Bipolaron Stabilization via Proton-Coupled Electron Transfer: The Case of n-Doped P(NDI2OD-T2)

Polaron and bipolaron formation upon doping plays a central role in determining the transport, optical, and magnetic properties of π-conjugated polymers. Using density functional theory, we model a series of P(NDI2OD-T2) chains that are n-doped via proton-coupled electron transfer (PCET) upon attaching a varying number of hydrogen atoms to the redox-active carbonyl groups of the naphthalene diimide (NDI) moieties. Our calculations demonstrate that the PCET-induced n-doping of P(NDI2OD-T2) gives rise to energetically stable bipolaron-based states associated with the effective protonation and bond reorganization of carbonyl groups; this situation is in marked contrast with the one found earlier in the case of P(NDI2OD-T2) n-doped by molecular dopants. Also, here, strong Coulomb repulsion and weak electronic coupling between adjacent bipolarons are observed, underlining the fact that these bipolaron-based states could be explored to realize charge density wave and Mott insulator phases. These findings not only provide a different perspective on the charge localization/delocalization characteristics in n-doped conjugated polymers but also expand their potential applications in emerging electronic and energy-related devices.