Raman spectra of carriers in ionic-liquid-gated transistors fabricated with poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)

We observed the Raman spectra of carriers, positive polarons and bipolarons, generated in a poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C14) film by FeCl₃ vapor doping. Electrical conductivity and Raman measurements indicate that the dominant carriers in the conducting state were bipolarons. We identified positive polarons and bipolarons generated in an ionic-liquid-gated transistor (ILGT) fabricated with PBTTT-C14 as an active semiconductor and an ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [BMIM][TFSI] as a gate dielectric using Raman spectroscopy. The relationship between the source-drain current (I_D) at a constant source-drain voltage (V_D) and the gate voltage (V_G) was measured. I_D increased above -V_G = 1.1 V and showed a maximum at -V_G = 2.0 V. Positive polarons were formed at the initial stage of electrochemical doping (-V_G = 0.8 V). As I_D increased, positive bipolarons were formed. Above V_G = -2.0 V, bipolarons were dominant. The charge density (n), the doping level (x), and the mobility of the bipolarons were calculated from the electrochemical measurements. The highest mobility (μ) of bipolarons was 0.72 cm² V^-1 s^-1 at x = 110 mol%/repeating unit (-V_G = 2.0 V), whereas the highest μ of polarons was 4.6 × 10^-4 cm² V^-1 s^-1 at x = 10 mol%.