Polaron formation mechanisms in conjugated polymers

• Published in: Physical chemistry chemical physics : PCCP Vol. 2; no. 1; pp. 317 - 331
• Main Authors: Bombile, Joel H, Janik, Michael J, Milner, Scott T
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 01.01.2018
• Subjects: Charge transport; Coupling; Crystal structure; Crystallinity; Current carriers; Deformation mechanisms; Density functional theory; Dielectric polarization; Dielectric strength; Polarons; Polymers; Spring constant
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c7cp04355d

In semiconducting polymers, interactions with conformational degrees of freedom can localize charge carriers, and strongly affect charge transport. Polarons can form when charges induce deformations of the surrounding medium, including local vibrational modes or dielectric polarization. These deformations then interact attractively with the charge, tending to localize it. First we investigate vibrational polaron formation in poly(3-hexylthiophene) [P3HT], with a tight-binding model for charges hopping between adjacent rings, coupled to ring distortions. We use density functional theory (DFT) calculations to determine coupling constants, including the "spring constant" for ring distortions and the coupling to the charge carrier. On single chains, we find only broad, weakly bound polarons by this mechanism. In 2d crystalline layers of P3HT, even weak transverse hopping between chains destabilizes this polaron. Then, we consider polarons stabilized by dielectric polarization, described semiclassically with a polarizable continuum interacting with the carrier wavefunction. In contrast to vibrational polarons, we find dielectrically stabilized polarons in P3HT are narrower, more strongly bound, and stable in 2d crystalline layers. In conjugated polymers, charge carriers interact with the dielectric medium to form polarons that are more strongly bound than vibrational polarons.