Terthiophene-based D-A polymer with an asymmetric arrangement of alkyl chains that enables efficient polymer solar cells.

We report a series of difluorobenzothiadizole (ffBT) and oligothiophene-based polymers with the oligothiophene unit being quaterthiophene (T4), terthiophene (T3), and bithiophene (T2). We demonstrate that a polymer based on ffBT and T3 with an asymmetric arrangement of alkyl chains enables the fabrication of 10.7% efficiency thick-film polymer solar cells (PSCs) without using any processing additives. By decreasing the number of thiophene rings per repeating unit and thus increasing the effective density of the ffBT unit in the polymer backbone, the HOMO and LUMO levels of the T3 polymers are significantly deeper than those of the T4 polymers, and the absorption onset of the T3 polymers is also slightly red-shifted. For the three T3 polymers obtained, the positions and size of the alkyl chains play a critical role in achieving the best PSC performances. The T3 polymer with a commonly known arrangement of alkyl chains (alkyl chains sitting on the first and third thiophenes in a mirror symmetric manner) yields poor morphology and PSC efficiencies. Surprisingly, a T3 polymer with an asymmetric arrangement of alkyl chains (which is later described as having an "asymmetric bi-repeating unit") enables the best-performing PSCs. Morphological studies show that the optimized ffBT-T3 polymer forms a polymer:fullerene morphology that differs significantly from that obtained with T4-based polymers. The morphological changes include a reduced domain size and a reduced extent of polymer crystallinity. The change from T4 to T3 comonomer units and the novel arrangement of alkyl chains in our study provide an important tool to tune the energy levels and morphological properties of donor polymers, which has an overall beneficial effect and leads to enhanced PSC performance.