Magnetophotocurrent in Organic Bulk Heterojunction Photovoltaic Cells at Low Temperatures and High Magnetic Fields

We study high-field (up to B∼8.5 T) magnetophotocurrent (MPC) related to photogenerated polaron pairs (PPs) in the temperature range T=10-320 K in organic bulk heterojunction photovoltaic cells. We find that in the high-field regime (B>1 T), MPC(B) response increases with B for temperature T>200 K but decreases with B at T<200 K. MPC(B) response does not saturate even at the highest field studied, at all T. We attribute the observed high-field MPC(B) response to two competing mechanisms within the PP spin states: (a) a spin-mixing mechanism caused by the difference in the donor-acceptor (or positive-negative polarons) g factors (the so-called "Δg mechanism"), and (b) the spin polarization induced by thermal population of the PP Zeeman split levels. The nonsaturating MPC(B) response at high fields and high temperatures indicates that there exist charge-transfer excitons (CTEs) with decay time in the subnanosecond time domain. With decreasing temperature, the CTE decay time sharply increases, thereby promoting an increase of the thermal spin-polarization contribution to the MPC(B) response.