Proceedings Article | 23 May 2018
KEYWORDS: Organic light emitting transistors, Electrodes, Quenching (fluorescence), Switching, Metals, Excitons, Switches, Electrical efficiency, Organic semiconductors, External quantum efficiency
Organic light-emitting transistors (OLETs) combining the dual functions of electrical switching and light emission are promising devices to push large amounts of charge carriers into an isolated recombination area. Most OLETs proposed so far, however, suffer from hole-electron current imbalance when driven at high currents. This results in a light emission zone very close to one of electrodes. The proximity of the electrode is detrimental to efficient light emission as the metal significantly quenches excitons. In addition, in single gate OLETs, the emission zone can unpredictably switch from one contact to the other upon small bias changes since electron and hole currents are not controlled individually. Dual-gate architectures have been proposed to independently control the transport of both types of charge carriers towards the recombination zone. In the split-gate OLET, where both gates lie side by side in the same plane, light is exclusively emitted from the center of the channel, far from the electrodes. But the unavoidable horizonal gap between the gates creates a highly resistive region in the vicinity of the recombination zone that lowers the electrical efficiency of the device.
Here, we introduce the overlapping-gate OLET, a novel dual-gate architecture in which one gate partially covers the other, leaving no horizontal gap between the gates. By accumulating charge carriers in the transport layer, each gate independently opens a unipolar gapless channel to transport and inject charge directly into the recombination zone, thereby avoiding transport through highly resistive ungated regions. For the active layer, we propose a vacuum-evaporated multi-layered structure of organic semiconductors: The transport layers are based on high mobility p- and n-type materials for efficient lateral transport. The recombination zone is made of a fluorescent host-guest emissive layer.
Thanks to this architecture, the red light emitted by the overlapping-gate OLET is precisely located along the edge of the top gate that overlaps with the bottom gate. Therefore, light emission stays localized in the center of the channel, isolated from the quenching electrodes. Besides, the independent control over the supply of both types of charge carriers enables balanced transport up to high current densities. As a result, high performance red-emitting OLETs are demonstrated with an external quantum efficiency of 5.6% at a high luminance over 2000 cd m−2. Furthermore, the device shows no efficiency roll-off up to a current density of 30 mA/cm2. The conditions for balanced transport are rationalized through the development of an equivalent-circuit model. This shows that balanced transport is achieved when both transport channels are biased in the linear regime and that charge densities at the boundaries of the emissive layer are equal.
The overlapping gate light emitting transistor is a promising step towards the development of bright organic light emitting devices that combine high efficiency at high current densities.
