  A universal mechanism for strong magnetic-field effects of nonmagnetic
organic semiconductors is presented. A weak magnetic field (less than hundreds
mT) can substantially change the charge carrier hopping coefficient between two
neighboring organic molecules when the magnetic length is not too much longer
than the molecule-molecule separation and localization length of electronic
states involved. Under the illumination of lights or under a high electric
field, the change of hopping coefficients leads also to the change of polaron
density so that photocurrent, photoluminescence, electroluminescence,
magnetoresistance and electrical-injection current become sensitive to a weak
magnetic field. The present theory can not only explain all observed features,
but also provide a solid theoretical basis for the widely used empirical
fitting formulas.
