Organic spin valves: hole injection from ferromagnetic materials into tris-8-hydroxyquinoline (Alq3) in the presence of interface states.

Abstract

This thesis presents the result of the charge carrier injection and the subsequent transport from ferromagnetic material into organic semiconductor in the Alq3 based organic spin valves. In order to study the dominant charge carrier polarity in the Alq3 based spin valves, a number of single (Alq3) layer and double (Alq3 and N,N’-bis(3-methylphenyl)-N,N’- diphenylbenzidine (TPD)) layer organic semiconductor diode devices are constructed using both conventional electrode materials as well as ferromagnetic electrodes. Single layer devices are characterised by time of flight (ToF) and dark injection (DI) transient techniques with or without ferromagnetic anodes. Double layer devices are characterised using current-voltage-luminescence (j-V-L) measurements with or without ferromagnetic cathodes. Despite Alq3 being considered an electron transport material, we measure long range hole transport within the devices with matched electron and hole mobility at large electric fields. The substitution of a conventional Al cathode with a ferromagnet drastically suppresses electroluminescence in double layer devices, due to poor electron injection from the large work function ferromagnet. DI measurements using a ferromagnetic anode display characteristic charge trapping consistent with the presence of hybridized interface states (HINTS) between anode and organic semiconductor. The temperature dependent DI and ToF measurements demonstrate a reduced hole injection barrier in the presence of the HINTS in the ferromagnetic/organic interface that enables Alq3 based organic spin valves operate at small bias. We conclude that the dominant charge carriers in Alq3 based spin valves are holes, contrary to conventional wisdom, and that hole injection under small bias conditions is aided by HINTS.