Unravelling the working mechanisms of homoeopathic organic solar cells

Organic solar cells (OSC) are a highly active, interdisciplinary field of research drawing together the expertise of chemists, physicists, material scientists and engineers. The research is exciting not only in terms of fundamental science, but also in terms of potential positive impact on the economy and society. OSC have the potential to become a very cost-competitive, large area and versatile photovoltaic technology. Academic and industrial research have produced efficiencies exceeding 10% and brought OSC close to commercialisation.
Until recently, the architecture used for all efficient OSC was based on the bulk heterojunction, a layer consisting of a mixture of donor and acceptor molecules. A mixing ratio between 1:4 and 1:1 (by weight or volume) was thought to be required for an efficient generation of free electron and holes at the interface between donor and acceptor, and for efficient transport to the electrodes. However, in 2011, a novel device architecture was introduced: OSC on the basis of fullerenes, the standard acceptor molecules, as absorbing layer were presented that only have a very small amount (5vol%) of donor molecules, yet worked very well. Up to then, the conventional understanding of OSC was that such OSC should not work at all, or at least not as well as they do; meanwhile they are reaching efficiencies of more than 6%. Their working mechanism is still far from understood. These unexpected results again show that the field of OSCs (and most likely organic electronics in general) holds some surprises and that its full potential is yet hard to estimate. To underpin further long-term technological innovations, fundamental studies are required. Unravelling the working mechanism of this novel architecture for OSC is the core of this project.
To achieve this goal, thin organic films and corresponding OSCs of this novel architecture will be made with systematic variations in the stack and processing conditions. For high control of the device preparation, vacuum processing of purified small molecules will be used. The key difference to other approaches is that this will be combined with the concept of molecular doping. Presently, this method is rarely used in OSCs, despite being the basis of all commercial organic light emitting diodes (OLED) and the current world record OSCs.
Through systematic variations of the OSC hole contact, here realised with doped transport layers, and varying mixing ratios of fullerene and donor and changing substrate temperature, the generation of photovoltage and free charge carriers will be investigated. I will measure the energy of the charge transfer states using Fourier-transform photocurrent spectroscopy (FTPS), quantify the barrier between the hole contact and the organic absorber layer using impedance spectroscopy, FTPS, and current-voltage measurements, as well as determine the microstructure of the mixed films using X-rays, all essential to probe their fascinating interplay. The charge carrier transport, in particular the hole transport, through the absorbing layer and its recombination dynamics will be studied using single-carrier devices and transient measurements. In addition to working efficiently, the solar cells investigated here can be considered of great interest in their own right. The highly diluted nature of the donor molecules is an excellent model system to experimentally study donor-acceptor interactions, something that is central to any OSC and still not fully understood. Discovering the working mechanisms of this novel architecture for OSC will also help to answer the question of why fullerenes are such special and successful acceptor molecules. The results of this project will stimulate the development of novel and better materials, enable researchers to further optimise this promising architecture for efficient and stable solar cells as well as explore new device concepts for other applications of organic electronics.

Organic electronic materials are already having an economic impact and many people will have used organic electronics without knowing it. Many mobile phone displays are made from organic light emitting diodes (OLED) and this technology is expected to become the leading mobile phone display technology in 2013 (DisplaySearch, 2013). The global market of organic electronics is predicted to reach more than 40 b$ by 2018, implying an annual growth rate larger than 30% (Transparency Market Research, 2012). This will include OLEDs for lighting application, organic solar cells (OSC), and possibly other applications like organic transistors.
OSCs are one of the emerging solar technologies and they are currently close to commercialisation. These solar cells have the potential to become a very cost-competitive, large area and versatile photovoltaic technology. Organic semiconductors are based on abundant and non-toxic raw materials and the manufacturing technologies are in principle capable of coating large areas inexpensively and fast. Hence, OSCs are one of the few renewable energy technologies that could be scalable at low cost to terawatt of installed capacity, which is necessary for any renewable energy technology to make a significant difference in the world's energy system.
The solar cell field is largely driven by power conversion efficiencies. 10%, as shown in the lab for OSC, are good, but to allow a really significant impact, pathways to 15% efficiency in single junction devices have to be found. One central part of this research aims at better understanding of what can be done to reduce energy losses during the conversion from light into electricity. If successful, this research has the potential to yield a wide range of long term economic and social benefits. It may ultimately lead to better OSCs by improved materials for solar energy conversion and/or through more efficient and stable device architectures to fulfil their potential for inexpensive, ubiquitous solar power. This in turn will lead to a significant reduction in CO2 emissions, help the UK to meet its commitment to CO2 reductions, reduce the dependence on energy imports, and eventually reduce electricity prices with huge benefits for both the economy and society. The better understanding of the investigated solar cells developed in this project will have benefits beyond OSCs to organic electronics in general, helping to come up with strategies to further improve devices made from organic semiconductors.
Furthermore, this project will strengthen the UK's activities in the field of vacuum processing organic electronics. The UK has a high-tech industrial cluster for vacuum technology in the area of Hastings that will benefit from my research, both by supplying equipment as well as by being able to further improve their equipment through close collaboration. If any of this work is taken towards commercialisation (licensing, spin-out companies) Isis Innovation, the dedicated technology transfer office of the University of Oxford, will help to maximise the impact.
Last but not least, it will deliver high-level scientific training in preparing, characterising, and simulating OSCs for one PhD student (Ivan Ramirez) with which the department supports starting my new group. He and subsequent PhD students that my group can attract will gain a broad understanding in the field of OSCs and organic electronics in general, benefit from the interdisciplinary environment, hence preparing them for their future careers in science or industry.