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Conjugated Polymers

Semiconductors are compounds whose electrical conductivity is midway between that of metals and that of insulating materials. The conductivity of inorganic semiconductors, such as silicon, is due to the introduction of charge into extended band states either by thermal excitation or by doping of the crystal structure with donor/acceptor atoms. Almost all organic solids are insulators. However, when their constituent atoms are bonded together in a special way known as π-conjugation, charge can be transported through these molecules. In the case of π-conjugated polymers, it can be seen that the p z orbitals of the constituent atoms are all parallel aligned, allowing charge delocalisation via the electron cloud overlaps (see figure). This gives rise to semiconducting properties in organic conjugated polymers.

There are many strong points of conjugated polymers, such as easy fabrication, mechanical flexibility, and low cost. Most notably the light emission properties of these polymers are very attractive, as they are readily available in a wide range of emitting colours, unlike their inorganic counterparts. The price of these flexible emissive properties is that these materials typically possess relatively poor charge transport capabilities, in comparison with inorganic semiconductors. Consequently one of the aims of this project is to combine the attractive properties of inorganic and organic semiconductors in hybrid devices.

Polymers in Diode Array Devices

One type of hybrid device is where an inorganic Gallium Nitride based device, which is emitting in the ultra violet portion of the spectrum, is used to optically pump an organic polymer layer. A schematic is shown here, where a UV micro-diode array is aligned beneath a conjugated polymer layer. The conjugated polymer is sandwiched between layers of a cross-linked epoxy polymer, which has been developed in this project specifically for high UV transparency. The polymer layers are prepared by the well established technique of spin casting. One of the great advantages of this epoxy polymer is that it is a solventless system, which allows us to fabricate complicated multilayer structures incorporating the conjugated polymer.

Shown beneath the schematic is an example of an array device. Similar such devices can readily be made using red and green emitting polymers.

Energy Transfer to Conjugated Polymers

Another type of hybrid device is where energy from the emissive states in the inorganic material is transferred to the organic material, without emission of intermediate photons. This occurs by the phenomenon known as Forster Resonant Energy Transfer (FRET). When the emission spectrum of the inorganic material strongly overlaps the absorption spectrum of the organic material and the two materials are in close proximity it is possible for energy to be transferred by means of FRET.

A schematic of a device utilising FRET is shown here. InGaN quantum wells are grown with capping layers as thin as 2nm. On top of this device a thin layer of conjugated polymer is prepared by spin casting. We have demonstrated devices where the efficiency of energy transfer from the quantum well to the conjugated polymer is up to 20 times greater than that where the polymer is photo-pumped alone. This presents us with the exciting prospect of fabricating hybrid devices which are highly efficient, and also incorporate the attractive emissive properties of conjugated polymers.

The Strathclyde Polymer Chemistry group focuses on delivering material solutions to the various project groups within the consortium. The advent of sub 350nm solid-state light sources has had important implications in terms of material science. Due to their molecular structure most polymer systems inherently absorb strongly in the ultra violet region.

Traditionally, inorganic materials such as quartz, diamond or sapphire have fulfilled the requirements of optics within this region. However, these materials are expensive to buy, form and integrate on the micro/nano level. The issue of integration is also difficult in thermally cured systems, where alignment over micro light emitting sources is problematic.

It is a desirable objective therefore to develop polymeric materials that have effective transmission and processing characteristics. Photo initiated polymerisation would provide greater flexibility than what can be achieved with inorganic or thermally processed material.

Several polymer systems have been developed which allow us to control many key attributes such as viscosity, miscibility, surface functionality and developing characteristics. All of the polymer systems are photo initiated whereby the primary photo event is the photolysis and degradation of a photo acid generator. The photo acid, which is the main absorbing component, degrades and bleaches to leave a highly transparent material down to 230nm.

The flexibility of the materials has allowed the systems to be processed using reactive ion etching, ink jet printing, electro hydrodynamic patterning and projection lithography techniques such as direct laser writing and two photon lithography. Also, the polymers are able to self align over the active optical elements.

The systems can also blend with a series of light emitting polymers, LEP’s, which allows the patterning of the polymers using standard lithographic techniques. This also allows self-alignment over the UV sources for down conversion.

The materials generated also exhibit a high degree of surface functionality and chemical resistance. Therefore, the materials have been used to attach fluorescently tagged oligionucleotides for bio analytical applications.