Role of structure and charge trapping on the bipolaron formation and magnetic-field response of gated conjugated polymers
Conjugated polymers exhibit unique spin-dependent phenomena arising from weak yet critical hyperfine interactions. Understanding these spin effects, particularly the spin-dependent formation and decay of correlated spin pairs, is important for advancing both organic electronics and polymer-based spintronics. Intrinsic magnetic-field responses such as magnetoresistance have primarily been investigated in diode architectures, where electrons and holes coexist. However, such systems are less suitable for probing bipolaron formation in unipolar transport, and the relationship between polymer structure and bipolaron formation in lightly doped polymers remains unclear. Here, we systematically investigate intrinsic magnetoresistance in representative conjugated polymers using field-effect transistors and observe a generally positive magnetoresistance. First-principles simulations reveal that bipolarons preferentially form on short conjugated segments associated with amorphous regions. Moreover, comparisons across these polymers show that enhanced charge trapping correlates with stronger magnetoresistance, implying promoted bipolaron formation. Bipolaron-incorporated energylevel-alignment modeling near metal/polymer interfaces suggests that charge traps can increase the bipolaron density.