Spin Injection into Organic Light-Emitting Devices with Ferromagnetic Cathode and Effects on Their Luminescence Properties

We investigated the luminescence properties of organic light-emitting devices under an external magnetic field. Circularly polarized light was observed from the devices with an Fe cathode, whereas those with an Al cathode did not show such behavior. The degree of circular polarization on the device with an Fe cathode increased with increasing applied magnetic field and decreased with increasing emissive layer thickness. These phenomena can be well explained in terms of the recombination of spin-polarized electrons injected from a ferromagnetic cathode. Consequently, spin diffusion length in the emissive layer, tris-(8-hydroxyquinolinato)-aluminum was estimated to be less than 60 nm from the thickness dependence of the degree of circular polarization