Spectral Signatures and Spatial Coherence of Bound and Unbound Polarons in P3HT Films: Theory Versus Experiment

The origin of the mid-infrared (IR) spectral features for hole absorption in doped poly(3-hexylthiophene) (P3HT) films is investigated theoretically and experimentally. Using a Holstein-style Hamiltonian to treat vibronic coupling involving the prominent vinyl stretching mode, the low-energy peak (A) occurring in the spectral interval 0.1–0.15 eV is found to contain a substantial (sometimes dominant) intrachain-polarized component, in contrast to the predictions of the more conventional treatments based on self-trapped, mid-gap polaron states where peak A is entirely interchain in origin. A higher-energy peak (B) located between 0.35 and 0.7 eV and largely intrachain-polarized is also obtained and associated with the conventional P1 polaron transition. Spectral signatures for polaron coherence based on peaks A and B are identified and used to analyze the molecular weight dependence of the IR spectral line shapes of P3HT doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane. Generally, the polaron coherence lengths along the chain and stacking axes increase with molecular weight, which is consistent with similar studies conducted on hole absorption in undoped P3HT.