crystallization of rods into fibrillar structures. Koppe et al. further assume that, after
aggregation, ordered domains link to each other and form a gel, which results in an
increase in viscosity (see Fig. 7) [51]. Aggregation and gelation occur most
intensely and quickly for high molecular weight chains and can be delayed or
even suppressed by “dilutants” such as PCBM and P3HT chains of low molecular
weight. Furthermore, side chain length and regioregularity play a crucial role in the
gelation process [58, 59].
For all the approaches described so far, crystal formation takes place in solution
before the deposition of the polymer on a substrate. The deposition of
pre-aggregated nanocrystals is typically achieved by means of drop-casting, spincoating, or dip-coating. The performance of nanofibril-based films of
polythiophenes has been intensely investigated in devices such as OFETs and
OSCs [50, 52, 55, 57, 60–66]. Merlo and Frisbie reported hole mobilities of up to
0.06 cm
2 /V s for single P3HT nanofibrils, demonstrating the high degree of order
within the fibrils [60, 61]. Samitsu et al. further showed for P3AT of different side
chain lengths that charge carrier transport in single fibrils is much more effective
than between fibrils in a network, suggesting a transport barrier between adjacent
fibrils (see Fig. 8) [63]. In contrast to thin films of P3AT cast from solutions
prepared using a good solvent, nanofibril-based films exhibit hole mobilities that
are relatively independent of the side chain length, which results from the high
in-plane order of the chains within the fibrils (edge-on), whereas the molecular
orientation in cast films is strongly affected by the alkyl chain length during
solution-casting [53]. In bulk-heterojunction solar cells, P3HT nanofibrils yield
high power conversion efficiencies of 3.9% [55] in combination with PCBM.
4 Thin Film Properties
Highly crystalline layers of P3HT can be obtained by pre-aggregation of nanofibrils
in solution followed by deposition of the aggregated nanostructures on a substrate,
which allows decoupling of crystallization from the film deposition process. However, device fabrication requires cheap and fast techniques to generate homogeneous films on a large scale. Consequently, controlled crystal formation in solution
is difficult to implement for device fabrication. Deposition of conjugated polymers
is typically pursued from solutions prepared using a good solvent like chloroform
via various techniques such as spin-coating, doctor-blading, dip-coating, and inkjetprinting. In these fast processes, crystal formation takes place during drying of the
Fig. 6 (continued) M n ¼ 6.0 kg/mol, 0.2 mg/mL in DMF/anisole (2:1 vol%) and (e) M n ¼ 10.2 kg/
mol, 0.05 mg/mL in anisole; scale bars: 500 nm. (f) Selected area electron diffraction of P3HT
nanoribbons (10.2 kg/mol) corresponding to (e). Molecular arrangement within P3HT nanoribbons
is shown on the right. (Reprinted with permission from Liu et al. [49]. Copyright (2009) American
Chemical Society)
52
K. Tremel and S. Ludwigs
aggregation, ordered domains link to each other and form a gel, which results in an
increase in viscosity (see Fig. 7) [51]. Aggregation and gelation occur most
intensely and quickly for high molecular weight chains and can be delayed or
even suppressed by “dilutants” such as PCBM and P3HT chains of low molecular
weight. Furthermore, side chain length and regioregularity play a crucial role in the
gelation process [58, 59].
For all the approaches described so far, crystal formation takes place in solution
before the deposition of the polymer on a substrate. The deposition of
pre-aggregated nanocrystals is typically achieved by means of drop-casting, spincoating, or dip-coating. The performance of nanofibril-based films of
polythiophenes has been intensely investigated in devices such as OFETs and
OSCs [50, 52, 55, 57, 60–66]. Merlo and Frisbie reported hole mobilities of up to
0.06 cm
2 /V s for single P3HT nanofibrils, demonstrating the high degree of order
within the fibrils [60, 61]. Samitsu et al. further showed for P3AT of different side
chain lengths that charge carrier transport in single fibrils is much more effective
than between fibrils in a network, suggesting a transport barrier between adjacent
fibrils (see Fig. 8) [63]. In contrast to thin films of P3AT cast from solutions
prepared using a good solvent, nanofibril-based films exhibit hole mobilities that
are relatively independent of the side chain length, which results from the high
in-plane order of the chains within the fibrils (edge-on), whereas the molecular
orientation in cast films is strongly affected by the alkyl chain length during
solution-casting [53]. In bulk-heterojunction solar cells, P3HT nanofibrils yield
high power conversion efficiencies of 3.9% [55] in combination with PCBM.
4 Thin Film Properties
Highly crystalline layers of P3HT can be obtained by pre-aggregation of nanofibrils
in solution followed by deposition of the aggregated nanostructures on a substrate,
which allows decoupling of crystallization from the film deposition process. However, device fabrication requires cheap and fast techniques to generate homogeneous films on a large scale. Consequently, controlled crystal formation in solution
is difficult to implement for device fabrication. Deposition of conjugated polymers
is typically pursued from solutions prepared using a good solvent like chloroform
via various techniques such as spin-coating, doctor-blading, dip-coating, and inkjetprinting. In these fast processes, crystal formation takes place during drying of the
Fig. 6 (continued) M n ¼ 6.0 kg/mol, 0.2 mg/mL in DMF/anisole (2:1 vol%) and (e) M n ¼ 10.2 kg/
mol, 0.05 mg/mL in anisole; scale bars: 500 nm. (f) Selected area electron diffraction of P3HT
nanoribbons (10.2 kg/mol) corresponding to (e). Molecular arrangement within P3HT nanoribbons
is shown on the right. (Reprinted with permission from Liu et al. [49]. Copyright (2009) American
Chemical Society)
52
K. Tremel and S. Ludwigs