Abstract

We have performed what we believe to be the first time-resolved absorption studies of C₆₀ solid films. The films were fabricated by the evaporation of highly purified C₆₀ onto a sapphire substrate in UHV conditions (<10^-10 Torr) at 550 degrees C. Measurements were performed using a CPM laser (620 nm, pulsewidth <100 fsec) in a standard pump-probe transmission geometry. The excitation wavelength corresponds to excitation of carriers across the HOMO-LUMO gap of solid C₆₀, a normally symmetry-forbidden transition in the isolated molecule. A nonexponential decay of optical transmission was observed on a picosecond timescale, with no subpicosecond transient characteristic of hot carrier thermalization. The nonexponential decay of the transmission signal is indicative of a geminate carrier recombination process, usually seen in amorphous semiconductors. The optical absorption spectrum of C₆₀ also possesses an exponential rise at the band edge (Urbach tail), also characteristic of amorphous semiconductors. The observed carrier recombination can be theoretically described by a diffusion/neutralization model from which a diffusion length can be obtained. The theoretically obtained mobility is in agreement with conductivity measurements for undoped 60C films on insulating substrates.

© 1991 Optical Society of America