Second, the growth of P3HT domains is certainly influenced by the diffusion
kinetics of the chains on the TCB surface during slow-DEC. Diffusion of longer
chains is likely to be more difficult on the TCB substrate as compared to oligomerlike chains. This may favor the growth of crystalline domains in the a P3HT direction
for short chains, resulting in domains with a high aspect ratio L a /L c .
Third, the polydispersity index of the low and high-M w fractions differ notably.
Low-M w P3HT has a rather low polydispersity index (1.07) that explains the
relative constant overall lamellar thickness (stem length) seen in the HR-TEM
images. As opposed to this, high-M w P3HT is more polydisperse (polydispersity
index of ~ 2) and the stem length distribution is very broad. For high-M w P3HT
films, lamellae show a strong tapering as observed for instance in epitaxied films of
polyfluorenes (PF2/6 and PFO) on oriented polytetrafluoroethylene (PTFE)
[71]. For poly[9,9-bis(n-octyl)fluorene-2,7-diyl] (PFO), lamellar tapering was
explained by segregation between short and long chains during crystallization.
A similar effect may also occur in the case of P3HT for high-M w samples, provided
that the crystallization kinetics is slow enough.
As a concluding remark, HR-TEM at low dose is a valuable method for observing P3HT nanocrystals at the scale of crystalline stems. Important informations on
the size and shape of the nanocrystals can be obtained. Recent results indicate that a
similar analysis can also be performed on spin-coated P3AT films [72]. In particular, the influence of thermal annealing on the nanocrystal dimensions of various
P3ATs (from n-pentyl to n-octyl side chains) was investigated as a function of the
annealing temperature.
4 Conclusions and Outlook
This contribution has illustrated the versatility of epitaxial crystallization to generate
highly crystalline and oriented films of P3HT. Different morphologies and
nanotextured patterns can be obtained by the proper choice of substrates used for
epitaxy, which often involves a 1D lattice matching, either in the plane (TCB, KAP)
or along the normal to the substrate (K-BrBz). The role of macromolecular parameters on the crystallization of P3HT could also be unveiled. Electron diffraction, dark
field, low dose HR-TEM imaging and GIXD provide highly complementary views of
the P3HT structure and nanomorphology. In particular, the important role of the
growth kinetics on the lamellar period and crystalline domain size was evidenced by
using a slow-directional epitaxial crystallization method. Electron diffraction on
epitaxied low-M w P3HT proved to be a valuable approach for generating structural
models of P3HT. Nevertheless, structural refinement of semiconducting polymers
remains a difficult task and more quantitative analyses of ED patterns in conjunction
with proper modeling tools will certainly allow the further improvement of existing
models. Despite the fact that epitaxy is still difficult to transpose to large-scale device
processing, the variety of results obtained by TEM and GIXD on epitaxied P3HT
films illustrates the unique possibilities offered by this approach in revealing
Understanding the Structure and Crystallization of Regioregular. . .
103
kinetics of the chains on the TCB surface during slow-DEC. Diffusion of longer
chains is likely to be more difficult on the TCB substrate as compared to oligomerlike chains. This may favor the growth of crystalline domains in the a P3HT direction
for short chains, resulting in domains with a high aspect ratio L a /L c .
Third, the polydispersity index of the low and high-M w fractions differ notably.
Low-M w P3HT has a rather low polydispersity index (1.07) that explains the
relative constant overall lamellar thickness (stem length) seen in the HR-TEM
images. As opposed to this, high-M w P3HT is more polydisperse (polydispersity
index of ~ 2) and the stem length distribution is very broad. For high-M w P3HT
films, lamellae show a strong tapering as observed for instance in epitaxied films of
polyfluorenes (PF2/6 and PFO) on oriented polytetrafluoroethylene (PTFE)
[71]. For poly[9,9-bis(n-octyl)fluorene-2,7-diyl] (PFO), lamellar tapering was
explained by segregation between short and long chains during crystallization.
A similar effect may also occur in the case of P3HT for high-M w samples, provided
that the crystallization kinetics is slow enough.
As a concluding remark, HR-TEM at low dose is a valuable method for observing P3HT nanocrystals at the scale of crystalline stems. Important informations on
the size and shape of the nanocrystals can be obtained. Recent results indicate that a
similar analysis can also be performed on spin-coated P3AT films [72]. In particular, the influence of thermal annealing on the nanocrystal dimensions of various
P3ATs (from n-pentyl to n-octyl side chains) was investigated as a function of the
annealing temperature.
4 Conclusions and Outlook
This contribution has illustrated the versatility of epitaxial crystallization to generate
highly crystalline and oriented films of P3HT. Different morphologies and
nanotextured patterns can be obtained by the proper choice of substrates used for
epitaxy, which often involves a 1D lattice matching, either in the plane (TCB, KAP)
or along the normal to the substrate (K-BrBz). The role of macromolecular parameters on the crystallization of P3HT could also be unveiled. Electron diffraction, dark
field, low dose HR-TEM imaging and GIXD provide highly complementary views of
the P3HT structure and nanomorphology. In particular, the important role of the
growth kinetics on the lamellar period and crystalline domain size was evidenced by
using a slow-directional epitaxial crystallization method. Electron diffraction on
epitaxied low-M w P3HT proved to be a valuable approach for generating structural
models of P3HT. Nevertheless, structural refinement of semiconducting polymers
remains a difficult task and more quantitative analyses of ED patterns in conjunction
with proper modeling tools will certainly allow the further improvement of existing
models. Despite the fact that epitaxy is still difficult to transpose to large-scale device
processing, the variety of results obtained by TEM and GIXD on epitaxied P3HT
films illustrates the unique possibilities offered by this approach in revealing
Understanding the Structure and Crystallization of Regioregular. . .
103