Neuromorphic Organic Electrochemical Transistors: High-Endurance Long-Term Potentiation induced by PEDOT:PSS Electrochemical Polymerization on the Gate Electrode

Organic electrochemical transistors (OECTs) are emerging devices in the field of artificial intelligence, as they are able to emulate several synapses functionalities. While short-term processes (short-term plasticity, spike-dependent plasticity, etc..) can be successfully simulated by exploiting peculiar features of OECTs ionic circuit, long-term potentiation (LTP) must be further investigated to increase the retention of the induced neuromorphic states [1]. A recent successful approach is based on electrodeposition occurring in the transistor channel [2]. This contribution describes the use of PEDOT:PSS electrodeposition on the gate electrode to obtain long-term potentiation [3]. The presynaptic signal (Vpre) is the potential applied to the gate electrode, which acts as a controller of the drain current, that represents the postsynaptic signal (Ipost). The neuromorphic behavior does not stem from the channel employed as a memory element but from an enhancement of the gating efficiency and switching properties. The deposition of PEDOT:PSS film raises the gate capacitance and thus the ability of the gate electrode in modulating the current flowing in the channel. LTP depends on both the number of pulses used and the Vpre, which generates LTP when a threshold of +0.7 V is overcome. The synapse weight is evaluated by measuring the transconductance, which varies from 0.3 μS for the native device to 30 μS for the neuromorphic OECT with the highest LTP. In-operando atomic force microscopy shed light on operating principle by showing the modifications of the gate electrode induced by Vpre. The structural strengthening of the artificial synapse is stable for at least two months, and the behavior can be reset by inducing long term depression by applying Vpre pulses that leads to a PEDOT:PSS overoxidation and to the formation of a nonconductive layer on the gate electrode. The artificial synapse also mimics short-term plasticity (STP), and in particular paired pulse depression, with two distinguishable exponential decay phases. The time constants associated with STP in our device, i.e., 0.4 and 2.0 ms, are considerably smaller than those characterizing the decay phases reported in some biological synapses. It is worth noting that PPD and LTP were induced using different shapes of Vpre waves with the same experimental setup. These results suggest that the proposed device could combine short-term plasticity and long-term plasticity in a hybrid process. The integration of these devices in neuromorphic circuits could open fascinating perspectives in the realization of advanced neuromorphic circuits based on OECTs.