Solution-Processed Study Of Two-Dimensional Organic Semiconductor Crystals And Their Study Of Field-Effect Transistors And Light Synaptic Devices

The organic semiconductor materials can be traced back to the 1980 s.Organic semiconductor materials(OSCs)have their unique advantages,such as low-cost solution preparation,adjustable performance,good flexibility and good biological properties,compared to inorganic semiconductor materials.OSCs have been favored by researchers and widely used in electronic devices,such as organic field effect transistors,organic light emitting diodes,and organic solar cells.In the past two or three decades,a large number of multifunctional,high-performance semiconductor materials have been designed and synthesized.On the other hand,as the development of solution-assembling technology,the poor electrical performance of electronics based on OSCs(organic electronics)has been alleviated.Based on this,organic electronic devices can well expand and complement in some specific applications where inorganic semiconductor devices are invalid.In the past ten years,two-dimensional(2D)organic crystals as an emerging material system have attracted wide attention,which are assembled by the intermolecular van der Waals forces drived self-assembly.The 2D organic crystals have molecular-level thickness,and ordered single crystalline arrangement that can provide good electrical performance.In addition,ultrathin structure enables the regulation of conductive channels under the two-dimensional limit,providing a good platform for the study of the transport mechanism of electronic devices.Here,a large-sized N-type 2D organic crystals was prepared by a solution method,and the growth mechanism was discussed.The N-type organic transistor was successfully fabricated.This work not only overcomes the urgent problem of solution-processed N-type 2D organic crystals,but also provides important guildlines for the scientific problems of solution-processing 2D organic crystals.Meanwhile,stacking with P-type semiconductors by solution process realizes the construction of heterojunction devices,which realizes the electron and hole two-channel transport.Based on the high-efficiency photoelectric performance of the 2D organic crystals,we built the light synaptic device with a planar Schottky diode structure.The ultrathin structure could effectively "bridge" the two interfaces in the device,which maked the Schottky barrier at metal/semiconductor interface being sensitively modulated by the photo-induced trapped charge at semiconductor/insulator interface.Finally,the device perfectly mimicked the function of optically programming synaptic devices.This work provides broader ideas for organic light programming neuromorphic devices.We also systematically studied the high tunability of the performance of the optical synaptic device.Through the study of the thickness of the semiconductor layer,the operating voltage,and the gating effect(gate voltage),we comprehensively understood the operating mode of this optical synaptic device,providing a solution for the development of multi-functional adjustable bionic electronic devices.The innovative research results of this paper mainly include the following points:1.The high-quality N-type two-dimensional organic crystals was prepared by the additive-assisted "metal-wire-gap" process.During this growth process,the additive can form a special nucleation interface,where the host molecule can spread more stably,leading to a lower nucleation barrier and triggering the growth of the two-dimensional crystalline film.In particular,the large-area and uniform monolayer of 2D organic crystals can be prepared by this process,which is undoubtedly of far-reaching significance for the study of device interface physics.Based on the monolayer and bilayer PDIF-CN2,we fabricated the planar transistor devices that showed excellent gate-controlled behaviors and good linear characteristics in the low VD region,indicating the low contact resistance;The average mobility of monolayer and bilayer PDIF-CN2 devices are 1.4×10-2 and 1.8×10-2 cm2 V-1 s-1,respectively,which is comparable to the few-layer PDIF-CN2 transistor devices reported by other research groups.Among them,the monolayer PDIF-CN2 device is the first monolayer PDIF-CN2 transistor device.This monolayer PDIF-CN2 device can provide a good research platform for electron transport and related interface issues because of complete screening of the intermolecular charge coupling.Additive-assisted crystallization technology has very good potentials in the solution-processed two-dimensional organic crystalline films.This work shows the great significance for the systematic study of two-dimensional organic system and its development of multi-functional electronic devices.2.The P3HT/PDIF-CN2 heterojunction film was prepared by additive-assisted "metal-wire-gap" process.AFM film characterization shows that the growth of PDIF-CN2 film is realized by the method of “filling the rough interface of the porous P3 HT film and then growing”.The upper film shows atomic flatness,indicating that the PDIF-CN2 is crystalline film with high quality.The transistor device was fabricated based on the prepared heterojunction.The transfer curve of transistors shows a typical V-shaped characteristic.The output curve shows a flipping behavior as the modulation of the gate voltage.By extracting the mobility,the device shows balanced bipolar transport characteristics,the hole and electron mobilities reach at 0.01 and 0.007 cm2 V-1 s-1,respectively.Such mobility performance is comparable to that of the single component transistor devices.Thus,This heterojunction stacking process can maintain the performance of each component well.This rough contact interface of heterojunction can facilitate the realization of more sufficient charge transfer,separation and recombination of excitons,and provides the possibility of realizing high-performance heterojunction functional devices.3.We used the "coffee ring" assembly technology to achieve the growth of C8-BTBT two-dimensional crystals.The spectral response and transistor device analysis show that the grown C8-BTBT two-dimensional crystalline film has high UV response and excellent photoelectric performance.Combined with the Schottky planar diode structure,we have developed optical synaptic devices with significant synaptic behaviors.The ultrathin semiconductor layer can effectively bridge the upper and lower interfaces of the device,obtainning the continuous modulation of the Schottky barrier by efficient interface capture.Based on this,the device shows the excellent photoelectric performance while achieving excellent memristive behavior.Through the study on the Schottky barrier of the device with or without light,we found that the bilayer C8-BTBT crystals has a sensitive optical modulation of Schottky barrier.The memristive mechanism is the photo-induced charge trapping at the silicon dioxide interface.The device achieves a sensitivity of ~9 ?W/cm2 and a low energy consumption of ~13.6 p J at a small reading voltage.In addition,the plasticity behaviors of the device exhibit multiple modulations,including light intensity modulation and pulse time interval modulation,pulse number modulation,etc.device shows flexible short-range and long-range transitions,indicating that our devices have great potential in mimicking optical cognition and information processing,and provide solutions for building high-performance organic photo-synaptic devices.4.We have deeply researched and verified the internal mechanism of optical synaptic devices,including change of light-induced Schottky barrier and modulation behavior of post-synaptic current by charge trapping on the silicon dioxide interface.We tested the I-V curve of the device under variable temperature conditions,and used Arrhenius equation to directly extract the Schottky barrier height of the device with and without light.We found that the light can significantly reduce the Schottky barrier(under our test conditions,the height of Schottky barrier under dark is 227 me V,the height of the Schottky barrier under light is 74 me V),which is the essential reason for the high-level photoelectric performance.We used three methods to verify the charge trapping effect on the silica interface,including the study of I-V curve hysteresis,the model fitting of CS current relaxation,and the control experiment of interface passivation.Among them,the study of the hysteresis of the IV curve found that device produces a large hysteresis window under the illumination,which means that a large amount of charge trapping is generated;The current relaxation behavior of the device after removing the light can be fitted well by the kinetic model of CS current relaxation on the silicon dioxide.And using Si3N4 to passivate the Si O2 interface,the memristive behavior of the device is completely disappeared.After these three methods,we can verify that the charge trapping modulation at the silica interface is the memristive mechanism of the photo-synaptic device.Based on this,we summarized the operation principle of the device,and further verified the correctness of the principle by different I-V test methods.5.By adjusting the device structure and test conditions,including the thickness and quality of the semiconductor layer,the operating voltage and the gating effect,we demonstrated the high tunability of the performance of the optical synaptic device based on two-dimensional organic crystals.Among them,by reducing the thickness of the semiconductor layer,both the photoelectric performance and memristive behavior of the device can be improved,thereby obtaining better synaptic performance with a lower power consumption.Through the study of device operating voltage modulation,we found that high operating voltage can comprehensively improve the working state of the device,including light response and synaptic performance,but high operating voltage will also bring greater power consumption problems.Through in-depth study,we found that the device can achieve a good balance between performance and power consumption at an operating voltage of-2 V.Thus,we can achieve the suitable working state towards various application requirements by the modulation of the operating voltage.The modulation of the gate voltage shows a clear directionality.The forward gate voltage improves the photosensitivity of the device,the reverse gate voltage reduces the photosensitivity of the device,and the reverse gate voltage can erase the state of the device,allowing the device state of "initialization".The study of the high tunability of device performance can allow us to deeply study the working mechanism and function realization of the device,which has important guiding significance for the development of multi-functional device applications.