about standing chains of P3HT, where the polymer backbone stands vertically on
the substrate (Fig. 16c) [97, 98]. The description of texture is not consistent in the
literature, with some groups referring to the orientation of the chains and others
denoting the orientation of crystalline grains with respect to the substrate. In this
chapter, we refer to the molecular orientation of the polymer chains.
In general, the thermodynamically favored texture is assumed to be formed by
edge-on oriented chains [1, 5, 68, 69]. This structure is obtained under conditions
close to the equilibrium realized for slow casting methods such as drop-casting
[1, 67, 68] and dip-coating, and by spin-coating from high boiling point solvents
[69]. The face-on orientation is regarded as a kinetically trapped morphology,
which is obtained by rapid drying of the layer [71]. This texture seems to be further
favored for P3HT of low regioregularity [1] and for layers prepared under shear
force, for instance via the friction transfer method [99] or mechanical rubbing [100]
(see Sect. 4.2.1).
It is generally believed that fast charge transport occurs along the chain axis
(c-axis) and in the π-stacking direction (b-axis), whereas the insulating side chains
(a-axis) lead to charge barriers and low mobilities. Therefore, an edge-on
orientation is highly desirable for OFETs since here high in-plane charge transport
between the source and drain electrode is essential. The importance of molecular
orientation for charge transport was first demonstrated by Sirringhaus and
coworkers, who studied transistor properties for different textures [1]. In edge-on
orientation, the field-effect mobility is more than 100 times higher than for chains in
face-on orientation, which is attributed to the excellent charge transport along the
chain axis and in the π-stacking direction.
The impact of the solvent evaporation rate on the texture was demonstrated by
Bao and coworkers, who prepared P3HT layers from chloroform solution using two
different methods, namely spin-coating and drop-casting (Fig. 17) [68]. In the slow
drop-casting process, polymer chains have sufficient time to self-assemble into
well-defined nanofibrils that are visible in AFM. In accordance with the morphology, the polymer chains adopt an edge-on orientation, as evidenced by grazing
incidence X-ray diffraction (GIXRD) measurements. In stark contrast, AFM measurements of spin-cast layers show an almost featureless microstructure, while
GIXRD reveals a predominant face-on texture.
DeLongchamp et al. studied the microstructure of P3HT spin-cast from chloroform for different spin speeds and found a remarkable change in the texture, which
is shown in Fig. 18 [71]. With decreasing spin speed, a transition from mainly faceon to edge-on was observed. A reduction in spinning speed led to a slower
evaporation of the solvent [101]. Therefore, this observation is in agreement with
various reports in the literature showing that slow evaporation typically results in a
higher degree of edge-on orientation [5, 70, 80, 102].
Besides the deposition conditions, the nature of the substrate can have a significant influence on the texture. Substrate treatment with self-assembled monolayers
(SAMs) such as octadecyltrichlorosilane, hexamethyldisilazane, and
γ-aminopropyltriethoxysilane results in an edge-on texture [72, 103–105]. By contrast, Chow and coworkers reported the formation of a face-on oriented P3HT
64
K. Tremel and S. Ludwigs
the substrate (Fig. 16c) [97, 98]. The description of texture is not consistent in the
literature, with some groups referring to the orientation of the chains and others
denoting the orientation of crystalline grains with respect to the substrate. In this
chapter, we refer to the molecular orientation of the polymer chains.
In general, the thermodynamically favored texture is assumed to be formed by
edge-on oriented chains [1, 5, 68, 69]. This structure is obtained under conditions
close to the equilibrium realized for slow casting methods such as drop-casting
[1, 67, 68] and dip-coating, and by spin-coating from high boiling point solvents
[69]. The face-on orientation is regarded as a kinetically trapped morphology,
which is obtained by rapid drying of the layer [71]. This texture seems to be further
favored for P3HT of low regioregularity [1] and for layers prepared under shear
force, for instance via the friction transfer method [99] or mechanical rubbing [100]
(see Sect. 4.2.1).
It is generally believed that fast charge transport occurs along the chain axis
(c-axis) and in the π-stacking direction (b-axis), whereas the insulating side chains
(a-axis) lead to charge barriers and low mobilities. Therefore, an edge-on
orientation is highly desirable for OFETs since here high in-plane charge transport
between the source and drain electrode is essential. The importance of molecular
orientation for charge transport was first demonstrated by Sirringhaus and
coworkers, who studied transistor properties for different textures [1]. In edge-on
orientation, the field-effect mobility is more than 100 times higher than for chains in
face-on orientation, which is attributed to the excellent charge transport along the
chain axis and in the π-stacking direction.
The impact of the solvent evaporation rate on the texture was demonstrated by
Bao and coworkers, who prepared P3HT layers from chloroform solution using two
different methods, namely spin-coating and drop-casting (Fig. 17) [68]. In the slow
drop-casting process, polymer chains have sufficient time to self-assemble into
well-defined nanofibrils that are visible in AFM. In accordance with the morphology, the polymer chains adopt an edge-on orientation, as evidenced by grazing
incidence X-ray diffraction (GIXRD) measurements. In stark contrast, AFM measurements of spin-cast layers show an almost featureless microstructure, while
GIXRD reveals a predominant face-on texture.
DeLongchamp et al. studied the microstructure of P3HT spin-cast from chloroform for different spin speeds and found a remarkable change in the texture, which
is shown in Fig. 18 [71]. With decreasing spin speed, a transition from mainly faceon to edge-on was observed. A reduction in spinning speed led to a slower
evaporation of the solvent [101]. Therefore, this observation is in agreement with
various reports in the literature showing that slow evaporation typically results in a
higher degree of edge-on orientation [5, 70, 80, 102].
Besides the deposition conditions, the nature of the substrate can have a significant influence on the texture. Substrate treatment with self-assembled monolayers
(SAMs) such as octadecyltrichlorosilane, hexamethyldisilazane, and
γ-aminopropyltriethoxysilane results in an edge-on texture [72, 103–105]. By contrast, Chow and coworkers reported the formation of a face-on oriented P3HT
64
K. Tremel and S. Ludwigs