Spin dynamics control of recombination current in organic semiconductors

The recombination rate of coulombically bound electron–hole pairs depends on their spin configuration. Because of low spin–orbit coupling the spin dynamics of these well separated coulombic pairs is determined by weak hyperfine and exchange interactions. In this case a weak magnetic field produces strong effect on the spin dynamics and hence on the recombination rate of e–h pairs. We have shown that the recombination current in organic semiconductors may have a maximum as a function of recombination constant. For high recombination constants the current is space charge limited and decreases with increasing the e–h recombination constant. This decay of current is due to decrease of the region where the recombination takes place. At a low recombination constant the recombination takes place in the whole volume and the current increases with increasing the recombination constant. The characteristic recombination constant separating those two regimes depends on the thickness of sample, applied voltage, and charge carrier mobilities. The model predictions are consistent with experimental data for magnetoresistance of organic semiconductors.