spectra of regioirregular P3HT resemble the solution spectra of regioregular P3HT
(shown in Fig. 3), consisting of a broad and almost featureless absorption band
[76]. This observation is related to the fact that regioirregular P3HT forms mainly
amorphous layers due to the weak tendency for intermolecular packing, which is
impeded by the sterically driven twist of the backbone. Therefore, the physical
conformation of the chains is similar to that of regioregular P3HT in the solution
state. By contrast, film spectra of regioregular P3HT are significantly red-shifted
and exhibit well-defined vibronic features in the low-energy part of the spectrum,
which is attributed to the presence of π-stacked aggregates in which chain twisting
is strongly constrained, leading to flat molecules with extended conjugation
[76]. Additionally, the optical properties are strongly dependent on the molecular
weight. This is illustrated in Fig. 9, which shows both photographs and absorption
spectra of P3HT thin films of different molecular weight after solution-casting and
thermal annealing, respectively [17]. The fact that the thin film spectrum of low
molecular weight P3HT resembles the solution spectrum indicates that a large
number of the chains adopt a twisted conformation in the solid state. Studies on
thermochromism strengthen the interpretation that the variation in optical absorption is connected to the conformation of the polymer chain [17]. Heating a thin
P3HT film of intermediate molecular weight (hexane fraction, M n ¼ 5.6 kg/mol) to
210
C leads to a strong color change from red to yellow/orange, most probably due
to a twisting of the backbone. By contrast, little and almost no change is observed
for high (M n ¼ 19 kg/mol) and low (M n ¼ 2.2 kg/mol) molecular weight P3HT,
respectively.
Fig. 9 Optical properties of P3HT thin films as a function of the molecular weight [17]: (a)
Optical photographs of P3HT layers spin-cast from the ethyl acetate (1), hexane (2),
dichloromethane (3), and chloroform (4) fractions. (b) Absorption spectra of P3HT thin films of
the ethyl acetate (open diamond, filled diamond), hexane (open triangle, filled triangle),
dichloromethane (open circle, filled circle), and chloroform (open square, filled square) fractions.
Open and filled symbols correspond to as-cast and thermally annealed layers (5 min. at 150
C),
respectively. (Reprinted with permission from Zen et al. [17]. Copyright (2004) Wiley-VCH)
Morphology of P3HT in Thin Films
55
(shown in Fig. 3), consisting of a broad and almost featureless absorption band
[76]. This observation is related to the fact that regioirregular P3HT forms mainly
amorphous layers due to the weak tendency for intermolecular packing, which is
impeded by the sterically driven twist of the backbone. Therefore, the physical
conformation of the chains is similar to that of regioregular P3HT in the solution
state. By contrast, film spectra of regioregular P3HT are significantly red-shifted
and exhibit well-defined vibronic features in the low-energy part of the spectrum,
which is attributed to the presence of π-stacked aggregates in which chain twisting
is strongly constrained, leading to flat molecules with extended conjugation
[76]. Additionally, the optical properties are strongly dependent on the molecular
weight. This is illustrated in Fig. 9, which shows both photographs and absorption
spectra of P3HT thin films of different molecular weight after solution-casting and
thermal annealing, respectively [17]. The fact that the thin film spectrum of low
molecular weight P3HT resembles the solution spectrum indicates that a large
number of the chains adopt a twisted conformation in the solid state. Studies on
thermochromism strengthen the interpretation that the variation in optical absorption is connected to the conformation of the polymer chain [17]. Heating a thin
P3HT film of intermediate molecular weight (hexane fraction, M n ¼ 5.6 kg/mol) to
210
C leads to a strong color change from red to yellow/orange, most probably due
to a twisting of the backbone. By contrast, little and almost no change is observed
for high (M n ¼ 19 kg/mol) and low (M n ¼ 2.2 kg/mol) molecular weight P3HT,
respectively.
Fig. 9 Optical properties of P3HT thin films as a function of the molecular weight [17]: (a)
Optical photographs of P3HT layers spin-cast from the ethyl acetate (1), hexane (2),
dichloromethane (3), and chloroform (4) fractions. (b) Absorption spectra of P3HT thin films of
the ethyl acetate (open diamond, filled diamond), hexane (open triangle, filled triangle),
dichloromethane (open circle, filled circle), and chloroform (open square, filled square) fractions.
Open and filled symbols correspond to as-cast and thermally annealed layers (5 min. at 150
C),
respectively. (Reprinted with permission from Zen et al. [17]. Copyright (2004) Wiley-VCH)
Morphology of P3HT in Thin Films
55