Excitons and the band gap in poly(phenylene vinylene)

The large extent of the singlet exciton in poly(phenylene vinylene) (PPV) and the small Stokes shift suggest an effective-mass calculation for determining the exciton binding energy ${\mathit{E}}_{\mathit{b}}$. We have carried out such a calculation using the electron and hole masses determined by a local-density-functional band-structure calculation. The resulting ${\mathit{E}}_{\mathit{b}}$ is 0.4 eV, which leads to a gap of ∼2.8 eV rather than the usually accepted 2.4 eV.