Solving the NIR dilemma for organic photovoltaics

Being able to convert sunlight efficiently into electrical energy would solve all the world's energy needs. It would also greatly reduce the production of carbon dioxide. Photovoltaic devices for achieving this conversion are becoming increasingly efficient, but the technology is still a long way off being put into broad application for large area collection of sunlight. One of the major problems is that the materials that currently work best in terms of photoinduced charge-separation are only able to collect a part of the solar spectrum. Sunlight incorporates a large amount of red and near-infrared light, and the proportion of such light reaching the earth increases when the sun is low in the sky (for example, in winter, and in mornings and evenings in summer), due to the scatter of light of shorter wavelengths. However, the photon energy content of such long-wavelength light is too low for current organic photovoltaic (OPV) devices.The proposed work will produce and study new materials which absorb such low-energy photons of light, and which then come together to form a single photon of twice the energy. This can be achieved by generating low-energy triplet excited states, which can then undergo a process of triplet-triplet annhilation (TTA) to generate higher energy singlet states. In this way, the normally useless red and NIR light is converted into useful green or orange light that can be used with the OPVs. The process is known as upconversion. A key new feature of our work will be that the singlet states once formed will be trapped by dopants that have lower-energy than the host materials and emit with high efficiency. This will circumvent the current problems such as wastage of the upconverted photons at defect sites.By preparing and studying new materials for these three purposes (triplet generation, hosts for TTA, and singlet traps), we shall optimise the process and identify the best systems to be used. The up-converting systems will be amenable for use with a whole range of OPV systems, by effectively bolting onto the OPV to collect the unharvested low-energy light and radiate it back into the OPV. This has the potential to lead to large increases in the efficiency of such systems.

The aim of the research is to develop an efficient solar up-converter, enabling IR photons to be turned into visible photons (of higher energy). Initially this is aimed at the photovoltaic market to enable use of the solar IR spectrum without the concomitant loss of open circuit potential of the PV cell; this is most relevant to organic PV, and companies working in this sector will be our first and main beneficiaries. However, this new technology is not limited to OPV, and so we have the potential to work with non-organic PV developers as well. Lastly, there are other potential solar-driven devices that can greatly benefit from solar IR up-conversion, namely fuel cells and photosynthetic mimics. Here, photons of specific energy are required as opposed to a broad spectrum, and solar up-conversion is perfect for enhancing these processes: new start up companies can be engaged throughout the project to develop this avenue as well. Within the OPV industries, the main technology lead companies currently working on large OPV projects are Konarka, G24i, BASF, Novaled, CDT, Nanoco and Plextronics. Of these we currently have research activity with the last four, and contacts with Konarka through our PVC Research. Thus, we are in an excellent position to discuss our results with these potential beneficiaries very quickly as we develop the technology. Once we have realistic performance figures to show them and have taken steps to initially protect new IP generated by the research, we will hold meetings with potential collaborators to discuss new collaboration specifically on the solar up-converters aimed at merging our technology with theirs to give greater impact to their devices. This route could give rather quick gains and impact on the 2-3 year time scale. We are also very close to PETEC (Printable Electronics Technology Centre). Our proposed up-converters will be ideal for reel-to-reel mass production. PETEC has just received 20m funding to set up a pre-production reel-to-reel line. Introducing our technology to them for scale-up would have a major impact for the UK and especially the UK public sector printable electronics centre of excellence. This would happen towards the end of the project on a 3-year timescale. In the mid to long term, via the Topless project (APM) (organic solid-state lighting with Thorn Lighting and Sumation CDT), we have been working on plans for integrated lighting systems for buildings. Because both organic solid-state lighting (based on PLEDs) and PV cells require and produce low voltage dc output, integration of a building's lighting to include PV, battery storage and organic lighting offers the most efficient lighting option for a building, also offering non-metered or off-grid lighting. With regard to these ideas, we are working with NIREC and so can bring our up-conversion technology to their attention as well, especially for integration for non-organic non-Si PV systems. Integrated PV lighting will produce a major impact for the UK and the up-conversion technology can be an important part of the equation. These implementations of our new technology will directly impact on the everyday lives of the general public. As stated above, we already have excellent collaboration with many of the main OPV players and routes into major start-up companies. At the appropriate time, when results are ready, we will hold meetings with potential partners (under NDA agreements) to discuss collaboration and trials. We aim to take the work forward by joint research and IP protection leading on, at the end of a successful EPSRC project, to a jointly funded project to develop the technology into product, via TSB funding or FP7/8. To address industry that we do not know, presentations at key Industrial Conferences will be given, such as IDTechX or Intertech PIRA Conferences and meetings organised by the UK Knowledge Transfer Networks. This will be in addition to more mainstream Academic Conferences.