Hendrik-Jan Brouwer

Novel high efficiency copolymer laser dye in the blue wavelength region

The lasing performance and wavelength tunability of a novel high efficiency copolymer poly[(2, 5, 2‘, 5‘‐tetraoctyl)‐p‐terphenyl‐4,4‘‐ylene vinylene‐p‐phenylene vinylene], TOP‐PPV, pumped with third harmonic radiation of a Nd:YAG laser, was studied in various organic solvents. The results were compared with Coumarin 120 and Coumarin 47 in ethanol under identical experimental conditions. The efficiency of the TOP‐PPV copolymer in hexane exceeds that of both coumarin dyes with more than 50%. The laser emission of the polymer dye in hexane is tunable in the wavelength region between 414 and 456 nm (Δλ=38 nm).

Conjugated block copolymers for light-emitting diodes

In this communication we report results obtained in our laboratory on the design, the synthesis, and the characterization of new light-emitting copolymers. We are interested in block copolymers containing well defined conjugated sequences, which give rise to luminescence. Those chromophores, which are (pi) -conjugated oligomers like oligo(phenylenevinylene)s, are linked to each other by non (pi) -conjugated oligomers, for example oligo(silanylene)s, or directly in a non-coplanar way. In such systems color tuning of the luminescence is achieved through precise control of the length of the conjugated blocks and by changing the chemical structure of those blocks. Variations of the chemical structures of both the luminescent and interrupting blocks also give control over properties like flexibility and solubility of the polymers, which are relevant for device fabrication and optimization.

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.

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.

Stimulated emission from vacuum-deposited thin films of a substituted oligo(p-phenylene vinylene)

The influence of the morphology on the light emission properties of vacuum-deposited polycrystalline oligo(p-phenylene vinylene) thin films under intense laser excitation was studied. The morphology of the thin films was varied by annealing just below the first melting point (crystal-mesophase transition) and by crystallization from the isotropic melt. Amplified spontaneous emission occurred within the individual domains and was only observed when the domain size was increased by thermal treatment. The amplified spontaneous emission threshold for the melt-crystallized thin films was found to be comparable to that for conjugated polymer thin films.

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.

LEDs based on conjugated PPV block copolymers

A way to control the bandgap in semi-conducting polymers is by preparing polymers with a partially conjugated backbone. In our laboratory, three conjugated copolymers containing PPV trimers as light emitting chromophores have been synthesized, which emit in the blue, green and orange wavelength region. The copolymers have a well defined conjugated backbone consisting of regularly alternating terphenylene and p-phenylene-vinylene blocks. The desired control of conjugation length is achieved through steric interaction induced by the side-chains in the terphenyl blocks of the copolymer. In this paper we evaluate the electrical and optical properties of LED devices based on these conjugated PPV block copolymers.

Effect of solid-state structure on optical properties of conjugated organic materials

Luminescence from PPV derivatives and related conjugated polymers in the solid state is strongly subject to intermolecular interactions. Model systems of oligomers have been investigated to explore such interactions in more detail. Direct electronic interaction is found in single crystals of a cyano-substituted five-ring oligo(phenylene vinylene), as a result of the π-stacking arrangement of the molecules. Luminescence spectra and PL-decay curves suggest that excimers are formed upon photoexcitation. This model may account for similar observations in films of related polymers. A second example is represented by a three-ring, stilbazole-type molecule, of which thin multilayer films have been obtained by means of the Langmuir-Blodgett technique. In this case, the features of the absorption and luminescence spectra are explained by dipole-(induced) dipole interactions within aggregates.

Stimulated emission from films of conjugated polymers and oligomers

Oligomers and block copolymers structurally related to PPV were investigated under intense optical excitation. Their well-defined molecular structure allows a better control of emission properties than is usually feasible in semiconducting polymers. Stimulated emission is demonstrated in single crystals of PPV-type oligomers, and also in thin films obtained by spin casting of copolymers containing PPV- type blocks. Waveguiding is shown to provide the length of interaction required for mirrorless laser generation. Thin films of oligomers obtained through deposition from the vapor phase are polycrystalline, and the optical losses in the as-deposited films are too large for lasing to be achieved. These films show stimulated emission only after the domain size has been increased by annealing.Lasing occurs within individual crystalline domains with a threshold value comparable to that found for optically clear amorphous films of conjugated polymers.

Preparation of photovoltaic cells from sexithiophene-C-60 blends

Large photovoltaic responses have been recently observed in devices based on conjugated polymer-C60 blends. Their enhanced performance, which relies on the formation of a bicontinuous network of donor-acceptor 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 length. A photovoltaic device with quantum efficiency of 1% is demonstrated.