Hybrids of semiconducting polymers and single-walled carbon nanotubes (SWNT) are interesting for organic electronic devices such as solar cells, light-emitting diodes and field-effect transistors (FETs). They are easily produced by selective dispersion of SWNTs in polymer solutions by ultrasonication followed by centrifugation. We demonstrate that nanotubes at concentration levels well below the percolation limit significantly improve charge injection of both holes and electrons into semiconducting polymers in top-gate FETs. This leads to lower contact resistances and reduced threshold voltages, thus the maximum ambipolar currents and visible light emission due to electron-hole recombination are considerably enhanced. The improved injection of holes and electrons allows for a much wider range of accessible polymers for ambipolar and light-emitting transistors. The same conjugated polymers can also be used to enrich specific semiconducting SWNT and to produce high-performance ambipolar nanotube network FETs. These show efficient nearinfrared electroluminescence. Mapping the emission from these networks during a gate voltage sweep allows us to visualize preferential current paths and investigate percolation models for purely semiconducting nanotube networks.
Ambipolar organic field-effect transistors (FET) are interesting as building blocks for low power complementary circuits in organic electronics. Another intriguing feature of ambipolar FETs is the recombination of holes and electrons within the channel, which leads to the formation of excitons that can relax radiatively and thus emit light. We have recently demonstrated that ambipolar charge transport is a generic feature in a wide range of polymer semiconductors when appropriate injection electrodes and trapfree dielectrics are used. Among these materials are those that are generally used in light-emitting diodes and thus show high photoluminescence efficiencies.
Here we demonstrate ambipolar light-emitting field-effect transistors based on the conjugated polymer OC<sub>1</sub>C<sub>10-</sub>PPV (poly(2-methoxy-5-(3,7-dimethyloctoxy)-p-phenylenevinylene)) as the semiconducting and emissive layer. OC<sub>1</sub>C<sub>10-</sub> PPV shows efficient electron and hole transport with field-effect mobilities of 3⋅10<sup>-3</sup> cm<sup>2</sup>/Vs and 6⋅10<sup>-4</sup> cm<sup>2</sup>/Vs, respectively. Electrons and holes are injected from calcium and gold source and drain electrodes, respectively, and recombine radiatively within the transistor channel leading to visible light emission. We can actively control the position of the recombination zone through the applied gate and source-drain bias in both constant and variable current mode and thus move the emission zone from the source through the channel to the drain electrode and vice versa. The intensity of light emitted from the channel is proportional to the drain current with efficiencies comparable to those of LEDs based on OC<sub>1</sub>C<sub>10-</sub>PPV.