Electron spin is a key consideration for the function of light-emitting diodes and solar cell devices. For example, the population of triplet excitons from recombining charges in organic solar cells is thought to be a main cause of photocurrent loss, but the exact mechanism of charge recombination to triplet states, whether geminate or nongeminate and whether it involves spin-state mixing is not well under-stood. In this contribution, the dynamics of free charge separation competing with recombination to polymer triplet states is studied in two closely related silaindacenodithiophene-based polymer/fullerene blends with differing photovoltaic performance. We use time-resolved laser spectroscopic techniques to show that the population of triplet states can be associated with the formation of bound electron-hole pairs at the polymer/fullerene interface. The pairs undergo spin-state mixing on the nanosecond time-scale and subsequent geminate recombination to the triplet exciton. We find that these bound electron-hole pairs can be dissociated by electric fields but still limit the overall generation of device photocurrent.