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.
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