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.
We report on the photovoltaic properties of solar cells containing a new discotic liquid crystalline material (DL-CuPc) based on copper phthalocyanine. In addition to being soluble, these materials can self-organize into highly ordered structures which can lead to good transport properties that can potentially be superior to those of amorphous materials. Increase in short-circuit current density and fill factor was obtained by thermal annealing of spin-coated DL-CuPc layer in bi-layer solar cells based on junction between DL-CuPc and C<sub>60</sub>. These improvements are explained by change in structure and morphology upon thermal annealing.