Spin-dependent exciton formation in pi-conjugated compounds.

The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly. We find an average singlet generation fraction of 22 +/- 1 per cent for the monomer, but 57 +/- 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.     

基于无机p型NiO缓冲层的钙钛矿发光二极管发光特性研究

【目的】研究如何提高溴基钙钛矿(CH3NH3PbBr3)发光二极管的发光亮度。【方法】通过在氧化铟锡(ITO)玻璃基底上引入无机p型NiO缓冲层与有机聚合物聚(3,4-亚乙二氧基噻吩)-聚(苯乙烯磺酸)(PEDOT∶PSS)形成有机/无机杂化空穴传输层,提高空穴注入效率,降低电子溢出。【结果】引入无机p型NiO缓冲层形成有机/无机杂化空穴传输层后,发光二极管的发光亮度提高了约1倍。【结论】该方法不仅可降低PEDOT∶PSS对ITO基底的腐蚀,还能显著提高空穴注入效率,提高发光二极管的发光亮度。     

有机电致发光器件的电极研究

介绍了有机电致发光器件的电极研究进展,着重分析了电极性能的改进机制。对于阳极,为了利干空穴的注入和可见光透射,一般应选择功函数较高的透明导电材料。由于铟锡氧化物(ITO)是目前使用最为广泛的阳极材料,所以着重探讨了对ITO膜各种处理方法的改进机理,认为虽然氧空位浓度、C污染、Sn浓度都对ITO的表面功函数有影响,但C污染是主要因素;一种较好的处理方法是既能有效清除C污染,同时又尽可能使ITO膜的电阻率降低,所以中度氧等离子体处理是一种较为理想的途径。对于阴极,为了利于电子的注入,应选择功函数较低且化学性质较为稳定的材料;层状阴极可以较好地实现这一目的,并分别从隧穿效应、界面能带弯曲及减少淬灭中心等方面解释了金属/绝缘层双层阴极的改进机理,认为这三个方面共同起了作用,但主次不一。     

基于双阳极界面修饰层有机太阳能电池研究

为提高有机太阳能电池的能量转换效率,提出一种基于双阳极界面修饰层的有机太阳能电池优化方案。该方法主要采用四氟乙烯( PTFE: Polytetrafluoroethylene) 和五氧化二钒( V2O5: Vanadiumpentoxide) 作为阳极界面修饰层,制备了器件结构为ITO/PTFE/V2O5 /PCDTBT ∶ PC71 BM/LiF/Al 的有机太阳能电池。测试结果表明,引入PTFE/V2O5双阳极界面修饰层的有机太阳能电池的能量转化效率最高可达6． 52%。相比于V2O5单阳极界面修饰层器件效率提高了11． 5%。测试结果证明双阳极界面修饰层的功函数与PCDTBT 材料的HOMO 能级更加匹配,有利于空穴的传输和提取。同时PTFE/V2O5 改善了氧化铟锡( ITO) 表面形貌,减少界面缺陷,抑制了界面处载流子的复合。     

Nitride semiconductors free of electrostatic fields for efficient white light-emitting diodes

Compact solid-state lamps based on light-emitting diodes (LEDs) are of current technological interest as an alternative to conventional light bulbs. The brightest LEDs available so far emit red light and exhibit higher luminous efficiency than fluorescent lamps. If this luminous efficiency could be transferred to white LEDs, power consumption would be dramatically reduced, with great economic and ecological consequences. But the luminous efficiency of existing white LEDs is still very low, owing to the presence of electrostatic fields within the active layers. These fields are generated by the spontaneous and piezoelectric polarization along the [0001] axis of hexagonal group-III nitrides--the commonly used materials for light generation. Unfortunately, as this crystallographic orientation corresponds to the natural growth direction of these materials deposited on currently available substrates. Here we demonstrate that the epitaxial growth of GaN/(Al,Ga)N on tetragonal LiAlO2 in a non-polar direction allows the fabrication of structures free of electrostatic fields, resulting in an improved quantum efficiency. We expect that this approach will pave the way towards highly efficient white LEDs.     

Flexible organic light-emitting diodes with ITO／Ag／ITO multi-layers as anodes

The transparent ITO/Ag/ITO multi-layers are developed as anodes on flexible PET (poly(ethylene terephthalate)) substrates. The influence of these anodes on FOLED (Flexible Organic Light-emitting Diodes) is investigated. From the results of research, it can be seen that the multi-layer anode has optimum characteristics, whose sheetresistance is 11 Ω and optical transmittance is about 80%,when the thickness of Ag sandwiched by two ITO layers is in the range of 14--18 nm. It is demonstrated that the OLED devices with multi-layer anodes give better luminescence and higher efficiency compared with those with single ITO anodes.     

Flexible organic light-emitting diodes with poly-3, 4-ethylenedioxythiophene as transparent anode

The flexible oragnic light-emitting diodes (OLEDs) fabricatedon poly-3, 4-ethylenedioxythiophene/polystyrenesulfonate (PEDOT/PSS) coated substrates were demonstrated. How the fabricating processes and the device structure will affect the device performance was studied and the atomic force microscopy was employed to analyze the mophorlogy of the conducting polymer anode. Under optimized conditions, flexible OLEDs with PEDOT anode showed the brightness up to 2760 cd/m² and maximum external quantum efficiency of 1.4%. These data are comparable to those of conventional flexible OLEDs with ITO anode.     

Graphene-based composite materials

Graphene sheets--one-atom-thick two-dimensional layers of sp2-bonded carbon--are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (approximately 3,000 W m(-1) K(-1) and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene-graphene composite formed by this route exhibits a percolation threshold of approximately 0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes; at only 1 volume per cent, this composite has a conductivity of approximately 0.1 S m(-1), sufficient for many electrical applications. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.     

Improving the performance of doped pi-conjugated polymers for use in organic light-emitting diodes

Organic light-emitting diodes (OLEDs) represent a promising technology for large, flexible, lightweight, flat-panel displays. Such devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode into the lowest unoccupied molecular orbital of the adjacent molecules (simultaneously, holes are injected by the anode into the highest occupied molecular orbital). The two types of carriers migrate towards each other and a fraction of them recombine to form excitons, some of which decay radiatively to the ground state by spontaneous emission. Doped pi-conjugated polymer layers improve the injection of holes in OLED devices; this is thought to result from the more favourable work function of these injection layers compared with the more commonly used layer material (indium tin oxide). Here we demonstrate that by increasing the doping level of such polymers, the barrier to hole injection can be continuously reduced. The use of combinatorial devices allows us to quickly screen for the optimum doping level. We apply this concept in OLED devices with hole-limited electroluminescence (such as polyfluorene-based systems), finding that it is possible to significantly reduce the operating voltage while improving the light output and efficiency.     

Efficient organic photovoltaic diodes based on doped pentacene

Recent work on solar cells based on interpenetrating polymer networks and solid-state dye-sensitized devices shows that efficient solar-energy conversion is possible using organic materials. Further, it has been demonstrated that the performance of photovoltaic devices based on small molecules can be effectively enhanced by doping the organic material with electron-accepting molecules. But as inorganic solar cells show much higher efficiencies, well above 15 per cent, the practical utility of organic-based cells will require their fabrication by lower-cost techniques, ideally on flexible substrates. Here we demonstrate efficiency enhancement by molecular doping in Schottky-type photovoltaic diodes based on pentacene--an organic semiconductor that has received much attention as a promising material for organic thin-film transistors, but relatively little attention for use in photovoltaic devices. The incorporation of the dopant improves the internal quantum efficiency by more than five orders of magnitude and yields an external energy conversion efficiency as high as 2.4 per cent for a standard solar spectrum. Thin-film devices based on doped pentacene therefore appear promising for the production of efficient 'plastic' solar cells.     

TiCl4处理和添加MgO对染料敏化太阳能电池性能的影响

利用TiCl4溶液对染料敏化太阳能电池(DSSC)阳极膜中导电基体和TiO2膜进行处理并研究了其对DSSC性能的影响.先用0.15mol/L TiCl4溶液处理ITO导电玻璃,涂膜后再用0.05mol/L的TiCl4水溶液处理TiO2膜以及在胶体中加入少量MgO,制备出光电转换效率高达8.47%的高效DSSC,而未经过任何处理的DSSC光电转换效率仅为5.75%,经过处理后的DSSC效率提高了47%.     

Management of singlet and triplet excitons for efficient white organic light-emitting devices

Lighting accounts for approximately 22 per cent of the electricity consumed in buildings in the United States, with 40 per cent of that amount consumed by inefficient (approximately 15 lm W(-1)) incandescent lamps. This has generated increased interest in the use of white electroluminescent organic light-emitting devices, owing to their potential for significantly improved efficiency over incandescent sources combined with low-cost, high-throughput manufacturability. The most impressive characteristics of such devices reported to date have been achieved in all-phosphor-doped devices, which have the potential for 100 per cent internal quantum efficiency: the phosphorescent molecules harness the triplet excitons that constitute three-quarters of the bound electron-hole pairs that form during charge injection, and which (unlike the remaining singlet excitons) would otherwise recombine non-radiatively. Here we introduce a different device concept that exploits a blue fluorescent molecule in exchange for a phosphorescent dopant, in combination with green and red phosphor dopants, to yield high power efficiency and stable colour balance, while maintaining the potential for unity internal quantum efficiency. Two distinct modes of energy transfer within this device serve to channel nearly all of the triplet energy to the phosphorescent dopants, retaining the singlet energy exclusively on the blue fluorescent dopant. Additionally, eliminating the exchange energy loss to the blue fluorophore allows for roughly 20 per cent increased power efficiency compared to a fully phosphorescent device. Our device challenges incandescent sources by exhibiting total external quantum and power efficiencies that peak at 18.7 +/- 0.5 per cent and 37.6 +/- 0.6 lm W(-1), respectively, decreasing to 18.4 +/- 0.5 per cent and 23.8 +/- 0.5 lm W(-1) at a high luminance of 500 cd m(-2).     

High efficient light-emitting diodes using polystyrene as matrix

High efficient polymer light-emitting diodes (PLEDs) were obtained by using a blend of conjugated polymer G-PF, a copolymer of fluorene and thiophene, and polystyrene (PS). The maximum electroluminescent (EL) efficiency of the device is 12 cd/A when G-PF/PS weight ratio is at 80/20, while that of pure G-PF device is 6.5 cd/A. Studies on photoluminescence and electroluminescence of the blends indicate that inter-chain interactions were tremendously suppressed due to the dilution effect. However, after PS concentration exceeds 20% the EL efficiency of the devices decreases with further increase of PS concentration. This may be due to the decrease of the recombination probability of electrons and holes with the excessive addition of PS insulator.     

Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers

Mechanisms for converting electrical energy into mechanical energy are essential for the design of nanoscale transducers, sensors, actuators, motors, pumps, artificial muscles, and medical microrobots. Nanometre-scale actuation has to date been mainly achieved by using the (linear) piezoelectric effect in certain classes of crystals (for example, quartz), and 'smart' ceramics such as lead zirconate titanate. But the strains achievable in these materials are small--less than 0.1 per cent--so several alternative materials and approaches have been considered. These include grafted polyglutamates (which have a performance comparable to quartz), silicone elastomers (passive material--the constriction results from the Coulomb attraction of the capacitor electrodes between which the material is sandwiched) and carbon nanotubes (which are slow). High and fast strains of up to 4 per cent within an electric field of 150 MV x m(-1) have been achieved by electrostriction (this means that the strain is proportional to the square of the applied electric field) in an electron-irradiated poly(vinylidene fluoride-trifluoroethylene) copolymer. Here we report a material that shows a further increase in electrostriction by two orders of magnitude: ultrathin (less than 100 nanometres) ferroelectric liquid-crystalline elastomer films that exhibit 4 per cent strain at only 1.5 MV x m(-1). This giant electrostriction was obtained by combining the properties of ferroelectric liquid crystals with those of a polymer network. We expect that these results, which can be completely understood on a molecular level, will open new perspectives for applications.     

垒层p型掺杂量的分布对InGnN基发光二极管性能的影响

采用APSYS软件分析了InGaN基发光二极管的垒层中p型掺杂量的分布及作用.研究结果表明当所有的p型掺杂量集中于最后一个垒层时，发光二极管的发光强度最大，大注入电流下的效率衰减量最小.其主要原因为优化垒层中p型掺杂量的分布有利于电子限制和空穴注入.     

低In组分量子阱垒层AlGaN对GaN基双蓝光波长发光二极管性能的影响

采用数值分析方法对含有低In组分AlGaN垒层InGaN/GaN混合多量子阱双蓝光波长发光二极管进行模拟分析. 结果表明，这种AlGaN量子阱垒层能有效改善电子和空穴在混合多量子阱活性层中的分布均匀性及减少电子溢出，实现电子空穴在各个量子阱中的平衡辐射，从而减弱了双蓝光波长发光二极管的效率衰减. 此外，通过改变AlGaN量子阱垒层的Al组分，可以调控双蓝光波长发光二极管发射光谱的稳定性:当Al组分为0.08时，双蓝光波长发光二极管的光谱在小电流和大电流下都比较稳定，而Al组分为0.09时，光谱只在40~100 mA电流范围内比较稳定.     

Alq3有机发光器件的研制及寿命研究

以铟锡氧化物 (ITO)为透明电极 ,8 羟基喹啉铝 (Alq3)为发光层 ,研制成ITO/Alq3/Al结构有机电致发光器件 (OLED) .测量表明其载流子的注入满足隧穿理论 ,发光阈值电压～ 1 2V ,所发绿光在正常室内环境下清晰可见 .通过电流随时间的变化测量了器件的工作寿命 ,并对影响器件寿命的原因作了分析 .     

Near infrared polymer light-emitting diodes

High efficiency of near infrared polymer light-emitting diodes with bilayer structure was obtained. The diode structure is ITO/PEDOT/LI/L2/Ba/Al, where L1 is phenyl-substituted poly [p-phenylphenylene vinylene] derivative (P-PPV), L2 is 9,9-dioctylfluorene (DOF) and 4,7-bis(3-hexylthiophen)-2-yl-2,1,3-naphthothiadiazole (HDNT) copolymer (PFHDNTI0). The electroluminescence (EL) spectrum of diodes from PFHDNTI0 is at 750 nm located inthe range of near infrared. The maximum external quantum efficiency is up to 2.1% at the current density of 35 mA/m^2. The improvement of the diode‘s performances was considered to be the irradiative excitons confined in the interface between L1 and L2 layers.     

Self-heating dependent characteristic of GaN-based light-emitting diodes with and without AlGaInN electron blocking layer

In this study, GaN-based light-emitting diodes （LEDs） with and without A1GaInN electron blocking layer （EBL） under self-heating effect are numerically studied. The energy band diagram, carrier transport and distribution characteristics, internal Joule heat and non-radiative recombination heat characteristics, and internal quantum efficiency are investigated. The effect of Auger recombination coefficient on efficiency droop under self-heating effect is also studied. The simulation results show that efficiency droop is markedly improved when an AlGaInN EBL is placed between p-type GaN layer and active region. However, the chip temperature of LED is significantly increased simultaneously. The results also indicate that Auger recombination can be neglected because it is not the major contributor for the internal heat source. The efficiency droop is unrelated to the internal heat source. However, both electron leakage and Auger recombination play important roles in efficiency droop mechanism under self-heating effect.     

掺杂的有机薄膜电致发光

用DCM染料和香豆素染料掺杂,成功地实现了有机薄膜电致发光颜色的改变,得到了从蓝绿到红色的发光,同时提高了器件的发光亮度,用不同区域掺杂方法,分析讨论了电致发光的机理,探讨了激子的产生区域和复合区域。