In Situ Observation of Alignment Templating by Seed Crystals in Highly Anisotropic Polymer Transistors
Nils E. Persson, Sebastian Engmann, Lee J. Richter, Dean M. DeLongchamp
Due to the highly directional nature of transport in polymer-based organic field-effect transistors (OFETs), alignment of the polymer backbone can significantly affect device performance. While many methods of alignment have been detailed, the mechanism of alignment is rarely revealedespecially in cases of flow-induced alignment. Polymer aggregates are often observed in highly aligned systems, but their role is similarly unclear. Here, we present a comprehensive characterization of blade-coated P(NDI2OD-T2) (N2200) for OFET applications, including a rigorous, multimodal characterization of its in-plane alignment. Film thickness follows the expected power-law dependence on coating speed, while bulk polymer backbone orientation transitions from perpendicular to parallel to the coating direction as speed is increased. Charge carrier mobility >2 cm2/(V s) is achieved parallel to the coating direction for aligned N2200 coated at 5 mm/s and is found to be strongly correlated with the in-plane alignment of the fibrillar morphology at the films surface, characterized with atomic force microscopy and near-edge X-ray absorption. We develop a model of N2200 crystal anisotropy through rotational scans of grazing incidence wide-angle X-ray scattering (GIWAXS) and use it to analyze simultaneous in situ GIWAXS and UVvis reflectance data from polymer solutions coated at 5 mm/s. A small population of crystals align early in the drying process, but bulk alignment occurs very late in the drying process, likely mediated by a lyotropic liquid crystal phase transition templated by the aligned crystals. Our characterization also suggests that the majority of material in N2200 thin films is noncrystalline at these conditions.
, Engmann, S.
, Richter, L.
and DeLongchamp, D.
In Situ Observation of Alignment Templating by Seed Crystals in Highly Anisotropic Polymer Transistors, Chemistry of Materials, [online], https://doi.org/10.1021/acs.chemmater.9b00888
(Accessed May 28, 2023)