Temperature effect on spin relaxation in organic semiconductors

Spin relaxation due to energy level fluctuation in organic semiconductors is investigated by solving the Schrِdinger equation of a two-level system with diagonal Hamiltonian elements being described by stochastic processes. When the polaron pair situates on one molecule, or an exciton, no spin relaxation is possible. In the opposite case when the coupled polaron pair distributes in two neighbor molecules, the relaxation occurs between spin singlet and one of the spin triplet states but not between spin triplet states. Based on our results we discuss the temperature effect in the exciton and bipolaron mechanisms proposed for organic magnetoresistance.