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. © 1995 The American Physical Society.