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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. 

Abstract When an electron is removed from a conjugated polymer, such as poly(3‐hexylthiophene‐2,5‐diyl) (P3HT), the remaining hole and associated change in the polymer backbone structure from aromatic to quinoidal are referred to as a polaron. Bipolarons are created by removing the unpaired electron from an already‐oxidized polymer segment. In electrochemically‐doped P3HT films, polarons, and bipolarons are readily observed, but in chemically‐doped P3HT films, bipolarons rarely form. This is explained by studying the effects of counterion position on the formation of polarons, strongly coupled polarons, and bipolarons using both spectroscopic and X‐ray diffraction experiments and time‐dependent density functional theory calculations. The counterion positions control whether two polarons spin‐pair to form a bipolaron or whether they strongly couple without spin‐pairing are found. When two counterions lie close to the same polymer segment, bipolarons can form, with an absorption spectrum that is blueshifted from that of a single polaron. Otherwise, polarons at high concentrations do not spin‐pair, but insteadJ‐couple, leading to a redshifted absorption spectrum. The counterion location needed for bipolaron formation is accompanied by a loss of polymer crystallinity. These results explain the observed formation order of single polarons, coupled single polarons, and singlet bipolarons in electrochemically‐ and chemically‐doped conjugated polymers.