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Order and structure in conjugated polymers for device applications

Objective


In terms of synthetic chemistry, a whole range of novel materials has been produced during the project. A series of PPV-based systems, including precursor PPV homopolymer and copolymers, plus soluble PPV homopolymers and copolymers have been synthesised (CDT, UCAM-C, MPIP-M). Ladder polymers have been made, as well as perylene-fluorene copolymers, polyindenofluorenes and polyfluorenes (MPIP-M). A series of side-chain chromophore polymers have been produced (TH-CSF). Di-block (rod-coil) and tri-block co-polymers have also been synthesised, using polyfluorene as the luminescent rod. Twisted fluorene analogue monomers have been prepared, and copolymers with fluorene synthesised, which exhibit blue fluorescence.
Modelling has been performed on interchain interactions, the electronic structure of precursor PPV copolymers, oxygen interaction with PPV, heterojunctions, and the electronic structure of polyfluorene and copolymers.
Polymers synthesised have been tested in LED and photodiode structures. Improvements in LED efficiency have been achieved by blending emitters with hole transport polymers and by the use of low workfunction cathode materials. Triplet-harvesting LEDs have also been successfully produced.
The conversion process of precursor PPV has been studied by XPS/UPS, as well as the band alignments and interactions of polymers with cathode materials. Homopolymer, copolymer and block copolymer thin films have been investigated by AFM techniques.
Microcavity LEDs have been successfully fabricated, using controlled doping of polymer layers to act as both charge injecting electrode and optical mirror.
Objectives and content
The electronic properties of conjugated polymers are
important, since these materials can be used in a range
of semiconductor devices such as light-emitting diodes
(LEDs), photodiodes and transistors. Conjugated polymers
have the advantages of easy control of the semiconducting
properties through chemical modification, and ease of
processing over large areas, leading to major potential
cost savings in device manufacture. LEDs based on
conjugated polymers are being actively developed for
commercial application in large-area emissive devices by
a number of companies Lifetimes of several thousand hours
have been achieved, and these figures continue to
improve.
Many important scientific questions remain about the
nature of the electronic states in conjugated polymers.
In particular, the role of interactions between chains in
determining the electronic properties is a matter of
intense debate. Inter-chain interactions have a major
impact on the operation of devices, since they not only
determine the transport properties, but can also
determine the spectrum and efficiency of emission in
LEDs, and the charge separation process in photodiodes.
This basic research programme combines synthesis,
characterisation, modelling and device fabrication to
investigate and control the effects of ordering and
structure on conjugated polymers. The programme aims to
achieve control three length regimes, ranging from the
nanometre scale, through the "meso" scale (10- 100 nm),
to the optical scale ( > 100 nm).
In the nanometre regime, the aim is to synthesise stable
materials with high luminescence efficiencies by control
of the intra-molecular and inter-molecular structure.
These materials will be characterised using a range of
techniques, and assessed for performance in LED devices.
The interfaces of these materials with electrode
materials, vital to the performance in LEDs and
photodiodes, will also be studied. In the meso regime,
the aim is to use polymer mixtures and block copolymers
to form phase separated materials. These materials will
be assessed in a range of devices, including:
photodiodes, where charge separation occurs at the
interface between two phases; light-emitting ionconducting devices, where the incorporation of an ionconducting phase allows low voltage emission to be
obtained; and light-emitting diodes, where control of
exciton transfer allows the emission colour to be tuned.
Materials capable of providing polarised emission will
also be investigated. In the optical regime, the aim is
to control the emission properties of polymer devices
using microcavity structures. Optical microcavities will
be fabricated with emission throughout the visible range,
and these structures will be incorporated into
microcavity LEDs. LEDs operating at high excitation
intensities under short pulse operation will be
developed, and the prospects for electrically driven
microcavity lasers will be assessed.
The consortium comprises 6 groups with a range of interdisciplinary research experience. Improved materials and
materials processing will be transferred via the two
industrial partners to industrial development,
particularly of polymer LEDs.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

THE CHANCELLOR, MASTERS AND SCHOLARS OF THE UNIVERSITY OF CAMBRIDGE
Address
Cavendish Laboratory, Madingley Road
CB3 0HE Cambridge
United Kingdom

Participants (5)

CAMBRIDGE DISPLAY TECHNOLOGY LTD
United Kingdom
Address
181A,greenwich House, Madingley Rise, Madingley Road
CB3 0HJ Cambridge
Linkfping University
Sweden
Address

581 83 Linkfping
MAX-PLANCK-GESELLSCHAFT ZUR FOERDERUNG DER WISSENSCHAFTEN E.V.
Germany
Address
Ackermannweg 10
55021 Mainz
Thomson-CSF
France
Address
Domaine De Corbeville
91404 Gometz La Ville
UNIVERSITE DE MONS HAINAUT
Belgium
Address
20,Place Du Parc 20
7000 Mons-bergen