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' -bis(phenethyl)-perylene-3,4:9,10-bis(dicarboximide) films with high-intensity synchrotron radiation. Layers deposited on glass or silicon dioxide substrates are shown to be highly oriented polycrystalline films with the [001] crystal axis normal to the substrate surface, and a mosaicity less than 0.02 degrees. Transverse x-ray scans through the (002) reflection reveal diffuse scattering from that the lateral domain sizes of the Films can be measured. 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 Angstrom. Slower evaporation onto heated substrate produces a film of preen color with a more than one order of magnitude larger domain:size of 3300 Angstrom. 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 lengths in polycrystalline DPEP films. [S0163-1829(99)00145-9].