(553b) Structural Characterization of Exceptionally Aligned Regioregular CDT-PT Based Copolymer with High Field-Effect Mobility

Polymer-based field-effect transistors typically have a charge mobility of less the 1 cm²/Vs.  The mobility can be further improved through alignment of the conjugated polymer chains in the direction of charge transport.  For our study, we use a regioregular conjugated copolymer (PCDTPT) based on cyclopenta[2,1-b:3,4-b′]dithiophene (CDT) and pyridal[2,1,3]thiadiazole (PT) structural units, which has a hole mobility around 0.6 cm²/Vs for isotropic spin-coated films.  When the regioregular PCDTPT is casted on an unidirectional nano-grooved substrate, the hole mobility has been measured to be greater than 20 cm²/Vs in the direction of the nano-grooves.  To provide further insight on the significance of the alignment in enhancing the mobility, this talk will focus on the detailed characterization of the crystalline orientation of the aligned regioregular PCDTPT films.  The characterization techniques include a combination of transmission electron microscopy (TEM), grazing incidence wide angle X-ray scattering (GIWAXS), and near edge x-ray absorption fine structure (NEXAFS) spectroscopy.  The results indicate elongated polymer fibers/chains along the alignment direction (i.e. parallel to nano-grooves) allowing for efficient intrachain transport between source-drain contacts.  Furthermore, the conjugated polymer backbone has preferential edge-on orientation resulting in pi-pi stacking in the plane of the film, thus allowing for efficient interchain charge transport.  Such efficient routes for charge transport between the source-drain contacts is responsible for the measured high mobility in the aligned films.