摘要： Predicting the interactions between a semi-conducting polymer and dopant is not straightforward due to the intrinsic structural and energetic disorder in polymeric systems. Although the driving force for e?cient charge transfer depends on a favorable o?set between the electron donor and acceptor, we demonstrate that the e?cacy of doping also relies on structural constraints of incorporating a dopant molecule into the semiconducting polymer ?lm. 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 in?ltration, the common charge transfer dopant, F4-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 F4-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 F4-TCNQ, suggests that steric hindrance plays a signi?cant role in the degree of charge transfer between dopant and polymer, even when the driving force for charge transfer is su?cient.