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 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)] 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.