Optical probes of photoexcitations in fullerene thin films from femtoseconds to milliseconds

We have studied photoexcitations in various fiillerene thin films using transient photomodulation and photoluminescence from 100 fs to 50 ms and absorption-detected magnetic resonance (ADMR). We show that singlet Frenkel type excitons are the primary photoexcitations; their recombination kinetics in the picosecond time domain are dispersive as a result of inhomogeneity. The long-lived photoexcitations, however, are shown to be triplet excitons and charged polarons, identified by the correlation found between their associated optical transitions and ADMR signals with spin 1 and Vz, respectively.