Electrical and magnetic characterization of doped conjugated polymers with pendent stable radicals*

Conjugated polymers such as poly(3-hexylthiophene-2,5-diyl) (P3HT) are being considered as potential backbone materials for organic radical battery electrodes to increase conductivity over unconjugated backbones, without sacrificing the electrochemical activity of radical groups. This is important for efficient current collection. Although conjugated polymers are among the most conductive organic materials at ambient temperatures, covalent attachment of stable radical such as such as 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to undoped, regioregular P3HT creates steric hindrances that result in an exponential decrease of film conductivity as a function of radical content. To recover the conductivity, we dope the conjugated backbone using either iodine or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). We characterize electrical properties of the doped conjugated radical polymer by varying both the pendent radical concentration and the doping level. Additionally, we used electron paramagnetic resonance to investigate the interplay between polarons and stable radical electrons. Transport measurements confirm that the conductivity increases by a few to several orders of magnitude, depending on the dopant.