For some polymers, post-deposition treatment is necessary or at least beneficial
to reach maximum field-effect mobilities. This is for example the case for PBTTT
(structure P2, see Fig. 11). After annealing within the liquid-crystal mesophase,
this polymer forms large crystalline areas and the hole mobility increases to
0.7 cm
2 V
À1 s
À1 [113]. For P3HT, simple annealing up to 150
C does not lead to
substantial improvement in the film morphology or hole mobility [63]. Prolonged
annealing above 160
C even leads to performance degradation [64]. However,
moderate annealing in inert atmosphere or under vacuum increases the on/off
ratio and shifts the onset voltage to more negative values (depending on the initial
preparation conditions) due to the removal of oxygen from the film [47, 63].
In order to substantially increase the crystallinity of a preformed P3HT film,
solvent–vapor annealing or controlled solvent swelling and deswelling is required.
Carbon disulfide (CS 2 ) is a good solvent for swelling polythiophene thin films under
controlled vapor pressure [65–67]. Within a swollen P3HT film the majority of
polymer chains adopt coil-like configurations and no indication for aggregates is
found. When the CS 2 vapor pressure is reduced, the film deswells and recrystallization can take place under controlled conditions. Using this technique, Crossland
et al. were able to tune nucleation density and grow large P3HT spherulites with
diameters of up to 100 μm [65]. Field-effect transistors produced with these
recrystallized P3HT films showed high hole mobilities (up to 0.11 cm
2 V
À1 s
À1 ),
which decreased with the number of spherulite boundaries within the channel [68].
In summary, the order and microstructure of P3HT thin films is extremely
sensitive to the solvents used, deposition conditions, and post-deposition treatment.
This can pose a problem for reproducibility but also enables wide tunability of
transport properties with the same polymer. Hence, careful control of all processing
parameters and complementary characterization techniques are crucial before
drawing any conclusions from device characteristics.
3.4 Dielectric Interface
In addition to regioregularity, molecular weight, and the solution deposition conditions of P3HT, the surface treatment of the dielectric has been found to be
extremely important for the orientation, crystallinity, and ultimately the carrier
mobility of P3HT thin films. As mentioned above, the typical device structure used
for studies on charge transport in P3HT is the bottom gate/top contact geometry
with doped silicon as the gate electrode and thermally grown SiO 2 as the gate
dielectric. Clean, untreated SiO 2 surfaces are terminated by –OH groups and are
thus very hydrophilic. These surface groups as well as adsorbed water can act as
charge traps [69, 70]. For most organic semiconductors, either solution-processed
or vapor-deposited, the passivation of the SiO 2 surface with hexamethyldisilazane
(HMDS) or self-assembled monolayers of alkylsilanes improves device performance with respect to both mobility and threshold voltage [71, 72]. By modifying
P3HT and Other Polythiophene Field-Effect Transistors
119
to reach maximum field-effect mobilities. This is for example the case for PBTTT
(structure P2, see Fig. 11). After annealing within the liquid-crystal mesophase,
this polymer forms large crystalline areas and the hole mobility increases to
0.7 cm
2 V
À1 s
À1 [113]. For P3HT, simple annealing up to 150
C does not lead to
substantial improvement in the film morphology or hole mobility [63]. Prolonged
annealing above 160
C even leads to performance degradation [64]. However,
moderate annealing in inert atmosphere or under vacuum increases the on/off
ratio and shifts the onset voltage to more negative values (depending on the initial
preparation conditions) due to the removal of oxygen from the film [47, 63].
In order to substantially increase the crystallinity of a preformed P3HT film,
solvent–vapor annealing or controlled solvent swelling and deswelling is required.
Carbon disulfide (CS 2 ) is a good solvent for swelling polythiophene thin films under
controlled vapor pressure [65–67]. Within a swollen P3HT film the majority of
polymer chains adopt coil-like configurations and no indication for aggregates is
found. When the CS 2 vapor pressure is reduced, the film deswells and recrystallization can take place under controlled conditions. Using this technique, Crossland
et al. were able to tune nucleation density and grow large P3HT spherulites with
diameters of up to 100 μm [65]. Field-effect transistors produced with these
recrystallized P3HT films showed high hole mobilities (up to 0.11 cm
2 V
À1 s
À1 ),
which decreased with the number of spherulite boundaries within the channel [68].
In summary, the order and microstructure of P3HT thin films is extremely
sensitive to the solvents used, deposition conditions, and post-deposition treatment.
This can pose a problem for reproducibility but also enables wide tunability of
transport properties with the same polymer. Hence, careful control of all processing
parameters and complementary characterization techniques are crucial before
drawing any conclusions from device characteristics.
3.4 Dielectric Interface
In addition to regioregularity, molecular weight, and the solution deposition conditions of P3HT, the surface treatment of the dielectric has been found to be
extremely important for the orientation, crystallinity, and ultimately the carrier
mobility of P3HT thin films. As mentioned above, the typical device structure used
for studies on charge transport in P3HT is the bottom gate/top contact geometry
with doped silicon as the gate electrode and thermally grown SiO 2 as the gate
dielectric. Clean, untreated SiO 2 surfaces are terminated by –OH groups and are
thus very hydrophilic. These surface groups as well as adsorbed water can act as
charge traps [69, 70]. For most organic semiconductors, either solution-processed
or vapor-deposited, the passivation of the SiO 2 surface with hexamethyldisilazane
(HMDS) or self-assembled monolayers of alkylsilanes improves device performance with respect to both mobility and threshold voltage [71, 72]. By modifying
P3HT and Other Polythiophene Field-Effect Transistors
119