Spectral Signatures\nof Polarons and Bipolarons in\nP3HT: Single Chains vs π‑Stacks

The impact of interchain interactions on the mid-IR absorption\nspectrum of polarons and bipolarons in π-stacked aggregates\nof p-doped conjugated polymers is investigated theoretically. These\nsystems are described using a Hamiltonian that allows for hole hopping\nin two dimensions, as well as the hole–hole and counterion–counterion\nrepulsive Coulombic interactions and the attractive hole–counteranion\ninteractions. Interchain interactions are found to dramatically increase\nthe bipolaron binding energy as well as the singlet–triplet\nenergy gap. They also lead to an additional near-IR peak in the bipolaron\nspectrum that is blue-shifted relative to the polaron P<sub>1</sub> band, thereby accounting for measurements on electrochemically doped\nP3HT. Ionized multipolaron complexes are also investigated and are\nshown to contribute additional red and blue-shifted spectral bands.