Light-Modulated Organic Synaptic Device Based On Pentacene

The brain is a complex biological neural network composed of numerous neurons and synapses.As the fundamental units of the brain,synapses play a critical role in information transmission and processing,endowing the brain with the ability to process and store information in parallel.Inspired by the structure of the brain,neuromorphic computing has evolved enormously in recent years,and the construction of synapse-like electronic devices has emerged as an important frontier research field.Approximately 80%of the external information acquired by humans comes from the visual system,making it crucial to develop synaptic devices with perception and processing capabilities of optical signals that can simulate the function of the visual system.In light-modulated synaptic devices,how to address the problem of bidirectional modulation of synaptic weights is one of the most serious challenges.In this work,by designing asymmetric top electrodes and introducing light-assisted charge trapping mechanism,we successfully fabricated an organic synaptic device that enables current modulation through light stimulation.We also explored the application of the device’s positive and negative photoconductivity effects in the emulation of synaptic plasticity.The main research contents are as follows:(1)A two-terminal organic synaptic device with asymmetric top electrode was fabricated using p-type small-molecular pentacene.Charge trapping and detrapping in poly(2-vinylnaphthalene)(PVN)by 254 nm UV illumination was induced to modulate the channel current.The mechanism of the asymmetric top electrode structure is demonstrated by the equivalent circuits analysis.With a simple device structure,the photonic synaptic device exhibited excellent retention characteristics and operational stability.Moreover,the dependance of hysteresis characteristic on the light wavelength and intensity was also explored.(2)Based on the positive photoconductivity effect observed in the device,we successfully simulated some important synaptic functions such as excitatory postsynaptic current(EPSC)and paired-pulse facilitation(PPF).The regulated current states are nonvolatile and exhibit a close dependence on external stimulation conditions(the retention and intensity),which is highly consistent with the biological results.More importantly,the device also successfully emulated Pavlov’s dog experiment,providing a useful experimental information for the research of the associative learning and memory mechanisms in the human brain.Finally,the mechanism of excitatory behavior is explored through energy band diagrams and in-situ monitoring on the floating electrode potential,as well as the COMSOL simulations.(3)The organic photonic synaptic device with simple device structure realizes a wide range of UV-stimulated inhibition over two orders of magnitude in a nonvolatile manner.Based on its excellent negative photoconductivity behavior,the device successfully emulated the inhibitory synaptic behavior and adaptive synaptic plasticity.The potential application of the device in the field of biological photoreceptors,information recognition and memory were further explored,which provides experimental information for the design and construction of high-performance neuromorphic visual system.Finally,the mechanism of inhibitory behavior was also confirmed by in-situ monitoring on floating electrode potential and COMSOL simulations.In summary,we successfully fabricated a high-performance organic synaptic device with asymmetric top electrode structure.The device current can be modulated by optical signals,and the architecture and working principles of the device are general,providing a feasible scheme for the design of light-modulated synapses.