Electron Tunneling in Organic Bilayer Light-Emitting Diodes with a Novel Electron-Transporting Polymer

Abstract

Bilayer light-emitting diodes have been fabricated by combining hole-transporting tri-stilbeneamine or poly[(2,5-bis((2-ethylhexyl)oxy)-1,4-phenylene)vinylene] (EH-PPV) with new electron-transporting polystyrene copolymer carrying tert-butyl or CF 3 -substituted quaterphenyl substituents as charge-transporting moieties. The latter are resistent against recrystallization and favor internal charge accumulation by virtue of low-lying HOMO and LUMO positions. When LEDs with interfacial electron barriers ≥0.5 eV are addressed by a rectangular voltage pulse, a step-function-like onset of the electroluminescence is observed after an extended delay time that depends on the time period between successive voltage pulses. It reflects the commencement of electron tunneling once the interfacial charge density has reached a critical value. The experimental results are in accordance with model calculations.