Effect of anions on bipolaron formation in ionic-liquid-gated transistors fabricated with poly(2, 5-bis(3-hexadecylthiophen-2-yl)thieno[3, 2-b]thiophene) (PBTTT-C16)

The type of carrier, polaron and bipolaron, generated in PBTTT-C16 ionic-liquid-gated transistors fabricated with [EMIM][TFSI] or [EMIM][FAP] was identified using Raman spectroscopy. Doping levels were obtained from electrochemical measurements. As the doping level increased, positive polarons were formed; positive bipolarons were then formed and dominant. The critical doping level of the polaron-to-bipolaron transition was 4.5 and 12mol %/π electron for [EMIM][TFSI] and [EMIM][FAP], respectively. The large anion FAP^- impeded the bipolaron formation even at a high doping level.