Polymer:fullerene photovoltaics have potential in small-scale power production but low open-circuit voltages limit their efficiency. Understanding the processes affecting the charge recombination rate is key to increasing device efficiency through optimizing open-circuit voltage. Most polymer-fullerene systems have an intramolecular triplet exciton state lower in energy than the interfacial charge-transfer state, and its formation can provide a terminal recombination pathway that may limit device performance. We used magnetic fields to modulate intersystem crossing in a prototypical system, and monitored the effect on the open-circuit voltage to infer changes in the steady-state carrier density and hence in the net recombination rate constant. We analyzed these effects using density-matrix modeling, and quantified the various recombination rate constants for a working device.