QUANTUM-SIZE EFFECTS OF CHARGE-TRANSFER EXCITONS IN NONPOLAR MOLECULAR ORGANIC THIN-FILMS

We use a quantum-mechanical model to analyze charge-transfer (CT) exci tons in closely packed, nonpolar organic molecular crystal thin films grown by the ultrahigh-vacuum process of organic molecular beam deposi tion. The exciton Hamiltonian includes both polarization effects and t he periodic pseudopotential of the crystal. This model takes into acco unt the very large anisotropy characteristic of many organic materials such as the archetype molecular crystal, 3,4,9,10-perylenetetracarbox ylic dianhydride (PTCDA) and PTCDA-based multilayers. Using a single-e xciton Hamiltonian, we quantitatively model experimental electroabsorp tion data and the absorption spectral shifts observed in ultrathin org anic multilayers or ''multiple quantum wells.'' The data analyzed from several such experiments give independent and consistent estimations of the anisotropic effective mass tensor and exciton radii for PTCDA a long different crystal axes. This treatment is general, and is found t o extend to other CT and Wannier exciton systems found in many interes ting, nonpolar organic molecular and inorganic semiconductor crystals such GaAs, suggesting similar physical origins for Wannier and CT exci tons in a wide range of materials.