Molecular Modelling of Charge Transport in Conjugated Materials.

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In recent years, the use of organic materials in electronics has become a very promising technological area. The attractive property of these materials is that they could allow us to use very cheap manufacturing techniques to produce electronic devices. This would open the way to applications with revolutionary potential: cheap solar cells would become ubiquitous, high efficiency light emitting diodes would drastically lower our energy bills, cheap transistors would allow integration of smart electronics in our living environments.So why have these applications not been put into practice? Devices manufactured from currently available materials are simply not efficient enough. In order to understand how to design the best possible materials for devices, we ought to first understand the fundamental electronic processes underlying the operation of a device. One of such processes is charge transport: it is at the heart of the operation of all these devices. We already know some things about charge transport: that it proceeds by charges hopping between molecules and that it is heavily affected by disorder, for example. Unfortunately, however, it is really tricky to model this hopping from the structure of the molecules. This is because charge hopping is affected by processes at all length scales: at the molecular length scale it is strongly dependent on the orientation and position of neighbouring molecules, and on the device length scale it is dependent on the formation of good pathways for conduction. This means that if we want to define a lattice of molecules that charges can hop on, we need to find a representation of the lattice that is accurate both on the length scale of an atom (0.1 - 1 nm) and on the length scale of a device (1mm): this is equivalent to drawing a golf course and getting the position and shape of all the blades of grass in it exactly right!To overcome these difficulties, I am going to use very efficient methods to simulate large clusters of molecules, thanks to a collaboration with colleagues at the Max Planck Institute for Polymer Research. I am then going to modify the methods to simulate charge transport to take full advantage of all the information from such morphology models, using not only the positions and orientations of molecules in these clusters, but also the way these vary in time. All these methods are going to be validated by comparison with sound experimental data on different material systems. I will look at three classes of material systems, all of which have technological relevance: liquid crystals, films of small molecules and finally polymers. This series will allow me to look at systems which are becoming increasingly complex. This research will allow me to model charge transport in a framework where all the steps of the simulation are soundly justified and based on molecular calculations. This will allow me to better understand the role that the chemical and physical properties of a materials have on charge mobility.