Photovoltage transients at fullerene-metal interfaces

Photovoltage (PV) transients are studied in C 60 - Pb and C 60 - Au thin films. The morphology of the C 60 layers is characterized by x-ray diffraction and atomic force microscopy, which evidence the formation of a nanocrystalline C 60 layer on polycrystalline Pb and Au underlayers. In contrast to Au substrate, Pb crystallites with a (111) texture are predominantly formed. The signs of the PV signals developed at the C 60 - Pb and C 60 - Au interfaces are found to be opposite due to very different workfunction values of the two metals. The evolution of the PV rise and decay curves with increasing light illumination intensity is completely different at the C 60 - Pb and C 60 - Au interfaces. The rise for the C 60 - Pb interface speeds up considerably with the increase in intensity, which is markedly different from the behavior at C 60 - Au , which exhibits nearly unchanged curve shapes. The PV decay time for C 60 - Au is also only weakly affected by varying light intensity. In contrast, increasing the illumination intensity causes the decay curves for C 60 - Pb to become multiexponential profiles, developing fast initial decays. The results are discussed in terms of different charge redistribution properties of C 60 molecules adsorbed on Pb(111) and Au interfaces. The observed transformation of the PV decay curves is explained by the presence of interface states and a subsequent interplay of the charge redistribution properties of C 60 molecules adsorbed on Pb(111). The results can be applied to sandwich structures containing organic-metal interfaces to account correctly for the interfacial charge transfer dynamics.