Direct Determination of the Exciton Binding Energy of Conjugated Polymers Using a Scanning Tunneling Microscope

We report scanning tunneling spectroscopy measurements of the threshold energy for injecting electrons or holes into thin, conjugated polymer films deposited on Au(111) substrates. Combining these results with optical absorption measurements, we estimate an exciton binding energy of $E_b=0.36\mathrm{}\pm 0.10\mathrm{eV}$ for poly[(2-methoxy-5-dodecyloxy)-1,4-phenylenevinylene-co-1,4-phenylenevinylene] and $E_b=0.30\mathrm{}\pm 0.10\mathrm{eV}$ for poly(9,9'-dioctylfluorene). In addition, we determine the alignment of the electronic levels of the polymers relative to the substrate.