Dynamics of photovoltaic processes

The development of new highly efficient solar cells is guided by insights gained with new powerful analytical tools that address dynamics of hot charge carriers and recombination kinetics directly in real time. In particular, by applying femtosecond two-photon-photoemission (fs-2PPE) the dynamics of photo-generated charge carriers are time-resolved and energy-resolved at surfaces. The method holds great promise also for studying dynamics at relevant interfaces of III-V materials. With the femtosecond 2PPE technique we address the dynamics of hot electrons in bulk and surface states of semiconductors. We also study injection and capture of hot electrons by adsorbed or chemically anchored molecular species. Transient absorption is the second powerful tool for studying the latter type of dynamic processes with femtosecond resolution. In addition we measure time-resolved photo-conductivity, luminescence, and stationary high-resolution excitation spectra of photocurrent and luminescence at He-temperatures. Since the introduction of dye-sensitized solar cells the department has maintained a leading role in their analysis. It appears meaningful to carry out basic research on related topics that make use of the unique femtosecond analytical tools combined with ultra-high-vacuum standards. A valuable tool is the UHV/solvent preparation chamber where an interface can be prepared with wet chemistry methods in an inert gas atmosphere. The goal of basic research in this field is reaching a satisfactory understanding of the basic processes. This will help in forming a well-founded prediction regarding the potential of this new type of solar cells, including solid-state analogues.