Excitons and exciton confinement in crystalline organic thin films grown by organic molecular-beam deposition

We study excitons in thin films of varying thickness of two organic molecular crystals grown by the ultrahigh-vacuum process of organic molecular-beam deposition; 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI). By using electroabsorption spectroscopy, low-temperature fluorescence, and absorption techniques, we find that nearly spatially symmetric charge-transfer/Wannier-like excitons are present in crystalline PTCDA films, and that these excitons can be confined in multilayer stacks of ultrathin layers of thicknesses ranging from 10 to 100 {}. In PTCBI films, however, the exciton has a smaller radius, and thus much weaker spectral and electric-field dependences on layer thickness are observed. In addition, we present theoretical and experimental evidence showing that exciton-lattice interactions may be modified by the growth of ultrathin layers and the subsequent confinement of the charge-transfer excitons.