Showing papers by "Andrew B. Holmes published in 1993"

Efficient light-emitting diodes based on polymers with high electron affinities

Abstract: CONJUGATED polymers have been incorporated as active materials into several kinds of electronic device, such as diodes, transistors1 and light-emitting diodes2. The first polymer light-emitting diodes were based on poly(p-phenylene vinylene) (PPV), which is robust and has a readily processible precursor polymer. Electroluminescence in this material is achieved by injection of electrons into the conduction band and holes into the valence band, which capture one another with emission of visible radiation. Efficient injection of electrons has previously required the use of metal electrodes with low work functions, primarily calcium; but this reactive metal presents problems for device stability. Here we report the fabrication of electroluminescent devices using a new family of processible poly(cyanoterephthalylidene)s. As the lowest unoccupied orbitals of these polymers (from which the conduction band is formed) lie at lower energies than those of PPV, electrodes made from stable metals such as aluminium can be used for electron injection. For hole injection, we use indium tin oxide coated with a PPV layer; this helps to localize charge at the interface between the PPV and the new polymer, increasing the efficiency of recombination. In this way, we are able to achieve high internal efficiencies (photons emitted per electrons injected) of up to 4% in these devices.

Chemical tuning of the electronic properties of poly(p-phenylenevinylene)-based copolymers

Abstract: Molecular engineering has allowed the control of the HOMO-LUMO gap energy in a family of conjugated polymers based on poly(p-phenylenevinylene) (PPV). Random copolymerization of 1 and 6 afforded precursor polymers 10 with two different leaving groups. Treatment under thermal conditions led to a partially conjugated polymer 11 with regions of high and low HOMO-LUMO gap energy, showing a 30-fold improvement in electroluminescence efficiency compared with PPV, while treatment under acidic and thermal conditions gave more substantially conjugated polymers 12 with lower HOMO-LUMO gap energies. This chemistry enabled the formation of lithographically patterned polymers exhibiting electroluminescence. The scope of these copolymerizations was also explored with other copolymers, 19 and 22b,c.

Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

Abstract: Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.

Electroluminescent device comprising a chromophoric polymeric composition

Abstract: An electroluminescent device is provided incorporating an emissive layer comprising a processible polymer matrix such as poly(methylmethacrylate) and a chromophoric component such as an asymmetric stilbene or distyrylbenzene The chromophoric component is blended with the polymer matrix or covalently attached thereto as a side chain and is selected to emit radiation in the region 400 nm to 500 nm when excited to luminesce

Large changes in optical response through chemical pre‐ordering of poly(p‐phenylenevinylene)

Abstract: Conjugated polymers such as poly(p-phenylenevinylene), (PPV, see Figure), show real promise in optoelectronic applications A new synthetic route to improved PPV is presented, involving the thermal conversion of a precursor polymer containing rigid rod conjugated segments joined by flexible spacer groups A high degree of interchain ordering results which influences the optical response of the material

Manufacture of electroluminescent devices

Abstract: In an electroluminescent device which comprises a layer of a semiconductive conjugated polymer between positive and negative charge carrier injecting electrodes, a barrier layer is arranged between the layer of semiconductive conjugated polymer and the charge carrier injecting layer for negative charge carriers. The barrier layer protects the layer of semiconductive conjugated polymer from for example mobile ions released by the reactive charge carrier injecting layer. The barrier layer can in some circumstances also itself be light-emissive.

Evolution of the electronic structure in a conjugated polymer series: polyacetylene, poly(p-phenylene), and poly(p-phenylenevinylene)

Abstract: Poly(p-phenylenevinylene) can be loosely viewed as a regular copolymer of polyacetylene and poly(p-phenylene). The electronic structure of polyacetylene, poly(p-phenylene), and poly(p-phenylenevinylene) have been studied using UV photoelectron spectroscopy and quantum chemical calculations based upon the valence effective Hamiltonian method. Excellent agreement between theory and experiment allows for a detailed description of the evolution of the electronic structure in this polymer series

Selective Cleavage of Benzyl Ethers

Abstract: An efficient and convenient method for the removal of benzyl ether protecting groups in the presence of other functionality has been developed. The method is sufficiently mild to allow the removal of trityl groups in the presence of benzyl ethers.

Molecularly Engineered Polymer Leds

Abstract: Polymeric light emitting devices may be fabricated from a simple structure consisting of a low work function cathode (typically calcium or magnesium), a conjugated semiconducting polymer and a transparent anode (typically indium-tin oxide). Optimum device efficiencies require the balanced injection of electrons and holes. This paper describes the application of molecular engineering in the design of a family of poly(cyanoterephthalylidenene)s which show increased electron affinity over the unsubstituted analogue [poly(p-phenylenevinylene) PPV]. In particular these polymers as the emissive layer in a bilayer device with indium tin oxide (ITO, positive transparent contact) and aluminum (stable negative contact) and PPV as a hole transporting layer exhibit internal efficiencies up to 4%.

Conjugated Polymer Light-emitting Diodes

Abstract: 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.

New Results on Metal-Polymer Interfaces

Abstract: New results on studies of the early stages of formation of the aluminum-poly(p−phenylenevinylene) interface are presented.

Electroluminescent devices made with conjugated polymers

Abstract: In this paper we discuss the principles of operation of polymer electroluminescent devices, and identify the factors which limit device efficiency. We identify how efficiencies can be improved by careful control of the polymer system, and by the use of multilayer structures to confine holes within the device. Using these techniques we can achieve efficiencies of better than 1% photons per electron in devices based on poly(p-phenylenevinylene). We also describe the use of induced absorption techniques to identify the excited states present within an electroluminescent device and to estimate their concentrations.