Enhanced charge transport by incorporating additional thiophene units in the poly(fluorene-thienyl-benzothiadiazole) polymer

We report a comparative study of optical properties, structure and morphology, field-effect transistor (FET) and solar cell performance between poly(4-(3,4′-dihexyl-2,2′-bithiophen-5-yl)-7-(5′-(9, 9-dioctyl-9H-fluoren-2-yl)-3,4′-dihexyl-2,2′-bithiophen-5-yl) benzo[c][1,2,5]thiadiazole) (F8TTBTT), and its predecessor poly((9,9- dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1, 3-benzothiadiazole]-2′,2″-diyl) (F8TBT). Compared to F8TBT, F8TTBTT has two more thiophene units incorporated in its monomer structure. Such a modification leads to a reduced optical band gap, improved charge injection and significantly enhanced ambipolar field-effect mobilities reaching 5 × 10^-2 cm² V^-1 s^-1 for holes and 4 × 10^-3 cm² V^-1 s^-1 for electrons. The enhanced carrier mobilities are most likely a result of an increased backbone planarization and interchain interaction. As a consequence of ambipolar transport, light-emission was observed from the transistor channel during operation. The reduced band gap and improved charge transport make F8TTBTT an interesting candidate also for solar cell applications. Unoptimized solar cells based on F8TTBTT:PCBM blends were found to exhibit power conversion efficiency under AM 1.5 illumination of ∼1.54%.