Injection and transport processes in organic light emitting diodes based on a silole derivative

Abstract

This paper reports on charge injection and transport in electroluminescent devices based on a silole derivative 1,1-dimethyl-2,5-bis(p-2,2′-dipyridylaminophenyl)-3,4-diphenylsilole (DMPPS). The devices are composed of tin doped indium oxide (In2O3:Sn or I TO)/poly(3,4-ethylene dioxythiophene doped with poly(styrene sulfonate)/DMPPS/metal. Current-voltage and luminance-voltage characteristics are first performed as a function of the electron injection barrier height and of the organic layer thickness. The voltage dependence of current and luminance varies with the metal cathode, i.e., Ca, Al, Cu, and Au. The effect of the DMPPS thickness in a double carrier device shows that electrons predominate and are bulk limited. An accurate investigation is carried out as a function of temperature for hole-only and bipolar devices, i.e., with gold and calcium cathodes. Hole-only devices (with Au cathode) exhibit an Ohmic behavior for low voltages. A hopping mechanism (thermally assisted tunnel transfer between localized states) agrees with experimental data, since activation energy is found close to 50meV. The electron transfer limitation is located at the DMPPS/cathode interface and the Fowler-Nordheim mechanism is qualitatively consistent with experimental data at high voltages. With a Ca cathode, electron conduction is preponderant and is bulk limited. A power dependence J∝Vm with m>2 is consistent with the model of trap charge limited conduction. The total electron trap density was estimated to be 2×1018cm−3.