In Situ Monitoring the Face-on to Bimodal Texture Transition of P(NDI2OD-T2) during Side-Chain Selective Solvent Vapor Annealing

Charge transport in conjugated semiconducting polymer films predominantly occurs along the intrachain backbone as well as interchain π–π stacking directions. The molecular orientation significantly affects the favorable charge transport direction, since it determines the alignment of π–π stacking. Bimodal texture exhibited π–π stacking in both out-of-plane and in-plane directions, thus forming a continuous 3D charge transport pathway in the films to achieve optimized electrical performance. However, the mechanism of the orientation transition has not been well-defined; thus, the construction of the bimodal texture remains an issue. Here, we achieved molecular reorientation in n-type high-molecular-weight polyN, N′-bis (2-octyldodecyl) -naphthalene-1, 4, 5, 8-bis (dicarboximide) -2, 6-diyl-alt-5, 5′- (2, 2′-bithiophene) P (NDI2OD-T2) films from predominately face-on to bimodal texture by employing side-chain selective solvent vapor annealing (SVA) post-treatment, which could be divided into two stages: film swelling process and film drying process. During the film swelling process, as observed by the in situ 2D-GIWAXS, the side chains were aligned perpendicular to the substrate due to the stronger solvent–side chain interactions than that of the main chain; thus, part of the molecules began to reorient from face-on to edge-on. As more and more side chains were selectively dissolved, a much greater free volume was provided for the further orientation transition. Eventually, the intermediate edge-on orientation formed. However, the swollen edge-on crystallites were not stable. During the following film drying process, some of the crystallites kept edge-on orientation, while some metastable ones reorientated to face-on orientation as the solvent–side chain interactions disappeared upon solvent evaporation, ultimately leading to a bimodal texture. Consequently, the mobility of the SVA-treated films was improved from 0. 092 (pristine film) to 0. 155 cm2 V–1 s–1 due to 3D charge transporting.