For applications in flexible electronics and thermoelectric devices, conjugated polymer films are often doped to introduce equilibrium charge carriers on the π-conjugated polymer backbone. If only one electron is removed, the remaining hole and the associated aromatic-to-quinoid structural changes are referred to as a polaron. When polarons are created, the conductivity of the films increases, but when bipolarons are created, the conductivity decreases. When the already-oxidized polymer segment is oxidized again, i.e., when the unpaired electron from a polaron is removed, a bipolaron is formed. In electrochemically-doped poly(3-hexylthiophene-2,5-diyl) (P3HT) films, both polarons and bipolarons are readily observed with distinct oxidation peaks in the cyclic voltammetry curve. However, in chemically-doped P3HT films, bipolarons are only observed for particular oxidation agents under specific conditions. We examined the formation of polarons and bipolarons using spectroscopic and structural experiments on electrochemically and chemically doped films in combination with time-dependent density functional theory (TD-DFT) calculations. We found that whether a spin-paired bipolaron is formed depends on the counterion positions. If the counterions are close to each other and to the polymer backbone, two polarons will spin-pair to form a bipolaron, which is characterized by a blue-shifted absorption spectrum. On the other hand, if the counterions are farther from the polymer backbone or each other, two polarons will not spin-pair, but instead J-couple, leading to a red-shifted absorption. X-ray diffraction shows that the counterion positions needed for the formation of bipolarons are associated with a loss of polymer crystallinity in chemically-doped films. Our combined experimental and simulation study explains the order of formation and the spectroscopic signature for polarons, coupled polarons, and bipolarons in chemically-doped and electrochemically-doped P3HT films.

Counterion positions and the formation of polarons, coupled polarons, and bipolarons in electrochemically doped and chemically doped P3HT film 

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