Formation of LiF4TCNQ Complexes During High Concentration Anion-Exchange Doping of Semiconducting Polymers

Anion-exchange doping, in which a semiconducting polymer is exposed to a solution containing both a dopant and an electrolyte, has become a popular method to create polarons on conjugated polymers to increase their electrical conductivity. Although many different initiator dopants can be used, a common dopant/salt combination for anion-exchange p-doping is 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F₄TCNQ) and lithium bis(trifluoromethane) sulfonimide (LiTFSI). Depending on the concentration of initiator dopant, it is usually presumed that all the F₄TCNQ^– ions that remain after doping are exchanged out by mass action for the salt anion, TFSI^–. When both LiTFSI and F₄TCNQ are present in excess, however, we find that two new peaks appear in the UV–visible absorption spectrum of anion-exchange-doped conjugated polymers that are not seen when conventionally doping without the addition of salt. We further see that these peaks appear in the same spectral regions, ∼1.95 and ∼3.65 eV, independent of the conjugated polymer being doped by F₄TCNQ/TFSI anion exchange, and that they do not appear when initiator dopants other than F₄TCNQ are used. With the aid of Resonance Raman spectroscopy and quantum chemistry calculations, we assign these peaks to a LiF₄TCNQ complex that forms during the exchange process of anion-exchange doping and remains in the doped polymer film. We estimate that roughly ∼25% of the F₄TCNQ^– ions present during the anion exchange doping process are converted into LiF₄TCNQ complexes, and show that the complex can be dissociated back into F₄TCNQ^– by washing with an appropriate solvent.