Effect of crystalline domain size on the photophysical properties of thin organic molecular films

We have studied two distinct morphologies of vacuum evaporated thin N,N' -b is(phenethyl)-perylene-3,4:9,10-bis(dicarboximide) films with high-intensit y synchrotron radiation. Layers deposited on glass or silicon dioxide subst rates are shown to be highly oriented polycrystalline films with the [001] crystal axis normal to the substrate surface, and a mosaicity less than 0.0 2 degrees. Transverse x-ray scans through the (002) reflection reveal diffu se scattering from that the lateral domain sizes of the Films can be measur ed. Fast evaporation of DPEP onto cooled substrates results in a bright red film that is found to have a grain structure with a domain size of 250 Ang strom. Slower evaporation onto heated substrate produces a film of preen co lor with a more than one order of magnitude larger domain:size of 3300 Angs trom. The domain sizes and shapes extracted from the x-ray data agree well with separately recorded atomic force microscopy images of the DPEP films. The obtained crystal structure and submicron domain size of the large-grain DPEP film explain the recently reported extremely long exciton transfer le ngths in polycrystalline DPEP films. [S0163-1829(99)00145-9].