Spin relaxation and magnetoresistance in disordered organic semiconductors

Because of the light elements involved, the spin–orbit coupling in organic materials is small. Therefore, the spin of charged polarons in these materials is expected to be a well conserved quantity. The conviction in the community grows that the main source of spin relaxation is in fact the coupling of the polaron spin to the random hyperfine fields of the hydrogen nuclei. By considering reactions between polarons forming bipolarons or excitons in the presence of these hyperfine fields we explain line shapes of the intrinsic magnetoresistance observed in disordered organic semiconductors. We also show how these hyperfine fields determine the spin-diffusion length in these semiconductors and how this affects the magnetoresistance line shapes of organic spin valves.