Photoinduced charge transfer across an organic/contact interface: polaronic state spectroscopy

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.