  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.
