Electron interaction and optical gap of conjugated polymers

The controversy regarding whether the optical gap of a polymer is increased or decreased by electron-electron interactions can be clarified by studying the effect of screened Coulomb interactions on the correlation function of the excited states. Since the competition between the diagonal and the off-diagonal parts of the electron interaction depends on the screening, the screening becomes the decisive factor to determine the dependence of the optical gap on the electron interaction. Our theory shows that the electron interaction with weak or normal screening increases the optical gap. However, if the screening is very strong, the optical gap can be reduced by the electron interaction.