Shift of absorption energy during thin dye film growth: interpretation by geometric models of the growth morphology

• Description of energetic shift of transition energy of organic thin films.
• Relation of differences in shift with film morphologies.
• Theoretical explanation based on geometric considerations.

We measure absorption spectra of two different perylene molecules in thin films with thicknesses varied between 1 and 30 nm physical vapor deposited on glass substrates. Atomic force microscopy (reveals the formation of crystallized needles for N, N`-dimethyl perylene tetracarboxylic acid diimide and amorphous round-shaped islands for 1,6,7,12-tetrachloro-N,N`-dimethyl perylene tetracarboxylic. For both molecules, a spectral shift of the lowest energy transition to lower energies was observed with increasing film thickness. Common models taking electric fields, electronic coupling or exciton confinement into account cannot completely describe the observed spectral shift. Here we show that the experimental values can be fitted with an advanced layering model based on the energetic difference between bulk and interface/surface materials. In contrast to the known simple d− 1 model, which describes the energetic shift for the special case of a layer-by-layer growth, we extend this idea to other film morphologies.