Research On Optoelectronic Neuromorphic Devices Based On Organic Transistors

With the advent of the information society,the data to be processed is growing explosively.The problem of"storage-wall"in the traditional von Neumann architecture is becoming more and more serious,and the demand for new computing architecture and system is also imminent.Benefiting from the characteristics of high bandwidth,no Joule heating,high-speed and low-interference transmission of optical signals,the photoelectric fusion system has the outstanding advantages of multithreading and high efficiency in signal processing and transmission.By simulating the processing and memory behavior of the human brain,the neuromorphic chip can extract and calculate non-structural data,and complete data processing with ultra-low power consumption and high efficiency.Therefore,owing to the advantages of photoelectric fusion system and neuromorphic chip,the neuromorphic system based on photoelectric fusion signal is expected to become the key to overcome the bottleneck of"storage-wall".The application scenario of the neuromorphic system is broadened by endowing its mechanical flexibility,which also gives the system a good application prospect in the era of artificial intelligence and the internet of everything.For the above applications,researchers carried out research on optoelectronic neuromorphic devices based on different device structures,such as memristors and transistors,and various semiconductor materials,such as organic semiconductors,oxide semiconductors and two-dimensional materials.From the perspective of semiconductor materials,with the advantages of rich species,easy modification of chemical structure,large-scale solution preparation and mechanical flexibility,organic semiconductors are suitable materials for flexible photoelectric fusion neuromorphic system.Considering the device structure,both memristors and transistors can realize simulating the signal processing and transmission function of synapse,the basic unit of nervous system.Whereas the memristors at both ends can only transmit signals through one path,the transistors at three ends can simultaneously adjust the synaptic weight and transmit signals,which makes the function of learning come true,and the realization of linear synaptic plasticity more likely.In terms of photoelectric dual modulation function and visual system simulation,the transistor has more advantages than the memristor structure.Therefore,neuromorphic optoelectronic devices based on organic transistors have important research significance。At present,the research of neuromorphic optoelectronic devices based on organic transistors is still in the early stage,and there are still many problems with the preparation process,device performance and function.In respect of the preparation process,the device mostly uses the solution spin coating method to prepare the organic semiconductor active layer.Although the solution method has the advantages of low cost and large-scale preparation,it is difficult to directly realize the patterned preparation of organic semiconductor films,which is not conducive to the definition of transistor channel size.In terms of device performance,the photoelectric performance of the device,especially the synaptic plasticity,is easily affected by water and oxygen in the environmental atmosphere,resulting in the lack of environmental stability.Finally,in terms of multi-functional integration,the integration of neuromorphic optoelectronic devices with other functional devices such as photodetectors and optoelectronic memories generally needs the combination of multiple different devices.Therefore,the integration of a multi-functional processing system needs to be further improved.In view of the above problems,this paper has carried out research on the printing preparation process,the working mechanism and environmental stability,the light wavelength modulation of the neuromorphic organic phototransistor,and the multi-functional integration based on a single device.The main contents and results of each research part are as follows:Firstly,the flexible and low-power neuromorphic organic phototransistor made by the printing preparation technology was studied.Printing technology provides a patterned,large-scale and low-cost approach to preparing flexible organic optoelectronic devices,but at the same time,it puts forward strict requirements for semiconductor materials,such as high mobility,high photosensitivity and appropriate solution viscosity.In this thesis,the ternary blend of organic semiconductors,perovskite nanocrystals and traditional polymers was used to overcome the above challenges,and a flexible printable optical synaptic transistor was successfully developed.In this paper,the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-B]benzothiophene（C₈-BTBT）with high crystallization performance was used as the carrier transport component,the Cs Pb Br₃ quantum dot with good stability was used as the light absorption component,and the polystyrene was used as the tackifier of the printing ink solution and the tunneling component of the active layer.Based on the solution ink of the ternary blend,the neuromorphic organic phototransistor was developed through the printing preparation process.The device successfully simulated a variety of synaptic and neural functions,including learning forgetting characteristics,moss code transmission,self-regulation of neural overstimulation,optical enhancement-electrical inhibition of bidirectional neural synaptic behavior and so on.Through curve fitting and parameter extraction,handwritten digital image training and recognition based on single-layer perceptron were realized,and the recognition rate was 75%.The design of single-layer blend semiconductor film in the device was conducive to the efficient absorption of the incident light,so that the device realized the optical response synaptic performance of ultra-low energy consumption,and the power consumption of a single optical synaptic behavior was only 0.11 f J.Furthermore,the device prepared on the polymer substrate could give the neural morphology organic phototransistor good mechanical flexibility,and its performance did not fade significantly after 200 times of bending.Secondly,the neuromorphic phototransistor based on photoinitiator/organic semiconductor thin film with a new working mechanism and good environmental stability was studied.The working mechanism of traditional organic optoelectronic devices is mostly based on the fact that organic semiconductors are excited by light to produce electron-hole pairs and separate under an electric field,to contribute to the photocurrent of the device.However,some devices in the separation process of electron-hole pairs based on light excitation are vulnerable to the influence of water and oxygen in the external environment,resulting in the influence of their photoelectric characteristics by water and oxygen,which is not conducive to the realization of a stable delayed decay process of light stimulation（that is,the process of light stimulated synapse）.In this paper,a new working mechanism of the neuromorphic organic optoelectronic device was studied.The composite film of Bisimidazole photoinitiator2,2’-bis（2-chlorophenyl）-4,4’,5,5’-tetraphenyl-1,2’-Bisimidazole（Cl-HABI）with stable photochemical properties and high mobility polymer organic semiconductor Poly[2,5-（2-octyldodecyl）-3,6-diketopyrrolopyrrole-alt-5,5-（2,5-di（thien-2-yl）thieno[3,2-b]thiophene）]（DPP-DTT）was used as the active layer of the device.The photoinitiator will undergo a photochemical reaction under ultraviolet light to generate free radicals and generate a local electric field,which causes the change of carrier concentration in organic semiconductor and changes the output current of the device.After removing the light,the photoinitiator returned to the initial structure and the current of the device restores.This process simulated the process of stimulation enhancement and the slow recovery of synapses not only in the signal transmission and processing of the device,but also in the microphysical process,which provided a new working mechanism for neuromorphic optoelectronic devices.More importantly,the above mechanism is weakly affected by water vapor and oxygen in the environment,so it can significantly improve the environmental stability of neuromorphic phototransistors.The prepared devices exhibited similar photoelectric modulation synaptic properties under atmospheric pressure and vacuum.In particular,the device maintained stable light response performance in a high humidity environment（80%relative humidity）,and the synaptic performance of the device did not change significantly when the ambient humidity increased from 30%to 80%。Thirdly,the multi-functional optical wavelength modulation based on a single transistor composed of the organic photochromic molecule and organic semiconductor was studied.Based on the wavelength selective isomerization response of organic photochromic molecules,a phototransistor based on the heterojunction structure of organic semiconductor Dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene（DNTT）and photochromic molecule diaryl ethylene（DAE）was constructed in this paper.Under the illumination of different wavelengths,DNTT and DAE molecules will change the separation and transfer behavior of electron-hole pairs induced by light,to realize the multi-functional integration based on a single device.The device showed highly sensitive detection performance under 450 nm visible light,with an I_light/I_dark ratio of10⁶,while it showed the reverse long-term optical memory characteristic of inhibiting current under 365 nm ultraviolet light,with an I_light/I_dark ratio of 10^-7.In addition,it could also realize the simulation of light response synaptic behavior.Benefiting from its high degree of functional integration,this multifunctional photoelectric device has a wide application prospects in the fields of photoelectric fusion storage and computing system,artificial intelligence and the interconnection of all things.