The primary stages of the charge carrier photogeneration in C₆₀ films studied by the 100-fs laser pulse pump-probe method

Abstract

The primary stages of photoinduced processes are studied in thin C₆₀ films by the femtosecond laser pump-probe method. The films were excited by 100-fs laser pulses with photon energies above (wavelengths 345 and 367 nm) and below (645 nm) the mobility threshold, the fraction of excited molecules being no more than several percent. Upon probing in the spectral range from 400 to 1100 nm, several regions with substantially different decay kinetics were observed in the difference spectrum, which is caused by the simultaneous presence of several relaxing components. The appearance of the 465-and 500-nm bleaching bands in the difference spectrum upon excitation by photons with energies both above and below the mobility threshold, which are typical for electroabsorption spectra, suggests that charge carriers are produced in both these cases. The observed dependence of relaxation on the oxygen amount in the sample volume suggests that during excitation both charged (electrons and holes) and neutral (excited molecules) components are produced. The fraction of charged components is greater upon excitation into the fundamental band. The appearance of the 500-nm absorption band delayed by 10⁻¹³–10⁻¹⁴ s, the delay being increased in the presence of oxygen, was attributed to the formation of excited anions due to the capture of electrons by C₆₀ molecules. It is concluded that upon excitation of the films by photons with the energy below the mobility threshold, charge carriers are produced due to two-photon absorption rather than due to singlet-singlet annihilation. When the films are excited by photons above the mobility threshold, the primary charge carriers are produced by direct optical excitation.