In addition, a zone melting technique is used to precisely control both the growth
rate (typically 20 μm/s) and the in-plane orientation of TCB crystals and thus to
achieve uniform surfaces of TCB crystals over several square centimeters, making
the films suitable for the epitaxial growth of P3HT. The improvement of the overall
in-plane orientation of the films leads to both the increase in the observed dichroic
ratio in the UV–visible absorption spectra (in excess of 13) and a substantially
narrowed angular spread of the in-plane orientation distribution of the P3HT
crystals [46]. The improved in-plane orientation of the films grown by slow-DEC
was particularly well observed by grazing incidence X-ray diffraction (GIXD),
which probes the orientation over a surface of the order of 1 cm
2 . Interestingly, by
using GIXD, it was possible to distinguish the 0 0 2 from the 0 2 0 reflection
(Fig. 5e) by changing the orientation of the incident X-ray beam with respect to the
in-plane chain direction. This is usually not possible because in spin-coated P3HT
films these two reflections overlap as they correspond to almost identical reticular
distances (0.38 nm). The observation of a strong 0 0 2 reflection highlights the high
crystallinity and 3D order observed in epitaxied layers.
Regarding the mechanism of orientation of P3HT on TCB, 1D epitaxy was
observed for a 17 kDa sample [43] with the epitaxial conditions: c P3HT //c TCB ,
(0 1 0) P3HT //(1 0 0) TCB and c P3HT /2 ~ c TCB . In this case, the crystalline lamellae of
P3HT grow perpendicular (i.e. edge-on) to the substrate of TCB (see Fig. 6). It is
worth noting that the terms ‘edge-on’ and ‘flat-on’ are used hereafter to refer to the
orientation of the crystalline lamellae on the substrate, not to the orientation of the
π-conjugated skeleton on the substrate (the terms ‘edge-on’ and ‘face-on’ are used
for the π-conjugated skeleton oriented perpendicular to and in the plane of the
substrate, respectively).
Interestingly, the orientation of P3HT on TCB was found to depend on the
molecular weight of the polymer. For a 7 kDa P3HT crystallizing with extended
chains, two different types of orientation of the crystalline lamellae on TCB were
evidenced. As well as edge-on lamellae, flat-lying P3HT lamellae consisting of
‘standing’ chains were also observed in BF (see Fig. 6). These lamellae are oriented
in the plane of the substrate with b P3HT //c TCB and (0 0 1) P3HT //(1 0 0) TCB . This
second population of oriented P3HT lamellae on TCB is also induced by 1D epitaxy
of P3HT on TCB. Indeed, for this population of flat-on lamellae, we observe the
epitaxial condition b P3HT /2 ~ c TCB [43]. To conclude, directional epitaxial crystallization is a powerful method for aligning P3HT and other poly(3-alkylthiophene)s
over large surfaces, provided that the growth of TCB is controlled, e.g., by a local
zone melting (slow-DEC).
Fig. 5 (continued) curve GIXD with q i //c P3HT ; lower black curve q i ⊥ c P3HT with q i the vector of
the incident X-ray beam. (f) 2D GIXD pattern recorded with the incident X -ray beam oriented
‘parallel’ to the chain direction. (g) TEM electron diffraction pattern. (Adapted with permission
from [46] © 2012, Royal Society of Chemistry)
92
M. Brinkmann et al.
rate (typically 20 μm/s) and the in-plane orientation of TCB crystals and thus to
achieve uniform surfaces of TCB crystals over several square centimeters, making
the films suitable for the epitaxial growth of P3HT. The improvement of the overall
in-plane orientation of the films leads to both the increase in the observed dichroic
ratio in the UV–visible absorption spectra (in excess of 13) and a substantially
narrowed angular spread of the in-plane orientation distribution of the P3HT
crystals [46]. The improved in-plane orientation of the films grown by slow-DEC
was particularly well observed by grazing incidence X-ray diffraction (GIXD),
which probes the orientation over a surface of the order of 1 cm
2 . Interestingly, by
using GIXD, it was possible to distinguish the 0 0 2 from the 0 2 0 reflection
(Fig. 5e) by changing the orientation of the incident X-ray beam with respect to the
in-plane chain direction. This is usually not possible because in spin-coated P3HT
films these two reflections overlap as they correspond to almost identical reticular
distances (0.38 nm). The observation of a strong 0 0 2 reflection highlights the high
crystallinity and 3D order observed in epitaxied layers.
Regarding the mechanism of orientation of P3HT on TCB, 1D epitaxy was
observed for a 17 kDa sample [43] with the epitaxial conditions: c P3HT //c TCB ,
(0 1 0) P3HT //(1 0 0) TCB and c P3HT /2 ~ c TCB . In this case, the crystalline lamellae of
P3HT grow perpendicular (i.e. edge-on) to the substrate of TCB (see Fig. 6). It is
worth noting that the terms ‘edge-on’ and ‘flat-on’ are used hereafter to refer to the
orientation of the crystalline lamellae on the substrate, not to the orientation of the
π-conjugated skeleton on the substrate (the terms ‘edge-on’ and ‘face-on’ are used
for the π-conjugated skeleton oriented perpendicular to and in the plane of the
substrate, respectively).
Interestingly, the orientation of P3HT on TCB was found to depend on the
molecular weight of the polymer. For a 7 kDa P3HT crystallizing with extended
chains, two different types of orientation of the crystalline lamellae on TCB were
evidenced. As well as edge-on lamellae, flat-lying P3HT lamellae consisting of
‘standing’ chains were also observed in BF (see Fig. 6). These lamellae are oriented
in the plane of the substrate with b P3HT //c TCB and (0 0 1) P3HT //(1 0 0) TCB . This
second population of oriented P3HT lamellae on TCB is also induced by 1D epitaxy
of P3HT on TCB. Indeed, for this population of flat-on lamellae, we observe the
epitaxial condition b P3HT /2 ~ c TCB [43]. To conclude, directional epitaxial crystallization is a powerful method for aligning P3HT and other poly(3-alkylthiophene)s
over large surfaces, provided that the growth of TCB is controlled, e.g., by a local
zone melting (slow-DEC).
Fig. 5 (continued) curve GIXD with q i //c P3HT ; lower black curve q i ⊥ c P3HT with q i the vector of
the incident X-ray beam. (f) 2D GIXD pattern recorded with the incident X -ray beam oriented
‘parallel’ to the chain direction. (g) TEM electron diffraction pattern. (Adapted with permission
from [46] © 2012, Royal Society of Chemistry)
92
M. Brinkmann et al.