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

Two D-A conjugated polymers, FBT-DThDT-1T and FBT-DThDT-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-DThDT-TT with relatively low molecular weight (MW) can be denoted as FBT-DThDT-TT-L and the second batch of FBT-DThDT-TT with much higher MW can be denoted as FBT-DThDT-TT-H. FBT-DThDT-1T possesses a low FET hole mobility of 2.6 x 10(-3) cm(2) (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-DThDT-1T, FBT-DThDT-TT with extended p-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-DThDT-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-DThDT-TT-L and FBT-DThDT-TT-H show improved hole mobilities of 0.38 and 0.20 cm(2) (V s)(-1), respectively. The i-PSCs based on FBT-DThDT-TT-L show a better PCE of 3.47%, and the i-PSCs based on FBT-DThDT-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-DThDT-TT-H-based devices. The impressive results make FBT-DThDT-TT a promising candidate for applications of large-scale solution-processable PSCs.