Light emitting field-effect transistor: simple model and underlying functional mechanisms

We report on light emission on organic thin film transistors of tetracene and polyfluorene (Poly(9,9-di(ethylhexyl)fluorene) (PF2/6)). The utilized transistor structure is a bottom gate configuration with interdigitated source and drain electrodes on a Si/SiO2 substrate with a channel length of 5 mum. Light emission occurs above a source drain voltage of 30V even if the gate voltage is higher than the drain voltage. The light output can be controlled by the gate voltage. The light emission occurs close to the drain electrode as observed by light microscope images of operating transistors. In order to understand the functional principle of alight emitting transistor a resistor capacitor equivalent circuit model has been utilized to describe charge carrier transport, carrier distribution and the electrical potential distribution in such a device. The model extends the common thin film transistor theory for unipolar charge transport to ambipolar charge transport. Analytical expressions for output and transfer characteristics as well as for the potential and charge carrier distributions are obtained. Further, the effect of contact resistors on the output and transfer characteristics are simulated. The model is used to explain the underlying mechanisms of the present devices. Imperfections on the contact electrodes, most probably due to under-etching of the electrodes are seen as the main reason for the electron injection.