Rational design of manufacturing processes for next generation optoelectronically active nanocomposite films and coatings

Our research aims to develop plastic films or coatings that change the colour and other characteristics of the light that passes through them, not by absorbing certain wavelengths of light, as a simple colour filter would, but by converting light of one wavelength to another without losing any energy. Solar cells offer an example of why this would be useful: conventional silicon solar cells are more efficient at collecting the energy of red light than they are of blue light. So if we coated the solar cell with a film that would convert every blue photon into two red photons, without losing any energy in the process, in principle we could make the silicon solar cells 30% more efficient.

Our previous research at Cambridge has shown in principle how this could be done. Certain organic semiconductors will absorb a blue photon to produce an electron-hole pair, which then splits into two. Normally these two electron-hole pairs would annihilate and the energy would be lost, but if we can arrange for the organic semiconductor to be in molecular contact with an inorganic semiconductor quantum dot, then the electron-hole pairs can migrate to the quantum dot, where they will recombine and emit two red photons.

The problem we now want to solve is to work out how to turn this idea into a practical product that we can manufacture on a large scale. We need to be able to make semiconductor nanocrystals that won't clump together, and to coat them with a very thin layer of the organic semiconductor so the two materials are in molecular contact. Then we have to disperse these tiny particles in a clear plastic film, which we can use to coat a solar cell - and the whole process has to be designed so that it doesn't increase the cost or complexity of making the solar cell too much.

This coating for solar cells is just one example of the potential there now is for taking the latest materials from the laboratory with novel and interesting optical properties and turning them into useful products. Another example is provided by thin sheets of semiconductors only a few atoms thick. These can be very efficient at absorbing light (for example from a light emitting diode) and reemitting it as a single, purer, colour. This will help us make better optical communication devices and display devices. But once again, we need to learn how to encapsulate and embed these tiny, ultrathin sheets into a plastic film without them sticking together in stacks.

The key to solving these manufacturing problems is understanding the factors that make these tiny particles and sheets stick together and what treatments could keep them apart - often this will involve sticking special molecules to their surfaces. In the final products, these particles and sheets will be dispersed in a plastic sheet, and we need to understand how, as the plastic film dries or sets hard, the drying process affects the particles, and whether the processes that take place in the drying film makes the optical effects we're looking for less effective. We will be studying the films we make with techniques that allow us to see the individual molecular layers around the particles, as well as how well the particles are dispersed. In this way we'll understand the rules for manufacturing these sorts of films.

By the end of the project, we aim to be able to work with solar cell manufacturers to test our idea in the real world and get to the point where a product can be commercialised. If we are successful, we'll have demonstrated that we can go from understanding the fundamental science of these optical and electronic effects in these new kinds of materials to make useful products that will benefit UK industry and help solve problems of climate change.

If successful, this project will create routes to market for a number of new concepts in optoelectronically active films, benefitting UK industry.

Our first target product is a "photon multiplier" film which will significantly boost the efficiency of silicon solar cells. This will add substantial value to the global PV industry, and our aim is to ensure that significant fraction of this value is captured by the UK, including through UK companies such as Eight19, our partner. Because our film is compatible with existing solar cell technology it does not have to displace an existing product to find a market; on the contrary it can directly benefit from the continued exponential growth of silicon solar cell manufacturing capacity.

Because the "photon multiplier" will increase the efficiency of silicon solar cells without significant increase in the capital cost of manufacturing plant, it will accelerate the adoption of solar energy worldwide, with substantial benefits in the reduction of carbon emissions.

Our second target is to create new phosphors utilising 2d materials; for both of these targets we anticipate creating opportunities for materials suppliers, both in the organic and inorganic nanoparticle spheres, and film converters. Finally, speciality chemical companies working in all areas that benefit from composite films of nanoparticles and polymers to give enhanced properties of all types will benefit from the manufacturing know-how generated in the project.