(507b) The Microstructure Foundation of High Carrier Mobility in Semiconducting Polymers

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.