Chain alignment and charge transport anisotropy in blade-coated P(NDI2OD-T2)/PS blend films

This study investigates semiconducting polymer chain orientation and alignment in blade-coated poly{[N,N’-bis(2- octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5’-(2,2’-bithiophene)} [P(NDI2OD-T2)] / polystyrene [PS] blends with a view to understanding how blending with an insulating polymer impacts upon P(NDI2OD-T2) chain alignment. Various characterization techniques are employed to probe bulk and surface microstructures, including polarized UV–vis absorption spectroscopy, near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy, grazing incidence wide-angle X-ray scattering (GIWAXS), and atomic force microscopy. While P(NDI2OD-T2) chains are found to be preferentially aligned parallel to the coating direction for an overall P(NDI2OD-T2) greater than 3 wt %; below 3 wt % P(NDI2OD-T2) a striking reversal of the preferential chain orientation from parallel to perpendicular to the coating direction is observed, captured by both bulk-sensitive (GIWAXS) and surface-sensitive (NEXAFS) techniques. Charge transport anisotropy is evaluated using top gate/bottom contact field-effect transistor measurements, from which higher charge-carrier mobility is achieved when P(NDI2OD-T2) chains are preferentially aligned with the channel length. Comparing the orientational order parameters extracted from the various techniques, the observed charge transport anisotropy trend most closely follows the degree of chain alignment at the surface obtained from NEXAFS spectroscopy. Overall our results indicate that the strong alignment can be achieved in P(NDI2OD-T2)/PS blends, with blending providing a route to modulate concentration effects in blade-coated films for the tuning and optimization of semiconductor chain alignment.