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.     

Influence of Dopant Concentration on Electroluminescent Performance of Organic White-Light-Emitting Device with Double-Emissive-Layered Structure

A novel phosphorescent organic white-light-emitting device （WOLED） with contiguration of ITO/NPB/CBP： TBPe：rubrene/Zn（BTZ）2：Ir（piq）2（acac）/Zn（BTZ）2/Mg：Ag is fabricated successfully, where the phosphorescent dye bis （1-（phenyl）isoquinoline） iridium （Ⅲ） acetylanetonate （Ir（piq）2 （acac）） doped into bis-（2-（2-hydroxyphenyl） benzothiazole）zinc （Zn（BTZ）2） （greenish-blue emitting material with electron transport character） as the red emitting layer, and fluorescent dye 2,5,8,11-tetra-tertbutylperylene （TBPe） and 5,6,11,12-tetraphenyl-naphthacene （rubrene） together doped into 4,4＇-N,N＇-dicarbazole-biphenyl （CBP） （ambipolar conductivity material） as the blue-orange emitting layer, respectively. The two emitting layers are sandwiched between the hole-transport layer N ,N＇-biphenyl-N , N＇-bis （1-naph thyl）-（1,1＇-biphenyl）-4, 4 Cdiamine （NP B） and electron-transport layer （Zn（BTZ）2 ） The optimum device turns on at the driving voltage of 4.5 V. A maximum external quantum efficiency of 1.53%. and brightness 15000 cd/m^2 are presented. The best point of the Commission Internationale de 1＇Eclairage （CIE） coordinates locates at （0.335, 0.338） at about 13 V. Moreover, we also discuss how to achieve the bright pure white light through optimizing the doping concentration of each dye from the viewpoint of energy transfer process.     

Improvement in performance of organic light-emitting devices by inclusion of multi-wall carbon nanotubes

Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was modified by different concentrations of multi-wall carbon nanotubes (MWNTs), and the nanocomposites of PEDOT:PSS and MWNTs were firstly used as hole-injection layer in fabrication of organic light-emitting devices (OLEDs) by using a double-layer structure with hole-injection layer of doped PEDOT:PSS and emitting/electron transport layer of tris(8-hydroxyquinolinato) aluminum (Alq₃). PEDOT:PSS solution doped with MWNTs was spin-coated on clean polyethylene terephthalate (PET) substrate with indium tin oxide (ITO). It was found that the electroluminescence (EL) intensity of the OLEDs were greatly improved by using PEDOT:PSS doped with MWNTs as hole-injection layer which might have resulted from the hole-injection ability improvement of the nanocomposites. Higher luminescence intensity and lower turn-on voltage were obtained by these devices and the luminance intensity obtained from the device with the hole-injection layer of PEDOT:PSS doped by 0.4 wt.% MWNTs was almost threefolds of that without doping.     

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     

Organic Light-Emitting Diodes with Magnesium Doped CuPc as an Efficient Electron Injection Layer

Bright organic electroluminescent devices are developed using a metal-doped organic layer intervening between the cathode and the emitting layer. The typical device structure is a glass substrate/indium-tin oxide （ITO）/copper phthalocyanine （CuPc）/N,N＇-bis-（1-naphthl）-diphenyl-1,1＇-biphenyl-4,4＇-diamine （NPB）/Tris（8-quinolinolato） aluminum（Alq3）/Mg-doped CuPc/Ag. At a driving voltage of 11 V, the device with a layer of Mg-doped CuPc （1：2 in weight） shows a brightness of 4312cd/m^2 and a current efficiency of 2.52cd/A, while the reference device exhibits 514 cd/m^2 and 1.25 cd/A.     

基于MADN空穴传输层的双层结构高效率黄绿光OLED及其阻抗谱分析

以MADN为空穴传输层,主-客掺杂体系[Alq₃∶0.7 Wt%rubrene]为发光兼电子传输层,构建了双层结构的高效率黄绿光OLED器件。该器件的黄绿光由主发光体Alq₃通过不完全能量转移到客发光体rubrene实现,电致发光峰值位于560 nm,1931CIE色坐标为(0.46, 0.52),最大发光效率达到了7.63 cd·A-1,比相应的NPB做空穴传输层的双层结构器件提高了30%。通过构建以MADN或NPB为空穴传输层的空穴单载流子器件并进行阻抗谱分析,结果表明MADN可以作为一种非常有效的空穴传输层,其空穴迁移性略低于NPB,这恰好弥补了OLED器件中空穴迁移比电子迁移快这一缺陷,为改善OLED发光层中载流子的平衡性创造了条件,从而提高了器件的发光效率。此外,MADN做空穴传输层的双层结构OLED的发光效率与传统三层结构器件(MADN和Alq₃分别作为空穴传输层和电子传输层)基本相当,表明了这种双层结构器件在简化器件结构的同时并不以牺牲发光效率为代价,发光层[Alq₃∶0.7 Wt%rubrene]兼具有优良的电子传输性能。     

Red and Near-Infrared Electroluminescences from Metal-Free Phthalocyanine

Organic light emitting diodes are fabricated based on metal-free phthalocyanine (H₂Pc) doped into tris-(8-hydroxyquinoline) aluminium (Alq₃). The device structure is ITO/NPB (30nm)/Alq₃: H₂Pc(30nm)/BCP(20nm)/Alq₃(20 nm)/Al. In the light-emitting layers, H₂Pc concentrations are varied from 0wt% to 100wt%. The emissions around 708nm and 800nm appear at low concentrations, while the emissions around 910nm and 930nm appear at high concentrations. The emissions around 708nm and 800nm are from H₂Pc monomers. The emissions around 910nm and 930nm are from H₂Pc aggregates. The dominant mechanism in the doped devices is direct chargetrapping.     

Organic LED device based on PtOEP phosphor without doping in host material

Electroluminescence (EL) and photoluminescence (PL) have been studied on multi-layer organic light-emitting diode (OLED) devices based on phosphorescent platinum octaethyl porphine (PtOEP) molecule. A multi-layer OLED (called Pt5) which has 100% PtOEP without doping in host as the emitting layer is investigated and compared its EL and PL characteristics with those of the other OLEDs (Pt2 and Pt3) with emitting layer of PtOEP doped in 4,4′-N,N′-dicarbazole-biphenyl (CBP) host material. It is observed that Pt5 shows a lower EL efficiency than Pt2 and Pt3. Three broad EL bands are observed at 500, 527 and 570 nm in the multi-layer device in addition to red sharp EL band due to PtOEP in Pt5, while only the red PtOEP EL is observed in Pt2 and Pt3. The 500, 527 and 570 nm EL peaks arise from absorption of the broad 525 nm Alq₃ emission band by PtOEP layer. The emission from the Alq₃ electron-transport layer is caused by the carrier leakage from the hole-blocking BAlq layer. The intensity of red EL due to PtOEP is much weaker in Pt5 than in Pt2. Taking into account the result of PL, it is suggested that highly efficient energy transfer from CBP host to PtOEP guest occurs in Pt2 and Pt3, giving rise to higher PtOEP luminance, while concentration quenching occurs in PtOEP layer in Pt5.     

Significant improvement of OLED efficiency and stability by doping both HTL and ETL with different dopant in heterojunction of polymer/small-molecules

This paper reports that the polymer/organic heterojunction doped light-emitting diodes using a novel poly-TPD as hole transport material and doping both hole transport layer and emitter layer with the highly fluorescent rubrene and DCJTB has been successfully fabricated. The basic structure of the heterostructure is PTPD/Alq₃. When hole transport layer and electron transport layer are doped simultaneously with different dopant, the electroluminescence quantum efficiencies are about 3 times greater than that of the undoped device. Compared with undoped device and conventional TPD/Alq₃ diode, the stability of the doping device is significantly improved. The process of emission for doped device may include carrier trapping as well as F\"{o}rster energy transfer.     

Investigation of an efficient YbF3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq₃)-based organic light-emitting diodes (OLEDs) using YbF₃ as the electron injection layer have been investigated. With an YbF₃ (3.0 nm)/Al cathode, the device with Alq₃ as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq₃-cathode interface could be well covered by YbF₃ at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq₃ and Al, and to reduce the destruction of Alq₃ by Al.     

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.     

Organic Light-Emitting Devices with a LiF Hole Blocking Layer

We introduce a thin LiF layer into tris-8-hydroxyquinoline aluminium （Alq3 ） based bilayer organic light-emitting devices to block hole transport. By varying the thickness and position of this LiF layer in Alq3, we obtain an electroluminescent efficiency increase by a factor of two with respect to the control devices without a LiF blocking layer. By using a 10nm dye doped Alq3 sensor layer, we prove that LiF can block holes and excitons effectively. Experimental results suggest that the thin LiF layer may be a good hole and exciton blocking layer.     

Influence of small-molecule material on performance of polymer solar cells based on MEH-PPV:PCBM blend

In this work,the influence of a small-molecule material,tris(8-hydroxyquinoline) aluminum (Alq 3),on bulk het-erojunction (BHJ) polymer solar cells (PSCs) is investigated in devices based on the blend of poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM).By dop-ing Alq 3 into MEH-PPV:PCBM solution,the number of MEH-PPV excitons can be effectively increased due to the energy transfer from Alq 3 to MEH-PPV,which probably induces the increase of photocurrent generated by excitons dissociation.However,the low carrier mobility of Alq 3 is detrimental to the efficient charge transport,thereby blocking the charge collection by the respective electrodes.The balance between photon absorption and charge transport in the active layer plays a key role in the performance of PSCs.For the case of 5 wt.% Alq 3 doping,the device performance is deteriorated rather than improved as compared with that of the undoped device.On the other hand,we adopt Alq 3 as a buffer layer instead of commonly used LiF.All the photovoltaic parameters are improved,yielding an 80% increase in power conversion efficiency (PCE) at the optimum thickness (1 nm) as compared with that of the device without any buffer layer.Even for the 5 wt.% Alq 3 doped device,the PCE has a slight enhancement compared with that of the standard device after modification with 1 nm (or 2 nm) thermally evaporated Alq 3.The performance deterioration of Alq 3-doped devices can be explained by the low solubility of Alq 3,which probably deteriorates the bicontinuous D-A network morphology;while the performance improvement of the devices with Alq 3 as a buffer layer is attributed to the increased light harvesting,as well as blocking the hole leakage from MEH-PPV to the aluminum (Al) electrode due to the lower highest occupied molecular orbital (HOMO) level of Alq 3 compared with that of MEH-PPV.     

Improved performances of the organic light-emitting devices by doping in the mixed layer

The brightness, efficiency and lifetime of organic light-emitting diodes (OLEDs) were remarkably improved by doping in a mixed layer. In this device, the emitting layer consists of a mixture of -naphthylphenybiphenyl amine (NPB), tris (8-hydroxyquinolinolate) aluminum (Alq₃) (referred to as the mixed layer) and an emissive dopant 5,6,11,12-petraphenylnaphthacene (rubrene), where the concentration of NPB declined while the concentration of Alq₃ was increased gradually in the deposition process. The device exhibited a maximum emission of 49,300 cd/m² at a destroy voltage of 35 V and a maximum efficiency of 7.96 cd/A at 10 V, respectively. The efficiency improves twofold in comparison with the conventional doped devices. Meanwhile, the device exhibited superior operational stability with a half-life time of 1000 h at a starting luminance of 1000 cd/m² by a constant current driver.     

Modification of anode surface in organic light-emitting diodes by chalcogenes

Influence of thin chalcogen X (S, Se, Te) interlayer between anode (indium-tin oxide, ITO) and a layer of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) used as a hole-transport layer (HTL) on the operating characteristics of organic light-emitting diodes (OLEDs) of composition ITO/X/TPD/Alq₃/Yb (Alq₃ - aluminum 8-quinolinolate) has been investigated. It was found that the sulphur layer decreases operating voltage and enhances operating stability of a device while the selenium or tellurium interlayers impair these characteristics.     

Improved performances of red organic light-emitting devices by co-doping a rubrene derivative and DCJTB into tris-(8-hydroxyquinoline) aluminum host

Performances of red organic light-emitting device were improved by co-doping 2-formyl-5,6,11,12-tetraphenylnaphthacene (2FRb) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetra-methyljulolidyl-9-enyl)-4H-pyran (DCJTB) in tris-(8-hydroxyquinoline) aluminum (Alq₃) host as the emitting layer. The device with 1 wt% DCJTB and 2.4 w% 2FRb in Alq₃ host gave a saturated red emission with CIE chromaticity coordinates of (0.65, 0.35) and a maximum current efficiency as high as 6.45 cd/A, which are 2 and 2.4 fold larger than that of the device with 1 wt% DCJTB (3.28 cd/A) in Alq₃ host and the device with 2.4 wt% 2FRb (2.72 cd/A) in Alq₃ host at the current density of 20 mA/cm², respectively. The improvement could be attributed to the effective utilization of host energy by both energy transfer and trapping in the electroluminescence process and the depression of concentration quenching between the dopants molecules.     

Surface and edge emission in organic light emitting devices

We report the studies on photoluminescence (PL) and electroluminescence (EL) spectra of organic light emitting diodes (OLED) based on most widely used light emitting material, i.e., Tris-(8-hydroxyquinoline) aluminum (Alq₃). PL studies from the edges and top surface were carried out on different thicknesses of single layer of Alq₃ coated on glass substrate and the PL intensity from the edges was found to be more than that from the top surface. On the other hand EL emission intensity from the surface was found to be larger than that from the edges of the OLED device. The discrepancy in the PL and EL emission from edge and the top surfaces is discussed. The effect of the thickness of Alq₃ layer on the PL intensity and the emission spectra are also investigated.     

White organic light-emitting diodes based on electroplex from polyvinyl carbazole and carbazole oligomers blends

White organic light-emitting diodes with a blue emitting material fluorene-centred ethylene-liked carbazole oligomer (Cz6F) doped into polyvinyl carbazole (PVK) as the single light-emitting layer are reported. The optical properties of Cz6F, PVK, and PVK:Cz6F blends are studied. Single and double layer devices are fabricated by using PVK: Cz6F blends, and the device with the configuration of indium tin oxide (ITO)/PVK:Cz6F/ tris(8-hydroxyquinolinate)aluminium (Alq₃)/LiF/Al exhibits white light emission with Commission Internationale de l'éclairage chromaticity coordinates of (0.30, 0.33) and a brightness of 402~cd/m². The investigation reveals that the white light is composed of a blue--green emission originating from the excimer of Cz6F molecules and a red emission from an electroplex from the PVK:Cz6F blend films.     

一种多层白色磷光有机电致发光器件的制备及性能研究

制备了一种结构为ITO/NPB/NPB:Ir(piq)₂(acac)/CBP:TBPe/BAlq:rubrene/BAlq/Alq₃/Mg:Ag的白色磷光有机电致发光器件.其中空穴传输型主体NPB掺杂磷光染料Ir(piq)₂(acac)作为红色发光层,双载流子传输型主体4,4′-N,N′-dicarbazole-biphenyl (CBP)掺杂TBPe作为蓝色发光层,电子传输型主体材料BAlq掺杂rubrene作为绿色发光层.以上发光层夹于 关键词： 电致发光 磷光染料 异质结 白光     

不同主体双发光层白色有机电致发光器件的性能研究

制备了结构为ITO/NPB/CBP:TBPe:rubrene/BAlq:Ir(piq)₂(acac)/BAlq/Alq₃/Mg:Ag的白色磷光有机电致发光器件.利用两种不同的主体材料,即用双载流子传输型主体材料CBP掺杂荧光染料TBPe及rubrene作为蓝光和橙黄光发光层;用电子传输型主体材料BAlq掺杂磷光染料Ir(piq)₂(acac)作为红色发光层.以上双发光层夹于空穴传输层NPB与具有电子传输性的阻挡层BALq之间.讨论了如何控制 关键词： 有机电致发光 磷光染料 掺杂 白光