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The Microstructure Foundation of High Carrier Mobility in Semiconducting Polymers

Dean M. DeLongchamp1, R. Joseph Kline2, Brandon M. Vogel3, Leah A. Lucas2, Daniel Fischer1, Lee J. Richter4, Martin Heeney5, Iain McCulloch5, and Eric K. Lin2. (1) National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8541, Building 224 Room A327, Gaithersburg, MD 20899-8541, (2) Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8541, Building 224 Room A327, Gaithersburg, MD 20899-8541, (3) Department of Chemical Engineering, Bucknell University, Lewisburg, PA 17837, (4) Surface and Microanalysis Science Division, National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8541, Building 224 Room A327, Gaithersburg, MD 20899-8541, (5) Merck Chemicals, Southampton, United Kingdom

Organic semiconductors have the potential to disrupt mainstream modes of electronics manufacturing because they can be deposited from solution in “bottom-up” additive fabrication processes.  The most critical property of an organic semiconductor is its carrier mobility; a high mobility enhances current density and increases switching speed to permit organic circuits to meet real applications.  Recently, new semiconducting polymers have been reported that are based on a poly(2,5-bis(3-alkylthiophen-2yl)thieno[3,2-b]thiophene (pBTTT) regiosymmetric monomer.  The pBTTT polymers (Mn≈30 kDa) can achieve charge-carrier field effect mobilities of up to 0.6 cm2/V·s, making them competitive with amorphous silicon.  The unusually high mobility of pBTTTs likely results from their exceptional molecular order, which is revealed upon examination of their microstructure.  When heated into a liquid crystalline state and then cooled, annealed pBTTT films exhibit unusually high crystallinity of a type that has not been reported before for polymers of this molecular mass. We develop a detailed picture of the microstructure of ≈ 20 nm thick films of pBTTTs using a combination of techniques to reveal the orientation and organization of all parts of the polymer chain.  The polymer long axis orientation can be characterized by spectral ellipsometry; the primary optical oscillator confined to the long axis exhibits minimal out-of-plane absorbance.  Specular X-ray diffraction indicates that these perfectly in-plane polymer chains are organized into lamellae with a regular vertical spacing.  These lamellae are organized laterally into large terraces of single molecule height that can be visualized with atomic force microscopy (AFM).  By Fourier transform infrared spectroscopy (FTIR), the aliphatic side chains appear fully extended, and tilt substantially relative to the lamellar plane normal.  Finally, near edge X-ray absorption fine structure (NEXAFS) spectroscopy reveals that the conjugated plane of the polymer backbone also tilts away from the lamellar plane normal.  We believe that this result reveals a true aromatic core plane tilt within the p-stacked crystal.  These structural aspects presumably allow high levels of p orbital overlap within large grains in the substrate plane, facilitating carrier transport.  The exceptional molecular order within pBTTT polymer films provides a clear foundation for the exceptionally high mobilities that they can achieve. The pBTTT motif may serve as a model system for understanding the behavior of other rigid alkylthiophene polymers.  In addition to the studies of the well-ordered, annealed films, we will describe the nature of the pBTTT liquid crystalline state, and the influence of surface chemistry on the ordering of this polymer.  Finally, we will report a new polymorph of the pBTTT polymer crystal that features multiple length scales of hierarchical ordering: terraced ribbons of molecular length (≈60 nm) and height (≈2 nm) that extend for tens of microns and exhibit uniaxial orientation over several millimeters.