Ferromagnetic mechanism in organic photovoltaic cells with closed-shell structures

We construct a model to reveal the spin polarization or ferromagnetism observed in organic composite nw-P3HT/C60 with closed-shell structures. Different from the organic ferromagnets with open-shell structures, the ferromagnetism of nw-P3HT/C60 comes from the charge transfers from the polymer to the small molecules. The transferred electrons become spin polarized and they are coupled together through the holes in the polymer. Finally, a ferromagnetic order appears in the pure organic composite. The magnetic moment of the system is mainly provided by the spin polarized small molecules. The magnetization is dependent upon the density of the transferred charges, which is consistent to our experimental observations. Our investigation also shows that some new spin phenomena may appear in excited states for organic semiconductors which is absent in the ground states.