Fixing the odds: Measuring electrical randomness to improve organic electronic devices

Electronic devices are ubiquitous in modern life. Transistors allow computers of all kinds to process information; a laser is at the heart of every CD and DVD player to read the information on the disk; solar panels convert the light produced by the sun directly into useful electrical energy; while light-emitting diodes are finding increasing use in lighting and displays of all kinds. Despite the diversity of the types of electronic devices and their uses, they are almost universally made from inorganic semiconductors. Inorganic materials require energy-intensive processing and careful handling during a production process that itself often results in toxic by-products. Organic materials, by contrast, offer ways of making electronic devices that are potentially much less energy-intensive and cheap. The organic materials of particular note are so-called 'conjugated polymers', that is, strings of largely carbon-containing molecules that have been specially designed to carry the electrons that constitute an electrical current. The key advantage of these polymers is that one can form an 'ink' by dissolving them in organic solvents and then manufacturing electronic devices using cheap, scalable roll-to-roll printing techniques like you would print a newspaper.

The problem with organic materials that is currently prohibiting their widespread use is the speed with which electrons can navigate their way through the material. If one had a powerful enough microscope, the electronic device made of polymers would look like a tangled mess of cooked spaghetti - with each sphagetti strand representing the backbone of a polymer chain. Electrons trying to move through this tangle can find it hard to move over the kinks of the polymer chain, and between chains. These sluggish electrons harm the performance of organic electronic devices to, instead levels below which they cannot find widespread commercial use. In order to remedy this we must understand the link between performance of the device and its structure, i.e. how the tangles of spaghetti are arranged. Defining this structure-performance relationship is especially difficult because, like in a bowl of cooked spaghetti, the arrangement of polymer chains is different everywhere. This randomness in the positions and orientations of the chains leads to randomness in the behaviours of the electrons. While this randomness in structure is well known, there are few techniques that specifically measure the electrical randomness that results, and exploit the information it contains.

In this proposal, we will pioneer the use of measurements of randomness in electrical current, or noise as it is sometimes called, produced by organic electronic devices. Noise can be understood quite well by the analogy of watching cars pass you whilst waiting at a bus stop - where electrons are the cars and the current is the number of cars that pass you in a given time. If you were to count the number of cars that pass you in successive minute intervals you will get a range of car flow rates. This fluctuation in the rate of cars passing you depends to some extent on the cars' history, i.e. what route they have taken. It is very similar for the case of electrons and current, only here the fluctuations in current inform you about the polymer chains the electrons have had to travel through en-route. For example if the current fluctuates quickly, it means the processes that are controlling the current also fluctuate quickly. Hence noise provides detailed information about what processes determine the electrical current. In the proposed research we will apply this powerful measurement to link the randomness in the behaviour of electrons with the structure of organic devices. In doing this we can make links between how the polymers and devices are made and their eventual performance, allowing chemists and device engineers to make better informed decisions, and enabling organic devices to fulfil their promise.

This proposal lies in the rapidly growing area of organic electronics. The key drivers for the growth in this industry stem from the prospect of reduced cost and new functionality compared to traditional inorganic devices. As such, organic electronics may establish new paradigms in renewable energy (organic photovoltaics) and display technology (organic light-emitting devices and backplanes). This potential has led to many UK based companies specialising in the commercialisation of organic electronic materials and devices, for example CDT, Eight19, Molecular Vision, PETEC, and Plastic Logic. A key concern for the organic electronics industry is to improve the performance of devices, which in many cases is related to the mobility of electrons and holes in the material. This proposal is uses a new type of measurement to gain information that is not easily accessible using standard techniques to allow better understanding of the processes limiting mobility.

The benefits of this proposal to industry, and therefore the economy through the wealth creation these companies provide, are twofold. Of prime importance is the additional fundamental knowledge that the proposed measurements can provide on the nature of charge transport in these materials. While the proposal can demonstrate the usefulness of these techniques over the timescale of the project, widespread exploitation is likely to take somewhat longer (5-10 years). Of secondary but more immediate impact (2-5 years) is the application of the proposed measurements to examine degradation mechanisms in organic devices. As organic technology is in its infancy, reliability issues are still important and so a new technique to better measure, predict and characterise failure would be timely. Exploitation of any industrially relevant information can be achieved through the project partner PETEC and Durham University's Business and Innovation Service.

The work is of particular relevance to renewable energy (organic solar cells) and so Dr Groves can make use of the Durham Energy Institute (DEI) to more widely disseminate the implications of his findings. The DEI facilitates cross-departmental research (for example with the social sciences), and commonly hosts public engagement events and advises government ministers on areas of energy policy. Dr Groves would utilise the events organised by the DEI to inform members of local and national government, together with the power industry in general, about how the proposed research impacts renewable energy.

Finally, the proposed research benefits people, both through the training of a PDRA and future students who may utilise the techniques developed, and through outreach events which Dr Groves participates in to inform the general public of how research can affect their lives.