The high carrier densities that are possible with electrolyte gating raise the
question of what happens if the carrier density is increased close to one charge
per monomer unit in the P3HT chain. Would an insulator-to-metal transition
be possible? Wang et al. demonstrated a stable Hall effect in electrolyte-gated
P3HT films [106], which was possible due to a crossover from a strongly
non-adiabatic, variable range hopping regime to an adiabatic, diffusive, band-like
transport regime at three-dimensional hole densities of up to 10
21 cm
À3 ,
corresponding to 0.2 holes per thiophene ring. However, probably due to dopinginduced disorder at these high hole densities, carrier wavefunction delocalization
and Coulomb gap collapse and thus insulator-to-metal transition did not occur.
Nevertheless, this experiment shows the potential of studying charge transport in
P3HT at the limits of carrier density and thus examining the fundamental properties
of conjugated polymers.
4 New Polythiophene Derivatives for FETs
Although interest in P3HT as a model system is still strong, many new versions of
thiophene-based semiconducting polymers have been developed over the past
decade. The main design criteria were stability against oxygen doping, large area
ordering, solution-processability, and high carrier mobility. To achieve these properties, the number, length, and position of alkyl side chains was varied and fused
thiophene moieties and electron-deficient comonomer units were introduced. The
resulting new materials can reach hole mobilities of up to 10 cm
2 V
À1 s
À1 in some
cases [107, 108] and can be used to study structure–property relationships. Detailed
synthetic approaches and molecular designs have been discussed in a number of
reviews [23, 109–112]; here we will only present a few examples of polythiophene
derivatives that are particularly promising for FET applications.
Improving the air stability of polythiophenes is crucial for their applicability in
commercial devices. To achieve this, the position of the highest occupied molecular orbital (HOMO) energy level has to be shifted further away from the vacuum
level, i.e., the ionization potential has to be increased to values larger than
À4.9 eV. One possible method is to reduce the coplanarity of the system and
thus the conjugation length, although this will also reduce π–π stacking and carrier
mobility. The ionization potential also depends on the electron density of the
conjugated system. Introducing electron-withdrawing groups to the conjugated
system or reducing the density of electron-donating groups will therefore shift the
HOMO level further away from the vacuum level. The first advance in this
direction was reported by Ong et al., who developed dialkylated quaterthiophene
polymers (usually named PQT; P1 in Fig. 11) [104]. Two thiophene units separate
the alkyl-bearing units and prevent them from interfering with each other. The
favorable orientation of the alkyl chains and their low density lead to close π–π
stacking and mobilities as high a 0.14 cm
2 V
À1 s
À1 . The central thiophene units
128
J. Zaumseil
question of what happens if the carrier density is increased close to one charge
per monomer unit in the P3HT chain. Would an insulator-to-metal transition
be possible? Wang et al. demonstrated a stable Hall effect in electrolyte-gated
P3HT films [106], which was possible due to a crossover from a strongly
non-adiabatic, variable range hopping regime to an adiabatic, diffusive, band-like
transport regime at three-dimensional hole densities of up to 10
21 cm
À3 ,
corresponding to 0.2 holes per thiophene ring. However, probably due to dopinginduced disorder at these high hole densities, carrier wavefunction delocalization
and Coulomb gap collapse and thus insulator-to-metal transition did not occur.
Nevertheless, this experiment shows the potential of studying charge transport in
P3HT at the limits of carrier density and thus examining the fundamental properties
of conjugated polymers.
4 New Polythiophene Derivatives for FETs
Although interest in P3HT as a model system is still strong, many new versions of
thiophene-based semiconducting polymers have been developed over the past
decade. The main design criteria were stability against oxygen doping, large area
ordering, solution-processability, and high carrier mobility. To achieve these properties, the number, length, and position of alkyl side chains was varied and fused
thiophene moieties and electron-deficient comonomer units were introduced. The
resulting new materials can reach hole mobilities of up to 10 cm
2 V
À1 s
À1 in some
cases [107, 108] and can be used to study structure–property relationships. Detailed
synthetic approaches and molecular designs have been discussed in a number of
reviews [23, 109–112]; here we will only present a few examples of polythiophene
derivatives that are particularly promising for FET applications.
Improving the air stability of polythiophenes is crucial for their applicability in
commercial devices. To achieve this, the position of the highest occupied molecular orbital (HOMO) energy level has to be shifted further away from the vacuum
level, i.e., the ionization potential has to be increased to values larger than
À4.9 eV. One possible method is to reduce the coplanarity of the system and
thus the conjugation length, although this will also reduce π–π stacking and carrier
mobility. The ionization potential also depends on the electron density of the
conjugated system. Introducing electron-withdrawing groups to the conjugated
system or reducing the density of electron-donating groups will therefore shift the
HOMO level further away from the vacuum level. The first advance in this
direction was reported by Ong et al., who developed dialkylated quaterthiophene
polymers (usually named PQT; P1 in Fig. 11) [104]. Two thiophene units separate
the alkyl-bearing units and prevent them from interfering with each other. The
favorable orientation of the alkyl chains and their low density lead to close π–π
stacking and mobilities as high a 0.14 cm
2 V
À1 s
À1 . The central thiophene units
128
J. Zaumseil