Martin Schlosser, Ida Zsuzsanna Kozma, Stefan Lochbrunner

Exciton Migration in Highly Doped Matrices In organic materials and in particular in thin layer systems designed for organic electronics applications complex energy transport and charge transfer processes govern the device performance. With transient absorption spectroscopy we investigate the ultrafast primary steps and the transport pathways. The understanding of these phenomena will lead to new optimization strategies for organic devices. Our main interests are thereby exciton diffusion, bandlike transport and charge separation in heterosystems. These processes occur for example in organic solar cells as indicated in the left figure. The absorption of light in the active layer leads to the formation of excitons which diffuse to the interface (heterojunction) between the absorbing and the next organic layer. Due to their different bandgaps the excitons can dissociate there and the charges are separated. In a first project we look into non dispersive exciton diffusion. In unordered polymers the electronically excited states of the segmental sites exhibit a large energy distribution. Excitons migrate to energetically favored sites till they are trapped at the site with the lowest excitation energy of the local enviroment. It results in a restricted diffusion length. To avoid this effect we investigate exciton diffusion in polymer matrices highly doped with dye molecules which are the active sites for exciton migration. If they interact weakly with the enviroment they should have a narrow energy distribution and trapping should be reduced. In this case exciton diffusion should be essentially loss free and large diffusion lengths are possible. For perylene bisimide dyes incoporated in PMMA we find a very fast energy transfer of 1.5 ps between the dye molecules allowing for efficient exciton migration. The transfer can be observed by measuring the decay of the excitation induced anisotropy (see figure). The lifetime of the excitons is limited in this system due to trapping by dimers. In the next step we want to avoid the dimer formation and to achieve diffusion lengths in the order of the layer thickness.

"Exciton Migration by Ultrafast Förster Transfer in Highly Doped Matrices" M. Schlosser and S. Lochbrunner J. Phys. Chem. B 110, 6001 - 6009 (2006) Details