Unraveling the Correlation between Crystallite Orientation in Conjugated Polymer Thin Film and Charge Transport Properties

Despite great progress in tailoring the molecular structures of donor−acceptor (D−A) copolymers, the ability to precisely tailor their solid-state chain packing and establish a comprehensive structure−property relationship remains a grand challenge. Here, three diketopyrrolopyrrole (DPP)-based copolymers adjusting alkyl side chains with various lengths are developed to control their aggregation behavior in different solvents and the corresponding crystallite orientation in thin films, establishing the structure−charge -transport −property relationship and elucidating the underlying mechanism. Specifically, DPPs with longer side chains exhibit weaker solution aggregation, favoring the formation of a face-on orientation in as-cast films, where the face-on to edge-on orientation can be further achieved by strengthening their solution aggregation via changing the solvent from chloroform to chlorobenzene and xylene. Remarkably, by using a floating-film transfer method, the initially face-on crystallites in as-cast DPP films can be transformed into narrower distributed edge-on crystallites, resulting in the charge mobility being nearly tenfold higher than that of the corresponding as-cast sample. This study highlights how molecular engineering and processing strategies collaboratively guide the aggregation and packing of DPPs in the solution and solid state, offering key insights into the structure−processing−property relationship of D−A copolymers for high-performance optoelectronic devices.