有机电致发光材料的能隙计算

有机电致发光材料具有可进行分子结构设计而改变其溶解性及光电性能等特点,以此为功能层、将电能转换为光能的有机电致发光二极管更是具备轻薄、高效、自发光、低能耗以及低成本可实现柔性器件等潜在优势。因此,研究合成高性能的有机电致发光材料并应用于制备可实用化的显示器及照明产品成了业界研究的热点。文中综述了以能带理论为基础,对有机电致发光材料进行结构设计改进发光性能时对将合成的材料的能隙计算的理论模型与计算方法及其应用进展,可望为聚合物发光材料的合成设计提供理论支持。     

Blue electroluminescence from oxadiazole grafted poly(phenylene-ethynylene)s

Blue poly(phenylene-ethynylene) (PPE) electroluminescence is achieved in a single layer organic light emitting device. The polymeric system consists of an oxadiazole grafted PPE, which combines the necessary charge transport properties while maintaining the desirable efficient, narrow light-emitting properties of the PPE. Incorporation of a pentiptycene scaffold within the PPE structure prevents ground-state and excited-state interactions between the pendent oxadiazole units and the conjugated backbone.     

Thermal annealing time effect on the performance of ambipolar organic light-emitting transistors based on conjugated polymer blends

Here we report the effect of thermal annealing time on the performance of ambipolar organic light-emitting transistors (OLETs) made using conjugated polymer blends. Regioregular poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorenyl-2,3-diyl-co-1,4-benzo-2,1',3-thiadiazole) (F8BT) were chosen as a p-type and a n-type component, respectively. As a gate insulator, poly(4,4'-oxydiphenylene-pyromellitimide) (PMDA-ODA PI) was employed due to its high solvent resistance and thermal stability. Results showed that the present OLETs exhibited ambipolar characteristics even after thermal annealing. All devices showed almost identical field-effect mobility for both holes and electrons. The highest field-effect mobility was achieved for the OLET annealed at 130 degrees C for 60 min, which was assigned to the improved polymer-metal contact by thermal annealing leading to better charge injection.     

Cyclodextrin-threaded conjugated polyrotaxanes as insulated molecular wires with reduced interstrand interactions

Control of intermolecular interactions is crucial to the exploitation of molecular semiconductors for both organic electronics and the viable manipulation and incorporation of single molecules into nano-engineered devices. Here we explore the properties of a class of materials that are engineered at a supramolecular level by threading a conjugated macromolecule, such as poly(para-phenylene), poly(4,4'-diphenylene vinylene) or polyfluorene through alpha- or beta-cyclodextrin rings, so as to reduce intermolecular interactions and solid-state packing effects that red-shift and partially quench the luminescence. Our approach preserves the fundamental semiconducting properties of the conjugated wires, and is effective at both increasing the photoluminescence efficiency and blue-shifting the emission of the conjugated cores, in the solid state, while still allowing charge-transport. We used the polymers to prepare single-layer light-emitting diodes with Ca and Al cathodes, and observed blue and green emission. The reduced tendency for polymer chains to aggregate allows solution-processing of individual polyrotaxane wires onto substrates, as revealed by scanning force microscopy.     

Organic/polymeric electroluminescent devices processed by hybrid ink-jet printing

Ink-jet printing (IJP) technology is a popular technology for desktop publishing. Since some of the conducting (or conjugated) organic molecules and polymers are solution processable, IJP becomes an ideal method for printing polymer/organic light-emitting diodes with high resolution. In this review article, we present the hybrid ink-jet printing technology (HIJP), which consists of an ink-jet printed layer in conjunction with another uniform spin-coated polymer layer, which serves as a buffer layer to seal the pin holes between the ink droplets. This HIJP technology has been successfully applied to the fabrication of polymer light-emitting logos, multicolor polymer/organic light-emitting diodes, and the built-in shadow mask for the cathode patterning for pixelated polymer LEDs.     

A hybrid light source with integrated inorganic light-emitting diode and organic polymer distributed feedback grating

We report a compact light source that incorporates a semiconductor light-emitting diode, nanostructured distributed feedback (DFB) Bragg grating and spin-coated thin conjugated polymer film. With this hybrid structure, we transferred electrically generated 390?nm ultraviolet light to an organic polymer via optical pumping and out-couple green luminescence to air through a second-order DFB grating. We demonstrate the feasibility of electrically driven, hybrid, compact light-emitting devices and lasers in the visible range.     

Patterning of light-emitting conjugated polymer nanofibres

Organic materials have revolutionized optoelectronics by their processability, flexibility and low cost, with application to light-emitting devices for full-colour screens, solar cells and lasers. Some low-dimensional organic semiconductor structures exhibit properties resembling those of inorganics, such as polarized emission and enhanced electroluminescence. One-dimensional metallic, III-V and II-VI nanostructures have also been the subject of intense investigation as building blocks for nanoelectronics and photonics. Given that one-dimensional polymer nanostructures, such as polymer nanofibres, are compatible with sub-micrometre patterning capability and electromagnetic confinement within subwavelength volumes, they can offer the benefits of organic light sources to nanoscale optics. Here we report on the optical properties of fully conjugated, electrospun polymer nanofibres. We assess their waveguiding performance and emission tuneability in the whole visible range. We demonstrate the enhancement of the fibre forward emission through imprinting periodic nanostructures using room-temperature nanoimprint lithography, and investigate the angular dispersion of differently polarized emitted light.     

A novel quasicrystal-resin composite for stereolithography

Laser stereolithography (SL) is an additive manufacturing technology which is increasingly being used to produce customized end-user parts of any complex shape. It requires the use of a photo-curable resin which can be loaded with ceramic powders or carbon fibers to produce composite parts. However, the range of available materials compatible with the SL process is rather limited. In particular, photo-curable resins reinforced by metal particles are difficult to process, because of fundamental limitations related to the high reflectivity of intermetallics in the UV–visible range. In this work, the unique properties of Al-based quasicrystalline alloys are being used to develop a new UV-curable resin reinforced by such metal particles. The optical properties of the quasicrystalline particles and of the filled resin are studied and they are found to be compatible with the SL process. The volume fraction of the filler particles in the liquid resin is optimized to increase the polymerization depth while preserving suitable rheological behaviour. Finally, 3D composite parts are being built by SL. The composite parts have improved mechanical properties compared to the unfilled resin (higher hardness, reduced wear losses and lower friction coefficient) and compete favourably with the other commercial photo-curable resins.     

A bright future for organic field-effect transistors

Field-effect transistors are emerging as useful device structures for efficient light generation from a variety of materials, including inorganic semiconductors, carbon nanotubes and organic thin films. In particular, organic light-emitting field-effect transistors are a new class of electro-optical devices that could provide a novel architecture to address open questions concerning charge-carrier recombination and light emission in organic materials. These devices have potential applications in optical communication systems, advanced display technology, solid-state lighting and electrically pumped organic lasers. Here, recent advances and future prospects of light-emitting field-effect transistors are explored, with particular emphasis on organic semiconductors and the role played by the material properties, device features and the active layer structure in determining the device performances.     

Linear and cyclic porphyrin hexamers as near-infrared emitters in organic light-emitting diodes

Here we report organic light-emitting diodes incorporating linear and cyclic porphyrin hexamers which have red-shifted emission (λ(PL) = 873 and 920 nm, respectively) compared to single porphyrin rings as a consequence of their extended π-conjugation. We studied the photoluminescence and electroluminescence of blends with poly(9,9'-dioctylfluorene-alt-benzothiadiazole), demonstrating a high photoluminescence quantum efficiency of 7.7% for the linear hexamer when using additives to prevent aggregation and achieving high color purity near-infrared electroluminescence.     

量子点材料应用于发光二极管的研究进展

量子点材料因具有独特的光学特性而被广泛应用于发光领域,用其作发光层可制成量子点发光二极管。与有机电致发光二极管相比,量子点发光二极管具有发光光谱窄、色域广、稳定性好、寿命长、制作成本低等优势。本文介绍了量子点发光器件在国内外的热点研究方向及取得的成果,并对其发展前景进行展望。     

Efficient charge carrier transfer from m-LPPP to C60 derivatives

The improvement of dissociation of singlet excitons by electron or hole capturing species like C₆₀ is essential for efficient photovoltaic cells based on conjugated systems. We investigate the charge transfer in blends of methyl-substituted ladder type poly-(paraphenylene) (m-LPPP) with new well soluble C₆₀ derivatives, which are characterized by sidegroups of different electronic properties (donor, acceptor nature). It is shown, that the electronic properties of the C₆₀ sidegroups strongly influence the efficiency of the charge transfer in these blends and hence significant different behaviours in the photocurrent and photoinduced absorption is observed.     

Carbon as an electronics material

Carbon is as natural an electronic material as silicon, and it has also the advantages that come from the multiplicity of forms available. We concentrate on the properties and potential uses of carbon in nanotubes, polymers and composites of the two. Unlike silicon, organic molecules can produce light of almost any wavelength and also multiple wavelengths including white. It is likely to be used initially for LCD backplanes. Nanotubes have shown potential for use in cold cathodes. Progress towards using them in displays is slow. However, commercial use in microwave resonators seems to be imminent. There has been substantial progress in understanding the use of conjugated polymers in photovoltaics, where polymer composites with nanotubes or blends of organic semiconductors promise to continue progress to greater efficiency and cost effectiveness. The understanding of fundamental processes has yet to develop to a point where it is of direct value to electronics. Simplifications based on semi-classical device physics appear to be able to short circuit these difficulties.     

Influence of relative humidity on the nanoscopic topography and dielectric constant of thin films of PPy:PSS

The morphological, electric, and dielectric properties of water-based conjugated polymer blends, such as polypyrrole:polystyrene sulfonate (PPy:PSS) or poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), are known to be influenced by the water content. These properties also influence the macroscopic performance when the conjugated polymer blends are employed in a device. An in situ humidity-dependence study on thin films of PPy:PSS by Kelvin probe force microscopy (KPFM) is presented. A particular KPFM mode, dielectric imaging, is used, which detects the second harmonic electrostatic force. Thin PPy:PSS films are drop-coated on hydrophobic graphite substrates. Upon increasing the relative humidity, the hydrophilic PSS is swelled and dewetted on the substrate, while the hydrophobic PPy remains almost unchanged. The swelling and dewetting of PSS results in irreversible morphological changes in the thin films, as well as nanoscopic rearrangement on the surface of the PPy:PSS films. The nanoscopic rearrangement can only be detected by dielectric imaging. It is also observed that relative humidity affects unannealed and thermally annealed PPy:PSS thin films differently.     

Synthesis and characterization of fluorescent poly(oxadiazole)s containing 3,4-dialkoxythiophenes

We report the synthesis, optical and electrochemical details, and properties of three conjugated copolymers (P1–P3) consisting of alternate hole-transporting thiophene derivative and electron-transporting 1,3,4-oxadiazole units. The polymers are prepared using the precursor polyhydrazide route. The polymers have well defined structure and exhibit good thermal stability with the onset decomposition temperature in nitrogen at around 300 °C. All the polymers are soluble in organic solvents such as DMF, DMSO, NMP and in strong organic acids like trifluoroacetic acid. The optical and charge-transporting properties of the polymers are investigated by UV–visible spectroscopy, fluorescence emission spectroscopy and cyclic voltammetry. The UV–visible absorption spectra of polymers in solution showed a maximum at around 400 nm. The polymers depicted green fluorescence both in solution and as thin films. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from −3.28 to −3.36 eV and high-lying HOMO energy levels ranging from −5.5 to −5.56 eV, which indicated that they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes.     

Enhanced Vertical Charge Transport of Homo- and Blended Semiconducting Polymers by Nanoconfinement

The morphology of conjugated polymers has critical influences on electronic and optical properties of optoelectronic devices. Even though lots of techniques and methods are suggested to control the morphology of polymers, very few studies have been performed inducing high charge transport along out-of-plane direction. In this study, the self-assembly of homo- and blended conjugated polymers which are confined in nanostructures is utilized. The resulting structures lead to high charge mobility along vertical direction for both homo- and blended conjugated polymers. Both semicrystalline and amorphous polymers show highly increased population of face-on crystallite despite intrinsic crystallinity of polymers. They result in more than two orders of magnitude enhanced charge mobility along vertical direction revealed by nanoscale conductive scanning force microscopy and macroscale IV characteristic measurements. Moreover, blends of semicrystalline and amorphous polymers, which are known to show inferior optical and electrical properties due to their structural incompatibility, are formed into harmonious states by this approach. Assembly of blends of semicrystalline and amorphous polymers under nanoconfinement shows charge mobility in out-of-plane direction of 0.73 cm² V⁻¹ s⁻¹ with wide range of absorption wavelength from 300 to 750 nm demonstrating the synergistic effects of two different polymers.     

PC-g-PS增容PC/PS共混体系的相容性和性能

研究了不同配比的ＰＣ／ＰＳ共混体系的物理和力学性能及其与相容性的关系,结果表明,ＰＳ含量越高,拉伸强度、冲击强度、密度和吸水率等越低,而表面硬度和耐溶剂开裂能力则提高。通过ＤＳＣ谱、ＳＥＭ等实验结果表明,少量的辐射接枝共聚物ＰＣ－ｇ－ＰＳ加入ＰＣ／ＰＳ共混体系,即能明显改善其相容性。加了共聚物以作为增容剂后,共混物的拉伸强度、冲击强度、密度、表面硬度和耐溶剂开裂能力均要比其相应的简单共混物的值来得高,而吸水率则有所下降。     

Self-assembly of highly ordered conjugated polymer aggregates with long-range energy transfer

Applications of conjugated polymers (CP) in organic electronic devices such as light-emitting diodes and solar cells depend critically on the nature of electronic energy transport in these materials. Single-molecule spectroscopy has revealed their fundamental properties with molecular detail, and recent reports suggest that energy transport in single CP chains can extend over extraordinarily long distances of up to 75 nm. An important question arises as to whether these characteristics are sustained when CP chains agglomerate into a neat solid. Here, we demonstrate that the electronic energy transport in aggregates composed of tens of polymer chains takes place on a similar distance scale as that in single chains. A recently developed molecular-level understanding of solvent vapour annealing has allowed us to develop a technique to control the CP agglomeration process. Aggregates with volumes of at least 45,000 nm(3) (molecular weight ≈ 21 MDa) maintain a highly ordered morphology and show pronounced fluorescence blinking behaviour, indicative of substantially long-range energy transport. Our findings provide a new lens through which the ordering of single CP chains and the evolution of their morphological and optoelectronic properties can be observed, which will ultimately enable the rational design of improved CP-based devices.     

Hybrid organic-inorganic light-emitting diodes

The demonstration of colour tunability and high efficiency has brought organic light-emitting diodes (OLEDs) into the displays and lighting market. However, high production costs due to expensive deposition techniques and the use of reactive materials still limit their market entry, highlighting the need for novel concepts. This has driven the research towards the integration of both organic and inorganic materials into devices that benefit from their respective peculiar properties. The most representative example of this tendency is the application of metal oxides in organic optoelectronics. Metal oxides combine properties such as high transparency, good electrical conductivities, tuneable morphology, and the possibility of deposition on large areas with low-cost techniques. The use of metal oxides as charge injection interfaces in OLEDs has also been investigated. Hybrid organic-inorganic light-emitting diodes (HyLEDs) are inverted OLEDs that employ air-stable metal oxides as the charge injection contacts. They are emerging as a potential competitor to standard OLEDs, thanks to their intrinsic air stable electrodes and solution processability, which could result in low-cost, large area, light-emitting devices. This article reviews the short history of this class of devices from its first solid state example published in 2006 to the present achievements. The data presented shed light on the electronic mechanism behind the functioning of HyLEDs and give guidelines for their further optimization.     

Dielectric properties depending on frequency in organic light-emitting diodes

Dielectric properties of organic light-emitting diodes were investigated using ITO/Alq₃/Al device. In spite of various advantages in organic light-emitting diodes, a fundamental study on physical properties is not yet sufficient. Dielectric properties are used for studying fundamental physical properties of materials through a frequency-dependent response. We have investigated magnitude and phase of impedance, electrical conductivity, and the dielectric loss depending on a bias-voltage variation using ITO/Alq₃(60 nm)/Al device. The device shows a frequency-dependent response such that a major contribution is resistive below time constant and capacitive above time constant. Also, the device shows a voltage-dependent electrical conductivity in low-frequency region. A bulk resistance rapidly decreases as the frequency increases above 1 MHz. The dielectric loss shows that there appears an interfacial polarization in low-frequency region, and an orientational polarization in high-frequency region.