Efficient and low potential operative host/guest concentration graded bilayer polymer electrophosphorescence devices

This study investigates enhanced electrophosphorescence and its mechanism in poly(N-vinyl carbazole) (PVK): N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1-biphenyl]-4,4′-diamine (TPD)/2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD): fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] concentration graded bilayer electroluminescence devices. The two layers are partially intermixed at the bilayer interface because the upper layer (composed of Ir(ppy)₃ and PBD) was spun cast from a solvent that slightly swells the bottom layer (composed of PVK and TPD). Moreover, PBD in the upper layer can act as an efficient electron transport layer as well as a hole blocking layer, resulting in greatly enhanced electron–hole recombination. An indium tin oxide (ITO)/3,4-polyethylenedioxythiophene–polystyrenesulfonate (PEDOT)/[PVK:TPD/Ir(ppy)₃:PBD] bilayer/LiF/Al device showed dramatically decreased turn-on and driving voltages, enhanced luminescence efficiency, and narrower emission spectra compared to those of conventional ITO/PEDOT:PSS/[PVK:TPD:Ir(ppy)₃:PBD] blend/LiF/Al devices.     

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.     

Enhanced performance of light-emitting diodes based on a nanocomposite of dehydrated nanotube titanic acid and poly(vinylcarbazole) (PVK)

Performance of light-emitting diodes (LEDs) based on a nanocomposite of dehydrated nanotube titanic acid (DNTA) and poly(vinylcarbazole) (PVK) as emissive layer, i.e., ITO/PVK:DNTA(2 wt%)(100 nm)/BCP(30 nm)/Alq₃(10 nm)/LiF(1 nm)/Al [device D₂], was reported. By investigation of the luminance and electric characteristics of the device, we found that the performance of device D₂ was greatly improved. The recombination zone changed evidently due to the improvement of holes mobility. Compared with those devices without incorporating DNTA, both the maximum luminance and the electroluminescent efficiency of the device D₂ can be improved by a factor of three. Furthermore, the turn-on voltage of the device decreased dramatically.     

稀土配合物Eu(asprin)3phen发光特性的研究

将稀土配合物Eu(asprin)3phen掺杂到导电聚合物PVK中，制成结构分别为ITO/PVK：RE配合物／LiF／Al(1)，ITO／PVK：RE配合物／PBD/LiF/Al(2)的电致发光(EL)器件。发现二者的电致发光谱存在着较大的差别：在器件(1)中，来自Eu^3 的位于594nm(^5D0→^7F1)和614nm(^5D0→^7F2)处的发光强度大致相当，而在器件(2)中，EL主要来自Eu^3 位于614nm的发光，594nm处的发光很弱，与薄膜状态下的光致发光谱(PL)一致。并针对此现象进行了初步讨论。     

铽配合物Tb(o-MBA)3phen与PVK掺杂体系的发光机理

合成了一种新型的稀土铽配合物材料Tb(o-MBA)₃phen,并把它作为发光材料应用于有机电致发光器件中。将铽配合物与PVK的混合溶液用旋涂法制得发光层,并利用Alq₃作为电子传输层制备了多种结构的电致发光器件:器件A:ITO/PVK:Tb(o-MBA)₃phen/LiF/Al;器件B:ITO/PVK:Tb(o-MBA)₃phen/BCP/Alq₃/LiF/Al;器件C:ITO/BCP/PVK:Tb(o-MBA)₃phen/Alq₃/LiF/Al。由器件A和C得到了纯正的、明亮的Tb³⁺的绿光发射,发射光谱中四个特征峰分别对应着能级⁵D₄→⁷F_J(J=6,5,4,3)的跃迁,而PVK的发光完全被抑制。在光致发光中PVK的发射光谱和铽配合物的激发光谱有一定的重叠,两者之间可能存在Frster能量传递。同时PVK与铽配合物掺杂体系的激发光谱与纯PVK的激发光谱非常相像,而与铽配合物的激发光谱差别很大,这也说明掺杂体系中铽的发光有一部分来源于PVK分子的激发,PVK与铽配合物之间存能量传递过程。研究了掺杂体系的电致发光性能,在电致发光中,铽的发光主要来源于稀土配合物直接俘获载流子形成激子并复合发光。通过优化选择得到了发光性能较好的器件,器件的最大亮度在17V时达到180cd/m²。     

稀土配合物Eu(asprin)_3phen发光特性的研究

将稀土配合物Eu(asprin)_3phen掺杂到导电聚合物PVK中,制成结构分别为ITO/PVK:RE配合物/LiF/AI(1),ITO/PVK:RE配合物/PBD/LiF/AI(2)的电致发光(EL)器件。发现二者的电致发光谱存在着较大的差别:在器件(1)中,来自Eu~(3 )的位于594nm(~5D_0→~7F_1)和614nm(~5D_0→~7F_2)处的发光强度大致相当,而在器件(2)中,EL主要来自Eu~(3 )位于614nm的发光,594nm处的发光很弱,与薄膜状态下的光致发光谱(PL)一致。并针对此现象进行了初步讨论。     

苯甲酰水杨酸铽的合成与发光特性研究

合成了一类以苯甲酰水杨酸(Benzoyl Salicylic Acid,BSA)为配体的稀土铽配合物,将导电高分子材料PVK引入到配合物中,制成了结构为ITO／PVK：Tb(BSA)4／LiF／Al的电致发光器件。并对该配合物的吸收特性及电致发光和光致发光性能进行了研究,实验数据表明,在PVK与Tb(BSA)4之间存在着能量传递,在电致发光中,PVK的发光完全被抑制,这与光致发光的表现不同,这是由于两种发光(光致和电致)机理不同造成的。章同时比较了几种不同PVK掺杂浓度对于器件性能的影响。     

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.     

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.     

Electroplex emission at PVK/Bphen interface for application in white organic light-emitting diodes

White organic light-emitting diode (WOLED) with a structure of ITO/poly(N-vinylcarbazole) (PVK)/4,7-diphenyl-1, 10-phenanthroline (Bphen)/tris(8-hydroxyquinoline)aluminum (Alq₃)/LiF/Al has been fabricated via the thermal evaporation technique. The electroluminescence (EL) spectrum of the as-fabricated WOLED covers from 380 to 700 nm of the visible light region with a wide blue emission from PVK and an interesting new red emission. The red emission at 613 nm in EL spectra of the WOLED was attributed to electroplex emission at PVK/Bphen interface since it was not observed in photoluminescence spectra. The WOLED showed a Commission International De l'Eclairage coordinate of (0.31, 0.32), which is very close to the standard white coordinate (0.33, 0.33).     

Spectral studies of thin films based on poly(N-vinylcarzole) and red dopant

Organic light-emitting diodes were fabricated with a structure of indium-tin-oxide (ITO)/poly(N-vinylcarzole)(PVK):4-(dicyanom-ethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)/8-tris-hydroxyquinoline aluminum (Alq₃)/lithium fluoride (LiF)/Al. The energy transfer from PVK to Alq₃ then to DCJTB and the charge trapping processes were investigated by employing the photoluminescence (PL) and electroluminescence (EL) spectra. With increasing thickness of the Alq₃ layer, the PL and EL emission from PVK were decreased gradually, which indicated that the effective energy transfer occurred from PVK to Alq₃ and then from Alq₃ to DCJTB. At the same time, we found that the exciton recombination zone could be adjusted by controlling the Alq₃ layer thickness and the applied voltages. The effects of different DCJTB concentrations on the optical and electrical characteristics of the devices were investigated, and an obvious red-shift was observed with the DCJTB dopant concentrations increasing in the PL and EL spectra.     

PBD的掺杂对PVK:Ir(ppy)3体系发光特性的影响

从三线态激子的发光机理入手,研究了PBD作为电子传输材料对PVK:Ir(ppy)3体系的影响。实验中制备了单层器件ITO/PVK:Ir(ppy)3/PBD/Al,ITO/PVK:Ir(ppy)3:PBD/Al和双层器件,ITO/PVK:Ir(ppy)3:PBD/BCP/Al,其中PVK:Ir(ppy)3的掺杂浓度比例不变,通过改变PBD的掺杂浓度,其变化范围是PBD与PVK的质量比从0:100到20:100,制得了一系列器件,研究了它们的光致发光(PL)光谱和电致发光(EL)光谱。发现PBD这种电子传输材料的加入对器件的亮度有很大提高,当PBD与PVK质量比为10%时,器件亮度最大。     

Synthesis and characterization of a solution processible deep blue light-emitting molecule composed of fluorene and pyrene units

A solution processible deep blue light-emitting molecule composed of pyrene and dialkylfluorene units, 1,6-bis(9,9′-dioctylfluorene-2-yl)pyrene (BDOFP) was synthesized and characterized. The synthesized compound was soluble in common organic solvents and the solution gave a smooth thin film after spin coating. The compound was characterized by using thermogravimetric analysis (TGA), differential calorimetry (DSC), UV–visible spectroscopy, fluorescence spectroscopy and cyclic voltammetry. The maximum UV–visible absorption and PL emission of BDOFP thin film were more red-shifted than those of BDOFP solution due to strong intermolecular interaction between flat segments. To improve color purity and film stability BDOFP was doped to a well-known charge-transporting polymer, poly(N-vinylcarbazole) (PVK). BDOFP thin film showed it maximum PL at 457 nm but the thin films of BDOFP doped PVK films showed it at 443 nm. Organic light-emitting diodes were fabricated with the simple structure of ITO/PEDOT:PSS/emitter/BmPyPB/LiF/Al configuration. BDOFP or three kinds of BDOFP:PVK blends with different ratios (10:90, 30:70, 50:50 by weight) were used as the emissive layers and [1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene] (BmPyPB) as the electron-transporting layer. All of light-emitting devices showed their electroluminescence in blue region of spectrum, especially EL using BDOFP: PVK (1:9) showed a deep-blue light emission with CIE coordinates of (0.14, 0.07). Maximum brightness, external quantum efficiency and current efficiency of the device were 500 cd/m², 0.7% and 0.44 cd/A, respectively.     

Ir(Fppy)_3掺杂PVK体系发光特性的研究

对蓝色磷光材料Ir(Fppy)3不同浓度掺杂PVK薄膜的光致发光(PL)和电致发光(EL)特性进行了研究。并制备了结构为ITO/PEDOT:PSS/PVK:Ir(Fppy)3/BCP/Alq3/LiF/Al的蓝色磷光有机电致发光器件。实验结果发现,磷光材料掺杂浓度不同,器件发光特性不同。当Ir(Fppy)3掺杂浓度比较低时,EL光谱中可以观察到PVK较弱的发光;当Ir(Fppy)3掺杂浓度较高时,会发生浓度猝灭;当Ir(Fppy)3掺杂浓度比较适中时,EL光谱中观察不到PVK的发光,只有Ir(Fppy)3的发光。通过I-V-L特性的比较,当掺杂浓度为4%时,器件的光电特性最好。     

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.     

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.     

Small-molecular white organic light-emitting devices employing 2, 5, 2′, 5′-tetra (<Emphasis Type="Italic">p</Emphasis>-trifluoromethylstyryl)-biphenyl as single-emitting component

We demonstrate a promising single layer white light-emitting device using a dimeric trimeric phenylenvinylene derivative as emitting layer. The broad electroluminescence emission band is composed of blue component from singlet excited state of individual 2, 5, 2′, 5′-tetra (p-trifluoromethylstyryl)-biphenyl molecule and long-wavelength electromer emission in electroluminescence. Therefore, white-light emission can also be obtained with a typical three-layer structure of ITO/NPB (50 nm)/TFM-TSB (50 nm)/Alq₃ (30 nm)/LiF/Al device. The maximum brightness of this device is 809 cd/m² at 217 mA/cm² and 13 V, and the maximum luminous efficiency is 1.49 cd/A at 11 mA/cm² and 8 V.      

PBD在稀土配合物与PVK混合体系电致发光中的作用

研究了PBD以较低浓度与铽配合物[Tb(m-MBA)₃phen]₂·2H₂O、PVK共掺杂体系的电致发光,制作了两类电致发光器件：ITO/PVK:Tb complex/PBD/LiF/Al,ITO/PVK：Tb complex:PBD/PBD/LiF/Al。在共掺杂的发光层中铽配合物的电致发光来源于两个途径,一个是由PVK到铽配合物的能量传递,另一个是电子和空穴在铽配合物上直接复合发光。改变PBD在发光层中的掺杂比例,制得一系列器件,通过对其光谱和亮度的研究,发现PBD在较低浓度掺杂时器件的稳定性和亮度随掺杂浓度的增加而降低。通过分析认为PBD的加入对给体(PVK)到受体(Tb complex)的能量传递效率影响较小,主要是由于PBD的加入使得电子和空穴在PVK链间的跳跃受到限制,使在由PVK、铽配合物和PBD三者掺杂组成的发光层中,注入的电子和空穴不能有效地在铽配合物上复合,这样就会减少激子在铽配合物上直接复合的概率,而造成器件的亮度和效率降低。     

Enhanced electron injection in organic light-emitting devices using Al/LiF electrodes

The temperature dependence of the current-voltage-luminescence characteristics in organic light-emitting diodes (OLEDs) with varying thickness of LiF layers are studied to understand the mechanism of the enhanced electron injection by inserting a thin insulating LiF layer at the tris(8-hydroxyquinoline) aluminum (Alq₃)–Al interfaces. At room temperature, the LiF/Al cathode enhances the electron injection and the quantum efficiency (QE) of the electroluminescence (EL), implying that the LiF thin layer lowers the electron-injection barrier. However, at low temperatures it is observed that the injection-limited current dominates and the barrier height for the electron injection in the device with LiF/Al appears to be similar with the Al only device. Thus, our results suggest that at low temperatures the insertion of LiF does not cause a significant band bending of Alq₃ or reduction of the Al work function.     

Influence of the electric characteristics of II–VI semiconductor material on the electroluminescence of lanthanide complex

The organic-inorganic combined structural device (ITO/PVK:Eu/ZnS/Al) is fabricated based on layered optimization scheme. II–VI semiconductor material ZnS is acted as an electron function (transporting and acceleration) layer. The hot electrons which have been accelerated in the ZnS layer directly impact excitation europium ions through resonant energy transfer and then recombine with injected holes to form excitons in PVK or EuTTA₂(N-HPA)Phen. Europium (Eu) ions may also be excited by intramolecular energy transfer from ligands. There are two kinds of excitation mechanisms: impacted excitation and injected recombination for the combined structural device. The electroluminescence (EL) intensity of the combined structural device is strongly improved and reaches up to 381 cd/m² at 20 V compared with the pure organic structural device. It may be an effective method to improve the EL intensity of the lanthanide complex by using electric characteristics of inorganic semiconductor materials.