Lay summary
Organic bulk-heterojunction photovoltaic cells rely on conductive organic polymers, organic solids or small organic molecules for light absorption and charge transport. The interest in organic solar cells has recently increased due to their advantages over conventional photovoltaics: they have a low impact on the environment, manufacturing is easy, and because they can be attached to flexible materials they can be put on many things (like plastic ID or bank cards, clothing, mobile phones and laptops). They are even sufficiently cheap for being associated with disposable objects. So far, these devices have shown rather modest power conversion efficiencies which could be related to inefficient charge transport or morphology problems resulting in fast charge recombination. The objective of this project is to study charge transport properties of unconventional photonic and charge transport materials, such as nanocrystalline inorganic, organic charge transport materials and nanocomposite hybrid systems and to identify the limiting factors in solar cell performance. Redox-active ionic liquids, such as imidazolium iodide, hole transporting molecular liquids, such as alkoxylated triarylamines, amorphous solid hole-conducting materials, such as spiro-MeOTAD, as well as cyanine dye layers will be more particularly scrutinized. Electron transfer dynamics at the junction between such materials will also be in the focus of this research.Terahertz time-domain spectroscopy (THz-TDS) is a very powerful technique for material studies, which covers the spectral range ~ 0.2-10 meV, bridging the gap between microwave and infrared experimental methods. Linear THz-TDS as a contact less, coherent optical technique allows for direct determination of the complex conductivity of materials. Both the absorption and the dispersion of the sample can be measured directly, and this is expected to give much more accurate values of the charge transport parameters with respect to other methods. By combining THz-TDS with synchronous optical excitation, one has optical-pump THz-probe spectroscopy (OPTP) available as a powerful tool with the ability to temporally resolve phenomena at the fundamental timescales of nuclear and electronic motion. Low frequency vibrations that are associated with the self trapping of charges in small polarons can as well be observed in the frequency range 0.2 - 3 THz. Application of OPTP spectroscopy is thus expected to provide invaluable information on the detailed mechanism of interfacial light-induced electron transfer and charge transport processes.