Graded doped structure fabricated by vacuum spray method to improve the luminance of polymer light-emitting diodes

An increase in luminance of a polymer light-emitting diode(PLED) was obtained by fabricating a graded doping structure using a vacuum spray method. The small electron transport molecule, Tris(8-hydroxyquinolinato) aluminum(III)(Alq3), was graded dispersed along the film in the direction of growth in the hole transport polymer poly(3-hexylthiophene-2,5-diyl)(P3HT, regiorandom) layer of the PLED, despite being dissolved in the same organic solvent as the polymer. The PLED reported here, which is composed of a graded structure, emitted brighter light than PLEDs composed of pure polymer or of a blend of active layers prepared by spin coating and/or vacuum spray methods.

Hole-Buffer Material Derived from Pyrene, Schiff Base and Tris to Enhance Emission Efficiency of Polymer Light-Emitting Diodes

Inserting a hole-buffer layer is an effective way to enhance emission efficiency of electroluminescence devices. We have successfully synthesized a new hole-buffer material PSB composed of pyrene, Schiff base and trihydroxy tert-butyl groups by the Suzuki-coupling reaction. The HOMO and LUMO lev-els were -6.33 eV and -2.55 eV, respectively, as estimated from cyclic volt-ammograms. In addition, homogeneous films (rms roughness ~2 nm) were readily obtained by spin-coating process. Multilayer polymer light-emitting diodes, ITO/PEDOT:PSS/PSB/SY/LiF/Al, have been fabricated using PSB as hole-buffer layer (HBL). Inserting PSB as HBL significantly enhances the per-formance (maximum luminance: 26,439 cd/m2, maximum current efficiency: 7.03 cd/A), compared with the one without PSB (9802 cd/m2, 2.43 cd/A). It is also superior to the device with conventional BCP as hole-blocking layer (ITO/PEDOT:PSS/SY/BCP/LiF/Al: 15,496 cd/m2, 5.56 cd/A). Current results strongly indicate that the PSB is a potential hole-buffer material for electrolu-minescent devices.

Solution Processable Material Derived from Aromatic Triazole, Azomethine and Tris: Preparation and Hole-Buffering Application in Polymer Light-Emitting Diodes

This work presents the synthesis of a new hole-buffering material TAZS and its successful application in polymer light-emitting diodes to enhance device performance. The TAZS is composed of aromatic 1,2,4-triazolylcore linked with three trihydroxy tert-butyl terminals via azomethine linkages. The TAZS forms ashomogeneous film deposited by spin-coating process. The HOMO and LUMO levels of TAZS are -5.23 eV and -2.40 eV, respectively, as estimated from cyclic voltammogram. The current density results of hole-only and electron-only devices confirm strong hole-buffering capability of TAZS layer. Multilayer PLEDs with different thickness of TAZS (ITO/PEDOT: PSS/TAZS (x nm)/SY/ETL/LiF/Al) have been successfully fabricated, using spin-coating process to deposit hole-injecting PEDOT: PSS, TAZS, and emissive SY layers. The PLED with 16 nm TAZS reveals the optimal device performance, with maximum luminance and maximum current efficiency of 19,046 cd/m2 and 4.08 cd/A, respectively, surpassing those without TAZS as HBL (8484 cd/m2, 2.13 cd/A). The hole-buffering characteristic of TAZS contributes greatly to improved charges’ recombination ratio and enhanced emission efficiency.

FLUORENE-BASED LIGHT-EMITTING POLYMERS

Several series of fluorene-based light-emitting polymers with the emphasis on achieving efficient and stable bluelight emission are reported. Spiro-functionalization may narrow the emission spectra (with smaller tail at Ionger wavelengths)of fluorene homopolymers to provide purer blue emission. The thermal spectral stability of the polymers could also beimproved because of the elevation of the glass transition temperature caused by the spiro-functionalization. However, theexcimer emission in fluorene homopolymers is not suppressed by the spiro-functionalization. Alternate copolymers of 9,9-dihexylfluorene and substituted phenylenes may emit efficient blue ligh both in solution and in film. The optical propertiesare dependent on the substituion on the phenylene ring. The alkoxy-substituted polymers displayed efficient PL and EL andgood thermal spectral stability. The HOMO and LUMO energy levels of the polymers based on the backbone structure couldbe tuned in a wide range by attaching different functional groups on the phenylene ring. By attaching europium(III) complexat the ends of the side chains in the alternate copolymers, we have demonstrated a new approach to achieving red emissionwith a very narrow spectrum. The copolymers of 9,9-dihexylfluorene and thiophene and bithiophene with differentsubstitutions were also synthesized to study the effect of substitution and regioregularity on the optical and other physicalproperties of the polymers.

Synthesis of Polyfluorene-Polytriarylamine Block Copolymer with Emitting Part at Junction Point for Light Emitting Applications

A block copolymer consisting of polyfluorene (PF) and polytriarylamine (PTAA) functionalized with green emitting phenoxazine moiety at the junction point of two blocks was designed and prepared for electroluminescent application. PF homopolymer was synthesized by Suzuki coupling polymerization, and was reacted with brominated phenoxazine. In the presence of the resulting PF functionalized with phenoxazine, C-N coupling polymerization of 4-(4’-bromophenyl)-4’’-butyldiphenylamine was carried out to afford a triblock copolymer, PTAA-phenoxazine-PF-phenoxazine-PTAA (PF-Ph-PTAA). Two types of random copolymers were also synthesized with fluorene and phenoxazine (PF2) by Suzuki coupling polymerization for comparison. All the polymers were soluble in common organic solvents and readily formed thin films by a solution processing. Prepared polymers exhibited similar UV absorption and PL emission in chloroform solutions. In a film state, the existence of phenoxazine unit drastically changed PL spectra. Although the content of phenoxazine unit in PF-Ph-PTAA was relatively high (13 mol%), it showed similar PL spectrum to that of PF2(phenoxazine content, 0.2 mol%) indicating that phenoxazine unit is isolated in single polymer chain nevertheless the high content. EL device based on PF-Ph-PTAA showed green-emission, suggesting that emission sites predominantly located in the vicinity of phenoxazine moiety because of its shallow HOMO level.

Alkali metal salts doped pluronic block polymers as electron injection/transport layers for high performance polymer light-emitting diodes

A series of alkali metal salts doped pluronic block copolymer F127 were used as electron injection/transport layers (ETLs) for polymer light-emitting diodes with poly[2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylenevinylene] (P-PPV) as the emission layer. It was found that the electron transport capability of F127 can be effectively enhanced by doping with alkali metal salts. By using Li2CO3 (15%) doped F127 as ETL, the resulting device exhibited improved performance with a maximum luminous efficiency (LE) of 13.59 cd/A and a maximum brightness of 5529 cd/m2, while the device with undoped F127 as ETL only showed a maximum LE of 8.78 cd/A and a maximum brightness of 2952 cd/m2. The effects of the doping concentration, cations and anions of the alkali metal salts on the performance of the resulting devices were investigated. It was found that most of the alkali metal salt dopants can dramatically enhance the electron transport capability of F127 ETL and the performance of the resulting devices was greatly improved.

Germafluorene conjugated copolymer——synthesis and applications in blue-light-emitting diodes and host materials

A germafluorene-fluorene copolymer was successfully obtained via Suzuki polymerization.The ger-manium containing copolymer has an efficient blue light emission under the ultraviolet irradiation and its single layer EL device showed the highest brightness of 2630 cd/m2 at 7.8 V and the highest effi-ciency of 0.301 lm/W at 6.2 V.The copolymer can also serve as the host material for phosphorescent metal complexes with the maximum brightness of 15600 cd/m2 and the quantum efficiency of 8.5%.The results are quite promising and promise that as its analogs of fluorene and silafluorene,germafluorene is an excellent building block for blue light-emitting polymers and host materials.

High-efficiency conjugated-polymer-hosted blue phosphorescent light-emitting diodes

Highly-efficient blue phosphorescent light-emitting diodes were fabricated based on a conjugated-polymer host by doping bis(2-(4,6-difluorophenyl)-pyridinato-N,C2') picolinate(FIrpic) into poly(9,9-dioctylfluorene)(PFO).Previously,conjugated polymers were not considered as potential hosts for blue phosphorescent dyes because of their low-lying triplet energy levels.Energy back transfer would occur and lead to poor luminescent efficiency in both photoluminescence(PL) and electroluminescence(EL) processes.However,by inserting a hole-transporting layer of poly(N-vinylcarbazole)(PVK),the energy back transfer was suppressed.At low FIrpic-doping concentrations,PFO emissions were completely quenched;with 8 wt% FIrpic,a maximum luminous efficiency of 11.5 cd/A was achieved.

Stability study of saturated red polymer light-emitting diodes

Saturated red polymer light-emitting diodes have been fabricated with a single emitting polymer blend layer of poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) and poly[9,9-dioc- tylfluorene-co-4,7-di-2-thienyl-2,1,3-benzothiadiazole] (PFO-DBT15). Saturated red emission with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.67, 0.33) was obtained. The device stability was investigated. The results showed that energy transfer occurred from MEH-PPV to PFO-DBT15, and MEH-PPV improved the hole injection and transportation.

Efficient polymer light-emitting diodes with violet blue emission based on blends of PSiF6-PPP and PSiFC6C6

有效聚合物有紫蓝排放的轻射出的二极管(PLEDs ) 用 paraphenylene-co-silafluorene (PSiF6-PPP ) 和聚合物的共聚物的混合被制作 poly (9,9''-alkyl-3,6-silafluorene )(PSiFC6C6 ) 。设备的表演对混合比率敏感。当到 PS1FC6C6 的 PS1F6-PPP 的集体比率是 1 时：3，最高的外部量效率在 105 cd-m~ 的发光性是 1.96%(-2), 它的电镀物品光(EL ) 光谱在 398 nm 达到顶点，在半最大值的完整的宽度是 67 nm。设备表演的改进由于从 PSiFC6C6 的精力转移到 PS1F6-PPP 和电子和洞的平衡注射。

Triphenyl Phosphine Oxide and Carbazole-based Polymer Host Materials for Green Phosphorescent Organic Light-emitting Diodes

Four novel polymers, poly（3,6-9-decyl-carbazole-alt-1,3-benzene） （PB13CZ）, poly（3,6-9-decyl-carbazole-alt- bis（4-phenyl） （phenyl） phosphine oxide） （PTPPO38CZ）, poly（3,6-9-decyl-carbazole-alt-2,4-phenyl（diphenyl） phosphine oxide） （PTPPO13CZ） and poly（3,6-9-decyl-carbazole-alt-bis（3-phenyl） （phenyl） phosphine oxide） （PTTPO27CZ） were synthesized, and their thermal, photophysical properties and device applications were further investigated to correlate the chemical structures with the photoelectric performance of bipolar host materials for phosphorescent organic light emitting diodes. All of them show high thermal stability as revealed by their high glass transition temperatures and thermal decomposition temperatures at 5% weight loss. These polymers have wide band gaps and relatively high triplet energy levels. As a result, the spin coating method was used to prepare the green phosphorescent organic light emitting diodes with polymers PTPPO38CZ, PTPPO13CZ and PTTPO27CZ as the typical host materials. The green device of polymer PTPPO38CZ as host material shows electroluminescent performance with maximum current efficiency of 2.16 cd.A-1, maximum external quantum efficiency of 0.7%, maximum brightness of 1475 cd.m-2 and reduced efficiency roll-off of 7.14% at 600 cd.m-2, which are much better than those of the same devices hosted by polymers PTTPO27CZ and PTPPO13CZ.

Polyfluorene-Polytriarylamine Block Copolymer as an Additive for Electroluminescent Devices Based on Polymer Blends

Electroluminescent characteristics were investigated for the blue emitting devices fabricated with the blend systems consisting of hole transporting polytriarylamine (PTAA), electron transporting polyfluorene (PF), and a block copolymer with both segments (PF-b-PTAA) as an active layer in order to elucidate the relationship between the chemical nature and morphology of the active layer, and EL performance. The addition of PF-b-PTAA to PF homopolymer afforded the hole injecting and/or electron blocking ability to increase the efficiency. The addition to PF/PTAA blend keeping the chemical composition constant also improved the performance by controlling the morphology and/or the domain size in phase-separated films.

To inhibit coffee ring effect in inkjet printing of light-emitting polymer films by decreasing capillary force

Polymer thin film with uniform thickness and flat surface profile is the key point for polymer light emitting diodes(PLEDs) by inkjet printing. However, the coffee ring effect is usually observed due to the mismatch between the evaporation of the solvent and the decrease of solution volume, which promotes the formation of radial flow from the interior of the drop to the edge. In this paper, coffee ring effects of inkjet printed poly(spirobifluorene) films were proposed to be restrained by decreasing capillary force by adding co-solvent with high boiling point and high viscosity to the main solvent. The low evaporation rate of the co-solvent can reduce the driving force of the radial flow; meanwhile the high viscosity of the co-solvent can increase the resistance of the radial flow. Thus, polymer films with improve uniformity can be obtained due to the suppression of the radial flow. The device performance was greatly improved under the condition of proper film thickness and film uniformity and the maximum luminous efficiency of devices with inkjet printed poly(spirobifluorene) can reach 80% of the spin-coated devices.

Hyperbranched polymer-cored star polyfluorenes as blue light-emitting materials

Hyperbranched polymer-cored star polyfluorenes with high molecular weights and narrow molecular weight distribution were prepared by palladium-catalyzed one-pot Suzuki polycondensation of multi- functional cores and an AB-type monomer. The optical, electrochemical and thermal properties of the hyperbranched polymer-cored star polymers were investigated. These polymers exhibited good ther- mal and color stability in solid state, and there was no significant blue-green emission after the poly- mers had been annealed in air for 2.5 h. Their three-dimensional hyperbranched structures could ef- fectively reduce the aggregation of the peripheral rigid linear conjugated polyfluorene chains.

New applications of poly(arylene ether)s in organic light-emitting diodes

Compared with conventional π-conjugated polymers,poly（arylene ether）s（PAEs） may take advantages of excellent thermal properties,well-defined effective conjugated length and no catalyst contamination.Recently,their applications have been extended from engineering plastics to optoelectronic materials.In this review,various kinds of functional PAEs used as fluorescent polymers,host polymers and phosphorescent polymers in organic light-emitting diodes（OLEDs） are outlined,and their molecular design,synthesis and device performance are overviewed.

High efficient light-emitting diodes using polystyrene as matrix

High efficient polymer light-emitting diodes(PLEDs) were obtained by using a blend of conjugatedpolymer G-PF, a copolymer of fluorene and thiophene, andpolystyrene (PS). The maximum electroluminescent (EL)efficiency of the device is 12 cd/A when G-PF/PS weight ratiois at 80/20, while that of pure G-PF device is 6.5 cd/A. Studieson photoluminescence and electroluminescence of the blendsindicate that inter-chain interactions were tremendouslysuppressed due to the dilution effect. However, after PS con-centration exceeds 20% the EL efficiency of the devices de-creases with further increase of PS concentration. This maybe due to the decrease of the recombination probability ofelectrons and holes with the excessive addition of PS insulator.

一步法合成固态荧光氮掺杂碳化聚合物点及多色LEDs的构建

以柠檬酸、尿素和丙烯腈为原料,乙醇为溶剂,通过一步溶剂热法制备了氮掺杂碳化聚合物点(NCPDs),其粒径分布范围约为12~22 nm.NCPDs粉末不仅可发射黄色荧光(λem=570 nm),而且荧光强度和发射峰位置具有显著的浓度依赖性.将NCPDs分散在硅溶胶中制备成NCPDs质量分数为0.15%~45%的NCPDs/硅溶胶复合薄膜,该复合材料的固态荧光发射波长随NCPDs的浓度增加逐渐从蓝色红移至红色(453~638 nm),且具有良好的稳定性.将NCPDs应用于发光二极管(LEDs),可构建蓝光到红光的多色LEDs.此外,NCPDs优异的固态荧光性能使其在指纹识别领域展示出潜在的应用前景,因此评估了其在指纹识别中的适用性.

基于PEDOT:PSS导电聚合物的透明电极制备及其光电性能研究

通过掺杂改性,在玻璃和柔性塑料衬底上采用旋涂法制备了高导电性和高透明性的PEDOT:PSS薄膜。然后以此为基础,研究了PEDOT:PSS为阳极的绿光OLED标准器件和黄光电致磷光器件性能。以CBP掺杂磷光材料(MPPZ)2Ir(acac)为发光层制备了柔性和平面OLED器件,考察了以ITO、PEDOT:PSS/玻璃、PEDOT:PSS/PET三种不同阳极器件的性能。实验结果表明,以PEDOT:PSS/玻璃阳极的器件启动电压为3.83 V,最大亮度可达18632 cd/m^(2),最大电流效率可达21.61 cd/A,显示了PEDOT:PSS透明导电薄膜作为OLED阳极材料具有很大的发展潜力。

聚乙烯吡咯烷酮链段长度依赖的钙钛矿结晶动力学和电致发光研究

基于低温溶液工艺的钙钛矿发光二极管由于低成本,高色纯度,宽色域等优势受到广泛关注。然而,在溶液结晶成膜过程中,钙钛矿晶粒无规形核、快速生长,导致薄膜存在大量的针孔和缺陷,严重影响器件性能。本文通过原位荧光和结构表征技术证实,聚乙烯吡咯烷酮(PVP)添加剂在特定的链段长度下,能优化调控钙钛矿结晶动力学,延缓晶粒生长速度,使得钙钛矿薄膜粗糙度由1.489 nm降低到0.954 nm,缺陷密度从1.55×10^(18)cm^(-3)降低至1.05×10^(18)cm^(-3)。相应地,薄膜荧光量子产率从16.9%提升到58.9%,器件外量子效率从8.55%提高到了18.00%。本文通过原位表征技术,全方位解析了添加剂分子结构对钙钛矿结晶动力学的调控机理,为添加剂分子的优化设计提供了直观依据。

Optimization of top polymer gratings to improve GaN LEDs light transmission

We present a grating model of two-dimensional （2D） rigorous coupled wave analysis （RCWA） to study top diffraction gratings on light-emitting diodes （LEDs）. We compare the integrated-transmission of the non-grating, rectangular-grating, and triangular-grating cases for the same grating period of 6 μm, and show that the triangular grating has the best performance. For the triangular grating with 6-μm period, the LED achieves the highest light transmission at 6-μm grating bottom width and 2.9-μm grating depth. Compared with the non-grating case, the optimized light transmission improvement is about 74.6%. The simulation agrees with the experimental data of the thin polymer grating encapsulated flip-chip （FC） GaN-based LEDs for the light extraction improvement.