Raman characterization and electrical properties of poly(3-hexylthiophene) doped electrochemically in an ionic liquid-gated transistor geometry

Using Raman spectroscopy, we observed carriers, polarons and bipolarons formed in an ionic-liquid-gated P3HT electrochemical transistor with an ionic liquid [BMIM][TFSI] as a gate dielectric. The relationships between the source-drain current (I_D), the gate voltage (V_G) at a constant source-drain voltage (V_D), and injected charges at each V_G were investigated. An increase in I_D is attributed to the formation of positive polarons, whereas a decrease in I_D corresponded to positive bipolarons. Thus, positive polarons are efficient carriers in P3HT electrochemical transistors. Charge densities, doping levels, electrical conductivities, and mobilities of polarons in P3HT were calculated from the electrochemical measurements. Only positive polarons exist below the dopant level x = 27 mol%, whereas at higher doping levels, polarons and bipolarons coexist. The mobility of polarons was dependent on the doping level. The highest mobility was 0.31 cm² V^-1 s^-1 at x = 15 mol%.