Electrochemical determination of the ionization potential and electron affinity of poly(9,9-dioctylfluorene)

We report cyclic voltammetry measurements for the blue electroluminescent conjugated polymer poly(9,9-dioctylfluorene). Both oxidation and reduction potentials are determined and thus estimates of both the ionization potential $Ip$ and electron affinity $Ea$ of the polymer are obtained for the same sample under the same experimental conditions. We estimate $Ip=5.80 eV$ and $Ea=2.12 eV.$ These results disagree with the common assumption that $Ea$ is, to good approximation, given by the difference between $Ip$ and the optical gap. Measurements on indium tin oxide/polyfluorene/calcium light emitting diode structures are consistent with the deductions from the electrochemical data.