An extended π-conjugated area of electron-donating units in D–A structured polymers towards high-mobility field-effect transistors and highly efficient polymer solar cells

Abstract

Two D–A conjugated polymers, FBT-DTh_DT-1T and FBT-DTh_DT-TT, using 5,6-difluoro-2,1,3-thiadiazole (FBT) as the electron-accepting unit, and terthiophene or 2,5-di(thiophen-2-yl)thieno[3,2-b]thiophene as the electron-donating unit, respectively, were synthesized. Among them, the first batch of FBT-DTh_DT-TT with relatively low molecular weight (MW) can be denoted as FBT-DTh_DT-TT-L and the second batch of FBT-DTh_DT-TT with much higher MW can be denoted as FBT-DTh_DT-TT-H. FBT-DTh_DT-1T possesses a low FET hole mobility of 2.6 × 10⁻³ cm² (V s)⁻¹ and a poor power conversion efficiency (PCE) of 0.91% in inverted polymer solar cells (i-PSCs) under the illumination of AM1.5G, 100 mW cm⁻² light. Compared with FBT-DTh_DT-1T, FBT-DTh_DT-TT with extended π-conjugation bears a TT replacing the middle thiophene of terthiophene on the backbone, which would increase the coplanarity of the polymer and thus facilitate both intermolecular packing and charge transport. FBT-DTh_DT-TT shows strong interchain aggregation in a room temperature solution, its absorption spectra in a room temperature solution and in a thin film were almost identical. The field-effect transistors based on FBT-DTh_DT-TT-L and FBT-DTh_DT-TT-H show improved hole mobilities of 0.38 and 0.20 cm² (V s)⁻¹, respectively. The i-PSCs based on FBT-DTh_DT-TT-L show a better PCE of 3.47%, and the i-PSCs based on FBT-DTh_DT-TT-H with a higher MW exhibit the best PCE up to 7.78%, with highly improved absorption capacity and miscibility with PC₇₁BM. Moreover, with a 355 nm thick active layer, a PCE of 6.72% with a high FF of 67.8% is still obtained for FBT-DTh_DT-TT-H-based devices. The impressive results make FBT-DTh_DT-TT a promising candidate for applications of large-scale solution-processable PSCs.