conjugation lengths. However, as the channel of a transistor is much longer than a
single polymer chain, interchain hopping is necessary for macroscopic transport.
This is facilitated by tie molecules that bridge adjacent crystallites. For optimal
bridging the boundary angle should be low, leading to mobility anisotropy in
aligned semicrystalline films. The need for tie molecules also explains the increase
in mobility with molecular weight, although P3HT films with lower molecular
weight show higher crystallinity and lower paracrystalline disorder than high
molecular weight films. The well-ordered crystalline regions are not connected to
each other and hence transport is strongly inhibited. Charge transport along the π–π
stacking direction is slower than perpendicular to it but faster than in the side-chain
stacking direction, in which long alkyl chains and minimal orbital overlap hinder
charge transfer.
In summary, obtaining maximum hole mobilities in P3HT transistors requires
long, well-planarized and π–π-stacked polymer chains that are well connected via
tie molecules at shallow angles. High crystallinity is no guarantee for high mobility;
instead, the connectivity of crystalline domains plays a major role in macroscopic
charge transport.
3.6 Blends and Copolymers
In contradiction to what one might expect, mixing P3HT with insulating polymers
can actually improve the carrier mobility and stability of P3HT-FETs substantially.
In the simplest case, the air stability of the device is improved by forming an
encapsulation layer of the insulating polymer on top of the P3HT. However,
through judicious choice of polymer and processing conditions it is possible to
affect the P3HT morphology in a way that leads to higher hole mobilities despite an
excess of the insulating polymer. This was first demonstrated by Goffri et al. for
blends of P3HT and semicrystalline isotactic polystyrene or polyethylene [89]. The
blends were processed in a way that allowed the P3HT to crystallize first (e.g., by
casting from a hot solution onto a hot substrate), followed by crystallization of the
semicrystalline insulating polymer. This propelled the already solidified P3HT to
the surface and interface of the film with the substrate. The resulting vertically
stratified structure creates continuous films of P3HT at the dielectric interface. Even
at blend compositions as low as 3 wt% of P3HT this procedure led to FETs with
hole mobilities of 0.01 cm
2 V
À1 s
À1 . In addition, these blend devices were selfencapsulating and thus more air-stable than pure P3HT-FETs. In other cases, the
formation of crystalline nanofibrils within an insulating matrix is observed by TEM
and AFM (e.g., for P3HT/PS blends in dichloromethane and poly
(3-butylthiophene)/PS in 1,2-dichlorobenzene) [56, 90]. Even at low volume fractions they can maintain continuous pathways for charge transport and mobilities
ranging from 0.006 to 0.014 cm
2 V
À1 s
À1 .
Although blends can show intriguing phase separation behavior, control over the
micro- and nanostructure of such films is limited. Predetermining the film’s
P3HT and Other Polythiophene Field-Effect Transistors
123
single polymer chain, interchain hopping is necessary for macroscopic transport.
This is facilitated by tie molecules that bridge adjacent crystallites. For optimal
bridging the boundary angle should be low, leading to mobility anisotropy in
aligned semicrystalline films. The need for tie molecules also explains the increase
in mobility with molecular weight, although P3HT films with lower molecular
weight show higher crystallinity and lower paracrystalline disorder than high
molecular weight films. The well-ordered crystalline regions are not connected to
each other and hence transport is strongly inhibited. Charge transport along the π–π
stacking direction is slower than perpendicular to it but faster than in the side-chain
stacking direction, in which long alkyl chains and minimal orbital overlap hinder
charge transfer.
In summary, obtaining maximum hole mobilities in P3HT transistors requires
long, well-planarized and π–π-stacked polymer chains that are well connected via
tie molecules at shallow angles. High crystallinity is no guarantee for high mobility;
instead, the connectivity of crystalline domains plays a major role in macroscopic
charge transport.
3.6 Blends and Copolymers
In contradiction to what one might expect, mixing P3HT with insulating polymers
can actually improve the carrier mobility and stability of P3HT-FETs substantially.
In the simplest case, the air stability of the device is improved by forming an
encapsulation layer of the insulating polymer on top of the P3HT. However,
through judicious choice of polymer and processing conditions it is possible to
affect the P3HT morphology in a way that leads to higher hole mobilities despite an
excess of the insulating polymer. This was first demonstrated by Goffri et al. for
blends of P3HT and semicrystalline isotactic polystyrene or polyethylene [89]. The
blends were processed in a way that allowed the P3HT to crystallize first (e.g., by
casting from a hot solution onto a hot substrate), followed by crystallization of the
semicrystalline insulating polymer. This propelled the already solidified P3HT to
the surface and interface of the film with the substrate. The resulting vertically
stratified structure creates continuous films of P3HT at the dielectric interface. Even
at blend compositions as low as 3 wt% of P3HT this procedure led to FETs with
hole mobilities of 0.01 cm
2 V
À1 s
À1 . In addition, these blend devices were selfencapsulating and thus more air-stable than pure P3HT-FETs. In other cases, the
formation of crystalline nanofibrils within an insulating matrix is observed by TEM
and AFM (e.g., for P3HT/PS blends in dichloromethane and poly
(3-butylthiophene)/PS in 1,2-dichlorobenzene) [56, 90]. Even at low volume fractions they can maintain continuous pathways for charge transport and mobilities
ranging from 0.006 to 0.014 cm
2 V
À1 s
À1 .
Although blends can show intriguing phase separation behavior, control over the
micro- and nanostructure of such films is limited. Predetermining the film’s
P3HT and Other Polythiophene Field-Effect Transistors
123