Branched Side Chains Govern Counterion Position and Doping Mechanism in Conjugated Polythiophenes

Abstract

Predicting the interactions between a semiconducting polymer and dopant is not straightforward due to the intrinsic structural and energetic disorder in polymeric systems. Although the driving force for efficient charge transfer depends on a favorable offset between the electron donor and acceptor, we demonstrate that the efficacy of doping also relies on structural constraints of incorporating a dopant molecule into the semiconducting polymer film. Here, we report the evolution in spectroscopic and electrical properties of a model conjugated polymer upon exposure to two dopant types: one that directly oxidizes the polymeric backbone and one that protonates the polymer backbone. Through vapor phase infiltration, the common charge transfer dopant, F 4 -TCNQ, forms a charge transfer complex (CTC) with the polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT), a conjugated polymer with the same backbone as the well-characterized polymer P3HT, resulting in a maximum electrical conductivity of 3 × 10 −5 S cm −1 . We postulate that the branched side chains of P3EHT force F 4 -TCNQ to reside between the π-faces of the crystallites, resulting in partial charge transfer between the donor and the acceptor. Conversely, protonation of the polymeric backbone using the strong acid, HTFSI, increases the electrical conductivity of P3EHT to a maximum of 4 × 10 −3 S cm −1 , 2 orders of magnitude higher than when a charge transfer dopant is used. The ability for the backbone of P3EHT to be protonated by an acid dopant, but not oxidized directly by F 4 -TCNQ, suggests that steric hindrance plays a significant role in the degree of charge transfer between dopant and polymer, even when the driving force for charge transfer is sufficient.

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

“…[45] We will argue below that this picture also holds true for the DDB dopants, and that dopants in conjugated polymers preferentially occupy the lamellar regions of their crystallites, when possible. Flat, small-molecule dopants like F 4 TCNQ can also π-stack with the polymer backbone, [62][63][64][65] but this only occurs under specialized circumstances such as carefully selected processing conditions that inhibit crystallization [62][63][64] or employing polymers with side chains to prevent the dopant from sitting in the lamellae and forcing it into the π-stacks. [65] Together, all of these studies suggest that π-stacking of the dopant and polymer is not a kinetically easily accessible structure.…”

“…Such measures include spinning the materials from a hot solvent that keeps the polymer dissolved and prevents undoped crystallites from forming, [62,63] or synthetically modifying the polymer with branched side chains to prevent lamellar intercalation. [65] Although the thermodynamics of the situation are not fully resolved, the kinetic problem is clear. A loss of polymer π-stacking to create partial charge transfer complexes is energetically unfavorable, and unless dopants can fully intercalate between the polymer chains, some π-stacking will be lost.…”

Structure and Doping Optimization of IDT-Based Copolymers for Thermoelectrics

“…During the doping process, the charge carrier transfers between dopants and polymers, and the concentration or mobility of the internal carriers will influence the Seebeck coefficient and electrical conductivity of CPs [15]. Recently, there are several reports of investigating the chemical doping process to improve the TE properties of CPs [16][17][18][19] For example, Jang et al reported the…”

“…During the doping process, the charge carrier transfers between dopants and polymers, and the concentration or mobility of the internal carriers will influence the Seebeck coefficient and electrical conductivity of CPs [ 15 ]. Recently, there are several reports of investigating the chemical doping process to improve the TE properties of CPs [ 16 , 17 , 18 , 19 ] For example, Jang et al reported the F4TCNQ-doped 4 H -cyclopenta[2,1- b :3,4- b′ ]dithiophene-based polymer (PCDTFBT) can achieve a high power factor up to 31.5 μW m −1 K −2 [ 20 ]. Yee et al synthesized poly(3-alkylselenophenen) (P3RSe) to achieve a high power factor of 13 μW m −1 K −2 at higher dopant concentrations (FeCl 3 ≈ 5 × 10 −3 M) [ 19 ].…”

Electrodeposited Copolymer Films with Tunable Conductivity

“…The discovery of polyacetylene [1] has generated a great interest for electrically conducting organic polymers such as polypyrroles, polyanilines, or polythiophenes [2,3]. The most outstanding property of this class of materials comes from the ease to electrochemically control and modulate their doping level to reversibly switch from an insulating to a metallic state [4]. Among heterocyclic conducting polymers, considerable attention has been drawn to electrodeposited polypyrrole and its derivatives because of their high conductivity, thermal and environmental stability, and ease of electrosynthesis [5,6].…”