Magneto-photocurrent in organic photovoltaic cells; the effect of short-lived charge transfer states

The spin degrees of freedom are responsible for the magnetic field effects in organic devices at low magnetic fields. The MFE is formed via a variety of spin-mixing mechanisms, such as the hyperfine (typical strength: B$_{\mathrm{hf}}$\textless 0.003 T), triplet-polaron or triplet-triplet (B$_{\mathrm{trip}}$\textless 0.1 T) interactions, that limit the response by their respective strength. We report on magneto-photocurrent (MPC) response of bulk hetero-junction organic photovoltaic cells in an extended field range B$=$0.00005 - 8 Tesla, and found that spin mixing mechanisms are still operative even at the highest fields. In fact, the response MPC(B) can be divided into three main regions, each with a different sign: sharp response that increases with B up to B$_{1}$ $\sim$ 0.04 T; broad response that decreases with B in the range from B$_{1}$ to B$_{2}$ $\sim$ 0.3-0.7 T; and even broader response that increases above B$_{2}$; this response does not saturate even at 8.5 T. We attribute the latter MPC component to short-lived charge transfer excitons (CTE) where spin-mixing is caused by the difference of the donor/acceptor g factors; a mechanism that is increasingly more effective at high magnetic field.