Electron spin resonance can provide unique insights into charge transport processes in organic semiconductors in a regime in which charge motion determines spin relaxation. In particular, electrically detected magnetic resonance (EDMR) probes directly the changes in charge transport properties that are sensitive to magnetic resonance excitation. Here, we present a systematic study of continuous-wave EDMR on conjugated polymer field-effect transistors (FETs) that can be operated in both unipolar as well as ambipolar regimes. We show that, in addition to a narrow, electron-hole recombination-induced EDMR signal that can only be detected in the ambipolar regime, there is also a broad EDMR signal when devices are operated in both unipolar and ambipolar regions. We attribute this signal to a spin blockade mechanism induced when mobile carriers encounter trapped charges along the charge transport percolation pathways and study its dependence on biasing conditions and temperature. The spin-blockade EDMR signature is also observed in conjugated polymer FETs that exhibit only unipolar operation. Our findings show that EDMR provides a powerful technique to study the role of spin blockade and bipolaron formation on the charge transport properties of a wide range of conjugated polymers.
