Arno Kraft

Poly(p‐phenylenevinylene) light‐emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement

We have fabricated light‐emitting diodes with poly(p‐phenylenevinylene) as the emissive layer, and with an electron‐transporting layer formed from a solid state dispersion of 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low‐voltage regime and an eightfold improvement in the high‐voltage regime over devices without the electron‐transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron‐transport layer is to confine holes to the emissive layer.

Electroluminescence from multilayer conjugated polymer devices: Spatial control of exciton formation and emission

Abstract We have constructed electroluminescent diodes using several layers of conjugated polymers with differing band gaps; these provide a range of different colour light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene, PPV, and derivatives have been used, with indium/tin oxide as hole-injecting layer and calcium as electron-injecting contact layer. For this selection of materials, we show that the ordering of the polymer layers allows control of the colour of device emission. Emission can be produced in more than one layer.

PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE IN CONJUGATED POLYMERIC SYSTEMS

The basic working principles and construction of electroluminescent polymer devices are described. The opportunities for combining creative synthetic chemistry and imaginative device physics to address issues of colour, efficiency, and control of processing of semiconducting polymeric materials are reviewed. New advances in the construction of multilayer devices and controlled syntheses of poly(p-phenylenevinylene) are reported.

Conjugated Polymer Light-emitting Diodes

It is now established that conjugated polymers can be used to provide charge transport and to act as the emissive layer in thin-film light-emitting diodes (LEDs). The operation of these devices provides important information about the semiconductor physics of these materials. We discuss here the progress made in the design, fabrication and measurement of these devices, and in the understanding of the basic properties that determine device performance.

Synthesis of a polyphenylene light-emitting polymer

Abstract A new poly(phenylene- co -dialkoxyphenylene) derivative was synthesized by the Suzuki coupling of aromatic bromoboronic acids. This showed strong violet photoluminescence (peak 415 nm) in the solid state and good solubility in organic solvents. A multilayer light-emitting diode was constructed with the structure indium-tin oxide/poly( p -phenylenevinylene)/polyphenylene/calcium. Efficient light emission was observed with a peak maximum in the blue region of the visible spectrum.

The synthesis and characterisation of some poly(2,5-dialkoxy-1,4-phenylene vinylene)s

We report here the synthesis of some new poly(2,5-dialkoxy-1,4-phenylenevinylene)s. By varying the length of the side chain of one of the alkoxy groups, polymers with similar physical but different rheological characteristics were produced. If a short side chain was incorporated, a brittle polymer resulted; a longer side chain resulted in a softer polymer, but one that produced better quality thin films.

Hole-transporting compounds for multi-layer polymer light-emitting diodes

Polymerisation of two mono-substituted triphenylamines gave 4a and 4b. Their use as a hole-transporting layer in poly(p-phenylenevinylene) electroluminescent devices resulted in a decrease of device efficiency compared to devices without the additional layer.

Blue-shifted electroluminescence from a stable precursor to poly(p-phenylene vinylene)

A partially conjugated polymer precursor to poly(p-phenylene vinylene) (PPV) is used to produce stable electroluminescence which enhanced efficiency over PPV in a simple device configuration with a blue-shift in both the electroluminescence and photoluminescence emission spectra

Extended π-conjugation in poly(p-phenylenevinylene) from a chemically modified precursor polymer

We have developed a synthetic method that allows a controlled increase in the one-dimensional character of poly(p-phenylenevinylene) by subtle chemical modification of the precursor polymer. Examples of the unoriented fully conjugated material exhibit room temperature optical absorption spectra (uncorrected for reflectivity) which have a peak in absorption at 2.45 eV with subsidiary shoulders at higher photon energies. The absorption maximum is red-shifted with respect to previously observed values. Luminescence spectra of these samples recorded at 10K are complementary with a peak in emission at 2.38 eV and accompanying subsidiary maxima at lower energies. The peaks in absorption and emission are identified as the transitions between the vibrational ground states of the electronic and the first excited singlet states which we term the (0,0) transitions.

The fabrication and assessment of optical waveguides in poly (p-phenylenevinylene/poly (2,5-dimethoxy-p-phenylenevinylene) copolymer

Abstract Optical waveguides have been fabricated using a copolymer of poly( p -phenylenevinylene) PPV and poly(2,5-dimethoxy- p -phenylenevinylene) PDMeOPV via a precursor route. Controlled conversion of the precursor copolymer can give partially conjugated higher band gap or fully conjugated lower band gap regions, and the difference in refractive index is used to define diffused-channel waveguides. The spatial resolution of the process is limited only by the diffusion of acid around the capping layer, and by the lithography. We have made lines in 0.15μm thick films as narrow as 1μm and have demonstrated optical waveguiding in 4μm wide lines. Slab waveguides and diffused-channel waveguides have been assessed by prism and end-fire coupling at 633nm and 780nm. The anisotropy in the refractive index, and optical propagation losses have been measured.