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 similar to 3%. Using internal field emission (Fowler-Nord heim tunneling) in single carrier devices, the top of the pi band and the bottom of the pi band have been determined for DO-PPP, and for po ly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV. F or DO-PPP, the single particle pi-pi energy gap is 3.4-3.6 eV; for ME H-PPV, the pi-pi energy gap is 2.2-2.3 eV. Comparison with optical ab sorption (the optical gap) implies that the exciton binding energy is small; for both polymers, E(exc) less than or equal to 0.2 eV. Such sm all residual values are probably not significant in the presence of di sorder broadening which is also 0.1-0.2 eV and in the presence of latt ice relaxation with self-localization energies of approximately 0.2 eV .