Power factor enhancement in benzodithiophene-based conjugated polymers through controlling solution-state aggregation

Abstract

Controlling solution-state aggregation of conjugated polymers is crucial for tuning solid-state morphology as well as electrical properties. However, the effect of solution-state aggregation on their thermoelectric performance is rarely reported. Using a series of poly(benzodithiophene-alt-thiophene) conjugated polymers (PBDT-Fm, m is the content of semi-fluorinated groups in polymer chain), the polymer solution aggregations were tuned by the evaporation temperature, which has achieved optimized P(BDT-Fm) films with high degree of molecular order, homogeneous polymer aggregates and small crystalline domains, leading to dramatic enhancement in thermoelectric performance. Compared with the naturally dried polymer films, the heated evaporating process led to at least 49-fold and 15-fold enhancement in the conductivity and power factor, respectively. Optical spectroscopy and microcosmic morphology have shown that the strong molecular motion of polymer chains at high evaporation temperature would disaggregate the polymer pre-aggregations in solution and assist the formation of orderly packed microstructures in solid state, leading to a significant increase in the power factor. Through controlling solution-state polymer aggregations, PBDT-F15 achieved the power factors up to 7.74 ± 0.01 µW m−1 K−2, which is among the highest values in the reported benzodithiophene-based conjugated polymers, highlighting the importance of controlling polymer solution aggregation in optimizing thermoelectric performance.