Conjugated polymers are usually doped by mixing an oxidizing agent with the polymer. Typically this is done in solution prior to casting a film of the doped polymer, but as the polymer becomes charged in solution it also becomes less soluble, leading to poor film morphology. We have developed an alternate method for chemically doping polymers that we refer to as sequential processing (SqP). In SqP, a film of the pristine conjugated polymer is cast first, and then the dopant is introduced in a second casting step using a solvent that swells but does not dissolve the underlying polymer. The degree of dopant penetration can be controlled by changing how the solvent in the second step swells the polymer, and the resultant doped film quality is as good as that of the original pristine polymer film. The excellent film quality allows us to easily make electronic measurements, including AC Hall Effect, for understanding carrier concentrations and mobilities. Moreover, since crystalline regions of the polymer do not swell, the crystallinity of the polymer can be controlled in the first casting step and then preserved through the doping process, allowing doping to be studied with a controllably tunable polymer morphology. We present results for a series of conjugated polymers, including P3HT, with a series of molecular dopants. We find that the degree to which the polarons delocalize depend directly on the crystallinity of the polymer film, providing a direct means to tune the carrier mobility and film conductivity. Spectroscopy measurements show that the polaron delocalization and mobilty depend on how strongly the dopant anions interact with the polarons; if we choose dopants where the countercharge is kept far from the polymer backbone, we can greatly improve polaron delocalization, mobility and conductivity. At the highest doping levels, we are able to drive many conjugated polymer systems to form bipolarons, whose properties we investigate with ultrafast transient absorption measurements.

Optoelectronic properties of sequentially-processed, chemically-doped, controlled crystallinity conjugated polymer fIlms

255th National Meeting (2018)

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