Reversibly tunable short-term plasticity (STP) of the channel current in organic neuromorphic devices is demonstrated with a three-terminal architecture. Electrolyte-gated organic transistorsEGOTsare driven with square voltage pulses at the drain electrodes, while the gate bias enables the modulation of the amplitude and characteristic time scale of the depressive STP spiking response up to 1 order of magnitude. The gate potential sets the baseline and the steady-state current, preluding multilevel memory writing. The fine-tuning of the STP response, which is not possible with two-electrode organic neuromorphic devices, is reversible and does not imply chemical modifications of the active layer.

Since the first reports of neuromorphic behavior in electronics, [2,3] a significant effort has been devoted to develop computing architectures based on such devices, with the aim to approach the computational and energetic efficiency of the human brain. [4] In neural (and neuromorphic) architectures, computation and data storage occur in the same physical space, overcoming the so-called "Von Neumann bottleneck." [5] Within this context, organic conductors and semi-conductors have been proposed as active materials for neuromorphic applications, giving rise to the field of organic neuromorphic electronics. [6,7] Such materials behave as organic mixed ionicelectronic conductors-OMIECs, [8] whose time response is dictated by the dynamic interplay between ions (slow carriers) and electrons (fast carriers) as well as the features of input signals. OMIECs are operated in aqueous environment and under driving voltages that are within the electrochemical stability window of water, which make them candidate for interfacing the living matter. [9][10][11] These features make OMIECs attractive for neuromorphic devices especially in comparison to siliconbased devices. [12] The first report of neuromorphic behavior in organic electronic devices was achieved by the NOMFET (nanoparticle organic memory field effect transistor) architecture. [13] In NOMFET, the slow kinetic phenomenon necessary to elicit a neuromorphic response was obtained by embedding gold nanoparticles that act as shallow traps in an organic semiconductor thin film. In an aqueous environment, the slow kinetics is inherent to the ion displacement in the electrolyte at the interface with the active OMIEC. A number of neuromorphic devices were demonstrated in aqueous electrolytes, from switchable nonvolatile memory elements [14] to devices emulating the main synaptic signal processing features, like spike-timing-dependent-plasticity, [15,16] shortterm-plasticity (STP), [17,18] and long-term-potentiation. [7,[19][20][21] These functions are the basis of processing, memorization, and learning mechanisms in the human brain. [22] Recently, such architectures were integrated with cultured neural cells with neither loss of functionality nor impairment of cell viability, [23] leading to the demonstration of the first bio-hybrid synapses. [24] The sensitivity of neuromorphic synapses to the composition of the ionic environment arises from the interplay of dynamic noncovalent interactions between molecular solutes and OMIEC, which establishes the timescale of the neuromorphic Organic neuromorphic devices mimic signal processing features of biological synapses, with short-term plasticity, STP, modulated by the frequency of the input voltage pulses. Here, an artificial synapse, made of intracortical microelectrodes, is demonstrated that exhibits either depressive or facilitative STP. The crossover between the two STP regimes is controlled by the frequency of the input voltage. STP features are described with an equivalent circuit where an inductance component is introduced i...

The role of pre-synaptic DC bias is investigated in three-terminal organic neuromorphic architectures based on Electrolyte gated Organic Transistors - EGOTs. By means of presynaptic offset it is possible to finely control the number of discrete conductance states in short-term plasticity experiments, to obtain, at will, both depressive and facilitating response in the same neuromorphic device and to set the ratio between two subsequent pulses in paired-pulse experiments. The charge dynamics leading to these important features are discussed in relationship with macroscopic device figures of merit such as conductivity and transconductance, establishing a novel key enabling parameter in devising the operation of neuromorphic organic electronics.

In the present work, five 2-styryl-1-benzopyrylium salts and their relative self-assembly processes towards TiO2 nanocrystalline layers were evaluated as photosensitizers in dye-sensitized solar cells (DSSCs). Integration of these 2-styryl-1-benzopyrylium salts with the semiconductor allow for the performance of highly specific functions suitable for smart applications in material science. Spectroscopic and photoelectrochemical measurements conducted on these five bio-inspired dyes, in solution and upon adsorption onto titanium dioxide films, allowed detailed discussion of the anchoring ability of the different donor groups decorating the 2-styryl-1-benzopyrylium core and have demonstrated their ability as photosensitizers. Our results suggest that the introduction of a dimethylamino group in position 4′ of the 2-styryl-1-benzopyrylium skeleton can alter the conjugation of the molecule leading to larger absorption in the visible region and a stronger electron injection of the dye into the conduction band of TiO2. Moreover, our experimental data have been supported by theoretical calculations with the aim to study the energy of the excited states of the five compounds. In this specific case, the simulations reported contributed to better describe the properties of the compounds used and to help create the necessary basis for the design of new and targeted bio-inspired molecules.