Nevertheless, as efficient hole transport necessitates the existence of a percolation pathway of crystalline P3HT throughout the entire layer, knowledge about the
degree of crystallinity and also about the 3D distribution of the different phases
forming the P3HT:fullerene blend are highly important. A well-established method
for obtaining information on these properties is electron tomography (ET) [79, 96,
155]. For example, Loos and coworkers applied ET to determine the amount and
distribution of the crystalline P3HT component in P3HT:PCBM blends [79]. These
samples were investigated directly after spin-coating or after thermal annealing for
20 min at 130
C. The overall degree of crystallinity (DoC) in the annealed samples
was between 40 and 55%, depending on layer thickness. The DoC from ET
compares nicely with the range of values given in Fig. 13 for annealed blends.
Notably, ET on thin layers (50–100 nm) revealed an enrichment of the crystalline
P3HT content close to the bottom side where hole extraction occurs [79]. On the
other hand, 200 nm thick layers had a homogeneous distribution of P3HT crystallites, which is highly beneficial for the efficient collection of holes throughout the
entire active blend.
Raman spectroscopy is another successful optical technique for study of the
degree of molecular ordering in the P3HT phase of the P3HT:PCBM blend. The
in-plane skeleton Raman modes of C¼C and C–C stretching were studied by Tsoi
et al. under excitation wavelengths ranging from resonant to nonresonant
[156]. Although direct electronic excitations in the resonant range could lead to a
strong fluorescent background that can cover Raman signals, this is still an important measurement technique because the resonant conditions provide information
about the molecular structure. In order to better understand the changes in the
morphology of the P3HT component, Raman spectra of regiorandom and
regioregular P3HT were compared where the regioregular polymer is known to
have a higher degree of ordering. These results were then compared with the Raman
spectra from a blend of regioregular P3HT with PCBM. The C¼C stretching mode
was found to be the superposition of the ordered and disordered P3HT characteristics and this was used to quantitatively estimate the degree of molecular ordering
in the blend. The degree of molecular ordering in the nonannealed blend compared
with the annealed blend was found to increase from 42 to 94%, relative to the
ordering of pristine regioregular P3HT. The percentage of crystallinity can then be
estimated based on a known percentage of crystallinity in the pristine P3HT. Tsoi
et al. assumes a crystallinity in the regioregular P3HT of 15% and hence a
percentage crystallinity of 6 and 14% in the P3HT phase of the annealed and
nonannealed blends, respectively. However, optical spectroscopy on pristine
P3HT layers consistently showed a degree of chain aggregation of approximately
40–50% [62, 150], and even higher degrees of crystallinity were reported in bulk
samples [157]. Using 50% as a reference value, the degree of aggregation as
estimated from the Raman spectra is 21 and 47% in the nonannealed and annealed
blends, respectively. This approximation seems to be more reliable, based on the
results of previous studies. Therefore, with an accurate method for determining the
percentage crystallinity in the pristine regioregular P3HT, the analysis of Raman
206
A.J. Moule ´ et al.
degree of crystallinity and also about the 3D distribution of the different phases
forming the P3HT:fullerene blend are highly important. A well-established method
for obtaining information on these properties is electron tomography (ET) [79, 96,
155]. For example, Loos and coworkers applied ET to determine the amount and
distribution of the crystalline P3HT component in P3HT:PCBM blends [79]. These
samples were investigated directly after spin-coating or after thermal annealing for
20 min at 130
C. The overall degree of crystallinity (DoC) in the annealed samples
was between 40 and 55%, depending on layer thickness. The DoC from ET
compares nicely with the range of values given in Fig. 13 for annealed blends.
Notably, ET on thin layers (50–100 nm) revealed an enrichment of the crystalline
P3HT content close to the bottom side where hole extraction occurs [79]. On the
other hand, 200 nm thick layers had a homogeneous distribution of P3HT crystallites, which is highly beneficial for the efficient collection of holes throughout the
entire active blend.
Raman spectroscopy is another successful optical technique for study of the
degree of molecular ordering in the P3HT phase of the P3HT:PCBM blend. The
in-plane skeleton Raman modes of C¼C and C–C stretching were studied by Tsoi
et al. under excitation wavelengths ranging from resonant to nonresonant
[156]. Although direct electronic excitations in the resonant range could lead to a
strong fluorescent background that can cover Raman signals, this is still an important measurement technique because the resonant conditions provide information
about the molecular structure. In order to better understand the changes in the
morphology of the P3HT component, Raman spectra of regiorandom and
regioregular P3HT were compared where the regioregular polymer is known to
have a higher degree of ordering. These results were then compared with the Raman
spectra from a blend of regioregular P3HT with PCBM. The C¼C stretching mode
was found to be the superposition of the ordered and disordered P3HT characteristics and this was used to quantitatively estimate the degree of molecular ordering
in the blend. The degree of molecular ordering in the nonannealed blend compared
with the annealed blend was found to increase from 42 to 94%, relative to the
ordering of pristine regioregular P3HT. The percentage of crystallinity can then be
estimated based on a known percentage of crystallinity in the pristine P3HT. Tsoi
et al. assumes a crystallinity in the regioregular P3HT of 15% and hence a
percentage crystallinity of 6 and 14% in the P3HT phase of the annealed and
nonannealed blends, respectively. However, optical spectroscopy on pristine
P3HT layers consistently showed a degree of chain aggregation of approximately
40–50% [62, 150], and even higher degrees of crystallinity were reported in bulk
samples [157]. Using 50% as a reference value, the degree of aggregation as
estimated from the Raman spectra is 21 and 47% in the nonannealed and annealed
blends, respectively. This approximation seems to be more reliable, based on the
results of previous studies. Therefore, with an accurate method for determining the
percentage crystallinity in the pristine regioregular P3HT, the analysis of Raman
206
A.J. Moule ´ et al.