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Modeling Polaron Excitations and Stabilization Mechanisms in Conjugated Polymers

Vishal Jindal, Scott T. Milner

TL;DR

The paper addresses the microscopic origin of polaron stabilization in conjugated polymers, using poly(3-hexylthiophene) (P3HT) as a case study. It introduces a dielectric-stabilized tight-binding model, parameterized from first-principles, to compute ground and excited polaron states and connect excitation energies to mid-IR spectroscopic features, particularly the chain-length dependence of peak B and the RR vs RRa distinctions. By contrasting this with a Holstein-like ring-distortion mechanism, the authors show that dielectric polarization provides sufficient binding and realistic excitation energies, while ring distortions yield weak stabilization incompatible with experiments. The work delivers a predictive framework linking polaron energetics to spectroscopic observables, advancing our understanding of charge transport in organic semiconductors.

Abstract

Charge carriers in organic semiconductors form polarons, which are self-localized states stabilized by interactions with their environment. Using a dielectric-stabilized tight-binding model parameterized from first-principles calculations, we compute ground and excited polaron states in poly(3-hexylthiophene) (P3HT). Our results quantitatively reproduce key mid-infrared absorption features, notably the chain-length-dependent shift of the intrachain polaron excitation peak (peak B) and its variation between regioregular and regiorandom P3HT. Comparison to an alternative stabilization mechanism based on local ring distortions reveals that dielectric polarization dominates polaron formation, as ring distortions yield insufficient binding and excitation energies inconsistent with experiments. These findings clarify the microscopic origin of polarons in conjugated polymers and provide a predictive framework linking polaron energetics to spectroscopic observables, advancing the understanding of charge transport in organic semiconductors.

Modeling Polaron Excitations and Stabilization Mechanisms in Conjugated Polymers

TL;DR

The paper addresses the microscopic origin of polaron stabilization in conjugated polymers, using poly(3-hexylthiophene) (P3HT) as a case study. It introduces a dielectric-stabilized tight-binding model, parameterized from first-principles, to compute ground and excited polaron states and connect excitation energies to mid-IR spectroscopic features, particularly the chain-length dependence of peak B and the RR vs RRa distinctions. By contrasting this with a Holstein-like ring-distortion mechanism, the authors show that dielectric polarization provides sufficient binding and realistic excitation energies, while ring distortions yield weak stabilization incompatible with experiments. The work delivers a predictive framework linking polaron energetics to spectroscopic observables, advancing our understanding of charge transport in organic semiconductors.

Abstract

Charge carriers in organic semiconductors form polarons, which are self-localized states stabilized by interactions with their environment. Using a dielectric-stabilized tight-binding model parameterized from first-principles calculations, we compute ground and excited polaron states in poly(3-hexylthiophene) (P3HT). Our results quantitatively reproduce key mid-infrared absorption features, notably the chain-length-dependent shift of the intrachain polaron excitation peak (peak B) and its variation between regioregular and regiorandom P3HT. Comparison to an alternative stabilization mechanism based on local ring distortions reveals that dielectric polarization dominates polaron formation, as ring distortions yield insufficient binding and excitation energies inconsistent with experiments. These findings clarify the microscopic origin of polarons in conjugated polymers and provide a predictive framework linking polaron energetics to spectroscopic observables, advancing the understanding of charge transport in organic semiconductors.
Paper Structure (5 sections, 21 equations, 9 figures, 1 table)

This paper contains 5 sections, 21 equations, 9 figures, 1 table.

Table of Contents

  1. Introduction
  2. Tight-binding polaron model
  3. Polaron excitation
  4. Alternative mechanism: ring distortion stabilization
  5. Discussion

Figures (9)

  • Figure 1: Experimental mid-infrared absorption spectra of regioregular (RR) and regiorandom (RRa) poly(3-hexylthiophene) (P3HT). Two distinct absorption features are observed: Peak A, a narrow low-energy band that appears in both RR and RRa P3HT, and Peak B, a broader high-energy band whose position and intensity vary significantly between RR and RRa-P3HT.
  • Figure 2: The charge carrier and its induced polarization in the surrounding medium modeled as smeared charge distributions.
  • Figure 3: Ground state (blue) and excited state (red) polaron wavefunctions $\Psi$ on straight P3HT chain consisting of 10 monomers. The inset shows the probability distribution $\Psi^2$ for the same polaron wavefunction.
  • Figure 4: Ground state (blue) and excited state (red) polaron wavefunctions $\Psi$ on straight P3HT chain consisting of 20 monomers. The inset shows the probability distribution $\Psi^2$ the same polaron wavefunction.
  • Figure 5: Polaron excitation energy as a function of oligothiophene (P3HT chain segment) length in terms of monomers, for $\epsilon_r = 2.5$.
  • ...and 4 more figures