Speaker: Moritz Riede, Oxford University
Time: September 24, 2015, 2:00 - 3:00
Organic solar cells (OSC) have attracted increasing attention from academia and industry in recent years. Power conversion efficiencies (PCE) have reached more than 10% and lifetimes of more than 10y are expected. The architecture used for the most efficient OSC is based on the bulk heterojunction, a layer consisting of a 1:4 to 1:1 mixture (by weight or volume) of donor and acceptor molecules. This mixing ratio was thought to be required for an efficient generation and extraction of photogenerated electron and holes. Very recently, however, OSC with very low concentrations of donor molecules (~1-10%) in a fullerene matrix have been shown to work remarkably well with PCEs of more than 5%. Given the diluted nature of the donor molecules this kind of OSC is sometimes called “homoeopathic” OSC, or “Organic Schottky Junction Diodes” following one hypothesis for their working mechanism. These OSC represent an exciting new device architecture. On the one hand, their working mechanisms are not yet well understood, on the other hand, they provide an excellent model system to learn more about the semiconducting organic materials, in particular about the interface between donor and acceptor molecules. As one example, by varying the concentration of transparent donor molecules in C60, we were able to show that the recombination scales with the amount of available interface between donor and C60, which in turn allowed us to vary the open circuit voltage Voc without changing the energy of the charge transfer state ECT. The energy loss ECT – eV_oc for the best performing TAPC-C60 combination (5%vol donor) was 0.52eV, i.e. lower than the typical energy loss of about 0.6±0.05eV in OSC. These findings give new design criteria for better organic materials and eventually higher efficiencies.