Energy level alignment and chemical interaction at Alq3/Co interfaces for organic spintronic devices

The electronic structure of the interface between tris(8-hydroxyquinoline) aluminum (Alq(3)) and cobalt was investigated by means of photoelectron spectroscopy. As demonstrated recently, this interface is characterized by efficient spin injection in organic spintronic devices. A strong interface dipole that reduces the effective work function of cobalt by about 1.5 eV was observed. This leads to a large barrier for hole injection into the highest occupied molecular-orbital (HOMO) level of 2.1 eV, in agreement with a previously proposed model based on electron transport in Co-Alq(3)-La0.7Sr0.3MnO3 spin valves. Further experimental results indicate that chemical interaction occurs between the Alq(3) molecules and the cobalt atoms, while the latter penetrate the Alq(3) layer upon vapor deposition of Co atoms. The data presented lead to significant progress in understanding the electronic structure of the Co-on-Alq(3) interface and represent a significant step toward the definition of the interface parameters for the efficient spin injection in Alq(3) based spin valves.