低效率滚降、发光颜色稳定的磷光白色有机电致发光器件

本文采用多发光层结构,制备了高亮度下具有高发光效率,同时在较宽亮度范围内发光颜色稳定的白色磷光有机电致发光器件(WOLED).在对双发光层结构磷光OLEDs的发光机制和载流子传输过程进行系统研究的基础上,将两种磷光OLEDs的发光层结构相结合,获得的多发光层结构磷光WOLED最大电流效率和外量子效率分别为34.6 cd/A和13.5%;当亮度为1000 cd/m^2时,其电流效率和外量子效率分别为33.9 cd/A和13.3%,外量子效率滚降仅为1.5%;亮度从1000 cd/m^2增至10000 cd/m^2的过程中,其CIE色度坐标从(0.342,0.403)变化至(0.326,0.392),变化量ΔCIE为(0.016,0.011).     

白光有机电致发光器件的载流子传输与复合特性

基于一种将具有电荷传输性的双极性主体材料与蓝、黄光客体材料共蒸的单层结构的同质结结构,制备了色温可调的白光有机电致发光器件(OLED)。由于不存在激子阻挡层,单层结构容易发生漏电流现象,致使OLED器件具有较高的工作电压和较低的电流/功率效率。在空穴/电子传输层进行p/n型掺杂的同质结结构则大大改善了器件性能。研究表明: 该种器件结构获得了相对较低的起压5.6 V,较高的电流效率2.64 cd/A和低效率滚降,其色坐标则随着亮度的增加沿着普朗克轨迹变化,产生类似于太阳光的发光特性。另外,对主体材料和共蒸层的电荷载流子的传输特性和复合机制也进行了一系列分析研究。     

Multiple roles of bathocuproine employed as a buffer-layer in organic light-emitting diodes

Efficiency and brightness and carriers injection have been obviously improved by using bathocuproine (BCP) as a buffer-layer in organic light-emitting diodes. Compared with the bufferless device, the quantum efficiency of device ITO/NPB (10 nm)/Alq₃ (10 nm)/BCP (2.4 nm)/Al has increased four times at the same current density (32 mA/cm²). Moreover, the buffer layer has changed the current-voltage properties and the turn-on voltage has obviously decreased. Considering BCP and Al³⁺ can react conveniently under room temperature, we suggest that a complex cathode structure of BCP/[(Al)_x(BCP)_y]^3x+/Al has formed under electric field and the new cation [(Al)_x(BCP)_y]^3x+ at the BCP/Al interface has improved the internal electric field and then enhanced the electrons injection. we conclude that: for a very thin (<1 nm) BCP buffer layer, improving electron injection will principally responsible to the improvement of the performance of the OLEDs; for a thicker BCP layer, there will be a synthetic function of BCP: improving electron injection, hole-blocking and electron-transporting.     

Ligand effect on the performance of organic light-emitting diodes based on europium complexes

A series of europium complexes were synthesized and their electroluminescent (EL) characteristics were studied. It was found by comparison that the different substituted groups, such as methyl, chlorine, and nitryl, on ligand 1,10-phenanthroline affect significantly the EL performance of devices based on these complexes. The more methyl-substituted groups on ligand 1,10-phenanthroline led to higher device efficiency. A chlorine-substituted group showed the approximate EL performance as two methyl-substituted groups, whereas a nitryl substituent reduced significantly the EL luminous efficiency. However, β-diketonate ligand TTA and DBM exhibited similar EL performance. The improved EL luminous efficiency by proper substituted groups on the 1,10-phenanthroline was attributed to the reduction of the energy loss caused by light hydrogen atom vibration, as well as concentration quenching caused by intermolecular interaction, and the match of energy level between the ligand and Eu³⁺.     

Stabilization of organic thin film transistors by ion implantation

We report on the effects of low energy ion implantation (N and Ne) in the reduction and control of the degradation of pentacene organic thin film transistors (OTFTs) due to the exposure to atmosphere (i.e. oxygen and water). We have observed that a controlled damage depth distribution preserves the functionality of the devices, even if ion implantation induces significant molecular structure modifications, in particular a combination of dehydrogenation and carbonification effects. No relevant changes in the pentacene thin film thickness have been observed. The two major transport parameters that characterize OTFT performance are the carrier mobility and the threshold voltage. We have monitored the effectiveness of this process in stabilizing the device by monitoring the carrier mobility and the threshold voltage over a long time (over 2000 h). Finally, we have assessed by depth resolved X-ray Photoemission Spectroscopy analyses that, by selectively implanting with ions that can react with the hydrocarbon matrix (e.g. N⁺), it is possible to locally modify the charge distribution within the organic layer.     

Fabrication and implementation of large-area organic light-emitting-diode devices on direct patterned backplanes

In this paper, we report on a new cost-effective fabrication technology for large-area organic light-emitting diode (OLED) devices that we have developed. We fabricated OLED lighting devices on a directly patterned backplane using a dry process without employing conventional photolithography patterning technology. The indium–tin-oxide (ITO) anode, metal cathode, insulator, and emissive organic layer patterns were all formed directly on the substrates during the sputtering and evaporation steps by using a shadow mask without using etching or lift-off methods. We fabricated and characterized green phosphorescent emission OLED devices with an emission area of 30 × 120 mm², based on backplanes formed by direct patterning technology. Application of direct patterning technology reduced the total number of processing steps to 4 from the 26 steps required by conventional photo-patterning technology, making it possible to reduce large-scale production cost. Although the process steps were reduced considerably, the typical characteristics were comparable to those of photo-patterned OLEDs. Furthermore, the lifetime of the OLEDs based on direct patterned backplane was observed to be 27,200 h, which is approximately 97% of the lifetime of conventionally patterned OLEDs.     

The effect of alkaline metal chlorides on the properties of organic light-emitting diodes

The multilayer organic light-emitting diodes (OLEDs) have been fabricated with a thin alkaline metal chloride layer inserted inside an electron transport layer (ETL), tris (8-hydroxyquinoline) aluminum (Alq₃). The alkaline metal chloride layer was inserted inside 60 nm Alq₃ at d=0, 10, 20 and 30 nm positions (d is the distance of the interlayer away from the Al cathode). The devices, with alkaline metal chlorides inserted at the Alq₃/Al interface, showed electron injection and electroluminescence (EL) intensity improvements. When the alkaline metal chlorides were inserted inside the Alq₃ layer at 10, 20 or 30 nm position apart from the Al cathode, both EL intensity and efficiency were enhanced for the devices with a thin potassium chloride (KCl) or rubidium chloride (RbCl) layer. On the contrary, the improvements were not observed for the OLEDs with a thin sodium chloride (NaCl) layer. A proposed insulator buffer layer model is employed to explain these characteristics of the devices.     

氯化锌修饰蓝光量子点发光二极管EI北大核心CSCD

采用氯化锌(ZnCl_(2))修饰镉基CdSe/ZnS蓝光量子点(B-QD)薄膜,发现与量子点表面结合力更强的ZnCl_(2)能够部分取代量子点长链配体油酸,有效钝化量子点表面缺陷,提高薄膜的荧光量子效率(PLQY)。此外,由于ZnCl_(2)具有偶极作用,使量子点真空能级提高0.2 eV,一方面可改善电子和空穴载流子注入平衡,另一方面可有效降低发光器件的启亮电压,提高器件的发光寿命。这种无机配体修饰量子点薄膜的方法可能为解决蓝光量子点发光二极管(B-QLEDs)因空穴注入困难和量子点表面缺陷导致器件性能不高的问题提供一个有效思路。     

Temperature-dependent electrical properties of organic light-emitting diodes depending on cathodes

We have studied temperature-dependent electrical properties of organic light-emitting diodes with a variation of cathode materials; Al, LiAl, and LiF/Al. The organic light-emitting diodes emit a light by a recombination of injected charge carriers such as holes and electrons. Thus, the charge transport is affected by the injection barrier at the interface. By varying the cathode materials, the electron injection at the interface could be controlled because of the work-function change at the cathode. Temperature-dependent current–voltage luminance characteristics of the organic light-emitting diodes were measured in the temperature range from 10 to 300 K. The current-voltage characteristics were analyzed in terms of Fowler–Nordheim tunneling model, and the energy-barrier height was obtained. A measured lifetime of device with LiF/Al cathode is relatively longer than the other cathodes at room temperature: 4.5 h for Al cathode, 12.4 h for LiAl, and 29.6 h for LiF/Al. The device with LiAl and LiF/Al cathode, in the aspect of lifetime and luminous efficiency, is superior to one of other cathodes.     

Improvement of characteristics of InGaN light-emitting diode by using a staggered AlGaN electron-blocking layer

The optical and physical properties of InGaN light-emitting diode (LED) with a specific design of staggered AlGaN electron-blocking layer (EBL) are investigated numerically in detail. The electrostatic field distribution, energy band, carrier concentration, electroluminescence (EL) intensity, internal quantum efficiency (IQE), and the output power are simulated. The results reveal that this specific design has a remarkable improvement of optical performance compared with the design of conventional LED. The lower electron leakage current, higher hole injection efficiency, and consequently mitigated efficiency droop are achieved. The significant decrease of electrostatic field at the interface between the last barrier and the EBL of LED could be one of the main reasons for these improvements.     

掺铒、镱铒共掺杂硅酸盐玻璃光致发光强度随泵浦波长的变化

利用半导体激光器的红移特性,测量了掺饵、镱铒共掺杂硅酸盐玻璃样品的光致发光强度随泵浦波长的变化。实验结果表明,掺铒硅酸盐玻璃样品的最佳泵浦波长为981.6nm,镱铒共掺杂硅酸盐玻璃样品的最佳泵浦波长为972.1nm;两种样品的-3dB带宽分别为3 6nm和7.4nm。     

Electroluminescence of organic light-emitting diodes consisting of an undoped (pbi)2Ir(acac) phosphorescent layer

The electroluminescence (EL) characteristics of phosphorescent organic light-emitting diodes (OLEDs) with an undoped bis(1,2-dipheny1-1H-benzoimidazole) iridium (acetylacetonate) [(pbi)₂Ir(acac)] emissive layer (EML) of various film thicknesses were studied. The results showed that the intensity of green light emission decreased rapidly with the increasing thickness of (pbi)₂Ir(acac), which was relevant to the triplet excimer emission. It suggested that the concentration quenching of monomer emission in the undoped (pbi)₂Ir(acac) film was mainly due to the formation of triplet excimer and partly due to the triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA). A green OLED with a maximum luminance of 26,531 cd/m², a current efficiency of 36.2 cd/A, and a power efficiency of 32.4 lm/W was obtained, when the triplet excimer emission was eliminated. Moreover, the white OLED with low efficiency roll-off was realized due to the broadened recombination zone and reduced quenching effects in the EML when no electron blocking layer was employed.     

Improvement of characteristics of an InGaN light-emitting diode by using a staggered AlGaN electron-blocking layer

The optical and physical properties of an InGaN light-emitting diode (LED) with a specific design of a staggered AlGaN electron-blocking layer (EBL) are investigated numerically in detail. The electrostatic field distribution, energy band, carrier concentration, electroluminescence (EL) intensity, internal quantum efficiency (IQE), and the output power are simulated. The results reveal that this specific design has a remarkable improvement in optical performance compared with the design of a conventional LED. The lower electron leakage current, higher hole injection efficiency, and consequently mitigated efficiency droop are achieved. The significant decrease of electrostatic field at the interface between the last barrier and the EBL of the LED could be one of the main reasons for these improvements.     

Comparative study of field-effect mobility with different expressions in organic thin film transistors

In organic thin film transistors (OTFTs), mobility generally exhibits field-effect dependence, and is strongly related to the disorder property of organic semiconductors used in OTFTs. Here, we compared three typical field-effect mobility models and used them to simulate the output characteristics of pentacene-based OTFTs. From the comparison of the theory and experiment, an analytic expression for the field-effect mobility to exactly describe the electrical characteristics of OTFTs was obtained. The better fit to the output characteristics of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB)-based OTFT and copper phthalocyanine (CuPc)-based OTFT by using the obtained analytic expression of the field-effect mobility further extended its applicability in OTFTs. This supplies a valuable manner to derive the field-effect mobility of carriers and understand the transport characteristics of carriers in OTFTs.     

Plasmon resonance modulated photoluminescence and Raman spectroscopy of diindenoperylene organic semiconductor thin film

In a comparison between a bare diindenoperylene (DIP) film and a DIP film spin-coated with a layer of gold nanoparticles, we have investigated the influence of plasmon resonances in the gold particles on spectroscopic properties of the molecular film. Under off-resonant excitation with a laser at 633 nm, the bare DIP film showed only weak photoluminescence (PL) and Raman signals, but after spin-coating gold nanoparticles on such a DIP film, we found an enhancement of both the PL and Raman signals by a factor of about 3, whereas no enhancement could be observed when the same sample was excited with laser light of 488 nm. This difference reveals that at 633 nm, plasmon resonances in the gold nanoparticles are excited, leading in turn to an enhancement of PL and Raman signals of the weakly absorbing DIP film via coupling between plasmons in the gold particles and exciton-polaritons in the molecular film. For the laser at 488 nm, due to a much larger absorption coefficient of DIP, excitons in the molecular film are directly excited, out-weighing the influence of an off-resonant coupling to the plasmon resonances in the gold particles occurring at much lower energy.     

TADF激基复合物主体材料提升溶液法制备蓝色荧光有机发光二极管的效率

为了提升溶液法制备的蓝色荧光有机发光二极管(OLEDs)的效率,采用了基于热激活延迟发光(TADF)的激基复合物作为主体材料。TADF激基复合物主体可以利用反向系间窜跃上转换形成单线态激子并将能量传递到客体,从而可以同时利用发光层中的三线态激子和单线态激子,以提升蓝色荧光器件的效率。选择蓝色荧光材料1-4-Di-[4-(N,N-diphenyl)amino]styryl-benzene(DSA-ph)作为客体发光材料,4,4′,4″-T-ris(carbazol-9-yl)triphenylamine(TCTA)掺杂1,3,5-Tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl)(TPBi)作为热激活延迟荧光激基复合物主体,通过溶液法制备了蓝色荧光OLEDs。通过测试TCTA,TPBi以及TCTA掺杂TPBi的光致发光光谱发现,与TCTA和TPBi相比,TCTA掺杂TPBi的光致发光谱(PL)发生了明显的红移(峰值波长变为437 nm),而且光谱变宽,证明了TCTA∶TPBi激基复合物的形成。通过对于DSA-ph掺杂激基复合物主体的薄膜与DSA-ph掺杂poly(methyl methacrylate)(PMMA)的薄膜进行PL测试发现,两者发光峰相同,都是来自DSA-ph的发光,说明激基复合物主体将能量传递到了DSA-ph;DSA-ph的吸收光谱与激基复合物主体的PL光谱存在很大重叠,说明激基复合物主体与DSA-ph的能量传递非常有效;通过对激基复合物主体掺杂不同浓度客体的薄膜进行瞬态PL衰减测试发现,与纯DSA-ph的寿命相比,DSA-ph掺杂激基复合物主体之后其寿命会延长,纯DSA-ph的寿命只有1.19 ns,DSA-ph掺杂激基复合物主体的荧光衰减曲线与激基复合物主体的荧光衰减曲线相似,这进一步证明了激基复合物主体将能量传递到了DSA-ph。研究了主体引入以及DSA-ph掺杂浓度对器件性能的影响。对于器件的亮度、电流密度、电压、电流效率、电致发光光谱等参数进行了测试,与不采用激基复合物主体的器件相比,采用激基复合物主体的器件性能明显改善,在DSA-ph掺杂浓度为10%时,器件亮度从2133.6 cd·m^-2提升到了3597.6 cd·m^-2,器件效率从1.44 cd·A-1提升到了3.15 cd·A-1,发光峰只有来自DSA-ph的发光。采用TADF激基复合物主体的方法有潜力实现溶液法制备的高效蓝色荧光OLEDs。     

Temporal characteristics of Ti: sapphire laser pumped synchronously by bounded pulse train

A Ti: sapphire laser pumped synchronously by a bounded frequency-doubled pulse train of a pulsed mode-locked Nd : YAG laser was presented. The temporal characteristics of the laser and optimal conditions of generating stable ultrashort pulses are also given.     

Micro‐LED pumped polymer laser: A discussion of future pump sources for organic lasers

Optical pumping conditions for organic solid‐state lasers (OSLs) are discussed with particular emphasis on the use of gallium nitride based light‐emitting diodes (LEDs) as pump sources. LEDs operate in a regime where the pump should be optimized for a short rise time and high peak intensity, whereas fall time and overall pulse duration are less important. Lasers pumped with this approach need to have very low thresholds which can now be routinely created using (one‐dimensional) distributed feedback lasers. In this particular case stripe‐shaped excitation with linearly polarized light is beneficial. Arrays of micron‐sized flip‐chip LEDs have been arranged in an appropriate stripe shape and the array dimensions were chosen such that the divergence of LED emission does not cause a loss in peak intensity. These micro‐LED arrays have successfully been used to pump OSLs with thresholds near 300 W/cm² (∼9 ns rise time, 35 ns pulse duration), paving the way for compact arrays of indirectly electrically pumped OSLs.     

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.     

Fabrication of Au thin film gratings by pulsed laser interference

We report that one-dimensional (1D) and two-dimensional (2D) metal thin film gratings can be directly fabricated by interfering Nd-YAG pulsed laser beams (wavelength = 1064 nm, pulse width = 6 ns) incident from the backside of glass substrate. This process utilizes a laser-induced thermo-elastic force which plays a role to detach the film from the substrate. Micro-scale Au transmission gratings with a minimum feature size of 1 μm could be generated by interference-driven periodic detachment. The fabrication of tube-structured patterns as well as stripes was also possible by adjusting the pulse power and this is explained with the effect of film cohesion.