Relaxation Dynamics of Photo-excited States and Generation and Transport Processes of Carriers in Pristine C_<60> Single Crystals.

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Aiming to understanding the whole relaxation processes following optical excitation in C_<60> crystals, we have investigated the carrier generation mechanism in connection with the photoluminescence process. In the first stage of the present research project, we have made clear that the carrier generation originates from thermal dissociation of Frenkel exciton and luminescence from localized exciton is enhanced by capturing the Frenkel exciton into the localized state. In order to confirm anti-correlation between the carrier generation and the luminescence process, photocurrent and photoluminescence were measured in one and the same crystal. The photocurrent was measured by Time-of-Flight method, and the photoluminescence through ITO electrodes and mica thin films was observed by a microspectroscopy method.Temperature dependence of photocurrent and photoluminescence intensities obeys to Arrhenius formula. In the range higher than 100K where the photocurrent becomes observable, the lumi … More nescence originates from radiative decay of localized exciton. The thermally activated behavior of the photocurrent exhibits anti-correlation to quenching of the photoluminescence. This relation is interpreted by that enhancement of carrier generation directly leads reduction of luminescence intensity. They can fit with the same activation energy of 99meV by assuming thermal activated population transfers from the Frenkel exciton. The estimated activation energy suggests that the localized state, which exists below 120meV from the Frenkel exciton state, doesn't contribute to the carrier generation. On the other hand, obtained time response of photocurrent is of "dispersive-type", and almost decays as t^<-□> (α〜1). Such response in TOF method has been attributed to anomalous dispersion of drift carriers. Carrier transport in C_<60> crystals seems to obey a hopping process, and the obtained response suggests the existence of continuous distribution of hopping sites in barrier height and distance. Less