Engineered bulk heterojunction inorganic:organic hybrid photovoltaics

The case for supporting clean, renewable technologies is strong with UK Government commitments to ensuring 15 % of our energy comes from renewable sources by 2020, this represents a seven fold increase in the market share for renewables in less than a decade. This target can only be achieved by implementing a combination of complementary solutions including biomass, wind, wave and solar. In particular solar energy harvesting has the potential to become competitive, in both economic and performance terms, if current limitations associated with next generation technologies can be overcome. In addition to environmental benefits there is the potential for significant economic development, recent analysis suggests that the entire renewable energy sector could support up to half a million jobs in the UK by 2020. The demand is present, evidenced by the increase in UK PV capacity from 10.9 Mw in 2005 to an estimated 26.5 Mw in 2009.

Inorganic-organic hybrid photovoltaic (h-PV) devices are a realistic prospect for the long-term development of entirely solution processable, scalable devices on rigid and flexible substrates. The pairing of a metal oxide (TiO2, ZnO) with a conjugated polymer to form a hybrid device is an attractive combination of materials. For example, ZnO provides efficient electron mobility, effective light-scattering, is of low cost and can be formed in a wide variety of (nano) structures from aqueous solution. The absorbing, hole-transporting conjugated polymers, such as poly(3-hexylthiphene)(P3HT), support a wide variety of processing routes and exhibit some of the best charge transport of all organic semiconductors. However progress made towards realising such h-PV technologies has been slow. Reported power conversion efficiency (PCE) values are typically < 1%, with some more recent publications reporting 2%. This compares with reported efficiencies of > 8% for commercial organic-PVs.

The nanostructured devices that will be prepared in this program will provide controlled bicontinuous networks for charge, and importantly will allow control of the polymer morphology - a parameter that has received little attention in h-PVs - although it is known to strongly influence exciton generation, free carrier transport and light absorption. This unique combination of materials and processing strategies presents an exciting opportunity for the development of h-PV devices that can overcome the current performance limitations by allowing control of the structural and morphological properties of the device not possible with other material combinations or processing techniques.

Importance of research to UK and EPSRC supported activities

Photovoltaic research is currently strongly supported in the UK, and continues to be an area where the UK is at the forefront of material and device development. Hybridphotovoltaics have enormous market potential due to the prospect for efficient, low-carbon energy production combined with low-costfabrication. The prospect of improving cell performance prompts highly favorable government policies and broadens commercial opportunities. Currently in the UK there is a tiny volume of research activity of hybrid cells and none in the novel areas outlined in this proposal hence there is an opportunity to support early stage research in this area and ensure the UK remains at the leading edge of development in renewable energy.

Collaboration and co-production

Links between the Martyn McLachlan and both the Centre for Plastic Electronics (CPE) and Energy Futures Laboratory (EFL) at Imperial College are already established. These interactions bring collaboration with UK manufacturing companies, end users, and other UK and European academics. The results of the proposed research will be communicated back through these centres - their input will ensure that a feedback loop exists between this project and technological advances relevant for industry. Support from Merck Chemicals ensures not only a supply of high quality materials and access to novel materials in the future, but importantly initiates a collaboration with an industrial partner at the forefront of materials development in a wide range of technologically relevant areas.

Capacity and Involvement

The tasks outlined in the proposed project will be managed and carried out by a skilled PDRA supported by the PI. Some polymer processing will be carried out by a MSc student during a 24 week project (Feb-Sep 2012). Additionally, the TAS and PL measurements will be carried out by an MRes student (Mr K. Prashanthan) in collaboration with Dr Saif Haque in the Department of Chemistry - both masters students will be provided with relevant training and learn in a technologically important and developing area. All of the researchers will benefit from working on a cutting-edge project in an exciting, growing field with an interdisciplinary team of materials scientists, chemists and physicists. Collectively, they will gain experience in synthesis, materials characterisation and processing, as well as device fabrication and analysis.