Optoelectronic synaptic devices utilizing ionic covalent organic frameworks exhibiting electrical and optical responses

Synaptic devices that mimic the behavior of biological synapses in electronic devices typically rely on electrical responses. However, synaptic devices that exhibit not only electrical responses but also visual responses are attracting increasing attention, as they enable the observation of memory processes without the need for external devices such as displays. In this study, an HPHI:PVA composite material is proposed for optoelectronic synaptic devices that mimic synaptic behavior in response to light signals. The composite consists of protonated poly(heptazine imide) (HPHI), a two-dimensional carbon nitride compound, and polyvinyl alcohol (PVA), a water-soluble polymer. The composite material exhibits both electrical responses and long-lasting color changes as visual responses, thereby mimicking synaptic behavior under pulsed light irradiation. These results suggest that HPHI:PVA, an active-layer metal-free material, can be applied to synaptic devices capable of performing multiple functions independently without external devices and without relying on inorganic materials in active-layer traditionally used in synaptic devices. Synaptic devices with electrical and visual responses enable direct memory observation without displays. Here, the authors developed a metal-free optoelectronic synaptic device with electrical and visual memory responses.