## 3 Organic Light Emitting Field Effect Transistors (OLEFETs)
OLEFETs have demonstrated superior external quantum efficiency (EQE) compared to OLEDs, with performance levels that exceed those of similar OLED devices. They also show higher current densities (1-10 A/cm\({}^{2}\) for a 1 nm-thick layer) compared to OLEDs (10\({}^{-3}\)-10\({}^{-2}\) A/cm\({}^{2}\)). The planar device structure, combined with the dual functionality of light emission and switching, positions OLEFETs as ideal candidates for next-generation flexible displays. This technology supports the development of transparent displays, which are crucial for applications in augmented reality, automotive displays, and wearable biomedical devices. Additionally, OLEFETs offer high degrees of integration with various optically active devices, allowing for more complex light manipulation and reducing the need for high-performance driving transistors. The inherent architecture of OLEFETs, along with their ability to reduce pinholes and shorts, potentially enhances production yields and lowers manufacturing costs, making them a promising alternative and complementary technology in the field of organic light-emitting devices [46]. In an OLEFET, optimal selection of S and D electrodes, with respective work functions, is essential to ensure efficient electron injection into the lowest unoccupied molecular orbital (LUMO) and hole injection into the highest occupied molecular orbital (HOMO) of the organic semiconductor. The magnitude of the current flowing between the source (S) and drain (D) electrodes (I\({}_{\text{SD}}\)) is controlled by the gate bias, which also switches the device between its off and on states. If the organic semiconductor exhibits electroluminescence, holes and electrons combine to form excitons within the transistor channel, resulting in the emission of light. In the context of OLEFETs, it is more accurate to refer to the electrodes as hole-injecting and electron-injecting electrodes rather than source and drain. This distinction arises because one electrode serves as the source of holes (and the drain for electrons), while the other electrode serves as the source of electrons (and the drain for holes). By adjusting the gate bias between the hole-injecting and electron-injecting electrodes, it becomes feasible to alter the location of the light emission region within the channel, where layers of accumulated electrons and holes converge. This capability is advantageous for enhancing  
Figure 4: Device architectures of (**a**) OLEFET (left) vs. (**b**) OLED (right). Simplified schematic representations of (left) an organic light-emitting transistor and (right) an organic light-emitting diode. The OLEFET is a three-terminal device featuring a dielectric layer and an active organic material, with charge transport primarily governed by lateral field-effect mechanisms. In contrast, the OLED consists of a vertical stack of various organic layers positioned between two electrodes, the cathode and the anode, with charge transport predominantly occurring through tunneling between adjacent layers. Each layer in the stack has a distinct function (e.g., HIL: hole injection layer, HTL: hole transport layer, EML: emissive layer where light emission happens, ETL: electron transport layer, EIL: electron injection layer). Figure taken from reference [46]. electroluminescence efficiency, as excitons can be generated farther away from the metallic electrodes, thereby minimizing the potential for light emission quenching. This underscores the significant advantage of the three-electrode configuration of OLEFETs compared to the two-electrode setup of OLEDs. Ideally, OLEFETs operate as ambipolar devices, meaning that both holes and electrons are injected and transported. However, gate-modulated electroluminescence is also observed in unipolar devices, where only one type of charge carrier (either electrons or holes) is transported. In unipolar OLEFETs, light emission occurs in close proximity to the electrode responsible for injecting the charge carriers that are not transported (Figure 5) [46].