P.F van Hutten

Sexithiophene-C60 blends as model systems for photovoltaic devices

Large photovoltaic responses have been recently observed in devices based on donor-acceptor blends. Their enhanced performance, which relies on the formation of a bicontinuous network of internal heterojunctions, is very sensitive to the morphology of the blend. In this paper, we propose sexithiophene-C60 blends, prepared by coevaporation, as a model system for the study of these devices. Preliminary results suggest a microphase-separated morphology with a domain size comparable to the exciton diffusion range. Photovoltaic cells based on the blends are demonstrated.

The plastic deformation of ultra-high molecular weight polyethylene

Gel-spun filaments of different initial morphologies have been subjected to controlled drawing at elevated temperatures. The drawn samples have been examined by high-resolution scanning electron microscopy. The deformation mechanism at temperatures up to 120° C is very similar to crazing, especially in the case of unoriented gel-spun filaments. Filaments exhibiting a shish-kebab morphology offer the opportunity of examining the deformation of elementary fibrils in a quantitative way. The transformation of individual lamellae into fibrils is the initial deformation mode, which is followed by slip of fibrils at a later stage. This is concluded from a comparison of experimental data and model calculations of the maximum draw ratio. Drawing at 144° C results in the formation of globular aggregates of lamellae, with a characteristic long period of 40 nm. This long period persists until all the globules have been converted, by micronecking, into aggregate fibrils of extended-chain character. On a molecular scale, the various processes can be described as the temperature-dependent flow behaviour of an entanglement network.

Excimer luminescence from single crystals and films of a cyano-substituted phenylene-vinylene model compound

Abstract In the context of luminescence from the solid state of poly( para -phenylene vinylene)-type materials, we report a study of three octyloxy-substituted five-ring oligo( para -phenylene vinylene)s, two of which bear cyano substituents on the innermost vinylene bonds. For each compound, the molecular arrangement in the single crystal has been derived from a crystallographic analysis. For the compound which has the cyano group not directly adjacent to the central ring, synthesis and structure are described here in detail. This compound, in contrast to the other two, has molecular π -stacking in the crystal lattice. Only for this compound do we observe a strongly red-shifted, long-lived emission, which is therefore ascribed to an excimer-type intermolecular excitation. The observations of excimer-type emission from cyano-substituted alkoxy–PPVs and in related copolymers fit in very well with the molecular picture derived from this study of model compounds.

Model compounds for light-emitting PPV's: Optical and structural data of substituted oligomers

Phenylene-vinylene oligomers are investigated as model compounds for the luminescent moiety of PPV-type homopolymers or copolymers having oligo(phenylenevinylene) units. Some of the rings carry substituents for improved solubility (as in PPV derivatives) and cyano groups have been substituted at various positions on the vinylene linkages. For several compounds single crystals have been obtained and their structure has been resolved, which allows the spectral response to be related to molecular conformation and packing. Thin film spectra demonstrate that cyano substitution results in a considerable red shift of the emission: a more planar conformation, π-stacking and, possibly, dipolar interactions cooperate with the electronic effect of the cyano group. The use of a cyano-substituted five-ring compound in a multilayer LED was found to strongly increase the luminescence efficiency.

A comparison between the morphology of semicrystalline polymer blends of poly(ε-caprolactone)/poly(vinyl methyl ether) and poly(ε-caprolactone)/(styrene-acrylonitrile)

The morphology of polymer blends of poly(epsilon-caprolactone) (PCL) and poly(vinyl methyl ether) (PVME) is compared with that of PCL and a random copolymer of styrene and acrylonitrile (SAN). The main objective is to determine the influence of the glass transition temperature of the amorphous component (T-g,T-a) on the morphology of the semicrystalline polymer blends. These blends represent the two extreme cases corresponding to T-c T-g,T-a, where T-c is the crystallization temperature. The morphology of these blends, with PVME and SAN representing the amorphous components, have been studied by small angle X-ray scattering. For both blends the long period increases with the addition of amorphous polymer, which is a strong indication for an interlamellar morphology. D.s.c. experiments, including enthalpy relaxation, are used to investigate the crystallinity and the interphases. The overall amount of crystallinity in both blends decreases with increasing content of amorphous polymer. However, the fraction of PCL that crystallizes decreases in PCL/SAN and increases slightly in PCL/PVME. Apparently, the addition of the low T-g,T-a PVME improves the crystallization of PCL in accordance with a simple Gamblers Ruin Model type argument. The high T-g,T-a of SAN means this does not occur in PCL/SAN blends. Conventional d.s.c. experiments show an interphase of pure amorphous PCL in PCL/SAN blends and enthalpy relaxation experiments demonstrate its presence in PCL/PVME blends as well.

Optical properties of single crystals and vacuum-deposited thin films of a substituted oligo(p-phenylene vinylene)

Abstract The optical properties and structural organization of a 5-ring n-octyloxy-substituted oligo(p-phenylene vinylene) have been investigated in the solid state. For this study solution-grown single crystals and vacuum-deposited thin films were used. The unit cell of the solution-grown single crystals was determined to be monoclinic, space group I2 a . The as-deposited oligomer films showed a polycrystalline structure. Annealing the film at a temperature of 120°C resulted in a morphological change, which manifested itself in an increase of crystalline domain size from approximately 4 μm to 20 μm. Both single crystals and thin films exhibit a high photoluminescence quantum yield of 0.5 and 0.7, respectively. In order to determine the relative importance of oxygen and water on photoluminescence quenching, we measured the luminescence decay of a vacuum-deposited film exposed to water and air. It was found that quenching is mainly due to contamination with oxygen from air rather than water and that this contamination is reversible to a great extent. We demonstrate that under intense laser excitation it is possible to obtain stimulated emission from both single crystals and thin films. Non-optimized single-layer light-emitting diodes prepared from the oligomer showed uniform green/yellow light emission with external electroluminescence efficiencies up to 0.15% using Ca as electron-injecting electrode. The electroluminescence efficiency is drastically increased by annealing.

Structural studies on thin films of an unsubstituted oligo(para-phenylenevinylene)

Abstract Typically, thin films of the conjugated oligomer p-bis[(p-styryl)styryl]benzene (P5V4) prepared by vacuum deposition on substrates like silicon wafers and quartz plates are polycrystalline. Atomic force and electron microscopy reveal elongated clusters with characteristic dimensions of 20×80 nm2. X-ray and electron diffraction (ED) measurements indicate a close correspondence between the crystal structure of these micro-crystallites and the reported structure of single crystals grown from solution. However, we argue on the basis of ED measurements that P5V4 micro-crystallites found in thin films vapor-deposited on amorphous carbon substrates adopt a morphology that differs from the single-crystal structure. In the proposed polymorph, the molecular axis is nearly perpendicular to the substrate plane, though the molecular arrangement is otherwise very similar to that in the single crystal. We suggest that several polymorphs may coexist in the thin films.

Organic Conductors and Semiconductors, Structure and Morphology of

Organic conductors and semi-conductors are a special class of polymers which may possess interesting optoelectronic properties. They are conjugated polymers with their structures containing at least one backbone chain of alternating double and single bonds (conjugated double bonds). In this article, the dependence of the properties of conjugated polymers on their structure has been reviewed.

POLYMER PHOTONIC MATERIALS: PHYSICAL AND CHEMICAL ASPECTS

This paper describes research activities aimed at developing polymer materials that will support and promote the transition from computing and communication based on electricity to optical handling of data streams. It deals with chemical synthesis of polymers possessing a well-defined, functional basic structure, and with physical characterization at the (sub)molecular, supramolecular and macroscopic levels, including surfaces and interfaces. The emphasis is on light-emission properties.