Impact of the dopant location in the semi-crystalline structure of alternated donor-acceptor copolymers on the polarity switching p → n mechanism

Strong p-type doping of alternated donor-acceptor copolymers induces p -> n polarity switching upon progressive band filling, the change of polarity coinciding with the maximum of charge conductivity. This study uncovers the role of the dopant's location in the semi-crystalline structure of the conjugated polymer on the mechanism of polarity switching in highly aligned polymer films of two alternated donor-acceptor copolymers. Using a combination of transmission electron microscopy and polarized UV-vis-NIR spectroscopy, the polarity switching is only observed when the dopants are present in both the crystalline and the amorphous domains of the polymer. This is observed for dopants such as FeCl3. In strong contrast, the dopant magic blue is not able to induce polarity switching because the dopants are not intercalated into the crystals but only in the amorphous phase of the polymer semiconductors. The precise location of dopants in the semi-crystalline structure of a polymer semiconductor is therefore a handle to induce or not the polarity switch and to tune the thermoelectric and charge transport properties of doped polymer semiconductors. The location of dopants in the crystalline and/or amorphous domains of D-A polymers determines the polarity switching of the Seebeck coefficient.