Lyotropic Liquid Crystalline Mesophase Governs Interfacial Molecular Orientation of Conjugated Polymer Thin Films

Interfacial out-of-plane molecular orientation critically influences the electronic performance of organic semiconductor thin films. The appearance of a lyotropic liquid crystalline (LC) mesophase during solution coating of conjugated polymers may directly determine the interfacial out-of-plane molecular orientation. However, a lack of studies on the packing structure of the liquid crystalline mesophase and its evolution to the solid state impedes the general understanding of the molecular orientation transformation from the liquid crystalline mesophase to solid-state thin films. This work addresses this unanswered question using poly[[2,5-bis(2-octadecyl)-2,3,5,6-tetrahydro-3,6-diketopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-(2-octylnonyl)-2,1,3-benzotriazole] (DPP-BTz) as a model compound. From near-edge X-ray adsorption fine structure spectroscopy and grazing incidence wide-angle X-ray scattering (GIWAXS) measurements, we observe distinct edge-on orientation at the top interface compared to a primarily face-on orientation in the bulk in solution-coated DPP-BTz thin films. Interestingly, the interfacial orientations in thin films are strongly correlated with those of the lyotropic liquid crystalline mesophase of DPP-BTz appearing during solution coating. Specifically, the LC mesophase adopts an edge-on orientation near the air-liquid interface and a face-on orientation in the bulk liquid layer. The multiscale structure and interfacial orientation of the mesophase are characterized by solution-state small-angle X-ray scattering and in situ GIWAXS measurements. We attribute the edge-on LC orientation at the top interface to surface energy minimization of alkyl side chains with air, while the anisotropic broad LC orientation distribution in the bulk arises from twisted molecular packing in the LC mesophase. The out-of-plane molecular orientation is preserved in the LC mesophase and is carried over to the solid-state thin film, creating the distinct edge-on interfacial alignment at the thin-film top surface.