## 2 Organic Electrochemical Transistors (OECTs)
It describes the rate at which ions can move through the channel when influenced by an electric field. High ion mobility ensures that ions from the electrolyte can quickly and efficiently penetrate the channel material, altering its oxidation state and thus modulating the conductivity of the OECT. This ion transport mechanism is fundamental to the operation  
Figure 1: Representative scheme of a classical OECT. Figure taken from reference [15]. of OECTs, as it directly impacts the speed at which the device can respond to changes in gate voltage. It is given by:  
\[\mu_{ion}=v_{ion}/E\]  
where \(v_{ion}\) is the ion velocity and \(E\) is the electric field. The infiltration of ions into the channel and, consequently, the material's doping degree (redox state) are controlled by the applied gate voltage (V\({}_{\text{G}}\)). The drain-source voltage (V\({}_{\text{DS}}\)), or the potential difference between the source and drain, determines the intensity or magnitude of the channel current (I\({}_{\text{D}}\)) observed in the drain (Figure 2). Thus, I\({}_{\text{D}}\) is proportional to V\({}_{\text{DS}}\) until saturation is reached. Consequently, the output characteristics will show a decrease in the drain current when V\({}_{\text{G}}\) is increased in the case of \"depletion mode\" operation (Figure 2a). Conversely, an increase in drain current would be observed in \"accumulation mode\" when anions are injected into the channel (Figure 2b). Applying a negative gate voltage will induce higher hole mobility in the semiconductor channel [15]. OECTs can also function as amplifiers by converting low gate voltage signals into more significant changes in drain current. The transfer curve (I\({}_{\text{D}}\) vs. V\({}_{\text{G}}\), with a constant V\({}_{\text{DS}}\)) illustrates the relationship between the drain current and the applied gate voltage. This curve provides a straightforward way to visualize how a transistor switches from its \"on state\" (characterized by high drain current) to its \"off state\" (very low current) in depletion mode (black curve, Figure 3). The effectiveness of this amplification, or ion-to-electron transduction, is represented by a key performance metric for OECTs known as transconductance (gm, blue curve, Figure 3) [15]. Figure 2: Representative output characteristics of OECT devices. All the represented lines reveal the different intensity curves applying different gate voltages (V\({}_{\text{G}}\)). For p-type materials (Blue, Top): (**a**) In depletion mode, applying a positive gate voltage (V\({}_{\text{G}}\)) allows cations to diffuse into the polymer, causing dedoping of the channel and switching the transistor to its “off” state; (**b**) in accumulation mode, applying a negative gate voltage (V\({}_{\text{G}}\)) results in anions entering the channel material, thereby switching the device to its “on” state. For n-type materials (Red, Bottom), both modes of operation are achieved by applying opposite gate voltages (V\({}_{\text{G}}\)): (**c**) negative for depletion mode and (**d**) positive for accumulation mode. Figure taken from reference [15]. The main advantage of OECTs is their ability to amplify the electrical signal through electrochemical modulation of the active channel. Small variations in the voltage applied between the gate electrode and the reference electrode can cause significant changes in the conductivity of the active channel, allowing for intrinsic amplification of the electrical signal. Among all the amount of different organic derivatives that can be used in OECTs, the most important are: p-type polymers, like PEDOT derivatives [16; 17], other thiophene derivatives [18; 19], poly(3-hexylthiophene-2,5-diyl) (P3 HT) derivatives [20], donor-acceptor (D-A) polymers [21; 22], n-type polymers [23; 24; 25] or small molecules [26]. In OECTs, electrolytes are essential for modulating the conduction channel, as they facilitate the injection or extraction of ions between the electrolyte and the transistor's active material, thereby altering its conductivity.