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Ultrafast Relaxation Dynamics after Optical Excitation of Hybrid Inorganic/Organic Systems


Bogner,  Lea
Physical Chemistry, Fritz Haber Institute, Max Planck Society;


Wolf,  Martin
Physical Chemistry, Fritz Haber Institute, Max Planck Society;

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Bogner, L. (2015). Ultrafast Relaxation Dynamics after Optical Excitation of Hybrid Inorganic/Organic Systems. PhD Thesis, Freie Universität, Berlin.

This work focuses on fundamental processes which influence the efficiencies of organic solar cells and LEDs, for instance the formation and decay dynamics of excitons, their diffusion, the charge transfer at interfaces between organic materials and inorganic electrodes and the correlated energy level alignment at these interfaces. These processes are investigated on the basis of four model systems, which represent different parts of a solar cell or an LED, by means of time-resolved photoelectron spectroscopy which facilitates the measurement of occupied and unoccupied states as well as the acquisition of ultrafast processes. ZnO is a promising material for transparent electrodes and as an active LED medium, therefore the processes in optically excited ZnO are of great interest. The investigations show that the electronic structure of the O-terminated ZnO surface is strongly influenced by the adsorption of hydrogen and that the exciton formation slows down at higher electron densities at the surface as the electron phonon coupling is screened. The SP6/ZnO interface can serve on the one hand as a model system for charge transfer processes, however SP6 in thick films represents a potential LED medium. In addition to the already known relaxation processes which have been observed before by time-resolved optical spectroscopy, photoelectron spectroscopy reveals another ultrafast component. The interaction of long-lived triplet states results in this system in the emission of electrons. Furthermore, photoelectron spectroscopy allows to draw conclusions on the absolute energies of the excited states. The energy level alignment at the interface between a metal electrode and pi- conjugated molecules is investigated on dicyanovinyl-substituted oligothiophenes on gold. The gold surface influences not only the electronic structure of the monolayer, it also significantly affects the lifetimes of excited states that increase with the distance to the metal surface. As a model system for a polymer semiconductor P3HT was established. The investigation of two films with different amounts of crystalline fractions shows that the relaxation dynamics proceeds faster in the film that features higher crystallinity and therefore superior transport properties. These results give an insight into the complex interrelated relaxation processes of optically excited states. The detailed comprehension of these processes promises their targeted utilization to optimize the efficiency of organic solar cells and LEDs.