Unfolding the Correlation between Solution Aggregation and Solid‐State Crystal Orientation in Donor–Acceptor Copolymers via Solvent Additive Processing

Abstract Tailoring the crystal orientation of donor–acceptor (D–A) copolymers is vital for boosting the performance of optoelectronic devices. Despite recent advances in controlling the crystal orientation of D–A copolymers in films, the investigation into their aggregates in solution and the correlation between the solution aggregates and solid‐state crystal orientation has been limited. Herein, an effective solvent additive strategy is reported for tuning solution aggregates and the consequent solid‐state structures of poly{[ N , N' ‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐ alt ‐5,5′‐(2,2′‐bithiophene)} (P(NDI2OD‐T2)). Specifically, the addition of 1‐decanethiol (10‐thiol) to the P(NDI2OD‐T2) chloroform solution promoted the aggregation of P(NDI2OD‐T2) chains because of the improved planarization of the backbones, which changed their crystal orientation in the film from coexisting edge‐on and face‐on to dominant edge‐on when produced by drop‐casting. The mechanism of this crystal orientation transformation is elucidated based on the interaction between 10‐thiol and the side chains of P(NDI2OD‐T2). The optical properties of P(NDI2OD‐T2) films with different crystalline structures are closely correlated. Notably, the 10‐thiol‐enabled facile tailoring of the crystal orientation in P(NDI2OD‐T2) can be readily applied to other D–A copolymers of interest. The findings of this study highlight a robust solvent additive strategy for regulating solution aggregates and crystal orientation in D–A copolymer films, which have applications in many optoelectronic devices.