Polymeric gate insulators to induce synaptic photoresponse of organic transistors

Photonic synapses have attracted increasing interest owing to their ultrafast signal transmission, high bandwidth, and low energy consumption. Dielectrics in organic photonic synaptic transistors (OPSTs) affect the photoinduced charge accumulated at the interface between the dielectrics and organic semiconductor (OSC) layer, modulating a synapse-like behavior. Herein, to investigate the effect of the interfacial properties of polymeric gate insulators on photosensitive synaptic characteristics, two types of polymers, i.e., poly(4-vinylphenol) and poly(styrene), were used as gate dielectrics of OPSTs. We discovered that the functional groups of the polymeric gate dielectrics that induce charge trapping primarily contribute to the synaptic properties of the OPSTs. This result was obtained by analyzing the morphological and physicochemical properties, including surface roughness, surface energy of the insulators, and grain size of the OSC layer on the dielectric layers. Further, the poly(4-vinylphenol)-based OPST with strong interfacial charge-trapping effect showed various synaptic characteristics, such as excitatory postsynaptic currents, paired-pulse facilitation, spike duration-dependent plasticity, spike number-dependent plasticity, and spike rate-dependent plasticity, according to the adjustment of various ultraviolet light information (i.e., exposure duration, number, and rate). In contrast, the poly(styrene)-based OPST did not show synaptic photoresponse. Furthermore, based on the potentiation/depression characteristics of the device, a recognition accuracy of 88% was achieved using handwritten digit recognition designed using datasets from the Modified National Institute of Standards and Technology. Therefore, this study reveals the understanding of the relation between the dielectric/OSC layer and photosensitive synaptic characteristics from the charge-trapping effect. It also provides a strategy for optimizing the photoresponsive synaptic characteristics of OPSTs.