## 3 Organic Light Emitting Field Effect Transistors (OLEFETs)
The optoelectronic characterization of OLEFETs necessitates the measurement of both optical and electrical gate-modulated characteristics, from which the EQE can be determined. Electrical characteristics enable the calculation of the mobility and the \(\mathrm{I_{ON}/OFF}\) ratio. Additional valuable insights are provided by the EL spectrum, which indicates the color of the emitted light, and by imaging the light emission region within the operational device, revealing the position of the recombination region as a function of the biasing conditions. Since 2003, when it was the first time they were described, a lot of advances have been developed in recent decades. For example, in 2021, the research conducted by [PERSON] and col. demonstrated a synergistic relationship between photoluminescence and charge transport through meticulous adjustments to the polymeric backbones in OLEFETs. The authors synthesized semi-ladder polymers possessing both luminescent and charge transport capabilities through the copolymerization of weak acceptors (such as 1 or 2) and weak donors (like fluorene (F) or carbazole (C)) (Figure 6). The study revealed that achieving a delicate  
Figure 5: OLEFET operation: (**a**) unipolar OLEFET, (**b**) ambipolar OLEFET. Figure taken from reference [46]. balance of high crystallinity, increased planarity, and interchain aggregation in the copolymer 2-F significantly enhances ambipolar charge mobility and photoluminescence quantum yield. 2-F demonstrated an external quantum efficiency (EQE) of 5.3% in multilayered OLEFETs. The primary application domain for area emission OLEFETs lies in panel display devices. Unlike active-matrix OLED display devices, this highly integrated structure not only enables higher resolution but can also be manufactured using cost-effective techniques like roll-to-roll processing. Nonetheless, the substantial challenge of attaining uniform regional emission in OLEFETs with robust stability and adaptable tunability has impeded their advancement in this field [47]. By integrating a charge transport buffer (CTB) layer between the conductive channel and the emitter layer, the redistribution of potential under the drain occurs within the protective influence of the CTB layer, leading to a significantly uniform current density. Ensuring the uniform recombination of balanced holes and electrons is essential for the establishment of area emission in OLEFETs. The OLEFETs demonstrated a remarkable \(\text{I}_{\text{ON}}/\text{OFF}\) ratio of up to 106, excellent stability over 200 cycles, and a high aperture ratio exceeding 80%. Thanks to the device's geometric versatility and its compatibility with traditional fabrication methods, it holds immense promise in display technology. In the same year, in 2021, [PERSON] and col. published a groundbreaking study on a high-mobility organic laser semiconductor, 2,7-diphenyl-9H-fluorene (**3**) (Figure 7), show-casing exceptional crystalline-enhanced emission achieved through meticulous control of its crystal growth process [48]. The one-dimensional nanowires of 3 exhibited an impressive absolute photoluminescence quantum yield of up to 80%, a high charge carrier mobility of approximately 0.08 cm\({}^{2}\)V\({}^{-1}\)s\({}^{-1}\), and remarkable lasing characteristics. Exploring organic electrically pumped lasers and other integrated electrically driven photonic devices holds significant importance in contemporary research endeavors, achieving a high-performance OLEFET. Figure 6: Chemical structures of derivatives 1 and 2 with fluorene (F) and carbazole (C) as donors described in this work [47]. In 2022, [PERSON] and col. developed emissive materials capable of emitting polarized light that are highly sought after for a diverse range of light-emitting electronic devices. In the realm of OLEFETs, attaining highly polarized emission directly from the device surface, known as surface emission, can be achieved by ensuring the in-plane co-linear alignment of molecular Transition Dipole Moments (TDMs) within the active layer of the device.