Photo-induced charge transfer from an Indium Tin Oxide (ITO) contact into [6,6]-phenyl-C61-butyric acid methyl
ester (PC60BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) is measured. Charge transfer peaks
are observed for a series of excitation energies below the PCBM absorption edge. If charge transfer is blocked
using a tunnel barrier or an applied electric field, the peaks disappear. The observed transitions are similar to
those predicted by theoretical calculations of the absorption spectra for negatively charged C60 and C70 chains. This observation suggests that charge transfer occurs preferentially at the polaronic transition energies in the
PCBM, providing a means for polaronic state spectroscopy.