Energy transfer over considerable long distances plays an important role in photosynthetic energy conversion processes. The occurrence of energy transfer over distances of typically 10-20 nm is in general attributed to the specific organization of the dye molecules in the light-harvesting complexes in photosynthetic organisms. In contrast to natural systems, energy transfer in artificial dye layers only occurs over distances of typically a few nanometers, which is often related to the amorphous nature of the dye layer. The aim of this research is to provide a fundamental understanding of the influence of the molecular organization of biomimetic dye molecules on the energy transfer dynamics. To this end, we have determined the exciton diffusion length in various structured porphyrin layers on top of a smooth electron-accepting semiconductor surface using the time-resolved microwave conductivity technique. In addition, layer structures have been characterized using optical spectroscopy including polarized optical microscopy and X-ray diffraction.
For ZnOP layers (see Figure) it is found that excitons diffusive between the self-assembled stacks, with the exciton diffusion length being as long as 15 nm. For more amorphous layers considerably shorter exciton diffusion lengths of less than 3 nm are found. Combining the exciton diffusion length with the exciton lifetime of 160 ps yields an exciton diffusion coefficients equal to 1.4×10-6 m2/s for the ZnOP layer. The large exciton diffusion coefficient originates from a strong excitonic coupling for interstack energy transfer. For this reason application of such self-assembled porphyrins in thin dye sensitized solar cells is promising.