Optimizing the Crystallization Behavior and Film Morphology of Donor–Acceptor Conjugated Semiconducting Polymers by Side-Chain–Solvent Interaction in Nonpolar Solvents

Abstract

The solution state of a conjugated polymer plays a critical role in the film morphology and charge carrier mobility of the solution process organic field-effect transistors (OFETs). However, it remains challenging to establish the relationship between solution-state and solid-state morphologies. Herein, we report an effective strategy to control the film formation process and film morphology by regulating the solution state, specifically, through the interaction between the branched side chain of poly[4-(4,4-bis(2-octyldodecyl)-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT-ODD) and the nonpolar solvent. A precise adjustment of the solution state using the Hansen solubility parameter distance (Ra) has been achieved. Compared to the polar solvent, ordered aggregates were formed in the solution formed by nonpolar solvents. Interestingly, a highly ordered chain arrangement was observed in solutions with suitable Ra’s (8.2 and 9.4 MPa1/2 for hexane and isooctane, respectively). Importantly, solution-state aggregation was maintained in the drop-casting film due to the memory effect; the as-prepared films were composed of different fiber sizes and crystal regions. The fiber consisted of a face-on region, with the polymer chains in the fiber aligned perpendicular to the long axis of the fibers. The crystallization process of the edge-on region was independent of the face-on region. The face-on and edge-on regions were produced through the aggregated and disordered parts, respectively. As the Ra increased, the fiber size and the face-on region content increased. Finally, the OFET device mobility was optimized with a significant increase in μ (from 9.7 × 10–5 to 1.3 × 10–3 cm2 V–1 s–1). These results showed that the precise regulations of the solution state by the side-chain–solvent interaction play an important role in the film formation process, film morphology, crystallization behavior, and device properties.