Energy Losses in Small-Molecule Organic Solar Cells

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Organic solar cells meanwhile reach power conversion efficiencies of more than 11%, however, in comparison to inorganic cells they lose disproportionately in energy. This means the so-called bandgap-voltage offset between the onset energy for light absorption and the open-circuit voltage of the solar cell is in most cases much bigger than the value of about 0.4 eV obtained for Si and GaAs. The goal of this project is to gain a deeper understanding of the origin of energy losses in organic solar cells and to develop strategies to reduce them. To achieve this goal we will use structurally well-defined donor-acceptor heterojunctions based on molecular materials as model systems and systematically investigate the influence of parameters like structural order, molecular orientation or intermixing of the two partners at their mutual interface. Applying temperature dependent electrical transport measurements and optical spectroscopy, we plan to elucidate the detailed role of the interfacial charge transfer state for power generation in organic solar cells.