doping by oxygen or other p-dopants like F4-TCNQ is necessary to obtain high
mobilities and good on/off ratios in such devices [7]. Blends of 5 wt% P3HT and
polystyrene that were processed under strict exclusion of oxygen and annealed
under vacuum initially showed almost no charge transport. The devices improved
after moderate doping by storing the devices in a nitrogen glovebox with an oxygen
concentration at the parts per million level. Mobilities reached 0.1–0.2 cm
2 V
À1 s
À1
and on/off ratios of 10
6 were accomplished. Similar behavior was found for other
thiophene-based polymers and for doping with F4-TCNQ. Lu et al. argued that the
specific morphology of the P3HT network was crucial for this performance
improvement. Most of the semiconductor formed a poorly interconnected network
of nanometer-sized crystallites at the top surface, while parts of this network
penetrated the insulating matrix closer to the gate dielectric in a bottom gate/top
contact configuration. In the doped state, the network paths penetrating into the
matrix were depleted when a positive voltage was applied and thus interrupted
charge transport. This led to low off-currents. When a negative voltage was applied,
holes accumulated in those parts and the entire network became conducting. The
FET was in its on-state. In the undoped state, charge accumulation occurred first in
the network parts close to the gate, leaving the larger parts on the top surface
screened. They were thus dominated by trap states and were essentially
non-conducting. The high mobilities could be explained by good interchain order
of the P3HT, as indicated by a strong 0–0 vibronic transition in the absorption
spectrum and narrow (020) X-Ray diffraction peaks. Given the excellent performance and stability of these devices containing only small amounts of semiconducting polymer, they are very interesting for various applications. A simple circuit
of a unipolar inverter based on two blend FETs was fabricated and showed good
switching behavior, with gains as high as 60. Thus, simple blends instead of
complicated and expensive copolymers might ultimately push the performance of
P3HT-FETs beyond the current limits.
3.7 Doping and On/Off Ratio
As already mentioned above, one of the major problems of P3HT-based FETs is the
low on/off ratio due to unintentional doping. A low off-current is important for
almost all electronic applications. For example, transistors that drive pixels should
have an on/off ratio of 10
9 to achieve maximum contrast. For digital logic, a low
off-current reduces unwanted power dissipation. The high off-currents in P3HT
transistors are due to p-doping of the films by oxygen. The additional positive
charge carriers have to be compensated by the gate voltage, which leads to positive
turn-on voltages in many P3HT-FETs. Also, the influence of the gate field extends
only few nanometers into the channel, whereas the whole P3HT film is doped. This
results in high bulk conductance and thus high off-currents (see Fig. 9).
Early on, the p-doping of P3HT by oxygen was attributed to the formation of a
reversible P3HT:O 2 charge transfer complex [100]. Interestingly, doping of P3HT
P3HT and Other Polythiophene Field-Effect Transistors
125
mobilities and good on/off ratios in such devices [7]. Blends of 5 wt% P3HT and
polystyrene that were processed under strict exclusion of oxygen and annealed
under vacuum initially showed almost no charge transport. The devices improved
after moderate doping by storing the devices in a nitrogen glovebox with an oxygen
concentration at the parts per million level. Mobilities reached 0.1–0.2 cm
2 V
À1 s
À1
and on/off ratios of 10
6 were accomplished. Similar behavior was found for other
thiophene-based polymers and for doping with F4-TCNQ. Lu et al. argued that the
specific morphology of the P3HT network was crucial for this performance
improvement. Most of the semiconductor formed a poorly interconnected network
of nanometer-sized crystallites at the top surface, while parts of this network
penetrated the insulating matrix closer to the gate dielectric in a bottom gate/top
contact configuration. In the doped state, the network paths penetrating into the
matrix were depleted when a positive voltage was applied and thus interrupted
charge transport. This led to low off-currents. When a negative voltage was applied,
holes accumulated in those parts and the entire network became conducting. The
FET was in its on-state. In the undoped state, charge accumulation occurred first in
the network parts close to the gate, leaving the larger parts on the top surface
screened. They were thus dominated by trap states and were essentially
non-conducting. The high mobilities could be explained by good interchain order
of the P3HT, as indicated by a strong 0–0 vibronic transition in the absorption
spectrum and narrow (020) X-Ray diffraction peaks. Given the excellent performance and stability of these devices containing only small amounts of semiconducting polymer, they are very interesting for various applications. A simple circuit
of a unipolar inverter based on two blend FETs was fabricated and showed good
switching behavior, with gains as high as 60. Thus, simple blends instead of
complicated and expensive copolymers might ultimately push the performance of
P3HT-FETs beyond the current limits.
3.7 Doping and On/Off Ratio
As already mentioned above, one of the major problems of P3HT-based FETs is the
low on/off ratio due to unintentional doping. A low off-current is important for
almost all electronic applications. For example, transistors that drive pixels should
have an on/off ratio of 10
9 to achieve maximum contrast. For digital logic, a low
off-current reduces unwanted power dissipation. The high off-currents in P3HT
transistors are due to p-doping of the films by oxygen. The additional positive
charge carriers have to be compensated by the gate voltage, which leads to positive
turn-on voltages in many P3HT-FETs. Also, the influence of the gate field extends
only few nanometers into the channel, whereas the whole P3HT film is doped. This
results in high bulk conductance and thus high off-currents (see Fig. 9).
Early on, the p-doping of P3HT by oxygen was attributed to the formation of a
reversible P3HT:O 2 charge transfer complex [100]. Interestingly, doping of P3HT
P3HT and Other Polythiophene Field-Effect Transistors
125