Photoexcited ultra-thin films of the organic semiconductor N,N'-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis dicarboximide (DPI8-CN2), grown on thermal Si/SiO2, exhibit an intense room temperature emission, strongly dependent on molecular coverage, even for sub-monolayer thicknesses. The luminescence spectra are characterized by a highly structured, isolated molecule emission in the sub-monolayer regime (coverage