R.H. Friend

Polymer electroluminesence in the near infra-red

Abstract We report the fabrication of polymer light emitting diodes from a cyano-substituted thienylene phenylene vinylene copolymer. Photoluminescence and electroluminescence studies of these devices show a broad emission spectrum which peaks in the near infra-red and extends out to a wavelength of 1000nm. Bilayer structures using poly(p-phenylene vinylene) as a hole transport layer and the cyano-copolymer as the emissive layer showed improved electroluminescence internal quantum efficiency of up to 0.2%..

Exciton dissociation at a poly(p-phenylenevinylene)/C60 heterojunction

Abstract We report measurements of the photovoltaic effect in heterojunction structures consisting of a layer of C 60 thermally evaporated onto a film of the conjugated polymer poly( p -phenylenevinylene) (PPV) sandwiched between electrodes of indium-tin oxide and Al. Peak quantum efficiencies of up to about 9% (electrons collected per incident photon) were measured under short-circuit conditions, with corresponding open-circuit voltages approaching 0.9 V. Increased quantum yields were obtained under forward and reverse biases. We model the response as arising from the dissociation of photogenerated excitons at the PPV/C 60 interface, with electron transfer to the C 60 . The C 60 layer acts as a dielectric spacer, providing a constructive enhancement of the optical fields at the PPV/C 60 interface. We have applied this model to estimate an exciton diffusion range of 60 to 80 A.

The effects of H2O and O2 on the photocurrent spectra of MEH-PPV

Abstract Photocurrent (PC) and current-voltage measurements were taken on MEH-PPV [poly(2-methoxy-5-(2′-ethyl-hexyloxy)-paraphenylene vinylene] in an as-made photocell, ITO / MEH-PPV (265 nm) / Al, in the photocell under ambient air, under a mixture of oxygen + nitrogen, and in vacuum after annealing. We discuss the different spectral responses and current-voltage characteristics obtained for the different atmospheres.

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.

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.

Synthesis and characterisation of doped and undoped poly(2,5-dimethoxy phenylene vinylene)

Abstract Conjugated polymers exhibit large changes in their optical and electronic properties upon either n- or p- type doping. We report details of the synthesis of poly(2,5-dimethoxyphenylene vinylene)s (DMPPV)s, prepared from sulphonium precursor polymers with a range of counter-anions, and optical measurements of their electronic transitions and infrared active vibrations. We have observed that the DMPPV produced when the sulphonium polyelectrolyte precursor polymer contained SbF 6 − as the counterion shows sub band-gap optical absorption bands. This was observed to take place during thermal conversion from the precursor to the conjugated polymer under acidic conditions.

Chemical control of colour and electroluminescent device efficiency in copolymeric poly(arylenevinylenes)

We report here that copolymeric poly(arylenevinylenes) have considerable advantages over their corresponding homopolymers in electroluminescent devices. We focus on the organic chemistry aspects of the design and properties of poly(p-phenylenevinylene)-based copolymer derivatives, especially poly(p-phenylenevinylene)/poly(2,5-dimethoxy-p-phenylenevinylene) copolymers and partially converted poly(p-phenylenevinylene).