Spectral Signatures of Polarons and Bipolarons in P3HT: Single Chains vs π-Stacks

The impact of interchain interactions on the mid-IR absorption spectrum of polarons and bipolarons in π-stacked aggregates of p-doped conjugated polymers is investigated theoretically. These systems are described using a Hamiltonian that allows for hole hopping in two dimensions, as well as the hole–hole and counterion–counterion repulsive Coulombic interactions and the attractive hole–counteranion interactions. Interchain interactions are found to dramatically increase the bipolaron binding energy as well as the singlet–triplet energy gap. They also lead to an additional near-IR peak in the bipolaron spectrum that is blue-shifted relative to the polaron P1 band, thereby accounting for measurements on electrochemically doped P3HT. Ionized multipolaron complexes are also investigated and are shown to contribute additional red and blue-shifted spectral bands.