The electronic structure of the interface between tris(8-hydroxyquinoline) aluminum $\left({\text{Alq}}_3\right)$ 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 ${\text{Co-Alq}}_3{\text{-La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_3$ spin valves. Further experimental results indicate that chemical interaction occurs between the ${\text{Alq}}_3$ molecules and the cobalt atoms, while the latter penetrate the ${\text{Alq}}_3$ layer upon vapor deposition of Co atoms. The data presented lead to significant progress in understanding the electronic structure of the Co-on-${\text{Alq}}_3$ interface and represent a significant step toward the definition of the interface parameters for the efficient spin injection in ${\text{Alq}}_3$ based spin valves.

• Received 19 March 2008

DOI:https://doi.org/10.1103/PhysRevB.78.045208