Comprehensive Analysis of Intermolecular Charge-Transfer Excited States in C₆₀ and C₇₀ Films

We have investigated intermolecular charge transfer (CT) excited states and demonstrated their contribution to the excitation-relaxation and photocarrier generation mechanisms for both C₆₀ and C₇₀ polycrystalline films. This has been done (1) experimentally, using UV-visible absorption (Abs) and electroabsorption (EA), luminescence and its modulation by external electric-field and steady-state photoconductivity; (2) semiempirically, analyzing the Abs and EA spectra in order to determine the CT state energies and to estimate the variation of the average polarizability tensor and the dipole moment; and (3) theoretically, by performing calculations of polarization energies and electrostatic stabilization energies. The most significant CT states have been identified at approximately 2.43 and 3.50 eV for C₆₀ films and at 2.26 and 3.60 eV for C₇₀ films (data from the semiempirical analysis). Their properties have been discussed in terms of crystallographic and electronic structure, with special emphasis on their mixing with Frenkel states.