Enhancing properties of organic light-emitting diodes with LiF inside the hole transport layer

A new device has been made by inserting thin LiF layer in N,N′-diphenyl-N,N′-bis(1-napthyl–phenyl)-1, 1′-biphenyl-4,4′-diamine (NPB), which has a configuration of ITO/NPB(20 nm)/LiF(0.5 nm)/NPB(20 nm)/Alq₃(60 nm)/LiF(0.5 nm)/Al. Compared with normal device, the device inserted LiF layer inside NPB (HTL) can improve its performance. The luminance and efficiency is about 1.4 and 1.3 folds high of the conventional structure, respectively. The suggestion mechanism is that the LiF in the NPB layer can block holes of NPB, and balance the holes and electrons. Consequently, there are more excitons formed to boost the diode’s luminance and efficiency. And it may offer some valuable references for OLED’s structure.     

基于ZnSe的有机-无机异质结电致发光器件

以电子束蒸发的方法制备硒化锌(ZnSe)薄膜，研究了基于ZnSe的有机-无机异质结电致发光器件.在双层器件ITO/ZnSe(50nm)/Alq₃(12nm)/Al中看到了峰值位于578nm的ZnSe电致发光，却很难得到单层器件ITO/ZnSe(50—120nm)/Al的电致发光；在此基础上进一步引入有机空穴传输层(HTL)，通过改变器件的结构，讨论了ZnSe对有机-无机异质结器件ITO/HTL/ZnSe/Alq₃/Al电致发光特性的影响.其电致发光光谱的研究结果证实了ZnSe在器件中的作用：ZnSe既起传输电子的作用，也起到传输空穴的作用，还作为发光层.并对ZnSe的发光机理进行了讨论. 关键词： 硒化锌 有机-无机异质结 电致发光 空穴传输层     

Pure red emission of dye-doped organic molecules from microcavity organic light emitting diode

Organic red emitting diode was fabricated by using 4-dicyanomethylene-2-methyl-6-[2-(2,3,6,7-tetrahydro-1 H,5H-benzo[ij]quinolizin-8-yl)vinyl]-4H-pyran (DCM)-doped tri-(8-quinolitolato) aluminum (Alq₃) as emitter with the structure of G/ITO/NPB(25 nm)/DCM:Alq₃(55 nm)/Alq₃(20 nm)/LiF (1.2 nm)/Al(84 nm), (glass/indium–tin-oxide/4,4-bis-[N-(1-naphthyl)-N-phenyl-amino]biphenyl, G/ITO/NPB), the wavelength of the maximal emission of which is 615 nm. By introducing cavity to Organic light emitting diode (OLED), we got pure red emitting diode with wavelength of the maximal emission of 621 nm and full-width at half-maximum (FWHM) of 27 nm. As far as we know, it is the best result in the dye-doped organic red emitting diode. We also made a device of G/ITO/NPB(25 nm)/DCM:Alq₃(29 nm)/DCM:PBD(26 nm)/Alq₃(20 nm)/LiF(1.2 nm)/Al(84 nm), in order to compare the performance of Alq₃ with that of 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD) as host material. It was found that the performance of device A is better than that of C both in brightness and color purity,as well as in EL efficiency.     

Highly efficient and stable organic light-emitting diode by balancing drift current of charge

Highly efficient and stable OLED device in which hole-drift current and electron-drift current are balanced was fabricated. Drift current characteristics according to the thickness of organic layer were examined using the device with ITO/m-MTDATA/NPB/Al structure that can only move the hole and the device with Al/LiF/Alq₃/LiF/Al structure that can only move the electron. Using the result of such examination, green device with balanced drift current was produced. Device with the structure of m-MTDATA (80 nm)/NPB (20 nm)/C-545T (3%) doped Alq₃ (5 nm)/Alq₃ (59 nm)/LiF (1 nm)/Al (200 nm) showed color purity of (0.309, 0.643) and high efficiency of 7.0 lm/W (14.4 cd/A). Most of light emission was observed inside the green emitting layer. Through the result of EL spectrum for the device also including red emitting layer, same result could be obtained. The device with balanced drift current also showed half life-time of 175 h for initial luminance of 3000 cd/m², which is more stable in comparison to the device without balanced drift current.     

The roles of zinc selenide sandwiched between organic layers and its applications in white light-emitting diodes

In this paper, the roles of zinc selenide (ZnSe) sandwiched between organic layers, i.e. organic/ZnSe/aluminum quinoline (Alq₃), have been studied by varying device structure. A broad band emission was observed from ITO/poly(N-vinylcarbazole)(PVK)(80 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al under electric fields and it combined the emissions from the bulk of PVK, ZnSe and Alq₃, however, emission from only Alq₃ was observed from trilayer device ITO/N,N^′-bis-(1-naphthyl)-N,N^′-diphenyl-1, 1^′-biphenyl-4, 4^′-diamine (NPB) (40 nm)/ZnSe(120 nm)/ Alq₃(15 nm)/Al. Consequently the luminescence mechanism in the ZnSe layer is suggested to be charge carrier injection and recombination. By thermal co-evaporating Alq₃ and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), we get white light emission with a Commission Internationale de l’E clairage (C.I.E) co-ordinates of (0.32, 0.38) from device ITO/PVK(80 nm)/ZnSe(120 nm)/ Alq₃:DCJTB(0.5 wt% DCJTB)(15 nm)/Al at 15 V and the device performs stably with increasing applied voltages.     

Low driving voltage in organic light-emitting diode using MoO3/NPB multiple quantum well structure in hole transport layer

Driving voltage of organic light-emitting diode (OLED) is lowered by employing molybdenum trioxide (MoO₃)/N, N'-bis(naphthalene-1-yl)-N,N'-bis(phe-nyl)-benzidine (NPB) multiple quantum well (MQW) structure in hole transport layer. For the device with double quantum well (DQW) structure of ITO/ [MoO₃ (2.5 nm)/NPB (20 nm)]₂/Alq₃(50 nm)/LiF (0.8 nm)/Al (120 nm)], the turn-on voltage is reduced to 2.8 V, which is lowered by 0.4 V compared with that of the control device (without MQW structures), the driving voltage is 5.6 V, which is reduced by 1 V compared with that of the control device at the 1000 cd/m². In this work, the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure, which is attributed not only to the reducing energy barrier between ITO and NPB, but also to the forming charge transfer complex between MoO₃ and NPB induced by the interfacial doping effect of MoO₃.     

室温下磁场对基于Alq3的有机电致发光器件的影响

制备了ITO/NPB/LiF/Alq₃/LiF/Al的器件,测量了该组器件效率和亮度的磁效应.结果表明,在50 mT磁场中,当LiF缓冲层厚度为0.8 nm时,器件的效率最大增加了12.4%,磁致亮度最大变化率17%.同时,制备的磷光器件ITO/NPB/LiF/CBP:6 wt% Ir(ppy)₃/BCP/Alq₃/ LiF/Al,在50mT磁场作用下,当LiF缓冲层的厚度为0.8 nm时,器件的效率最大增加12.1%.在Alq_{3 关键词： 有机发光 磁场 效率 磁致亮度}     

The influence of exciton behavior on luminescent characteristics of organic light-emitting diodes

We used N,N′-bis-(1-naphthyl)-N,N′-1-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB), 4,4′-N,N′-dicarbazole-biphenyl (CBP) and tris(8-hydroxyquinoline) aluminum (Alq₃) to fabricate tri-layer electroluminescent (EL) device (device structure: ITO/NPB/CBP/Alq₃/Al). In photoluminescence (PL) spectra of this device, the emission from NPB shifted to shorter wavelength accompanying with the decrease of its emission intensity and moreover the emission intensity of Alq₃ increased relatively with the increase of reverse bias voltage. The blue-shifted emission and the decrease in emission intensity of NPB were attributed to the polarization and dissociation of NPB excitons under reverse bias voltage. The increase of emission intensity of Alq₃ benefited from the recombination of electrons (produced by the dissociation of NPB exciton) and holes (injected from the Al cathode).     

Bright electroluminescent devices with tunable spectra obtained by strictly controlling the doping concentration of electron injection sensitizer

In this paper, we report an efficient strategy to design bright blue and blue-green electroluminescent (EL) devices by slightly doping tris(8-hydroxyquinoline) aluminum (Alq₃) into N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB) as the light-emitting layer (EML). Bright EL devices with tunable spectra were obtained by strictly controlling the doping concentration of Alq₃. With increasing current density, EL efficiencies of these devices increase first and then decrease gradually after reaching the maximum. Analyzing the current density-voltage (J−V) characteristics of hole-only and electron-only devices, we found the presence of Alq₃ molecules in EML not only facilitates the injection of electrons from hole block layer (HBL) into EML but also stays the transport of holes in EML, thus causing significant enhancement of EL efficiency and brightness due to improved carriers balance and broadening of recombination zone. More interestingly, the doping concentration of Alq₃ strongly influences the injection and transport processes of electrons, thus determining the distribution of holes and electrons on NPB and Alq₃ molecules.     

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.     

Charge carriers bulk recombination instead of electroplex emission after their tunneling through hole-blocking layer in OLEDs

Charge carriers bulk recombination instead of forming electroplex after their tunneling through a hole-blocking layer, i.e. 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), in organic electroluminescence (EL) device ITO/poly-(N-vinyl-carbazole)(PVK)/BCP/tris(8-hydroxyquinoline) aluminum (Alq₃)/Al is reported. By changing the thickness of BCP layer, one can find that high electric fields enhance the tunneling process of holes accumulated at the PVK/BCP interface into BCP layer instead of forming “electroplex emission” as reported earlier in literatures. Our experimental data show that charge carriers bulk recombination takes place in both PVK layer and BCP layer, and even in Alq₃ layer when BCP layer is thin enough. Further, it is suggested that PVK is the origin of the emission shoulder at 595 nm in the EL spectra of trilayer device ITO/PVK/BCP/Alq₃/Al.     

Reduction of driving voltage in organic light-emitting diodes with molybdenum trioxide in CuPc/NPB interface

A novel structure of organic light-emitting diode was fabricated by inserting a molybdenum trioxide (MoO₃) layer into the interface of hole injection layer copper phthalocyanine (CuPc) and hole transport layer N,N′-diphenyl-N,N′-bis(1-napthyl-phenyl)-1,1′-biphenyl-4,4′-diamine (NPB). It has the configuration of ITO/CuPc(10 nm)/MoO₃(3 nm)/NPB(30 nm)/ tris-(8-hydroxyquinoline) aluminum (Alq₃)(60 nm)/LiF(0.5 nm)/Al. The current density-voltage-luminance (J-V-L) performances show that this structure is beneficial to the reduction of driving voltage and the enhancement of luminance. The highest luminance increased by more than 40% compared to the device without hole injection layer. And the driving voltage was decreased obviously. The improvement is ascribed to the step barrier theory, which comes from the tunnel theory. The power efficiency was also enhanced with this novel device structure. Finally, “hole-only” devices were fabricated to verify the enhancement of hole injection and transport properties of this structure.     

1.53 μm electroluminescence from ErF3-doped organic light-emitting diodes

Near-infrared (NIR) organic light-emitting devices (OLEDs) are demonstrated by employing erbium fluoride (ErF₃)-doped tris-(8-hydroxyquinoline) aluminum (Alq₃) as the emitting layer. The device structure is ITO/N,N′-di-1-naphthyl-N,N′-diphenylbenzidine (NPB)/Alq₃: ErF₃/2,2′,2″-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole) (TPBI)/Alq₃/Al. Room-temperature electroluminescence around 1530 nm is observed due to the ⁴I_13/2–⁴I_15/2 transition of Er³⁺. Full width at half maximum (FWHM) of the electroluminescent (EL) spectrum is ～50 nm. NIR EL intensity from the ErF₃-based device is ～4 times higher than that of Er(DBM)₃Phen-based device at the same current. Alq₃–ErF₃ composite films are investigated by the measurements of X-ray diffraction (XRD), absorption, photoluminescence (PL) and PL decay time. Electron-only devices are also fabricated. The results indicate that energy transfer mechanism and charge trapping mechanism coexist in the NIR EL process.     

High-efficiency Simple Structure White Organic Light-emitting Devices Based on Rubrene Ultrathin Layer

White organic light-emitting devices (WOLEDs) were fabricated with an ultrathin layer of rubrene inserted between NPB and TPBI. With a simple three-layer structure of ITO/NPB(50 nm)/rubrene(0.1 nm)/TPBI(50 nm)/LiF/Al, a white light with CIE coordinates of (0.31, 0.30) were generated. The device gave a maximum luminance efficiency of 2.04 lm/W at 5 V. Furthermore, with a multilayer structure of ITO/m-MTDATA(30 nm)/NPB(20 nm)/rubrene(0.1 nm)/TPBI(40 nm)/Alq₃(10 nm)/LiF/Al, the device reached a maximum luminance efficiency of 4.29 lm/W at 4 V and the luminance could exceed 10 000 cd/m² at 10 V.     

Ohmic contact and space-charge-limited current in molybdenum oxide modified devices

The effect of indium-tin oxide (ITO) surface treatment on hole injection of devices with molybdenum oxide (MoO₃) as a buffer layer on ITO was studied. The Ohmic contact is formed at the metal/organic interface due to high work function of MoO₃. Hence, the current is due to space charge limited when ITO is positively biased. The hole mobility of N, N′-bis-(1-napthyl)-N, N′-diphenyl-1, 1′biphenyl-4, 4′-diamine (NPB) at various thicknesses (100–400 nm) has been estimated by using space-charge-limited current measurements. The hole mobility of NPB, 1.09×10⁻⁵ cm²/V s at 100 nm is smaller than the value of 1.52×10⁻⁴ cm²/V s at 400 nm at 0.8 MV/cm, which is caused by the interfacial trap states restricted by the surface interaction. The mobility is hardly changed with NPB thickness for the effect of interfacial trap states on mobility which can be negligible when the thickness is more than 300 nm.     

微腔有机电致发光器件的角度依赖性

设计并制作了两个器件,一个是微腔有机电致发光器件(MOLED):G/DBR/ITO/NPB(46 nm)/DPVBi(20 nm)/Alq₃(56 nm)/LiF(1 nm)/Al(150 nm);一个是无腔器件(OLED):G/ITO/NPB(46 nm)/DPVBi(20 nm)/Alq₃(56 nm)/LiF(1 nm)/Al(150 nm)。测试并分析了器件性能。OLED在电流密度30 mA/cm²时的电致发光(EL)光谱随观测角度由0°~70°都是一宽谱带,是发光层DPVBi的特征发光谱,峰值都在452 nm处,半峰全宽均为70 nm,色坐标均为(x=0.18,y=0.19),无腔器件没有角度依赖性。相同电流密度下,微腔器件的EL谱随观测角度由0°~70°,发光峰值蓝移,由472 nm逐渐移至428 nm;峰值相对强度渐弱,由0.32变至0.02;半峰全宽由14 nm增加至120 nm;色坐标由(x=0.14,y=0.10)变至(x=0.19,y=0.25),颜色由紫蓝变成蓝白到接近白色。微腔器件具有明显的角度依赖性。     

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.     

White organic light-emitting diodes based on benzothiazole derivative

This paper describes the white organic light-emitting diodes (WOLEDs) made from a benzothiazole derivative, N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylnaphthalen-1-amine (BPNA). The bright yellowish-white emission was obtained from a non-doped triple-layer device: ITO/NPB (40 nm)/BPNA (50 nm)/Alq₃ (40 nm)/LiF/Al. The Commission Internationale de L’Eclairage (CIE) coordinates of the device were (0.24, 0.36) at 10 V. The maximum brightness of the device was 9225 cd/m² at 14.4 V. A current efficiency of 3.08 cd/A, a power efficiency of 1.21 lm/W and an external quantum efficiency of 1.18% at a driving current density of 20 mA/cm² were achieved. WOLED with a DCJTB-doped structure of ITO/TcTa/BPNA/BPNA: DCJTB (0.5%)/BPNA/BCP/Alq₃/LiF/Al was fabricated in comparison with the non-doped device. The device emitted bright white light with the CIE coordinates of (0.33, 0.29) at 10 V and a maximum luminance of 7723 cd/m² at 14.8 V.     

Low driving voltage in an organic light-emitting diode using MoO3/NPB multiple quantum well structure in a hole transport layer

The driving voltage of an organic light-emitting diode(OLED) is lowered by employing molybdenum trioxide(MoO3)/N,N’-bis(naphthalene-1-yl)-N,N’-bis(phe-nyl)-benzidine(NPB) multiple quantum well(MQW) structure in the hole transport layer.For the device with double quantum well(DQW) structure of ITO/[MoO3(2.5 nm)/NPB(20 nm)]2/Alq3(50 nm)/LiF(0.8 nm)/Al(120 nm)],the turn-on voltage is reduced to 2.8 V,which is lowered by 0.4 V compared with that of the control device(without MQW structures),and the driving voltage is 5.6 V,which is reduced by 1 V compared with that of the control device at the 1000 cd/m2.In this work,the enhancement of the injection and transport ability for holes could reduce the driving voltage for the device with MQW structure,which is attributed not only to the reduced energy barrier between ITO and NPB,but also to the forming charge transfer complex between MoO3 and NPB induced by the interfacial doping effect of MoO3.     

使用PTB7作为阳极修饰层提高有机发光二极管的性能

采用poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3-?uoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7)作为有机发光二极管器件的阳极修饰层,制备了结构为indium tin oxide(ITO)/PTB7(不同浓度)/N,N’-Bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine(NPB,40 nm)/8-hydroxyquinoline(Alq3,60 nm)/LiF(1 nm)/Al的系列器件,同时研究了不同浓度的PTB7对器件性能的影响.PTB7的最佳浓度为0.25 mg/mL,器件性能得到明显的改善,起亮电压为4.3 V.当驱动电压为14.6 V时,最大亮度为45800 cd/m2,最大电流效率为9.1 cd/A.与没有PTB7修饰的器件相比,其起亮电压降低了1.9 V,最高亮度提升了78.5%.器件性能提高归因于PTB7的插入使得空穴注入和传输能力大大改善.