Dynamics of photoexcited carrier relaxation inC60films

The relaxation of photoexcited carriers in solid ${\mathrm{C}}_{60}$ films has been investigated through the measurement of their time-resolved transmissivity. A nonexponential temporal relaxation approaches an exponential decay as the ambient temperature of the sample is cooled from room temperature to 5 K. In addition, an Arrhenius-type dependence of the carrier relaxation time on the ambient temperature has been observed. This result implies that carrier trapping may play an important role in the relaxation mechanism. The dependence of the decay time on the carrier density also supports this assumption. A comparison with a simple model of the nonexponential (‘‘stretched’’) decay suggests that temperature-dependent structural changes of the films also influence carrier relaxation.