL), 1,4,5,8,9,11‐hexaazatriphenylene hexacarbonitrile (HATCN) and n‐doped electron‐transporting layer (n‐ETL), where 1,1‐bis‐(4‐bis(4‐methyl‐phenyl)‐amino‐phenyl)‐cyclohexane (TAPC) was used as the HTL is reported. Energy level alignment determined by the capacitance–voltage (C–V) measurements and the current density–voltage characteristics of the structure clearly show that the electron injection at the HATCN/n‐ETL junction limits the charge generation in the CGUs rather than charge generation itself at the p‐HTL/HATCN junction. Consequently, the CGUs with 30 mol% Rb₂CO₃‐doped 4,7‐diphenyl‐1,10‐phenanthroline (BPhen) formed with the HATCN layer generates charges very efficiently and the excess voltage required to generate the current density of ±10 mA cm^?2 is around 0.17 V, which is extremely small compared with the literature values reported to date.     

Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

2,2′‐(perfluoronaphthalene‐2,6‐diylidene)dimalononitrile (F6‐TCNNQ) is investigated as a molecular p‐type dopant in two hole‐transport materials, 2,2′,7,7′‐tetrakis(N,N‐diphenylamino)‐9,9‐spirobifluorene (Spiro‐TAD) and tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA). The electron affinity of F6‐TCNNQ is determined to be 5.60 eV, one of the strongest organic molecular oxidizing agents used to date in organic electronics. p‐Doping is found to be effective in Spiro‐TAD (ionization energy = 5.46 eV) but not in TCTA (ionization energy = 5.85 eV). Optical absorption measurements demonstrate that charge transfer is the predominant doping mechanism in Spiro‐TAD:F6‐TCNNQ. The host–dopant interaction also leads to a significant alteration of the host film morphology. Finally, transport measurements done on Spiro‐TAD:F6‐TCNNQ as a function of dopant concentration and temperature, and using a highly doped contact layer to ensure negligible hole injection barrier, lead to an accurate measurement of the film conductivity and hole‐hopping activation energy.     

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.     

A Multifunctional Bipolar Luminogen with Delayed Fluorescence for High‐Performance Monochromatic and Color‐Stable Warm‐White OLEDs

Increasing exciton utilization and reducing exciton annihilation are crucial to achieve high performance of organic light‐emitting diodes (OLEDs), which greatly depend on molecular engineering of emitters and hosts. A novel luminogen (SBF‐BP‐DMAC) is synthesized and characterized. Its crystal and electronic structures, thermal stability, electrochemical behavior, carrier transport, photoluminescence, and electroluminescence are investigated. SBF‐BP‐DMAC exhibits enhanced photoluminescence and promotes delayed fluorescence in solid state and bipolar carrier transport ability, and thus holds multifunctionality of emitter and host for OLEDs. Using SBF‐BP‐DMAC as an emitter, the nondoped OLEDs exhibit maximum electroluminescence (EL) efficiencies of 67.2 cd A^?1, 65.9 lm W^?1, and 20.1%, and the doped OLEDs provide maximum EL efficiencies of 79.1 cd A^?1, 70.7 lm W^?1, and 24.5%. A representative orange phosphor, Ir(tptpy)₂acac, is doped into SBF‐BP‐DMAC for OLED fabrication, giving rise to superior EL efficiencies of 88.0 cd A^?1, 108.0 lm W^?1, and 26.8% for orange phosphorescent OLEDs, and forward‐viewing EL efficiencies of 69.3 cd A^?1, 45.8 lm W^?1, and 21.0% for two‐color hybrid warm‐white OLEDs. All of these OLEDs can retain high EL efficiencies at high luminance, with very small efficiency roll‐offs. The outstanding EL performance demonstrates the great potentials of SBF‐BP‐DMAC in practical display and lighting devices.     

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

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

Highly Efficient Red Phosphorescent OLEDs based on Non‐Conjugated Silicon‐Cored Spirobifluorene Derivative Doped with Ir‐Complexes

A novel host material containing silicon‐cored spirobifluorene derivative (SBP‐TS‐PSB), is designed, synthesized, and characterized for red phosphorescent organic light‐emitting diodes (OLEDs). The SBP‐TS‐PSB has excellent thermal and morphological stabilities and exhibits high electroluminescence (EL) efficiency as a host for the red phosphorescent OLEDs. The electrophosphorescence properties of the devices using SBP‐TS‐PSB as the host and red phosphorescent iridium (III) complexes as the emitter are investigated and these devices exhibit higher EL performances compared with the reference devices with 4,4′‐N,N′‐dicarbazole‐biphenyl (CBP) as a host material; for example, a (piq)₂Ir(acac)‐doped SBP‐TS‐PSB device shows maximum external quantum efficiency of η_ext = 14.6%, power efficiency of 10.3 lm W^?1 and Commission International de L'Eclairage color coordinates (0.68, 0.32) at J = 1.5 mA cm^?2, while the device with the CBP host shows maximum η_ext = 12.1%. These high performances can be mainly explained by efficient triplet energy transfer from the host to the guests and improved charge balance attributable to the bipolar characteristics of the spirobifluorene group.     

Stable,Glassy, and Versatile Binaphthalene Derivatives Capable of Efficient Hole Transport,Hosting, and Deep‐Blue Light Emission

Organic light‐emitting diodes (OLEDs) have great potential applications in display and solid‐state lighting. Stability, cost, and blue emission are key issues governing the future of OLEDs. The synthesis and photoelectronics of a series of three kinds of binaphthyl (BN) derivatives are reported. BN_1–3 are “melting‐point‐less” and highly stable materials, forming very good, amorphous, glass‐like films. They decompose at temperatures as high as 485–545 °C. At a constant current density of 25 mA cm^?2, an ITO/BN₃/Al single‐layer device has a much‐longer lifetime (>80 h) than that of an ITO/NPB/Al single‐layer device (8 h). Also, the lifetime of a multilayer device based on BN₁ is longer than a similar device based on NPB. BNs are efficient and versatile OLED materials: they can be used as a hole‐transport layer (HTL), a host, and a deep‐blue‐light‐emitting material. This versatility may cut the cost of large‐scale material manufacture. More importantly, the deep‐blue electroluminescence (emission peak at 444 nm with CIE coordinates (0.16, 0.11), 3.23 cd A^?1 at 0.21 mA cm^?2, and 25200 cd m^?2 at 9 V) remains very stable at very high current densities up to 1000 mA cm^?2.     

Luminescent Poly(p‐phenylenevinylene) Hole‐Transport Layers with Adjustable Solubility

The active part of present polymer light‐emitting diodes (PLEDs) consists of only a single layer. Multilayer devices have the advantage that the electron and hole transport can be balanced and that the recombination can be removed from the metallic cathode, leading to higher efficiencies. A major problem for polymer‐based multilayer devices is the solubility of the materials used; a multilayer can not be fabricated when a spin‐cast layer dissolves in the solvent of the subsequent layer. We demonstrate the development of high‐mobility poly(p‐phenylenevinylene) (PPV)‐based hole‐transport layers with tunable solubility by chemical modification. Enhanced charge‐transport properties are achieved by using symmetrically substituted PPVs; copolymers of long and short side chains enable us to tune the solubility without loss of the enhanced charge transport. Dual‐layer PLEDs, in which the holes are efficiently transported via this copolymer towards the luminescent layer, exhibit an enhanced efficiency at high voltages (> 10 V) and a strongly improved robustness against electrical breakdown.     

Highly Efficient Non‐Doped Blue‐Light‐Emitting Diodes Based on an Anthrancene Derivative End‐Capped with Tetraphenylethylene Groups

A novel blue‐emitting material, 2‐tert‐butyl‐9,10‐bis[4‐(1,2,2‐triphenylvinyl)phenyl]anthracene ( TPVAn ), which contains an anthracene core and two tetraphenylethylene end‐capped groups, has been synthesized and characterized. Owing to the presence of its sterically congested terminal groups, TPVAn possesses a high glass transition temperature (155 °C) and is morphologically stable. Organic light‐emitting diodes (OLEDs) utilizing TPVAn as the emitter exhibit bright saturated‐blue emissions (Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of x = 0.14 and y = 0.12) with efficiencies as high as 5.3 % (5.3 cd A^–1)—the best performance of non‐doped deep blue‐emitting OLEDs reported to date. In addition, TPVAn doped with an orange fluorophore served as an authentic host for the construction of a white‐light‐emitting device that displayed promising electroluminescent characteristics: the maximum external quantum efficiency reached 4.9 % (13.1 cd A^–1) with CIE coordinates located at (0.33, 0.39).