  A theory is introduced for spin relaxation and spin diffusion of hopping
carriers in a disordered system. For disorder described by a distribution of
waiting times between hops (e.g. from multiple traps, site-energy disorder
and/or positional disorder) the dominant spin relaxation mechanisms in organic
semiconductors (hyperfine, hopping-induced spin-orbit, and intra-site spin
relaxation) each produce different characteristic spin relaxation and spin
diffusion dependences on temperature. The resulting unique experimental
signatures predicted by the theory for each mechanism in organic semiconductors
provide a prescription for determining the dominant spin relaxation mechanism.
