Light and its interaction with physical matter plays a crucial role in several processes that are at the base of the life on Earth[1]. Such interaction was deeply studied and exploited in bioelectronics allowing for several application like biosensing[2] and visual-pathway emulation[3]. In humans, light stimulus is transduced in the retina and directly sent to the brain as spiking signals[4].

In this context, light-driven neuromorphic devices were demonstrated, offering advantages of optical systems, such as negligible signal delay, no power loss and wide bandwidth[5]. On the other hand, such devices rely on metal oxides and photovoltaic materials, crucially impeding a their usage in biointerfaces and preventing from their application[6] in case of loss of vision or other sight-related pathologies .

In light of this, we demonstrate a fully organic photo electrochemical transistor (OPECT) able to conjugate light-induced and voltage-controlled neuromorphic features, while also exploiting key features of conducting polymers (CPs), as biocompatibility and stability in aqueous environment[6]. In addition, the presented platform allows for multimodal signal integration, combining different stimuli independently.

A photoresponsive PEDOT:PSS covalently bonded to azobenzenes moieties (azo-tz-PEDOT:PSS) is synthesized though an ad hoc “click” reaction and used as gate electrode in planar-gated depletion-mode OPECT (azo-OECT). Here, by shining UV light on the gate a charge transfer/trapping mechanisms is induced, light-modulating the conductance of the transistor. Firstly, the conductance modulation is exploited to prove the biomimetic potential of the azo-OECT, by implementing a fully organic OFF vertical pathway of the human retina.

Subsequently, by exploiting the multimodal signal processing capability of the azo-OECT, neuromorphic features are investigated. By applying positive voltage pulses at the gate terminal, it is in fact possible to elicit a transient conductance modulation of the azo-OECT, that is reversed after few seconds, mirroring the short-term synaptic plasticity (STP). On the other hand, when the gate electrode is lit by light pulses, a stable conductance modulation is observed, mimicking long-term synaptic plasticity (LTP) and proving learning capabilities. The opposite reaction (forgetting) is then induced by applying negative voltage pulses at the gate terminal.

Ultimately, the combination of all the input signals allows the azo-OECT to learn sensory information following Atkinson-Shiffrin memory model[7].

The neuromorphic features demonstrated in azo-OECTs, along with intrinsic properties of CPs, are what makes the proposed architecture an ideal candidate for a neurohybrid interface, in which the artificial neuromorphic device could cooperate with biological circuitry, independently computing biological, electrical and optical signals.