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Heterojunctions between an inorganic semiconductor and organic molecules are of interest for light harvesting and optoelectronic devices such as solar cells and light emitting diodes (LEDs), because they combine the optical absorption and emission properties of molecules with the high charge carrier mobility provided by semiconductors. The functionality of these devices is based on the charge or energy transfer across the heterojunction and their efficiency depends critically on the energy level alignment at the interface because it determines, for example, the energy barrier that charge carriers have to overcome. To investigate the ultrafast processes at the interface we use time- and angle-resolved photoelectron spectroscopy, where the time delay between a pump and a probe pulse can be varied with femtosecond precision.
In my doctoral studies I want to look for a way not only to induce modifications to the surface or interface electronic properties, but also control them by an all-optical method. The basic idea is to make e.g. the work function a time-dependent quantity by choosing adsorbate molecules that have dipole moments which can be changed by a short laser pulse. The charge and energy transfer dynamics across the interface will change when the work function is altered. To observe these changes a challenge will be to add a third laser pulse for activation to the present pump-probe scheme.
This work will be carried out within the Sfb 951 “Hybrid Inorganic/Organic Systems for Opto-Electronics” and contribute to a deeper understanding of the fundamental electronic processes at inorganic/organic heterojunctions.