The exciton dynamics on flat (001) rubrene crystal surfaces have been compared with those under confined pyramidal geometry by time-resolved photoluminescence with micrometer spatial resolution. The luminescence spectra can be interpreted in terms of generation of a free and a self-trapped exciton. Their ratio depends significantly on the structural size which we explain by the optical absorption profile of the pyramids in combination with the exciton diffusion constant. For the latter a lower limit of 0.2 cm2/s at 4 K has been estimated. Temperature-dependent decay times reveal activation barriers between free and self-trapped exciton of 3 meV and 14 meV.
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