The aim of this project is to contribute to the fundamental insights into the mechanisms and criteria that affect the efficiency and stability of 'exciton' solar cells, based on the 'bulk heterojunction' concept. In this type of photovoltaic cells, foto-induced excitons (hole-electron pairs) that are formed in a donor phase of a phase separated system, must diffuse towards the (nano)interface between the donor and acceptor phase, where they must dissociate to create free charge carriers with sufficient mobility. This requires fast electron transport towards the acceptor phase through an excited complex or 'exciplex'. The efficiency of the system is of course determined by a suitable choice of the donor (conjugated polymer, e.g. P3HT or MDMO-PPV), the acceptor (fullerene derivative, e.g. PCBM) and the formed nanomorphology, ensuring optimal charge transfer at the interface through the exciplex intermediary. Photovoltaic cells as described above offer a potential technological alternative to inorganic cells for efficient and cost effective conversion of solar energy to electrical energy.
At a practical level, the research will be performed on two fronts. On the one hand an experimental study will be conducted on the phase behaviour of blends composed of semi-conducting, conjugated polymers and fullerene derivatives, performed at the Physical Chemistry and Polymers research department (FYSC). The creation and stabilisation of a nanostructured, co-continuous donor and acceptor morphology is paramount for the stable and efficient performance of solar cells based on these organic thin layers. The phase and crystallization behaviour and the position of glass transition(s) of selected systems will be studied in a concentration range that is as broad as possible, using advanced thermal analysis techniques such as modulated temperature DSC, 'Rapid-scanning DSC' and chip calorimetry, but also surface characterisation techniques such as AFM and nanothermal analysis. On the other hand a theoretical time-dependent DFT study will be performed on the 'exciplex'-based charge transfer at the interface between the polymer, which is excited by light, and the fullerene derivative. This study will be performed in the General Chemistry research department (ALGC). A first goal here is to investigate whether an efficient charge transfer can take place in the experimentally studied systems. Afterwards, the analysis of the charge transfer mechanisms will make it possible to determine which characteristics of the polymer and fullerene derivative are important in order to realise a successful charge separation. This concerted research strategy aims for a more fundamental understanding of the 'synthesis-structure-processing-characteristic' relations of these systems.