Doping-level dependent mobilities of positive polarons and bipolarons in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT-C16) based on an ionic-liquid-gated transistor configuration

Abstract

We studied the relationship between the electrical properties and type of carrier, polarons and/or bipolarons, in unannealed and annealed PBTTT-C16 in an ionic-liquid-gated transistor (ILGT) configuration using Raman spectroscopy and electrochemical measurements. The types of carriers generated in the ILGT were identified by Raman spectroscopy. Electrical conductivity and mobility were obtained as a function of the doping level or injected charge density from the electrochemical measurements. The electrical conductivity increased rapidly due to the formation of positive polarons. Bipolaron formation started at 7.3 and 3.4 mol%/π electrons for unannealed and annealed PBTTT-C16, respectively. Bipolarons are the dominant carriers in high-conductivity regions for unannealed and annealed PBTTT-C16. The highest bipolaron mobility in unannealed PBTTT-C16 was 0.92 cm2 V−1 s−1 with 11 mol% doping (charge density, 1.3 × 1021 cm−3), whereas the highest bipolaron mobility in annealed PBTTT-C16 was 1.2 cm2 V−1 s−1 with 4.8 and 5.2 mol% doping (charge density, 5.7 and 6.2 × 1020 cm−3).