Efficient blue light-emitting diodes from a soluble poly (para-phenylene) internal field emission measurement of the energy gap in semiconducting polymers

Polymer light-emitting diodes with poly (2-decyloxy-1,4-phenylene), DO-PPP, as the luminescent polymer emit blue light with external quantum efficieny of ∼3%. Using internal field emission (Fowler-Nordheim tunneling) in single carrier devices, the top of the π band and the bottom of the π* band have been determined for DO-PPP, and for poly (2-methoxy-5-(2ʹ-ethyl-hexyloxy)-1,4phenylene vinylene), MEH-PPV. For DO-PPP, the single particle π-π* energy gap is 3.4–3.6 eV; for MEH-PPV, the π-π* energy gap is 2.2–2.3 eV. Comparison with optical absorption (the optical gap) implies that the exciton binding energy is small; for both polymers, Eexc ≤ 0.2 eV. Such small residual values are probably not significant in the presence of disorder broadening which is also 0.1–0.2 eV and in the presence of lattice relaxation with self-localization energies of approximately 0.2 eV.