发光层和空穴传输层对白色电致发光器件性能的影响

利用电子传输性能良好的苯并噻唑螯合锌(Zn(BTZ)₂)作为蓝光层，通过设计不同类型的空穴传输层并试验不同厚度的发光层后，制作了一种最佳厚度的双发光层白色电致发光器件：氧化铟锡(ITO)/N-N′-双(3-甲基苯基)-N-N′-二苯基-1-1′-二苯基-4-4′-二胺(TPD)∶N,N′-二(1-萘基)-N,N′-二苯基-1,1′-联苯-4-4′-二胺(NPB)(1∶0.0 关键词： 厚度 空穴传输层 白光 载流子     

Blue organic light-emitting diodes with 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as hole transport and emitting layer and the impedance spectroscopy analysis

2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) based fluorescent blue organic light-emitting diodes (OLEDs) are demonstrated. With MADN as emitting layer, experiments indicate that thick MADN (40–60 nm) is preferable for constructing efficient blue OLED. With MADN as hole-transport and emitting layer and tris(8-hydroxy-quinolinato)aluminium (Alq₃) as electron-transport layer, the OLED electroluminescent characteristics show a mixture emission of MADN and Alq₃ with Commission Internationale d'Eclairage (CIE) color coordinates of (0.25, 0.34), indicating feasible hole transporting in MADN. Using 4,7-diphenyl-1,10-phenanthroline (BPhen) replacing Alq₃ as electron-transport layer, the OLED shows deep blue emission with a maximum luminous efficiency of 4.8 cd/A and CIE color coordinates of (0.16, 0.09). The hole transport characteristics of MADN are further clarified by constructing hole-only device and performing impedance spectroscopy analysis. The results indicate that MADN shows superior hole-transport ability which is almost comparable to typical hole-transport material of N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB), suggesting a promising application for constructing efficient blue OLED with integrated hole-transport layer and emitting layer.     

Electroluminescence enhancement in ultraviolet organic light‐emitting diode with graded hole‐injection and ‐transporting structure

Electroluminescent intensity and external quantum efficiency (EQE) in ultraviolet organic light‐emitting diodes (UV OLEDs) have been remarkably enhanced by using a graded hole‐injection and ‐transporting (HIT) structure of MoO₃/N,N ′‐bis(naphthalen‐1‐yl)‐N,N ′‐bis(phenyl)‐benzidine/MoO₃/4,4′‐bis(carbazol‐9‐yl)biphenyl (CBP). The graded‐HIT based UV OLED shows superior short‐wavelength emis‐ sion with spectral peak of ～410 nm, maximum electroluminescent intensity of 2.2 mW/cm² at 215 mA/cm² and an EQE of 0.72% at 5.5 mA/cm². Impedance spectroscopy is employed to clarify the enhanced hole‐injection and ‐transporting capacity of the graded‐HIT structure. Our results provide a simple and effective approach for constructing efficient UV OLEDs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

High-efficiency organic light-emitting diodes (OLEDs) with a mixed layer structure

Improved performance of organic light-emitting diodes (OLEDs) as obtained by a mixed layer was investigated. The OLEDs with a mixed layer which were composed of N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB), tris-(8-hydroxyquinolato) aluminum (Alq₃) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) showed the highest brightness and efficiency, which reached 19048 cd/m² at 17 V and 4.3 cd/A at 10 mA/cm², respectively. The turn-on voltage of the device is 2.6 V. Its Commission Internationale del’Eclairage (CIE) coordinate is (0.497, 0.456) at 17 V, and the CIE coordinates of the device are largely insensitive to the driving voltages, which depicts stabilized yellow color.     

Improved performance of organic light-emitting devices with plasma treated ITO surface and plasma polymerized methyl methacrylate buffer layer

Transparent indium-tin-oxide (ITO) anode surface was modified using O₃ plasma and organic ultra-thin buffer layers were deposited on the ITO surface using 13.56 MHz rf plasma polymerization technique. A plasma polymerized methyl methacrylate (ppMMA) ultra-thin buffer layer was deposited between the ITO anode and hole transporting layer (HTL). The plasma polymerization of the buffer layer was carried out at a homemade capacitively coupled plasma (CCP) equipment. N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-1,1′-diphenyl-4,4′-diamine (TPD) as HTL, Tris(8-hydroxy-quinolinato)aluminum (Alq₃) as both emitting layer (EML)/electron transporting layer (ETL), and aluminum layer as cathode were deposited using thermal evaporation technique. Electroluminescence (EL) efficiency, operating voltage and stability of the organic light-emitting devices (OLEDs) were investigated in order to study the effect of the plasma surface treatment of the ITO anode and role of plasma polymerized methyl methacrylate as an organic ultra-thin buffer layer.     

An organic electroluminescent device made from a gadolinium complex

A gadolinium ternary complex, tris(1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) (phenanthroline) gadolinium [Gd(PMIP)₃(Phen)] was synthesized and used as a light emitting material in the organic electroluminescent (EL) devices. The triple layer device with a structure of indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (20 nm)/Gd(PMIP)₃(Phen) (80 nm)/2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (bathocuproine or BCP) (20 nm)/Mg: Ag(200 nm)/Ag(100 nm) exhibited green emission peaking at 535 nm. A maximum luminance of 230 cd/m² at 17 V and a peak power efficiency of 0.02 lm/w at 9 V were obtained.     

Optimization of microcavity OLED by varying the thickness of multi-layered mirror

We optimized the emission efficiency from a microcavity OLEDs consisting of widely used organic materials, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq₃) as emitting and electron transporting layer. LiF/Al was used as a cathode, while metallic Ag was used as an anode material. A LiF/NPB bi-layer or NPB layer on top of the cathode was considered to alter the optical properties of the top mirror. The electroluminescence emission spectra, electric field distribution inside the device, carrier density, recombination rate and exciton density were calculated as a function of the position of the emission layer. The results show that for optimal capping layers thicknesses, light output is enhanced as a result of the increase in both the reflectance and transmittance of the top mirror. Once the optimum structure has been determined, the microcavity OLED devices were fabricated and characterized. The experimental results have been compared to the simulations and the influence of the thickness of the mirror layers, emission region width and position on the performance of microcavity OLEDs was discussed.     

Evidence for the changes in hole injection mechanism with a CoPc hole injection layer

The hole injection in hole-only devices with the structures of Al/N,N′-bis(1-naphthyle)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/ITO and Al/NPB/cobalt phthalocyanine (CoPc)/ITO were analyzed. With the combined analysis of current density–voltage and impedance measurement, the charge injection mechanism based on the injection limited current model was investigated. The NPB single layer device shows Richardson–Schottky type thermionic emission in the entire applied bias range. On the other hand, the device with the CoPc hole injection layer shows thermionic emission until the applied bias reaches 3.7 V. Increasing the bias further, Fowler–Nordheim tunneling dominates the charge injection. The changes of hole injection mechanism were discussed by evaluating the energy level changes with internal field distributions.     

LiF插层对有机发光二极管磁场效应的调控

制备了有LiF插层的有机发光二极管,以八羟基喹啉铝（Alq₃）作为电子传输层,N, N′-二苯基-N, N′-二(1-萘基)-1,1′-联苯-4,4′-二胺（NPB）作为空穴传输层.通过改变Alq₃与NPB间LiF插层的厚度,研究了不同温度下器件的光电特性及电致发光的磁场效应.测量结果表明:LiF插层可以影响器件内部载流子的输运和激发态的形成.较厚的插层阻碍了空穴的传输,使器件的电流效率变低.但实验中发现, 关键词： LiF插层结构 磁场效应 三重态激子     

New spiro[benzotetraphene-fluorene] Derivatives: Synthesis and Application in Sky-Blue Fluorescent Host Materials

Blue light-emitting spiro[benzotetraphene-fluorene] (SBTF)-based host materials, 3-(1-naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (1), 3-(2-naphthyl)-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (2), and 3-[2-(6-phenyl)naphthyl]-10-naphthylspiro[benzo[ij]tetraphene-7,9′-fluorene] (3) were designed and prepared via multi-step Suzuki coupling reactions. Introducing various aromatic groups into SBTF core lead to a reduction in band gap and a determination of the color purity and luminescence efficiency. Typical sky-blue fluorescent organic light emitting diodes with the configuration of ITO/N,N′-di(1-naphthyl)-N,N′-bis[(4-diphenylamino)phenyl]-biphenyl-4,4′-diamie (60 nm)/N,N,N′,N′-tetra(1-biphenyl)-biphenyl-4,4′-diamine (30 nm)/host: dopant (30 nm, 5 %)/LG201 (electron transporting layer, 20 nm)/LiF/Al were developed using SBTF derivatives as a host material and p-bis(p-N,N-diphenyl-aminostyryl)benzene (DSA-Ph) as a sky-blue dopant material. A device obtained from three materials doped with DSA-Ph showed color purity of 0.148 and 0.239, a luminance efficiency of 7.91 cd/A, and an external quantum efficiency >4.75 % at 5 V.     

Effect of desiccant on the performance of green organic light-emitting device

A green organic light-emitting diodes (OLED) with a multilayer structure of indium-tin oxide (ITO)/copper-phthalocyanine (CuPc) (200Å)/N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (α-NPD) (600Å)/N′- diphenyl-N,N′-tris(8-hydroxyquinoline) aluminium (Alq₃) (400Å):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7- tetrahydro-1H,5H,11H-(l)benzopyropyrano(6,7,8-i, j)quinolizin-11-one (C545T) (2%)/Alq₃ (200Å)/LiF (10Å)/Al (1000Å) was prepared via vacuum thermal evaporation. To reduce the impact of water vapor and oxygen on the device, we encapsulated it with a kind of specific and efficient desiccant, called DESIPASTE, under the protection of high-purity nitrogen. By analyzing a series of optical characteristics of OLEDs, the results showed that this desiccant can improve the brightness about 500 and 250 cd/m² at same driving voltage and current density, respectively. The electroluminescent (EL) spectra were hardly affected except a very weak blue shift of broadband emission peak. It turns out that encapsulation with DESIPASTE is a simple and efficient way to improve the performance of OLED.     

Reduced hole loss in organic light emitting diode incorporating two p-doped hole transport layers

The hole-injecting structure of 15 nm MoO₃-doped 4,4-N,N-bis [N-1-naphthyl-N-phenyl-amino]biphenyl (NPB:MoO₃)/5 nm MoO₃-doped 4,4′-N,N′-dicarbazole-biphenyl (CBP:MoO₃) has been used in organic light emitting diodes (OLEDs). It was found that a device using the 15 nm NPB:MoO₃/5 nm CBP:MoO₃/NPB combination was superior to one adopting the 20 nm NPB:MoO₃/NPB combination due to two major causes: the NPB:MoO₃/CBP:MoO₃ interface is a quasi-ohmic contact, and the hole transport barrier from CBP:MoO₃ to NPB is smaller than that from NPB:MoO₃ to NPB; moreover, it outperformed the device employing the 20 nm CBP:MoO₃/NPB combination, mostly because of the higher conductivity of NPB:MoO₃ compared to CBP:MoO₃. We demonstrate that using a structure resulting from uniting two p-doped hole transporters is a beneficial, simple method of achieving an improved trade-off between high conductivity and small hole transport barrier, thereby leading to a significantly reduced ohmic loss in the hole current conduction in the OLEDs, relative to the single p-doped layers.     

Highly efficient blue fluorescent OLEDs with doped double emitting layers based on p—n heterojunctions

We fabricate a kind of novel efficient blue fluorescent organic light emitting device(OLED) based on p-n heterojunctions composed of hole transporting layer(HTL) N,N ’-bis(naphthalen-1-yl)-N,N ’-bis(phenyl)-benzidine(NPB) and electron transporting layer(ETL) 4,7-diphnenyl-1,10-phenanthroline(BPhen),into which a new blue material,DNCA(a derivation of N 6,N 6,N 12,N 12-tetrap-tolylchrysene-6,12-diamine),is partially doped simultaneously,and double emitting layers are configured.With a turn-on voltage of 2.6 V at 1 cd/m 2,this type of OLED presents a maximum luminance efficiency(η max) of 8.83 cd/A at 5.818 mA/cm 2 and a maximum luminance of over 40000 cd/m 2.Meanwhile,the Commission Internationale De L’Eclairage(CIE) coordinates of this device change slightly from(0.13,0.27) to(0.13,0.23) as the driving voltage increases from 3 V to 11 V.This improvement in the electroluminescent characteristics is attributed mainly to the ideal p-n heterojunction which can confine and distribute excitons evenly on two sides of the heterojunction interface so as to improve the carrier combination rate and expand the light-emitting region.     

Enhanced brightness of organic light-emitting diodes based on Mg:Ag cathode using alkali metal chlorides as an electron injection layer

Different thicknesses of cesium chloride (CsCl) and various alkali metal chlorides were inserted into organic light-emitting diodes (OLEDs) as electron injection layers (EILs). The basic structure of OLED is indium tin oxide (ITO)/N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1.1′-biphenyl-4.4′-diamine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃)/Mg:Ag/Ag. The electroluminescent (EL) performance curves show that both the brightness and efficiency of the OLEDs can be obviously enhanced by using a thin alkali metal chloride layer as an EIL. The electron injection barrier height between the Alq₃ layer and Mg:Ag cathode is reduced by inserting a thin alkali metal chloride as an EIL, which results in enhanced electron injection and electron current. Therefore, a better balance of hole and electron currents at the emissive interface is achieved and consequently the brightness and efficiency of OLEDs are improved.     

同时掺杂磷光和荧光染料的白色有机电致发光器件性能研究

以磷光染料Ir(piq)₂(acac)作为发光掺杂剂,掺入空穴传输性主体材料NPB中得到红色发光层,荧光材料TBP掺入到主体CBP中作为蓝色发光层,制备了结构为ITO/NPB/NPB：Ir(piq)₂(acac)/CBP/CBP：TBPe/BCP/ALq/Mg：Ag的双发光层白色有机电致发光器件.其中ALq₃、未掺杂的NPB和CBP及BCP层分别作为电子传输层、空穴传输层和激子阻挡层.实验中通过调节发光层厚度及Ir(piq)_{2关键词： 磷光 激子阻挡层 有机电致发光}     

Enhancement of the efficiency and the color stabilization of organic light-emitting devices fabricated utilizing stepwise doped hole transport layers

The electrical and the optical properties of organic light-emitting devices (OLEDs) consisting of aluminum (Al)/lithium quinolate/tris (8-hydroxyquimoline) Al/5,6,11,12-tetraphenylnaphthacene (rubrene)-doped N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1-biphenyl-4,4′-diamine (NPB)/indium-tin-oxide/glass structures fabricated with uniformly doped and stepwise-doped hole transport layers (HTLs) were investigated. The turn-on voltage of the OLEDs fabricated utilizing a stepwise-doped HTL was smaller than that of the OLEDs fabricated with a uniformly doped HTL, and the corresponding luminance at the same voltage was higher. The Commission Internationale de l'Eclairage (CIE) chromaticity coordinates of the OLEDs fabricated utilizing a stepwise-doped HTL became stabilized, and the CIE chromaticity coordinates of the OLEDs at 12 V was (0.43, 0.53), indicative of a yellow emission corresponding to the rubrene layer. The luminescence mechanisms of the OLEDs fabricated utilizing a stepwise-doped HTL are described on the basis of the experimental results.     

Direct observation of the potential distribution within organic light emitting diodes under operation

We show the first direct measurement of the potential distribution within organic light emitting diodes (OLEDs) under operation and hereby confirm existing hypotheses about charge transport and accumulation in the layer stack. Using a focused ion beam to mill holes in the diodes we gain access to the cross section of the devices and explore the spatially resolved potential distribution in situ by scanning Kelvin probe microscopy under different bias conditions. In bilayer OLEDs consisting of tris(hydroxyquinolinato) aluminum (Alq₃)/N, N ′‐bis(naphthalene‐1‐yl)‐N,N ′‐bis(phenyl) benzidine (NPB) the potential exclusively drops across the Alq₃ layer for applied bias between onset voltage and a given transition voltage. These findings are consistent with previously performed capacitance–voltage measurements. The behavior can be attributed to charge accumulation at the interface between the different organic materials. Furthermore, we show the potential distribution of devices with different cathode structures and degraded devices to identify the cathode interface as main culprit for decreased performance. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Theoretical design study on multifunctional triphenyl amino‐based derivatives for OLEDs

The use of triphenyl amino‐based derivatives in organic light‐emitting diodes (OLEDs) can significantly improve their efficiency and stability and especially their electroluminescence characteristics – most of the new hole‐transport materials have this feature. In this study, a series of triphenyl amino‐based compounds were computed, including two newly designed molecules. They can function as charge transport materials and emitters with high efficiency and stability. To reveal the relationship between the properties and structures of these bifunctional and multifunctional electroluminescent materials, the ground and excited state geometries were optimized at the B3LYP/6‐31G(d), HF/6‐31G(d), TD‐B3LYP/6‐31G(d), and CIS/6‐31G(d) levels, respectively. The ionization potentials (IPs) and electron affinities (EAs) were computed. The lowest excitation energies, the maximum absorption, and emission wavelengths of these compounds were calculated by employing the time‐dependent density functional theory (TD‐DFT) method. Also, the mobilities of holes and electrons were studied computationally based on the Marcus electron transfer theory. The CH₂Cl₂ solvent effect on the absorption spectra of N,N′‐di‐1‐naphthyl‐N,N′‐diphenylbenzidine ( NPB ) was considered by polarizable continuum model (PCM). The results obtained for these compounds are in good agreement with the experimental values. These data show that the proposed compounds 1 and 2 (N,B‐di‐1‐naphthyl‐N,B‐diphenylbenzidine and Mes₂N[p‐4,4′‐biphenyl‐NPh(1‐naphthyl)]), are multifunctional and bifunctional materials similar to Mes₂B[p‐4,4′‐biphenyl‐NPh(1‐naphthyl)] ( BNPB ) and NPB , respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

Electrical characteristics and efficiency of organic light-emitting diodes depending on hole-injection layer

In a device structure of ITO/hole-injection layer/N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphthyl]4,4′-diamine(NPB)/tris(8-hydroxyquinoline)aluminum(Alq₃)/Al, we investigated the effect of the hole-injection layer on the electrical characteristics and external quantum efficiency of organic light-emitting diodes. Thermal evaporation was performed to make a thickness of NPB layer with a rate of 0.5–1.0 Å/s at a base pressure of 5 × 10⁻⁶ Torr. We measured current–voltage characteristics and external quantum efficiency with a thickness variation of the hole-injection layer. CuPc and PVK buffer layers improve the performance of the device in several aspects, such as good mechanical junction, reducing the operating voltage, and energy band adjustment. Compared with devices without a hole-injection layer, we found that the optimal thickness of NPB was 20 nm in the device structure of ITO/NPB/Alq₃/Al. By using a CuPc or PVK buffer layer, the external quantum efficiencies of the devices were improved by 28.9% and 51.3%, respectively.     

Low voltage red phosphorescent organic light-emitting devices with triphenylphosphine oxide and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl electron transport layers

We have developed red phosphorescent organic light-emitting devices operating at low voltages by using triphenylphosphine oxide (Ph₃PO) and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) electron transport layers. 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and tris-(1-phenylisoquinolinolato-C²,N) iridium(III) [Ir(piq)₃] were used as host and guest materials, respectively. Small voltage drops across the electron transport layers and direct injection of holes from 4,4′,4″-tris[N-(2-naphthyl)-N-phenyl-amino]-triphenylamine (2-TNATA) hole transport layer into the Ir(piq)₃ guests are responsible for the high current density at low voltage, resulting in a high luminance of 1000 cd/m² at low voltages of 2.8–3.0 V in devices with a structure of ITO/2-TNATA/CBP:Ir(piq)₃/DPVBi/Ph₃PO/LiF/Al.