Metal Halide Semiconductors-based Photosensors And Light-stimulated Synaptic Devices

Metal halide semiconductors（MHSCs）have been used in photoelectric devices such as solar cells,light-emitting diodes,photosensors,and light-stimulated synapses because of their excellent photoelectric properties,such as high charge carrier mobility,high absorption coefficient,tunable band gap,low exciton binding energy,long charge carrier diffusion length,and low defect density.Among them,MHSCs-based photosensors and light-stimulated synapses,as a kind of devices that can convert optical signals into electrical signals,can be applied to image recognition,information processing and other fields.Therefore,it has theoretical and practical significance to investigate this kind of device under the trend of rapid development of information technology.At present,scientists have carried out a series of studies on MHSCs-based photoelectronic devices.However,there are still some unsolved problems in photosensors and light-stimulated synaptic devices based on MHSCs previously reported.For example,（1）most of the active materials used are organic-inorganic hybrid lead halide perovskites,which exhibit toxicity and poor air-stability.（2）Some devices exhibit complexed fabricated processes and require large voltage to drive.Therefore,if one or both of the above problems can be solved,the development of MHSCs-based photosensors and light-stimulated synaptic devices will be promoted.Therefore,we take the stable all inorganic MHSCs as the research object,and studies the application of MHSCs in air-stable and low-energy photosensors and light-stimulated synapses by regulating material preparation method,material composite mode and device structure in the paper.Some research progress has been made and the main research contents are as follows:1.Preparation and performance study of photosensor based on lead-free double perovskite single crystal.Lead-free double perovskite Cs₂Ag Bi Br₆ single crystals with millimeter-size were synthesized by the slow cooling of supersaturated solution.Cs₂Ag Bi Br₆ single crystal had good crystallinity and wide spectral absorption by the analysis of X-ray diffraction and UV-Vis absorption spectrum.The trap density and charge carrier mobility of the Cs₂Ag Bi Br₆ single crystal were measured by the space-charge-limited current method,which were 1.37×10¹⁰ cm^-3 and 8.84 cm² V^-1 s^-1,respectively.Then,symmetric Au electrodes were evaporated on the surface of Cs₂Ag Bi Br₆ single crystal to prepare lead-free perovskite single crystal-based photosensors.The device showed high optical sensing performance with responsivity（R）of 4.88 A W^-1,external quantum efficiency（EQE）of 1100%,and detectivity（D*）of 12.2×10¹² Jones,respectively.In addition,when asymmetric electrodes（Au/Ag）were evaporated on the surface of Cs₂Ag Bi Br₆ single crystal,we obtained the first reported self-driven photosensor based on the lead-free perovskite single crystal and investigated optical sensing performance of the device.Owing to the high crystal quality of single crystal,both two types of devices exhibited fast optical switching performance with the response and recovery time at the scale ofμs.Moreover,the unencapsulated Cs₂Ag Bi Br₆ single crystal-based photosensors had good long-term stability,which was proved by good optical response of the device after being stored in air for several months.2.Preparation and performance study of the self-driven photosensor based on lead-free perovskites and metal oxide heterojunction.Cs Bi₃I₁₀/Sn O₂self-driven photosensors with a vertical structure were prepared by the solution process.In the device,Cs Bi₃I₁₀ possesses high light absorption property,which can ensure the generation of photogenerated charge carriers.A typical typeⅡenergy band structure is formed between Cs Bi₃I₁₀ and Sn O₂,which is conducive to promote the effective extraction and transfer of electrons and holes,enhancing the device performance.Therefore,Cs Bi₃I₁₀/Sn O₂ self-driven photosensors exhibited high optical sensing performance,such as high light dark switching ratio（2.33×10⁵）,high R(0.2 A W^-1),short response/recovery time（7.8/8.8μs）,and low light detection limit(10 p W cm^-2).Moreover,the self-driven photosensor showed outstanding environmental stability and high temperature stability.The unpackaged devices still had good photosensitivity after being stored in air for 73 days.The device performance had not changed significantly after continuous operation at 353 K for 1 h.Moreover,the Cs Bi₃I₁₀/Sn O₂ self-driven photosensor array can realize weak light imaging.3.Preparation and performance study of light-stimulated synaptic devices based on organic semiconductors and perovskite quantum dots.Cs Pb Br₃ QDs were prepared by the hot injection method,which were blended with the organic semiconductor（Poly[2,5-（2-octyldodecyl）-3,6-diketopyrrolopyrrole-alt-5,5-（2,5-di（thien-2-yl）thieno[3,2-b]thiophene）],DPPDTT）.Then,light-stimulated synaptic devices based on DPPDTT/Cs Pb Br₃ QDs were prepared by simple spin-coating method.Here,DPPDTT/Cs Pb Br₃ QDs film acted as the charge-transporting material and the light absorbing material.The optical response performance of the device can be improved via the effective charge transfer between DPPDTT and Cs Pb Br₃ QDs.Meantime,the photogating effect caused by charge transfer effect and charge capture in the channel or at the interface between DPPDTT and Cs Pb Br₃ QDs can make the device realize the simulation of biological synaptic behaviors.DPPDTT/Cs Pb Br₃ QDs light-stimulated synaptic devices simulated successfully some important biological synaptic properties,such as excitatory postsynaptic current,paired-pulse facilitation,short-term memory,long-term memory,and“learning-forgetting-relearning”experience.Moreover,we investigated the effects of applied voltages and incident light parameters on the performance of synaptic devices.Experiments proved that the device can work at a voltage as low as-0.0005 V,generating an electronic energy consumption of 0.5 f J for a synaptic event.Cs Pb Br₃ QDs could be wrapped by stable DPPDTT owing to the design of the blend system,reducing the direct contact between perovskite and air.Therefore,the device exhibited considerable environmental stability.Finally,DPPDTT/Cs Pb Br₃ QDs light-stimulated synaptic devices were applied preliminarily to mimic logic function and human visual memory.4.Preparation and performance study of the light-stimulated synaptic device based on metal halides and single-walled carbon nanotubes（SWCNTs）.Bi I₃/SWCNTs light-stimulated synaptic devices were fabricated by spinning Bi I₃ layer on the SWCNTs-coated substrates via drop-coating method.Here,Bi I₃ and SWCNTs are the light absorption layer and the conductive channel layer,respectively.Because the light absorption intensity of Bi I₃/SWCNTs film is consistent in the the whole absorption spectrum of Bi I₃,the device can achieve almost same synaptic behaviors under the light pulses with different wavelengths.Meantime,Bi I₃/SWCNTs light-stimulated synaptic devices can simulate basic biological synaptic functions,such as excitatory postsynaptic current,paired-pulse facilitation,etc.Moreover,by studying the effects of various parameters such as bias voltage,pulse time and intensity on the device,we found that Bi I₃/SWCNTs light-stimulated synaptic devices can work at a voltage of-0.001 V,achieving the minimum power consumption of 7.5 f J for a synaptic event.Owing to the superior air-stability of SWCNTs and Bi I₃,the unpackaged device had good synaptic response after being stored in air for 170 days,indicating outstanding environmental stability.Finally,a synaptic array with 3×3 devices were fabricated to simulate the learning and forgetting process of biological synapses by regulating the number of light pulses to the array.In summary,a series of MHSCs-based photosensors and light-stimulated synaptic devices with superb performance have been designed to achieve stable and low energy consumption optoelectronic devices and arrays in this paper.The applications of the devices in optical information imaging and visual information processing were preliminatively studied.Therefore,the research content of this paper has great application potential the fields of image recognition and information processing in the future,and is expected to promote the development of low-power intelligent sensing system in the future.