Temperature effect on spin relaxation in organic semiconductors

Spin relaxation due to energy level fluctuation in organic semiconductors is investigated by solving the Schrödinger 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.

► At finite temperature molecular vibration induces spin relaxation.
► Relaxation happens between spin singlet and triplet states but not triplet states.
► The relaxation does not happen between spin states of an exciton.
► At room temperature the relaxation is much quicker than the interconversion.
► The Zeeman splitting has weak effect on the relaxation when temperature is high.