Theory of Organic Magnetoresistance in Disordered Organic Semiconductors

The understanding of spin transport in organics has been challenged by the discovery of large magnetic field effects on properties such as conductivity and electroluminescence in a wide array of organic systems. To explain the large organic magnetoresistance (OMAR) phenomenon, we present and solve a model for magnetoresistance in positionally disordered organic materials using percolation theory. The model describes the effects of singlet-triplet spin transitions on hopping transport by considering the role of spin dynamics on an effective density of hopping sites. Faster spin transitions open up 'spin-blocked' pathways to become viable conduction channels and hence produce magnetoresistance. We concentrate on spin transitions under the effects of the hyperfine (isotropic and anisotropic), exchange, and dipolar interactions. The magnetoresistance can be found analytically in several regimes and explains several experimental observations.