Chain conformation and aggregation structure regulation of an efficient acceptor-acceptor type photovoltaic polymer

In recent years, donor copolymers with an acceptor₁-acceptor₂ (A₁-A₂) structure have attracted great attention in the field of organic solar cells (OSCs). Compared to the traditional D-A type donor, the double acceptor type polymer not only expands photovoltaic donor diversity, but also further deepens the energy level due to the stronger electron affinity. However, the general A₁-A₂ structure with non-linear polymeric conformations limited the efficient charge transportation and photovoltaic performance. Herein, fluorine substituted benzotriazole (FBTZ) was selected as an acceptor₁ (A₁) unit and copolymerized with a more planar 1,3-bis(thiophen-2-yl) benzodithiophene-4,8-dione (BDD) as an acceptor₂ (A₂) unit to construct two polymer donors P94 and P95 with an A₁-A₂ structure. The two copolymer donors show a deep-lying energy level and the P95 film exhibited a higher hole mobility of up to 1.0 × 10⁻³ cm² V⁻¹ s⁻¹. Interestingly, although P94 and P95 had the same main chains, it was found that less steric hindrance induced less H-aggregation and more linear polymer conformations. Correspondingly, the dominating J-aggregation and linear conformation of P95 exhibited a better power conversion efficiency of 13.51%. To the best of our knowledge, it is the highest efficiency of OSCs based on A₁-A₂ type copolymer donors in recent years. Therefore, the development of an A₁-A₂ type donor has great potential in constructing efficient photovoltaic donors.