Polymer light‐emitting diode prepared with an ionomer and polyaniline

Light‐emitting diodes of poly[2‐methoxy‐5‐(2′‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) have been fabricated from sodium sulphonated polystyrene (SSPS) ionomer as an electron‐injecting and hole‐blocking material and polyaniline doped with camphor sulphonic acid (PANI‐CSA) as a polymer anode. The presence of PANI in an ITO/PANI/MEH‐PPV/AI device makes little difference to its optical output and quantum efficiency. SSPS shows good electron injection in an ITO/MEH‐PPV/SSPS/AI device. When SSPS ionomer is introduced in an ITO/PANI/MEH‐PPV/SSPS/A1 device, a larger enhancement in optical output is obtained and its quantum efficiency is also enhanced because of its balanced charge injection. The electroluminescent device associated with SSPS ionomer and PANI is a well‐designed structure to improve stability and efficiency. Copyright © 2000 John Wiley & Sons, Ltd.     

Anodic and Cathodic Electrochemically Generated Chemiluminescence in Conjugated Polymers

In the present contribution, we show that long‐lived electrochemically generated chemiluminescence (ECL) from conjugated polymers can be achieved in both anodic and cathodic polarizations of poly[2‐(2′‐ethylethoxy)‐5‐methoxy‐1,4‐phenylene vinylene] (MEH–PPV), when the appropriate electrochemical conditions are adopted and an adequate degree of purity for the active polymer, solvent, and supporting electrolyte is achieved. The quantum efficiency is generally higher during the anodic polarization of MEH–PPV, whereas the ECL vs. time profile depends on the nature of the supporting electrolyte. Such findings led to the conclusion that the kinetics of the doping/undoping processes in MEH–PPV represents a crucial factor in determining the emissive properties of the conjugated polymer. A comparison of the ECL emission from analogous polymeric systems, namely poly[2,5‐bis‐(triethoxymethoxy)‐1,4‐phenylene vinylene] (BTEM–PPV) and poly[2,3‐dibutoxy‐1,4‐phenylene vinylene] (DB–PPV), is also reported.     

Controlled Pattern Formation of Poly[2‐methoxy‐5‐(2′‐ethylhexyloxyl)–1,4‐phenylenevinylene] (MEH–PPV) by Ink‐Jet Printing

A detailed survey on the processing of poly[2‐methoxy‐5‐(2′‐ethylhexyloxyl)–1,4‐phenylenevinylene] (MEH–PPV) solutions via ink‐jet printing and the subsequent characterization of the resulting films is reported. The printability of MEH–PPV dissolved in different solvents, and with varied concentrations, is studied. Limitations of the printability of highly concentrated polymer solutions are overcome by using ultrasonication. The pattern formation of the resulting lines is explained in relation to the contact angle formed by the MEH–PPV solution on the substrate and interchain interactions. A uniform thickness distribution of MEH–PPV films is obtained when toluene is used as the solvent. Further improvement on the surface quality of the lines is achieved by optimizing the printing parameters. The line stability as a function of the print‐head velocity is also studied. Additionally, current–voltage (I–V) characteristics and the morphology of the MEH–PPV films, as determined by atomic force microscopy, are discussed.     

Stabilization of Semiconducting Polymers with Silsesquioxane

Polyhedral oligomeric silsesquioxanes (POSS) anchored to poly(2‐methoxy‐5‐(2‐ethylhexyloxy)‐1.4‐phenylenevinylene) (MEH‐PPV) (MEH‐PPV–POSS), and to poly(9,9‐dihexylfluorenyl‐2,7‐diyl) (PFO) (PFO–POSS) were synthesized. Compared with the corresponding parent polymers, MEH‐PPV and PFO, MEH‐PPV–POSS and PFO–POSS have better thermal stability. MEH‐PPV–POSS and MEH‐PPV have identical absorption and photoluminescent (PL) spectra, both in solution and as thin films. They also have identical electroluminescent (EL) spectra. Devices made from MEH‐PPV–POSS exhibit higher brightness (1320 cd m^–2 at 3.5 V) and higher external quantum efficiency (η_ext = 2.2 % photons per electron) compared to MEH‐PPV (230 cd m^–2 at 3.5 V and η_ext = 1.5 % ph el^–1). Compared with PFO in the same device configuration, PFO–POSS has improved blue EL emission and higher η_ext.     

Triplet Exciton and Polaron Dynamics in Phosphorescent Dye Blended Polymer Photovoltaic Devices

The triplet exciton and polaron dynamics in phosphorescent dye (PtOEP) blended polymer (MEH‐PPV) photovoltaic devices are investigated by quasi‐steady‐state photo‐induced absorption (PIA) spectroscopy. According to the low‐temperature PIA and photoluminescence (PL) results, the increase in strength of the triplet‐triplet (T₁‐T_n) absorption of MEH‐PPV in the blend system originates from the triplet‐triplet energy transfer from PtOEP to MEH‐PPV. The PtOEP blended MEH‐PPV/C₆₀ bilayer photovoltaic device shows a roughly 30%–40% enhancement in photocurrent and power‐conversion efficiency compared to the device without PtOEP. However, in contrast to the bilayer device results, the bulk heterojunction photovoltaic devices do not show a noticeable change in photocurrent and power‐conversion efficiency in the presence of PtOEP. The PIA intensity, originating from the polaron state, is only slightly higher (within the experimental error), indicating that carrier generation in the bulk heterojunction is not enhanced in the presence of PtOEP. The rate and probability of the exciton dissociation between PtOEP and PCBM is much faster and higher than that of the triplet‐triplet energy transfer between PtOEP and MEH‐PPV.     

Laterally Ordered Bulk Heterojunction of Conjugated Polymers: Nanoskiving a Jelly Roll

This paper describes the fabrication of a nanostructured heterojunction of two conjugated polymers by a three‐step process: i) spin‐coating a multilayered film of the two polymers, ii) rolling the film into a cylinder (a “jelly roll”) and iii) sectioning the film perpendicular to the axis of the roll with an ultramicrotome (nanoskiving). The conjugated polymers are poly(benzimidazobenzophenanthroline ladder) (BBL, n‐type) and poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV, p‐type). The procedure produces sections with an interdigitated junction of the two polymers. The spacing between the phases is determined by spin‐coating (～15 nm to 100 nm) and the thickness of each section is determined by the ultramicrotome (100 to 1000 nm). The minimum width of the MEH‐PPV layers accessible with this technique (～15 nm) is close to reported exciton diffusion lengths for the polymer. When placed in a junction between two electrodes with asymmetric work functions (tin‐doped indium oxide (ITO) coated with poly(3,4‐ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS), and eutectic gallium‐indium, EGaIn) the heterostructures exhibit a photovoltaic response under white light, although the efficiency of conversion of optical to electrical energy is low. Selective excitation of BBL with red light confirms that the photovoltaic effect is the result of photoinduced charge transfer between BBL and MEH‐PPV.     

Loading Fullerene into a Conjugated Polymer Without Chemical Modification

Conjugated polymers doped with fullerenes show excellent photoconversion efficiencies. However, due to the poor solubility of pure C₆₀ in common organic solvents, it was thought that some chemical modifications to C₆₀ were required in order to load conjugated polymers with comparable weights of fullerene. Here, we report a novel way to load large amounts of unmodified C₆₀ fullerene into the conjugated polymer poly(2‐methoxy‐5‐(2′‐ethylhexoxy)‐1,4‐phenylenevinylene), MEH‐PPV. The principle of our approach is to separate the film‐deposition stage from the material‐solidification stage. That is, materials are quickly solidified from dilute solution into colloidal particles, which are large enough to be collected by electrophoretic deposition. Deposition from a mixture of separate suspensions of MEH‐PPV and C₆₀ fullerene resulted in films consisting of isolated particles. In contrast, by using a suspension made of a dilute solution of MEH‐PPV and C₆₀, which was so dilute that traditional techniques such as spin‐coating were not applicable, an MEH‐PPV film doped with 20 mol‐% C₆₀, corresponding to an almost 1:1 ratio by weight of molecularly dispersed C₆₀, was easily obtained. To the best of our knowledge, this is the first report on the successful loading of an equivalent weight ratio of unmodified C₆₀ molecules into MEH‐PPV.     

Micro‐Nanostructured Interfaces Fabricated by the Use of Inorganic Block Copolymer Micellar Monolayers as Negative Resist for Electron‐Beam Lithography

This paper introduces an approach where the match of two different length scales, i.e., pattern from self‐assembly of block copolymer micelles (< 100 nm) and electron‐beam (e‐beam) writing (> 50 nm), allow the grouping of nanometer‐sized gold clusters in very small numbers in even aperiodic pattern and separation of these groups at length scales that are not accessible by pure self‐assembly. Thus, we could demonstrate the grouping of Au nanoclusters in different geometries such as squares, rings, or spheres.     

Controllable Molecular Doping and Charge Transport in Solution‐Processed Polymer Semiconducting Layers

Here, controlled p‐type doping of poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐p‐phenylene vinylene) (MEH‐PPV) deposited from solution using tetrafluoro‐tetracyanoquinodimethane (F4‐TCNQ) as a dopant is presented. By using a co‐solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current–voltage characteristics of MEH‐PPV‐based hole‐only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4‐TCNQ dopant per 600 repeat units of MEH‐PPV leads to a free carrier density of 4 × 10²² m^?3. Neglecting the density‐dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4‐TCNQ doped MEH‐PPV and a silver electrode.     

Solvation‐Induced Morphology Effects on the Performance of Polymer‐Based Photovoltaic Devices

Polymer‐based photovoltaic devices have been fabricated by blending the conjugated polymer, poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV) with the buckminsterfullerene, C₆₀. The photo‐induced current and the open‐circuit voltage show a strong dependence on the polymer processing conditions. It was found that the photovoltaic devices fabricated with tetrahydrofuran or chloroform (non‐aromatic solvents) have smaller photocurrents under same reverse bias as well as higher open circuit voltages than the devices fabricated with xylene, dichlorobenzene, or chlorobenzene (aromatic solvents). The device performance dependence on the processing solvent is attributed to the different solvation‐induced polymer morphology.     

Polymeric Alkoxy PBD [2‐(4‐Biphenylyl)‐5‐Phenyl‐1,3,4‐Oxadiazole] for Light‐Emitting Diodes

The syntheses are reported of the title polymeric alkoxyPBD derivative 5 and the dipyridyl analogue 12 using Suzuki coupling reactions of 1,4‐dialkoxybenzene‐2,5‐diboronic acid with 2,5‐bis(4‐bromophenyl)‐1,3,5‐oxadiazole, and its dipyridyl analogue, respectively. Thermal gravimetric analysis shows that polymers 5 and 12 are stable up to 370 °C and 334 °C, respectively. Films of polymer 5 spun from chloroform solution show an absorption at λ_max = 367 nm, and a weaker band at 312 nm, and strong blue photoluminescence at λ_max = 444 nm. The photoluminescence quantum yield (PLQY) was found to be 27 ± 3 %. For polymer 12 , the absorption spectra reveal bands of equal intensity at λ_max = 374 and 312 nm, with PL at λ_max = 475 nm. Device studies using polymer 12 were hampered by its instability under illumination and/or electrical excitation. Polymer 5 is stable under these conditions and acts as an efficient electron‐transporting/hole‐blocking layer. For devices of configuration ITO/PEDOT/MEH‐PPV/polymer 5 /Al an external quantum efficiency of 0.26 % and brightness of 800 cd/m² was readily achieved: orange emission was observed, identical to the MEH‐PPV electroluminescence.     

Fibrillar Constructs from Multilevel Hierarchical Self‐Assembly of Discotic and Calamitic Supramolecular Motifs

We here report on polymeric solid‐state self‐assembly leading to organization over six length scales, ranging from the molecular scale up to the macroscopic length scale. We combine several concepts, i.e., rod‐like helical and disc‐like liquid crystallinity, block copolymer self‐assembly, DNA‐like interactions to form an ionic polypeptide–nucleotide complex and packing frustration to construct mesoscale fibrils. Ionic complexation of anionic deoxyguanosine monophosphate (dGMP) and triblock coil–rod–coil copolypeptides is used with cationic end blocks and a helical rod‐like midblock. The guanines undergo Hoogsteen pairing to form supramolecular discs, they π‐stack into columns that self‐assemble into hexagonal arrays that are controlled by the end blocks. Packing frustration between the helical rods from the block copolymer midblock and the discotic motif limits the lateral growth of the assembly thus affording mesoscale fibrils, which in turn, form an open fibrillar network. The concepts suggest new rational methodologies to construct structures on multiple length scales in order to tune polymer properties.     

MEH—PPV夹层结构的430nm蓝色电致发光

本文首次报道了不同于MEH－PPV本征发光的蓝色发光。我们制备了以SiO2为加速层、MEH－PPV为发光层的有机／无机复合电致发光器件,用交流电压驱动时,除MEH－PPV的本征发光外,还得到了430nm的蓝色发光。我们认为430nm的蓝色发光是由被SiO2加速的电子直接碰撞MEH－PPV所致。     

Efficient Infrared Electroluminescent Devices Using Solution‐Processed Colloidal Quantum Dots

We report efficient electroluminescence in the near‐infrared from PbS–MEH‐PPV (poly(2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene)) large‐area, solution‐cast nanocomposite devices. We employ multivariate optimization of the structural and materials components that govern the radiative, energy‐transfer, and bipolar‐injection efficiencies into the devices. As a result, we report an external electroluminescence quantum efficiency of 0.27 %, which corresponds to an internal electroluminescence quantum efficiency of 1.9 %. The very best devices exhibit internal‐radiative‐efficiency‐limited performance and not transport‐ or capture‐limited performance, indicating that further gains in efficiency may be achieved if the internal radiative efficiency of the nanocrystal–polymer composite can be further increased without compromising transfer and device bipolar‐injection efficiency.     

Impedance Spectroscopy of π‐Conjugated Organic Materials

AC electrical properties of organic light‐emitting diodes with poly(2‐methoxy‐5‐(2'‐ethyl‐hexyloxy)‐1,4‐phenylenevinylene) (MEH‐PPV), poly[2,5‐bia(dimethyloctylsilyl)‐1,4‐phenylene‐vinylene] (BDMOS‐PPV), and tris‐(8‐hydroxyquinolate)‐aluminum (AlQ₃) as light‐emitting materials are studied. The frequency‐dependent real and imaginary parts of impedance were fitted using an equivalent circuit. We found that the conduction mechanism is a space‐charge limited current with exponential trap distribution.     

High‐Nanofiller‐Content Graphene Oxide–Polymer Nanocomposites via Vacuum‐Assisted Self‐Assembly

Highly ordered, homogeneous polymer nanocomposites of layered graphene oxide are prepared using a vacuum‐assisted self‐assembly (VASA) technique. In VASA, all components (nanofiller and polymer) are pre‐mixed prior to assembly under a flow, making it compatible with either hydrophilic poly(vinyl alcohol) (PVA) or hydrophobic poly(methyl methacrylate) (PMMA) for the preparation of composites with over 50 wt% filler. This process is complimentary to layer‐by‐layer assembly, where the assembling components are required to interact strongly (e.g., via Coulombic attraction). The nanosheets within the VASA‐assembled composites exhibit a high degree of order with tunable intersheet spacing, depending on the polymer content. Graphene oxide–PVA nanocomposites, prepared from water, exhibit greatly improved modulus values in comparison to films of either pure PVA or pure graphene oxide. Modulus values for graphene oxide–PMMA nanocomposites, prepared from dimethylformamide, are intermediate to those of the pure components. The differences in structure, modulus, and strength can be attributed to the gallery composition, specifically the hydrogen bonding ability of the intercalating species     

一种新型可溶性PPV的电致发光特性的研究

合成了一种新型聚对苯撑乙稀（PPV）衍生物聚{1,6－已烷二氧代－（2－甲基－1,4－亚苯基）－氰代－[2－甲氧基－5－正幸氧基苯－1,4－二甲基]}－3－甲基－1,4－亚苯基。利用光电子能谱及UV－vis吸收光谱确定了其HOMO和LUMO能级,在此基础上,研制了结构为ITO／Polymer/Al的单层器件,起亮电压4V,电致发光峰值540nm。     

Hybrid Solar Cells Using a Zinc Oxide Precursor and a Conjugated Polymer

We describe a new method towards bulk‐heterojunction hybrid polymer solar cells based on composite films of zinc oxide (ZnO) and a conjugated polymer poly[2‐methoxy‐5‐(3′,7′‐dimethyloctyloxy)‐1,4‐phenylene vinylene] (MDMO‐PPV). Spin‐coating diethylzinc as a ZnO precursor and MDMO‐PPV from a common solvent at 40 % humidity and annealing at 110 °C provides films in which crystalline ZnO is found to be intimately mixed with MDMO‐PPV. Photoluminescence and photoinduced spectroscopy demonstrate that photoexcitation of these hybrid composite films results in a fast and long‐lived charge transfer from the polymer as a donor to ZnO as ato be obtained n acceptor. Using the ZnO‐precursor method, hybrid polymer solar cells have been made with an estimated air‐mass of 1.5 (AM 1.5) energy conversion efficiency of 1.1 %. This new method represents a fivefold improved performance compared to similar hybrid polymer solar cells based on amorphous TiO₂.     

Non‐Covalent Functionalization of Graphene Using Self‐Assembly of Alkane‐Amines

A simple, versatile method for non‐covalent functionalization of graphene based on solution‐phase assembly of alkane‐amine layers is presented. Second‐order Møller–Plesset (MP2) perturbation theory on a cluster model (methylamine on pyrene) yields a binding energy of ≈220 meV for the amine–graphene interaction, which is strong enough to enable formation of a stable aminodecane layer at room temperature. Atomistic molecular dynamics simulations on an assembly of 1‐aminodecane molecules indicate that a self‐assembled monolayer can form, with the alkane chains oriented perpendicular to the graphene basal plane. The calculated monolayer height (≈1.7 nm) is in good agreement with atomic force microscopy data acquired for graphene functionalized with 1‐aminodecane, which yield a continuous layer with mean thickness ≈1.7 nm, albeit with some island defects. Raman data also confirm that self‐assembly of alkane‐amines is a non‐covalent process, i.e., it does not perturb the sp² hybridization of the graphene. Passivation and adsorbate n‐doping of graphene field‐effect devices using 1‐aminodecane, as well as high‐density binding of plasmonic metal nanoparticles and seeded atomic layer deposition of inorganic dielectrics using 1,10‐diaminodecane are also reported.