The use of poor solvents or solvent mixtures can further allow aggregation and
crystallization directly from solution, which is the subject of the following section.
This aggregation is accompanied by a bathochromic shift of the absorption maxima
and the appearance of a vibronic fine structure (see also Sect. 3). In the extreme case
of solid-state spectra of P3HT films, these effects are even more pronounced, with
the spectra also becoming more structured for high molecular weights (see
Sect. 4.1.1).
3 Crystallization from Solution
The rigidity and planar conformation of the conjugated backbone of many semiconducting polymers such as regioregular P3HT allows efficient packing and
crystallization. However, due to the difficulty of forming ordered crystalline structures from interpenetrating and entangled polymer chains of high rigidity, there are
very few publications on single crystals of polymeric semiconductors [43]. Only
recently has the growth of P3HT single crystals from solution using a self-seeding
approach been reported [44]. The single crystalline nature of the deposited material
and its identification as the form II polymorph was performed by Brinkmann via
transmission electron microscopy coupled with electron diffraction (TEM/ED). For
a more detailed crystal structure and polymorph discussion we refer to Sect. 4.1.2
and the article by Brinkmann et al. in this book [45].
Although insulating polymers such as PE crystallize in 2-dimensional
(2D) lamellar sheets, conjugated polymers like P3HT typically form high aspect
ratio, 1D nanocrystals (nanofibrils, nanowhiskers, nanofibers, and nanorods)
[46]. Aggregation is driven by strong π–π interactions perpendicular to the conjugated backbone, as well as by hydrophobic interactions of the side chains.
In 1993, Ihn et al. were the first to report crystallization of P3HT nanocrystals
from a dilute solution of a poor solvent [47]. Nanofibrils were grown from cyclohexanone (0.05%) by slow cooling of the solution from 50
C to room temperature
(25
C/h). After deposition from solution on TEM grids, nanofibrils with a width of
around 15 nm, a height of 5 nm, and a length of around 10 μm were evidenced by
TEM. Figure 4a shows the bright-field image and the corresponding electron
diffraction pattern of P3HT nanofibrils grown from cyclohexanone. A representative AFM height image of similar nanofibrils is given in Fig. 4b. The precise
packing of the chains within the nanofibrils was determined by electron diffraction
and X-ray analysis and is schematically presented in Fig. 4c. The polymer backbone
packs normal to the long axis of the fibril (i.e., the π-stacking is oriented along the
fibril direction), whereas the alkyl chains point perpendicular to the substrate. This
arrangement is also referred to as edge-on orientation (see also Sect. 4.1.2).
Although the height of the fibrils relates to only two or three layers of P3HT chains,
the fibril length of around 10 μm demonstrates the strong packing tendency in the
π-stacking direction. The fact that the (number) average contour length of the
chains (around 65 nm), which was calculated on the basis of the (number) average
Morphology of P3HT in Thin Films
47
crystallization directly from solution, which is the subject of the following section.
This aggregation is accompanied by a bathochromic shift of the absorption maxima
and the appearance of a vibronic fine structure (see also Sect. 3). In the extreme case
of solid-state spectra of P3HT films, these effects are even more pronounced, with
the spectra also becoming more structured for high molecular weights (see
Sect. 4.1.1).
3 Crystallization from Solution
The rigidity and planar conformation of the conjugated backbone of many semiconducting polymers such as regioregular P3HT allows efficient packing and
crystallization. However, due to the difficulty of forming ordered crystalline structures from interpenetrating and entangled polymer chains of high rigidity, there are
very few publications on single crystals of polymeric semiconductors [43]. Only
recently has the growth of P3HT single crystals from solution using a self-seeding
approach been reported [44]. The single crystalline nature of the deposited material
and its identification as the form II polymorph was performed by Brinkmann via
transmission electron microscopy coupled with electron diffraction (TEM/ED). For
a more detailed crystal structure and polymorph discussion we refer to Sect. 4.1.2
and the article by Brinkmann et al. in this book [45].
Although insulating polymers such as PE crystallize in 2-dimensional
(2D) lamellar sheets, conjugated polymers like P3HT typically form high aspect
ratio, 1D nanocrystals (nanofibrils, nanowhiskers, nanofibers, and nanorods)
[46]. Aggregation is driven by strong π–π interactions perpendicular to the conjugated backbone, as well as by hydrophobic interactions of the side chains.
In 1993, Ihn et al. were the first to report crystallization of P3HT nanocrystals
from a dilute solution of a poor solvent [47]. Nanofibrils were grown from cyclohexanone (0.05%) by slow cooling of the solution from 50
C to room temperature
(25
C/h). After deposition from solution on TEM grids, nanofibrils with a width of
around 15 nm, a height of 5 nm, and a length of around 10 μm were evidenced by
TEM. Figure 4a shows the bright-field image and the corresponding electron
diffraction pattern of P3HT nanofibrils grown from cyclohexanone. A representative AFM height image of similar nanofibrils is given in Fig. 4b. The precise
packing of the chains within the nanofibrils was determined by electron diffraction
and X-ray analysis and is schematically presented in Fig. 4c. The polymer backbone
packs normal to the long axis of the fibril (i.e., the π-stacking is oriented along the
fibril direction), whereas the alkyl chains point perpendicular to the substrate. This
arrangement is also referred to as edge-on orientation (see also Sect. 4.1.2).
Although the height of the fibrils relates to only two or three layers of P3HT chains,
the fibril length of around 10 μm demonstrates the strong packing tendency in the
π-stacking direction. The fact that the (number) average contour length of the
chains (around 65 nm), which was calculated on the basis of the (number) average
Morphology of P3HT in Thin Films
47