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 12 mol%/pi electron for [EMIM][TFSI] and [EMIM][FAP], respectively. The large anion FAP (1) impeded the bipolaron formation even at a high doping level.