Photoluminescence as a probe of molecular organization in PDI8-CN2 ultra-thin films

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 <30%) and by a condensed-state singlet exciton fluorescence temporally evolving (within 0.5 ns) toward an unstructured, energetically relaxed, excimer-like emission, for thicker films. Once a complete monolayer is formed, only the unstructured excimer emission can be detected. The experimental findings are interpreted in terms of progressive deposition of nearly not interacting molecules, followed by islands formation where a strong dimeric coupling takes place, upon increasing the coverage. A thorough investigation by means of AFM and in-situ X-ray diffraction confirms the proposed picture.