Branched Side Chains Influence the Efficacy of Doping in Conjugated Polymers

The principles that govern effective charge transfer between dopants and semiconducting polymers are poorly understood. It is currently unclear how the position of the dopant in the thin film can affect carrier mobility. Here, we report the evolution in spectroscopic and electrical properties of a model conjugated polymer upon exposure to two types of dopants: a strong oxidant (F₄TCNQ) and a strong acid (HTFSI). The model polymer was poly(3-(2′-ethyl)hexylthiophene) (P3EHT), a branched side chain analogue of the well-characterized polymer P3HT. We find that F₄TCNQ forms a charge transfer complex (CTC) with P3EHT resulting in a maximum electrical conductivity of 3×10^–5 S cm^–1. We postulate that the branched side chains of P3EHT constrain the position of F₄TCNQ within the P3EHT crystallites, resulting in partial charge transfer between the donor and acceptor. Conversely, protonation of the polymeric backbone from HTFSI increases the electrical conductivity of P3EHT to 4×10^–3 S cm^–1, two orders of magnitude higher than when F₄TCNQ is used. This work shows that a favorable energetic offset between the donor and acceptor is not sufficient to predict the charge transfer mechanism, but also relies on structural constraints of incorporating a dopant molecule into the polymer film.