observation is in agreement with results obtained by Heil et al., who focused on
transistor measurements in rubbed layers of P3HT [114].
4.2.2 Order in Patterned Thin Films
Besides chain orientation, the shape of micro- and nanostructures plays a crucial
role in device performance. For instance, bulk-heterojunction organic solar cells
require phase separation of the donor and acceptor on the nanometer length scale in
order to allow for efficient exciton separation into free charges. Within these phaseseparated functional layers, charge transport to the respective electrodes is highly
affected by both crystallinity and chain orientation. For applications, it is therefore
essential to gain control of both the shape of nanostructures and the chain orientation within the nanostructures. Patterning of polymer layers in micro- or nanometersized structures is mainly achieved by lithographic methods, particularly
nanoimprint lithography (NIL). In the standard NIL process, a rigid mold with
nanostructured features is pressed into the polymer film at high pressure and
enhanced temperature (above the glass transition temperature of the polymer).
The mold is removed after cooling of the film below the T g . The mechanism of
chain orientation within confined geometries is complex and, according to Hu and
Jonas, different factors must be considered [115]: reduced nucleation probability
within cavities, graphoepitaxial alignment, rheological orientation, confinement,
and pressure during the patterning process. It is not yet fully understood how each
parameter affects a specific material.
NIL-induced orientation was first found for polyvinylidene fluoride (PVDF) and
the liquid crystalline polymer poly(9,9-dioctylfluorene-co-benzothiadiazole)
(F8BT) [116–118]. Structural anisotropy was also reported for P3HT after patterning by NIL. For instance, Cui et al. described strong optical birefringence for
gratings of P3HT (700 nm periodicity) produced by NIL, most probably the result
of stretching of chains in amorphous domains and reorientation of microcrystals
[119]. A detailed investigation of the molecular orientation in nanostructures via
GIXRD was given by Hu and coworkers and Ocko and coworkers [97, 120]. Hu and
coworkers found that the molecular orientation is highly sensitive to the geometry
of the nanostructures achieved via NIL. For both nanopillars and nanogratings, a
vertical alignment of the backbone is found because the polymer chains orient
along the flow during the imprinting process (see Fig. 25) [97]. In P3HT
nanogratings, the π-stacking direction points along the grating axis. The orientation
mechanism is explained by a combination of flow-induced chain alignment, attractive π–π interactions between neighboring chains, and hydrophobic interactions
between the alkyl chains and the SAM-treated mold surface. By contrast, Ocko and
coworkers reported a face-on orientation of chains in nanogrooves produced by
NIL, while the polymer backbone preferentially aligns along the groove axis
[120]. The variance of these results illustrates the importance of the mold geometry
comprising the width and height of the nanostructures as well as the thickness of the
residual layer, all parameters having a significant impact on the flow of material
74
K. Tremel and S. Ludwigs
transistor measurements in rubbed layers of P3HT [114].
4.2.2 Order in Patterned Thin Films
Besides chain orientation, the shape of micro- and nanostructures plays a crucial
role in device performance. For instance, bulk-heterojunction organic solar cells
require phase separation of the donor and acceptor on the nanometer length scale in
order to allow for efficient exciton separation into free charges. Within these phaseseparated functional layers, charge transport to the respective electrodes is highly
affected by both crystallinity and chain orientation. For applications, it is therefore
essential to gain control of both the shape of nanostructures and the chain orientation within the nanostructures. Patterning of polymer layers in micro- or nanometersized structures is mainly achieved by lithographic methods, particularly
nanoimprint lithography (NIL). In the standard NIL process, a rigid mold with
nanostructured features is pressed into the polymer film at high pressure and
enhanced temperature (above the glass transition temperature of the polymer).
The mold is removed after cooling of the film below the T g . The mechanism of
chain orientation within confined geometries is complex and, according to Hu and
Jonas, different factors must be considered [115]: reduced nucleation probability
within cavities, graphoepitaxial alignment, rheological orientation, confinement,
and pressure during the patterning process. It is not yet fully understood how each
parameter affects a specific material.
NIL-induced orientation was first found for polyvinylidene fluoride (PVDF) and
the liquid crystalline polymer poly(9,9-dioctylfluorene-co-benzothiadiazole)
(F8BT) [116–118]. Structural anisotropy was also reported for P3HT after patterning by NIL. For instance, Cui et al. described strong optical birefringence for
gratings of P3HT (700 nm periodicity) produced by NIL, most probably the result
of stretching of chains in amorphous domains and reorientation of microcrystals
[119]. A detailed investigation of the molecular orientation in nanostructures via
GIXRD was given by Hu and coworkers and Ocko and coworkers [97, 120]. Hu and
coworkers found that the molecular orientation is highly sensitive to the geometry
of the nanostructures achieved via NIL. For both nanopillars and nanogratings, a
vertical alignment of the backbone is found because the polymer chains orient
along the flow during the imprinting process (see Fig. 25) [97]. In P3HT
nanogratings, the π-stacking direction points along the grating axis. The orientation
mechanism is explained by a combination of flow-induced chain alignment, attractive π–π interactions between neighboring chains, and hydrophobic interactions
between the alkyl chains and the SAM-treated mold surface. By contrast, Ocko and
coworkers reported a face-on orientation of chains in nanogrooves produced by
NIL, while the polymer backbone preferentially aligns along the groove axis
[120]. The variance of these results illustrates the importance of the mold geometry
comprising the width and height of the nanostructures as well as the thickness of the
residual layer, all parameters having a significant impact on the flow of material
74
K. Tremel and S. Ludwigs