aggregates from 55 to 59%. The reduction of entanglements should also reduce
disorder within the π-stacks and thus lead to improved interchain transport and
higher mobilities.
As shown above, aggregates are crucial for obtaining high mobility P3HT films.
Another way of promoting the creation of ordered aggregates during film formation
is by adding poor solvents with a higher boiling point than the main solvent to a
P3HT solution. The lower volatility of the poor solvent leads to longer drying times,
which again promote controlled aggregation. This was demonstrated by Park
et al. who added acetonitrile (0–10%, boiling point 81
C) to solutions of P3HT in
chloroform (boiling point 61
C) [51]. Absorption spectra of the mixed solutions
started to show aggregation-related red-shifts and characteristic vibronic features
for acetonitrile volume fractions of 17%. The crystallinity of spincast films
increased. However, the maximum field-effect mobility of 0.015 cm
2 V
À1 s
À1
was found for films spincast from solutions of 1 wt% P3HT in chloroform with
only 3.3 vol% acetonitrile. Higher volume fractions of acetonitrile led to precipitation and, thus, inhomogeneous films and worse FET performance. For low
concentrations of acetonitrile in the precursor solution, aggregation started during
the evaporation process as the volume fraction of the non-solvent increased. This
also explains why adding acetonitrile to high boiling point, good solvent solutions
like dichlorobenzene (boiling point 174
C) and chlorobenzene (boiling point
132
C) did not show any improvement in crystallinity or mobility.
Chang et al. added acetone, which has a lower boiling point (56
C), to chloroform
solutions of P3HT and observed an increase in hole mobility from 0.0043 to
0.017 cm
2 V
À1 s
À1 for 2 vol% of acetone [52]. Absorption spectra of the solutions
showed no sign of aggregation, whereas absorption spectra of the spincast films
started to show peaks associated with π–π stacking in P3HT aggregates. Also, the
grazing incidence X-ray diffraction of spincoated films showed an increase of the
(100) peak intensity, associated with lamellar packing along the crystallographic
direction perpendicular to the polymer backbone. Chloroform and acetone form a
pressure-minimum (temperature-maximum) azeotrope at about 68 vol% of chloroform and 32 vol% of acetone, with a boiling point of 64.7
C [53]. Hence, during the
drying process the concentration of the non-solvent acetone increased gradually up to
that volume fraction and aggregate formation was facilitated. This led to the observed
increase in field-effect mobilities without any additional post-deposition treatment.
The pre-aggregation can be driven to its extreme (i.e., formation of P3HT
nanowires) by cooling a solution of a moderate solvent (e.g., 1,2-dichlorobenzene)
[54–58] or by templating their growth from solution by carbon nanotubes [59,
60]. These nanowires can be several micrometers long and 10–20 nm wide. They
show a high degree of order but transport can only occur along the π–π stacking
direction. FETs based on single P3HT nanowires show reasonable mobilities of about
0.02 cm
2 V
À1 s
À1 [61, 62], whereas networks of nanowires generally show lower
overall mobilities due to the limited fill-factor and necessary nanowire-to-nanowire
contacts. The lower mobility in P3HT nanowires compared to the highest possible
mobilities in P3HT thin films indicates that transport along the π–π stacking direction
is slower than along the polymer backbone.
118
J. Zaumseil
disorder within the π-stacks and thus lead to improved interchain transport and
higher mobilities.
As shown above, aggregates are crucial for obtaining high mobility P3HT films.
Another way of promoting the creation of ordered aggregates during film formation
is by adding poor solvents with a higher boiling point than the main solvent to a
P3HT solution. The lower volatility of the poor solvent leads to longer drying times,
which again promote controlled aggregation. This was demonstrated by Park
et al. who added acetonitrile (0–10%, boiling point 81
C) to solutions of P3HT in
chloroform (boiling point 61
C) [51]. Absorption spectra of the mixed solutions
started to show aggregation-related red-shifts and characteristic vibronic features
for acetonitrile volume fractions of 17%. The crystallinity of spincast films
increased. However, the maximum field-effect mobility of 0.015 cm
2 V
À1 s
À1
was found for films spincast from solutions of 1 wt% P3HT in chloroform with
only 3.3 vol% acetonitrile. Higher volume fractions of acetonitrile led to precipitation and, thus, inhomogeneous films and worse FET performance. For low
concentrations of acetonitrile in the precursor solution, aggregation started during
the evaporation process as the volume fraction of the non-solvent increased. This
also explains why adding acetonitrile to high boiling point, good solvent solutions
like dichlorobenzene (boiling point 174
C) and chlorobenzene (boiling point
132
C) did not show any improvement in crystallinity or mobility.
Chang et al. added acetone, which has a lower boiling point (56
C), to chloroform
solutions of P3HT and observed an increase in hole mobility from 0.0043 to
0.017 cm
2 V
À1 s
À1 for 2 vol% of acetone [52]. Absorption spectra of the solutions
showed no sign of aggregation, whereas absorption spectra of the spincast films
started to show peaks associated with π–π stacking in P3HT aggregates. Also, the
grazing incidence X-ray diffraction of spincoated films showed an increase of the
(100) peak intensity, associated with lamellar packing along the crystallographic
direction perpendicular to the polymer backbone. Chloroform and acetone form a
pressure-minimum (temperature-maximum) azeotrope at about 68 vol% of chloroform and 32 vol% of acetone, with a boiling point of 64.7
C [53]. Hence, during the
drying process the concentration of the non-solvent acetone increased gradually up to
that volume fraction and aggregate formation was facilitated. This led to the observed
increase in field-effect mobilities without any additional post-deposition treatment.
The pre-aggregation can be driven to its extreme (i.e., formation of P3HT
nanowires) by cooling a solution of a moderate solvent (e.g., 1,2-dichlorobenzene)
[54–58] or by templating their growth from solution by carbon nanotubes [59,
60]. These nanowires can be several micrometers long and 10–20 nm wide. They
show a high degree of order but transport can only occur along the π–π stacking
direction. FETs based on single P3HT nanowires show reasonable mobilities of about
0.02 cm
2 V
À1 s
À1 [61, 62], whereas networks of nanowires generally show lower
overall mobilities due to the limited fill-factor and necessary nanowire-to-nanowire
contacts. The lower mobility in P3HT nanowires compared to the highest possible
mobilities in P3HT thin films indicates that transport along the π–π stacking direction
is slower than along the polymer backbone.
118
J. Zaumseil