Artificial Neurons And Synaptic Devices Based On Organic Electronic Materials

In recent years,using the electronic device to simulate the characteristics of information storage and processing in the nervous system of biology,and the realization of neuromorphic computing(also known as brain-like/inspired computing)have received extensive attention from researchers.A series of artificial synapses and neuron devices have been developed and reported.With the development of neuromorphic electronics and the further expansion of artificial intelligence application scenarios,neuromorphic devices will be used in many emerging industries,e.g.,wearable health detection,medical care,virtual reality,and augmented reality,which puts forward higher requirements for neuromorphic devices.For example,wearable neuromorphic devices,in addition to having good neuron-like and synaptic properties,also needs to have mechanical properties,such as skin-like flexibility and stretchability,to meet the requirements for wearing comfort.Although artificial neurons and synaptic devices based on inorganic materials have achieved considerable research progress,the flexibility,biosafety,stretchability,degradability,and wearability of such devices are often limited by the inherent properties of inorganic materials to a certain extent,which further limits the application scope of inorganic neuromorphic devices.In addition,two-terminal inorganic neuromorphic devices often exhibit a highly nonlinear evolution of conductance,which will lead to errors in weight update during neural network training.Compared with inorganic electronic materials,organic electronic materials have many advantages,such as good flexibility,light weight,good biocompatibility,and electrical/chemical/mechanical properties that can be easily adjusted by molecular structure and morphology.Therefore,the use of organic materials to fabricate neuromorphic devices can achieve ideal device performance by tuning the properties of organic materials.Also,organic thin films can be prepared by rapid film-forming processes,such as spin coating,blade coating,dip-coating,printing,etc.,which can effectively reduce the fabrication cost of organic electronic devices.Compared with inorganic neuromorphic devices,organic neuromorphic devices are still in their infancy.Nevertheless,it is still very attractive to combine organic neuromorphic devices with wearable electronic devices,bioelectronic devices,consumer electronic devices,etc.to build new application scenarios.At present,the research on using the characteristics of organic materials to achieve the excellent performance of neuromorphic devices while taking into account the characteristics of environmental friendliness,biosafety,and extensibility of the devices is relatively scarce,which limits the practical process of organic neuromorphic devices.On the other hand,most of the organic sensory-neuromorphic devices are realized by using the special stimulus-response characteristics of semiconductors or the special design of materials/device structures,and there is a lack of simple implementation strategies for sensory neuromorphic devices;Moreover,the existing neuromorphic devices based on organic materials often show nonlinear changes in resistance,which is not conducive to the construction of high-performance neural networks.To solve the above problems,this thesis first prepares a green electronic insulating material with good ionic conductivity and uses its ion migration effect to realize environmentally-friendly green organic neuromorphic devices,which can effectively reduce the e-waste pollution caused by the renewal of consumer electronics;secondly,we use the organic semiconductor/insulating layer interfacial effect to realize light-controlled sensory-neuromorphic devices and study the influence of the interfacial effect on the plasticity of synaptic devices.It proves the feasibility of using the organic photosensitive synaptic array based on the interface effect to realize image sensing and pattern learning,which provides a simpler method for the preparation of intelligent perceptual neuromorphic devices.Moreover,in future research,we can combine the interfacial effect with the ion migration effect to prepare green neuromorphic devices with sensing ability,to expand the application prospect of the devices in the field of consumer electronics and intelligent sensing;Through the selection and preparation of intrinsically stretchable organic materials and using the structure of the electrochemical transistors,we have realized high performance stretchable neuromorphic devices with the characteristics of linear weight updating ability,good state retention,and excellent cycle stability.This part of the research provides a good material and device design basis for stretchable organic neuromorphic devices and breaks the ground for bringing neuromorphic devices into skin-like wearable electronics for achieving human-integrated/mimetic intelligent systems.The core contents of this thesis are as follows:(1)We first developed an insulating material with ionic conductivity,biological safety,and environmental friendliness.In this part of the study,we used cellulose as a raw material to prepare an ion-conducting and electron insulating layer.Cellulose is a typical biomass material with a wide range of sources.At the same time,cellulose has better biological safety and is environmentally friendly.The cellulose that exists in nature has a multi-level structure.We use the method of chemical oxidation to treat cellulose.On the one hand,it can effectively disperse cellulose to a nanometer level.On the other hand,it can introduce movable ions into the molecular structure of cellulose.By air-drying the nano-cellulose pulp naturally,we obtained cellulose-based nanopaper,which has higher transparency,lower surface roughness,better mechanical and temperature stability,excellent ionic conductivity,and easy degradation characteristics.A series of excellent properties of cellulose nanopaper provide a basis for us to further use cellulose nanopaper to prepare electronic devices.With the cellulose nanopaper simultaneously serve as the insulating layer(electronic insulation)and substrate,organic field-effect transistors(OFETs)have been prepared.The research results indicate that the cellulose nanopaper reported by us is suitable for preparing a series of organic semiconductor-based flexible FETs,including p-type polymer semiconductor-based transistors,n-/p-type small molecule.We also proved that cellulose nanopapers can also be used for different organic semiconductor device manufacturing processes including solution spin coating and thermal evaporation.The dissociation of sodium carboxylate on the side chain of cellulose under the action of trace water is further proved to be main the reason for the ionic conductivity of cellulose nanopaper.Through adjusting the washing times,encapsulation,and other means,the ionic conductivity of cellulose nanopaper can be modulated which is beneficial to reduce the operating voltage of FETs.The cellulose nanopaper we developed is expected to be used in biodegradable,flexible,transparent,and low-voltage green electronic devices and products,including but not limited to organic electronic devices and products.(2)Furthermore,we fabricated organic synaptic transistors with lateral structure using cellulose nanopaper with ion conductivity.The lateral structure-based synaptic transistor prepared by using cellulose nanopaper can exhibit outstanding transistor performance under 1.5 V operating voltage.Importantly important synaptic behaviors,e.g.,EPSC,dendritic integration and signal filtering characteristics have been successfully implemented using organic synaptic transistors with lateral structures made from cellulose nanopaper.These neuromorphic characteristics are vital for the implementation of neuromorphic computing functions in the future.Cellulose nanopaper has good environmental safety and biodegradability characteristics.This work may pave a promising path towards the development of"green"synaptic electronics.(3)The interfacial effect has always been regarded as an unfavorable factor in transistor devices.In this part of the research,polyacrylonitrile(PAN)is served as the dielectric material to demonstrate that the interfacial charge trapping effect(ICTE)of organic semiconductor(OSC)and an insulating layer can be used to achieve synaptic transistors with light as stimulation signal.The cyano group in PAN is generally negatively neutral.When the cyano group is in contact with the OSC at the interface,it will adsorb holes due to static electricity.Therefore,when there is no light,part of the holes is adsorbed in the interfacial trap;when light occurs,light can be used as energy to excite holes in the trap and make them able to participate in conduction.When the light is removed,because the trap still exists,the holes in the semiconductor will be gradually trapped until it fills the trap.The photo-generated charge trapping and de-trapping process that occurs at the interface provides the basis for the prepared organic synaptic transistors with light as the stimulation signal.A variety of typical neuromorphic device characteristics,e.g.,EPSC have been successfully simulated using organic synaptic transistors based on interfacial effects.By adjusting the light stimulation parameters,our device also exhibits learning and memory behavior similar to the human brain.All experimental curves obtained from our devices can be expressed well by using mathematical equations,which proved that the ICTE in transistors can work well as a facile means to induce synapse-like behavior.Thus,through experimental demonstration,our work shows the possibility of using the ICTE as a facile and broadly applicable approach for the preparation of light-stimulated synaptic transistors,and this work holds promises for the future design of light-control and in-sensor neuromorphic devices.(4)The artificial intelligence data processing unit used for wearables also needs to have skin-like flexibility and stretchability,to achieve good wearing comfort and avoid unreliable but potentially massive physical interconnections between soft and hard components.Not only that,the research and development of skin-like wearable neuromorphic devices will help to achieve proximity-sensing computing,and will help to further integrate wearable sensors to improve the efficiency of health data acquisition and processing.In this part of the research,the semiconductor layer is served by the first-time report of stretchability on ionic electron mixed semiconducting polymers,which is based on the polythiophene backbone and tri-ethylene glycol(TEG)side chain,namelypoly-[3,3′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-2,2′-bithiophene](p(g T2)).Based on p(g T2),a set of materials and device design strategies are reported to realize intrinsic stretchable neuromorphic devices based on organic electrochemical transistor(OECT)structure.The devices prepared by us show more than 800 levels of different storage states,highly linear/symmetric weight updates and low switching changes,excellent switching durability(>10~8),good state retention characteristics(>10~4 s),together with the high stretchability of 100%strain over 100 repeated cycles.We further integrated the device into the prototype array and successfully implemented vector-matrix multiplication(VMM).Even under 100%strain,the array can perform VMM operations.Finally,based on the parameters extracted from the device,we performed a large-scale neural network simulation to prove the feasibility of realizing artificial intelligence-based health signal classification.This work breaks the ground for combining AI data analysis into skin-like wearable electronics for achieving human-integrated/mimetic intelligent systems.In conclusion,this paper develops a series of neuromorphic devices with excellent performance based on organic materials,realizing environmentally friendly green synaptic devices,light-controlled sensing synaptic devices,wearable,and stretchable high-performance neuromorphic devices.The research content of this paper not only expands the material system of organic neuromorphic devices,enriches their application scenarios,but also provides a simple preparation method of light-controlled intelligent sensing neuromorphic devices.In the future,through the further design of organic electronic materials and device structures,intelligent perceptual neuromorphic device arrays and chips with the characteristics of environmental friendliness,biosafety,flexibility,and extensibility can be realized.The research content of this thesis is expected to be used in many fields such as health detection and visual information processing in the future,which is conducive to promoting the application of artificial intelligence.