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Abstract The relaxation of photoexcited polarons in doped conjugated polymers is studied with ultrafast transient absorption (TA) spectroscopy to examine the effect of polymer morphology and counterion size on polaron mobility. Processing conditions are first used to create F₄TCNQ‐doped (2,3,5,6‐tetrafluoro‐tetracyanoquinodimethane) poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) films with different morphologies and thus free and trapped polarons in different ratios. We find that less crystalline films have a higher fraction of trapped polarons, but, remarkably, that free and trapped polarons have the same relaxation times in all films. Films doped with a large dodecaborane (DDB) cluster‐based dopant are then used to show that trapping is based on Coulomb interactions between polarons and counterions; no trapped polarons are observed in TA due to the reduced Coulomb interaction between the polarons and the DDB counterion. Indeed, the relaxation of polarons in these films is an order of magnitude faster than that in F₄TCNQ‐doped films, consistent with reduced trapping. Finally, the results are used to argue that counterion size has a greater effect on polaron mobility than polymer morphology and crystallinity. All of the experiments show that pump/probe spectroscopy provides a straightforward way to determine the local mobilities and degree of carrier trapping in doped conjugated polymer films. 

To improve their electrical conductivity for various applications, semiconducting polymer films are often chemically doped to increase their equilibrium charge carrier density. Recently, a novel doping method involving anion exchange has provided control over the identity of the counterions that reside in such films, leading to increased stability under ambient conditions. In this work, however, we show that by ion-exchanging 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane-doped poly(3-hexylthiophene-2,5-diyl) films with hygroscopic salts like bis(trifluoromethane)sulfonimide lithium or LiPF6, the doped film's electrical conductivity drops significantly when exposed to ambient humidity. The change in electrical conductivity depends directly on the degree of hygroscopicity of the counterion and can be over 50% with relatively modest changes in relative humidity (RH), and up to a factor of four between ambient and completely dry conditions. The film's humidity response is entirely reversible when adsorbed water is removed, potentially allowing the doped semiconducting polymer films to function as humidity sensors. Hall effect measurements show that the cause of the drop in conductivity with increasing RH is due to a decrease in carrier mobility and not due to de-doping. Our results emphasize that it is important to control the sample environment when making electrical measurements on anion-exchange doped semiconducting polymer films.