Study of UV excited white light‐emitting diodes for optimization of luminous efficiency and color rendering index

Wavelength down‐converted white‐light sources excited by near ultraviolet light‐emitting diodes require specific phosphor properties in order to generate high‐quality white light (namely, light with good color rendition and stability of color coordinate). Simulation and experimental results are discussed, with particular emphasis on the spectral distribution property of red phosphor required to realize high values of luminous efficiency and color rendition. A peak wavelength of 610 nm and an FWHM of 80 nm for the spectral power distribution were proposed, and co‐doped phosphate materials were synthesized successfully. This can contribute to a white‐light source with a luminous efficiency of 45 lm/W and color rendering index greater than 90 at a color temperature of 5600 K and an operational current of 20 mA. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of thickness variation of hole injection and hole blocking layers on the performance of fluorescent green organic light emitting diodes

In this paper we present the effect of thickness variation of hole injection and hole blocking layers on the performance of fluorescent green organic light emitting diodes (OLEDs). A number of OLED devices have been fabricated with combinations of hole injecting and hole blocking layers of varying thicknesses. Even though hole blocking and hole injection layers have opposite functions, yet there is a particular combination of their thicknesses when they function in conjunction and luminous efficiency and power efficiency are maximized. The optimum thickness of CuPc (Copper(II) phthalocyanine) layer, used as hole injection layer and BCP (2,9 dimethyl-4,7-diphenyl-1,10-phenanthroline) used as hole blocking layer were found to be 18 nm and 10 nm respectively. It is with this delicate adjustment of thicknesses, charge balancing is achieved and luminous efficiency and power efficiency were optimized. The maximum luminous efficiency of 3.82 cd/A at a current density of 24.45 mA/cm² and maximum power efficiency of 2.61 lm/W at a current density of 5.3 mA/cm² were achieved. We obtained luminance of 5993 cd/m² when current density was 140 mA/cm². The EL spectra was obtained for the LEDs and found that it has a peaking at 524 nm of wavelength.     

Analysis on the high luminous flux white light from GaN-based laser diode

We fabricated a phosphor-conversion white light using an InGaN laser diode that emits 445 nm and phosphor that emits in the yellow region when excited by the blue laser light. At 500 mA injection current the luminous flux and the luminous efficacy were 113 lm and 44 lm/W, respectively. The relationship of the luminous flux and the luminous efficacy of the white light with an injection current were discussed. Based on the evaluation method for luminous efficacy of light sources established by the Commission International de I’Eclairage (CIE) and the phosphor used in this experiment, a theoretical analysis of the experiment results and the maximum luminous efficacy of this white light fabrication method were also presented.     

White light emission from fiber pigtailed laser module

We fabricated a phosphor-conversion white light using an InGaN laser diode that emits 445 nm and phosphor that emit in the yellow. The InGaN laser diode was coupled to an optical fiber firstly and the phosphor was excited by the laser light output from the fiber. At 350 mA injection current the luminous flux and the luminous efficacy was 73 lm and 42.7 lm/W, respectively. The luminance was estimated to be 50 cd/mm². The relationship of the luminous flux and the luminous efficacy of the white light with injection current were measured and discussed.     

基于CuInS2/ZnS核壳结构量子点的高光效暖白光LED

采用逐步热注射法合成了用于白光LED的CuInS₂/ZnS(CIS/ZnS)核壳结构量子点.通过调整Cu/In的比率, 在CuInS₂(CIS)量子点的基础上, 合成了发射波长在570~650 nm之间可调的CIS/ZnS量子点.与CIS量子点的低量子产率相比, 具有核壳结构的CIS/ZnS量子点的量子产率达到了78%.通过在黄光荧光粉YAG :Ce³⁺表面旋涂CIS/ZnS量子点的方式制备了暖白光LED器件.在工作电流为10 mA时, 暖白光LED的发光效率达到了244.58 lm/W.由于CIS/ZnS量子点的加入, 所制备的白光LED器件的显色指数达到86.7且发光颜色向暖色调发生了转移, 相应的色坐标为(0.340 6, 0.369 0).     

White light emission from blue and near ultraviolet light-emitting diodes precoated with a Sr3SiO5:Ce3+,Li+ phosphor

White-light-emitting diodes (WLEDs) were fabricated by combining a yellow Sr3SiO5:Ce3+, Li+ phosphor with a blue light-emitting diode (LED) (460 nm chip) or a near ultraviolet (n-UV) LED (405 nm chip), respectively. Color temperature (Tc) of Sr3SiO5:Ce3+, Li+-based WLEDs could be tuned from 6500 to 100,000 K (blue LED pumping) and from 4900 to 50,000 K (n-UV LED pumping) without mixing with other phosphors. The blue LED-pumped WLED showed excellent white light (luminous efficiency=31.7 lm/W, Tc=6857 K) at 20 mA. This WLED showed a stable color coordinates property against an increase of the forward current. An n-UV LED-pumped WLED also showed bright white light (25.0 lm/W, 5784 K) at 20 mA.     

基于新型空穴传输材料的有机电致发光器件的研究

利用真空蒸镀方法以N2,N7-二(间甲苯胺基)-N2,N7-二苯基-2,7-二胺基-9,9-二甲基芴[2,7-bis(pmethoxyphenyl-m'-tolylamino)9,9-dimethylfluorene,TPF-OMe]为空穴传输层、8-羟基喹啉铝[tris(8-hydroxyquinolinato)aluminum,Alq3)]作为发光层及电子传输层,制备了双层器件.与制作的典型双层结构N,N'-二苯基-N,N'-二(3-甲基苯基)-1,1'-联苯-4,4'-二胺[N,N'-biphenyl-N,N'-bis-(3-methylphenyl)-1,1'-biphenyl-4,4'diamine,TPD/Alq3]器件相比,电流密度较大,发光效率低,发光谱峰为516 nm,色坐标为(0.30,0.53),为Alq3材料发光.以TPF-OMe为发光层兼空穴传输层,2,9-二甲基-4,7-二苯基-1,10-菲罗啉(2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline,bathocuproine或BCP)为空穴阻挡层,Alq3为电子传输层,制作三层有机电致发光器件.结果表明,光谱峰值在414 nm,色坐标为(0.20,0.24),为蓝色光,是TPF-OMe材料本身发光,器件在15 V电压下电流密度为1137 mA/cm2,亮度为900 cd/m2,在3 V偏压下有最大流明效率,为0.11 lm/W.基于TPF-OMe材料的器件的击穿温度比基于TPD材料的器件高近20℃,原因可能在于TPF-OMe材料比TPD材料高19℃的玻璃化转变温度(Tg).     

Affect of the electrical characteristics depending on the hole and electron injection materials of red organic light-emitting diodes

This study examined the electrical and optical properties of red OLEDs (organic light-emitting diodes) with a four-layer structure, ITO/amorphous fluoropolymer (AF)/N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1-biphenyl-4,4′-diamine (TPD)/R-H:R-D/lithium fluoride (LiF)/Al, containing a hole injection material, AF (amorphous fluoropolymer) and an electron injection layer material, LiF. Compared to the basic structure (two-layer structure), the brightness and luminous efficiency of the four-layer structure, ITO/TPD/R-H:R-D/Al, increased approximately 100 times (30,000 lm/m²) and 150 times (51 lm/W), respectively, with an applied voltage. The excellent efficiency of the external proton was also increased 150 times (0.51%). That is, the hole and electron injection layers improved the surface roughness of ITO and Al, and the interfacial physical properties. In addition, these layers allowed the smooth injection of holes and electrons. The luminance, luminous efficiency and external quantum efficiency were attributed to an increase in the recombination rates.     

高效率荧光粉转换型白光有机发光二极管的制备和性能研究

制备了一种荧光粉转换型白光有机发光二级管(pc-WOLEDs),器件由发蓝光的荧光装置和发红光的无机荧光粉Sr₂Si₅N₈∶Eu²⁺颜色转化层(CCL)复合组成,前者通过真空镀膜得到,后者采用旋涂法制备。CCL发射的红光和未被吸收的蓝光复合产生白光,最大发光效率为22.1 cd/A,最大能量效率为11.26 lm/W,外量子效率为10.2%。该pc-WOLEDs的色坐标为(0.32, 0.34),非常接近标准白光(0.33, 0.33)。稳定性实验结果显示,该器件在不同电流密度下表现出了良好的稳定性。     

Effect of dye dopants in poly(methylphenyl silane) light-emitting devices

To investigate the inter-molecular energy transfer between polysilane and dye dopants, poly(methylphenylsilane)(PMPS) was used as a host material and perylene as the blue dopant. The structure of the devices is indium–tin oxide (ITO)/PEDOT:PSS(30 nm)/PMPS:perylene(dye dopant 0.1–1.0 mol%)(60 nm)/Alq₃(20 nm)/LiF(0.5 nm)/Al(100 nm). Poly(3,4-ethylenedioxythiophene) (PEDOT):poly(4-styrenesulfonate) (PSS) is used as a buffer layer, tris(8-hydroxyquinoline)aluminum (Alq₃) as hole transporting layer, LiF as hole injection layer. The device shows a luminance 810 cd/m² at current density of 28 mA/cm², luminous efficiency of 0.14 lm/W. The external quantum efficiency (EQE) is about 0.5% and EQE increased up to 0.52% by doping with single wall carbon nanotubes (SWNT) into the emissive layer. We found an efficient inter-molecular energy transfer from polysilane to dye dopants. Furthermore, using the polysilane and energy-matched dye dopants enable to fabricate the electroluminescence devices through wet processes.     

NaCl/Ca/Al as an efficient cathode in organic light-emitting devices

An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency.     

基于铱配合物材料的高效高稳定性有机发光二极管

使用基于重金属Ir的新磷光材料(tpbi)₂Ir(acac)，制备了多层结构有机发光二极管器件: ITO/CuPc (40 nm)/α-NPD (45 nm)/CBP: (tpbi)₂Ir(acac) (3%, 30nm)/BCP(20 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al (100 nm).测试了材料的寿命、光谱吸收性质和器件的I-V-L特性.器件在低电压下电流符合热发射注入模型，高电压下I-V呈线形关系.不同偏压下器件发光光谱稳定，多峰拟合结果表明器件光谱由α-NPD发光峰(450 nm)，(tpbi)₂Ir(acac)主发光峰(518 nm)和肩峰(543 nm)构成.驱动电压为6 V时，器件效率达到最大12.1 lm/W，此时亮度为136 cd/m²，器件亮度最大为13500 cd/m²，此时效率为0.584 lm/W. 关键词： 有机发光二极管 磷光 效率 I-V-L特性')" href="#">I-V-L特性 光谱     

Solution-processed WOx hole injection layer for efficient fluorescent blue organic light-emitting diode

Solution-processed tungsten oxide (s-WO_x) interfacial layer for efficient hole injection in fluorescent blue organic light-emitting diode (OLED) is demonstrated. The OLED using 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as emitter shows luminous efficiency of 3.3 cd/A, power efficiency of 2.5 lm/W and external quantum efficiency of 4.6% with Commission Internationale d'Eclairage (CIE) color coordinates of (0.154, 0.102). Using MADN doped 1-4-di-[4-(N,N-diphenyl)amino]styryl-benzene as emitter, luminous efficiency of 10.8 cd/A, power efficiency of 6.4 lm/W and external quantum efficiency of 7.2% with CIE color coordinates of (0.167, 0.283) are achieved. Atomic force microscopy and X-ray photoelectron spectroscopy show that s-WO_x features superior film morphology and non-stoichiometry with slight oxygen deficiency. Current-voltage characteristics and impedance spectroscopy analysis indicate that s-WO_x behaves slightly enhanced hole injection and accordingly contributes to improved device performance in comparison with conventional vacuum thermal evaporation WO_x. Our results pave an alternative way for broadening WO_x application with solution process and advancing fluorescent blue OLEDs.     

新型高色纯度弱电流猝灭性蓝色有机发光器件

以ADN为基质,分别以不同掺杂剂制备了四种蓝色有机发光器件,器件结构为:CuPc(12 nm)/NPB(40 nm)/AND∶Dopant(50 nm)/Alq(12 nm)/LiF(4 nm)/Al。掺杂剂有:BCzVB(amino-substituted distyrylarylenederivatives)、TBPe、BCzVBi和DSA-ph四种。研究了最佳掺杂浓度以及器件的亮度、电流密度、效率和色坐标等电学特性和光学特性。其中掺杂BCzVB制备了色纯度高、低电流猝灭性的蓝色有机发光器件,色坐标达到x=0.146,y=0.162,最大亮度为11600 cd/m2(15 V),电流效率为2.8 cd/A,流明效率为1.79 lm/W;以ADN为基质,分别以TBPe、BCzVBi和DSA-ph为掺杂剂,制备了另外三种对比器件。器件ADN∶TBPe色坐标为x=0.162,y=0.222(蓝绿光),效率随电流的增加而降低很快;器件ADN∶BczVBi有较好的色纯度(色坐标:x=0.164,y=0.146),但电流效率较低:2.03 cd/A,效率随电流的增加降低幅度也较快。器件ADN∶DSA-ph效率较高为8 cd/A,效率随电流增加变化幅度不大,但色纯度比较差(x=0.153,y=0.306),适合于做白色有机发光器件。     

High-performance organic red-light-emitting device based on DCJTB and a new host material

An efficient red-light-emitting device using a new host material (DPF) and a red dopant (DCJTB) with a configuration of ITO/NPB (50 nm)/DCJTB:DPF (2%, 10 nm)/TPBI (30 nm)/LiF (0.5 nm)/Mg:Ag has been fabricated and investigated. The red OLED yields a brightness of 9270 cd/m² at 10 V, a maximum current efficiency of 4.2 cd/A and a maximum power efficiency of 3.9 lm/W. Using DPF as host material, the performance is much better than that of a prototypical Alq₃-based device, which has a maximum efficiency of 1.9 cd/A and 0.6 lm/W. The performance is even comparable with red OLEDs using an assist dopant or a cohost emitter system. Results of this work indicate that DPF is a promising host material for red OLEDs with high efficiency and simple device structure.     

The use of a perylenediimide derivative as a dopant in hole transport layer of an organic light emitting device

A perylene diimide (PDI) derivative was used as a dopant in the hole transport layer (HTL) of an organic light emitting device. The HTL examined was poly (N-vinylcarbazole) (PVK) and the PDI used was N,N′-di-dodecylperylene-3,4,9,10-bis-(dicarboximide), (N-DODEPER). The structure of the device was ITO/PEDOT:PSS (70 nm)/PVK:N-DODEPER(0, 0.2, 0.4, 0.8 wt.%) (65 nm)/Alq₃ (35 nm)/LiF (1.3 nm)/Al (100 nm). 0.8 wt.% N-DODEPER presence exhibited a luminous efficiency of 7.87 cd/A and an external quantum efficiency of 0.78% at 21 mA/cm² and a power efficiency of 3l m/W at 12 mA/cm². The luminous and power efficiency values were significantly enhanced by a factor of 15 with respect to that of undoped device.     

高效率共轭聚合物主体绿光磷光发光二极管

因电致发光效率高和器件制备工艺简单,聚合物为主体的绿色磷光电致发光成为一个研究热点.共轭聚合物的三线态能级一般低于绿色磷光材料的三线态能级,易对磷光的发光引起猝灭导致低的发光效率,所以较少被用作绿色磷光材料的主体.通过增加聚乙烯基咔唑（PVK）作为空穴传输层,获得了高发光效率的共轭聚合物聚芴（PFO）作主体绿色磷光发射,甚至高于相同条件下以PVK为主体的绿色磷光发射.究其原因,PVK的电子阻挡作用使发光中心靠近PVK与PFO的界面,界面处PVK因为其高的三线态能级增强了绿色磷光的发光.当三-（2-苯基吡啶）-Ir（Ir（ppy）₃）掺杂浓度为2％时得到了最高的亮度效率24.8 cd/A,此时的电流密度为4.65 mA/cm²,功率效率为11 lm/W,最高亮度达到35054 cd/m²,色坐标是（0.39,0.56）. 关键词： 共轭聚合物 磷光 绿光发光     

以联苯乙烯衍生物为发光层的蓝色有机发光二极管特性的研究

以一种新型联苯乙烯衍生物NPVBi作为发光层，制备了结构为：ITO／TPD／NPVBi/Alq3/LiF/Al的有机薄膜电致发光器件，其中TPD厚度保持为50nm,NPVBi与Alq3厚度之和保持为50nm。通过调节NPVBi与Alq3的厚度，获得了色纯度较好的NPVBi蓝色电致发光，最高亮度为708cd/m^2，最大流明效率为1.13lm/W。结果表明，发光层NPVBi和电子传输层Alq3的厚度对器件的发光特性有显著的影响。     

Improvement of the outcoupling efficiency of an organic light-emitting device by attaching microstructured films

In this paper, we present and analyse the optical characteristics, such as spectral shift, CIE coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency, of an organic light-emitting device (OLED) with a commercial diffuser film or a brightness-enhancement film (BEF) attached. Compared to a planar green OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 29% and 23%, respectively. The overall luminous power efficiencies are enhanced by 28% and 7%. Compared to the planar green device, we observe blue shifts at different viewing angles when microstructured films are attached, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifts and the full width at the half maximum (FWHM) decrease with increasing viewing angles due to the microcavity effect. When the diffuser is attached, the spectral peak has a constant blue shift (6 nm) compared to that of the planar OLED. On the other hand, in the BEF case, the spectral shift depends on the viewing angle (2-12 nm blue shifts from 0 to 80°). This is due to the different operating principles (scattering and redirected light) of the diffuser and BEF. Since the transmittance spectra of both the diffuser film and the BEF are flat over the visible range, it is suitable for lighting applications by using white OLED. When attaching the films on a commercial white OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 34% and 31%, respectively. The overall luminous power efficiencies are enhanced by 42% and 8%.     

Improvement of electroluminescent property of blue LED coated with highly luminescent yellow-emitting phosphors

White light-emitting diodes (WLEDs) were fabricated by combining InGaN-based blue light-emitting diodes (LEDs) with highly luminescent Tb₃Al₅O₁₂:Ce³⁺ (TAG:Ce), Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce), and Sr₃SiO₅:Eu²⁺ (SS:Eu). The TAG:Ce-based WLED showed a color rendering index (R _a ) of 79 and a luminous efficiency (η _L ) of 34.1 lm/W at 20 mA. The YAG:Ce-based WLED and the SS:Eu-based WLED showed low R _a values of 75 and 57 but high luminous efficiency values of 38.9 and 41.3 lm/W at 20 mA, respectively. When a mixture of YAG:Ce and SS:Eu was coated on a blue LED and the resultant WLED operated at 20 mA, the WLED showed a highly bright white light similar to daylight (η _L =40.9 lm/W, color temperature T _c =5,716 K, and R _a =76). Moreover, the WLED showed stable color coordinates against a considerable variation of applied current.