Wavelength-Selective Bidirectional Photoconductance in a Fully Optically Modulated Organic Heterojunction Synapse for Energy-Efficient Neuromorphic Vision

Optoelectronic synaptic devices are promising hardware for biomimetic vision systems. Fully optical modulation with wavelength selectivity enables direct mapping from optical stimuli to synaptic responses for in-sensor neuromorphic processing. However, realizing controllable carrier transport for wavelength-dependent bidirectional photoconductance under low-energy-consumption constraints remains challenging. Here, we report a fully optically modulated artificial synapse based on a C8-BTBT/F 16 CuPc organic heterojunction that exhibits wavelength-selective bidirectional photoconductance. Under 350 nm illumination, the device shows positive photoconductivity (PPC), whereas 578 nm illumination induces negative photoconductivity (NPC), corresponding to synaptic potentiation and depression, respectively. This dual-mode behavior is attributed to spectrally selective excitation and wavelength-dependent interfacial charge redistribution enabled by the type-II heterojunction. The device operates at voltages down to 1 mV and achieves an electrical energy consumption of 220 fJ per synaptic event. By tuning optical pulse parameters, the device successfully emulates representative synaptic functions, including paired-pulse facilitation (PPF), multi-level conductance modulation, and pulse-duration-dependent relaxation dynamics. By incorporating the experimentally derived optical conductance modulation, the device supports both artificial neural network (ANN)- and convolutional neural network (CNN)-based image-recognition tasks, achieving performance close to that of the corresponding ideal synaptic-weight models. This work advances the development of wavelength-encoded, energy-efficient neuromorphic vision systems based on fully optical modulation.