Despite the recent interest in “organic spintronics,” the dominant spin relaxation mechanism of electrons or holes in an organic compound semiconductor has not been conclusively identified. There have been sporadic suggestions that it might be hyperfine interaction caused by background nuclear spins, but no confirmatory evidence to support this has ever been presented. Here, we report the electric-field dependence of the spin-diffusion length in an organic spin-valve structure consisting of an $\text{Al}{q}_3$ spacer layer, and argue that these data, as well as the available data on the temperature dependence of this length, contradict the notion that hyperfine interactions relax spin. Instead, they suggest that the Elliott-Yafet mechanism, arising from spin-orbit interaction, is more likely the dominant spin relaxing mechanism.

• Received 23 September 2009

DOI:https://doi.org/10.1103/PhysRevB.81.153202