ELECTRON-TUNNELING IN ORGANIC BILAYER LIGHT-EMITTING-DIODES WITH A NOVEL ELECTRON-TRANSPORTING POLYMER

Bilayer light-emitting diodes have been fabricated by combining hole-t ransporting tri-stilbeneamine or 2,5-bis((2-ethylhexyl)oxy)-1,4-phenyl ene)vinylene] (EH-PPV) with new electron-transporting polystyrene copo lymer carrying tert-butyl or CF3-substituted quaterphenyl substituents as charge-transporting moieties. The latter are resistent against rec rystallization and favor internal charge accumulation by virtue of low -lying HOMO and LUMO positions. When LEDs with interfacial electron ba rriers greater than or equal to 0.5 eV are addressed by a rectangular voltage pulse, a step-function-like onset of the electroluminescence i s observed after an extended delay time that depends on the time perio d between successive voltage pulses. It reflects the commencement of e lectron tunneling once the interfacial charge density has reached a cr itical value. The experimental results are in accordance with model ca lculations.