order to verify this notion experimentally, a number of transport measurements on
P3HT films with predetermined alignment were carried out. The earliest study of
charge transport in aligned P3HT on friction-transferred polytetrafluoroethylene
(PTFE) by Amundson et al. indicated in-plane anisotropy of the mobility [84], but
the roughness of the PTFE alignment layer and its impact on mobility did not allow
for a clear distinction of the origin of the mobility differences. Later, Jimison
et al. investigated the influence of grain boundaries on transport in aligned P3HT
using a directional crystallization technique to form films with an anisotropic fiber
structure containing crystalline regions that were separated by disordered grain
boundaries [85]. The polymer chains within crystallites were oriented parallel to the
fiber axis. Top-contact FETs with charge transport either parallel or perpendicular
to the fiber direction showed mobilities that were an order of magnitude higher
along the fibers than across. Jimison et al. concluded that the effective device
mobility was dominated by the mobility in the grain boundaries and, thus, the
observed mobility anisotropy in the aligned P3HT films was mainly due to different
grain-boundary orientations. That is, holes crossed the grain boundaries along the
fiber direction much more easily than the boundaries across fibers. Along a fiber, the
different crystallites can be bridged by tie molecules, which maintain connectivity.
This is not possible for crystallites in neighboring fibers. Efficient π–π-overlap is
also not likely due to disorder at the fiber edge. Thus, not just the number of grain
boundaries but also their placement and relative orientation are important for
maximizing the overall charge transport.
A convenient way of aligning P3HT thin films without introducing defects and
independent of the substrate is the application of strain by stretching a pre-cast film
on a rubbery (polydimethylsiloxane, PDMS) substrate before transferring it onto
the FET substrate. This was demonstrated by O’Connor et al. [86]. The crystalline
parts of the films aligned in the direction of applied strain. The degree of alignment
increased proportionally with strain while the amorphous regions remained highly
disordered. In short-channel (5 μm) FETs, the mobility increased by almost a factor
of 2 in the direction of strain but decreased by a factor of 4–5 perpendicular to it,
reaching a maximum ratio of 9 at a strain of 100%. Importantly, the strained films
exhibited an increasing amount of plane-on orientation instead of the usually
observed edge-on orientation, which enables transport along the π–π stacking
direction. In agreement with theory, these results suggest that transport along the
polymer backbone is fastest, followed by transport in the π–π stacking direction and
side-chain stacking direction.
This direction dependence was further corroborated by studies of mobility
anisotropy within P3HT films without the influence of grain boundaries. Crossland
et al. [68] grew large spherulites (diameters > 100 μm) of P3HT by controlled
solvent swelling with carbon disulfide and deposited arrays of electrodes on top.,
This arrangement enabled FET measurements for various angles with respect to the
radial spherulite growth direction. Mobilities were again higher along the polymer
backbones than along the π–π stacking direction despite periodic amorphous
interlamellar interruptions. Fisher et al. tested this further by producing aligned
films of P3HT with different molecular weights by applying an electric field during
P3HT and Other Polythiophene Field-Effect Transistors
121
P3HT films with predetermined alignment were carried out. The earliest study of
charge transport in aligned P3HT on friction-transferred polytetrafluoroethylene
(PTFE) by Amundson et al. indicated in-plane anisotropy of the mobility [84], but
the roughness of the PTFE alignment layer and its impact on mobility did not allow
for a clear distinction of the origin of the mobility differences. Later, Jimison
et al. investigated the influence of grain boundaries on transport in aligned P3HT
using a directional crystallization technique to form films with an anisotropic fiber
structure containing crystalline regions that were separated by disordered grain
boundaries [85]. The polymer chains within crystallites were oriented parallel to the
fiber axis. Top-contact FETs with charge transport either parallel or perpendicular
to the fiber direction showed mobilities that were an order of magnitude higher
along the fibers than across. Jimison et al. concluded that the effective device
mobility was dominated by the mobility in the grain boundaries and, thus, the
observed mobility anisotropy in the aligned P3HT films was mainly due to different
grain-boundary orientations. That is, holes crossed the grain boundaries along the
fiber direction much more easily than the boundaries across fibers. Along a fiber, the
different crystallites can be bridged by tie molecules, which maintain connectivity.
This is not possible for crystallites in neighboring fibers. Efficient π–π-overlap is
also not likely due to disorder at the fiber edge. Thus, not just the number of grain
boundaries but also their placement and relative orientation are important for
maximizing the overall charge transport.
A convenient way of aligning P3HT thin films without introducing defects and
independent of the substrate is the application of strain by stretching a pre-cast film
on a rubbery (polydimethylsiloxane, PDMS) substrate before transferring it onto
the FET substrate. This was demonstrated by O’Connor et al. [86]. The crystalline
parts of the films aligned in the direction of applied strain. The degree of alignment
increased proportionally with strain while the amorphous regions remained highly
disordered. In short-channel (5 μm) FETs, the mobility increased by almost a factor
of 2 in the direction of strain but decreased by a factor of 4–5 perpendicular to it,
reaching a maximum ratio of 9 at a strain of 100%. Importantly, the strained films
exhibited an increasing amount of plane-on orientation instead of the usually
observed edge-on orientation, which enables transport along the π–π stacking
direction. In agreement with theory, these results suggest that transport along the
polymer backbone is fastest, followed by transport in the π–π stacking direction and
side-chain stacking direction.
This direction dependence was further corroborated by studies of mobility
anisotropy within P3HT films without the influence of grain boundaries. Crossland
et al. [68] grew large spherulites (diameters > 100 μm) of P3HT by controlled
solvent swelling with carbon disulfide and deposited arrays of electrodes on top.,
This arrangement enabled FET measurements for various angles with respect to the
radial spherulite growth direction. Mobilities were again higher along the polymer
backbones than along the π–π stacking direction despite periodic amorphous
interlamellar interruptions. Fisher et al. tested this further by producing aligned
films of P3HT with different molecular weights by applying an electric field during
P3HT and Other Polythiophene Field-Effect Transistors
121