Preparation Of Lead-free Halide Perovskite Film And Its Resistive Switching Properties

With the rapid development of the information age,the demand for information storage capacity and miniaturization of memory units has been increased.However,the commonly used silicon-based Flash memory has nearly approached its physical limit.Therefore,new types of storage devices,such as ferroelectric random-access memory,magnetic random access memory,phase-change random access memory,and resistive switching random access memory(Re RAM)were developed.Among them,the Re RAM is widely regarded as one of the most powerful competitors as the next-generation non-volatile memory due to its simple structure,fast operation speed,excellent flexibility,and long endurance.The core material inside of the Re RAM device is active layer.Those active layer materials are insulators or semiconductors.The commonly used active layer materials include metal oxides,organics materials,and perovskites oxide.However,the synthesis process for metal oxides usually requires reactions at high temperature or sputtering,and the cost is relatively high.On the other side,the organic materials probably would be decomposed in ambient atmosphere easily.The perovskite oxides need to be prepared at high temperature and demonstrate poor mechanical properties,which is not suitable to fabricate flexible devices.In recent years,the halide perovskites have been widely used in optoelectronic devices,such as solar cells,light-emitting diodes,field-effect transistors,Re RAM,and artificial synapses.However;the lead used in the halide perovskite structure is heavy metal,which would bring a huge threat to human health and environmental protection,thus hindering its practical applications in the field of data storage.This thesis focused on the preparation of low-lead or lead-free halide perovskite active layer materials and its resistive switching performance:(1)The Sn-doped CsPbBr₃ quantum dots with optoelectronic properties were studied.Firstly,the low-lead Sn-doped CsPbBr₃ quantum dots were synthesized by hot-injection method.The TEM and XRD results showed that a small amount of Sn-doping could partially replace the lead elements,and which could passivate the CsPbBr₃ quantum dots,reduce the surface defects of the quantum dots and improve the photoluminescence quantum yield(PLQY)of quantum dots.The PLQY of quantum dots increased from 21.0%to 40.4%when the doped ratio of Pb:Sn was 9:1.With the increasing amount of tin doping concentration(Pb:Sn=8:2,Pb:Sn=6:4),the heterogeneous phase would exist,and the PLQY would gradually decrease from 40.4%to 10.4%.In all,the Cs Pb_0.9Sn_0.1Br₃ demonstrated the strongest photoluminescence and electroluminescence,and the PL peak located at 520 nm with PLQY of 40.4%.The corresponding QLED device based on Cs Pb_0.9Sn_0.1Br₃ was constructed,and the electroluminescence peak position was 512 nm with luminescence brightness of 343.0cd/m².In addition,the resistive switching performance of tin-doped Cs Pb_0.9Sn_0.1Br₃quantum dots was studied.It was not observed resistive switching behavior in Cs Pb_0.9Sn_0.1Br₃based memory device due to the film of Cs Pb_0.9Sn_0.1Br₃quantum dots is too thin.(2)The preparation method of environmental-friendly Cs₃Cu₂I₅ perovskite films was studied.Firstly,lead-free Cs₃Cu₂I₅ perovskite films were synthesized by spin-coating and anti-solvent assisted methods,respectively.The crystal structure,surface morphology,bandgap,and optical property of the Cs₃Cu₂I₅perovskite films were investigated.1)The Cs₃Cu₂I₅perovskite films prepared with or without using anti-solvent demonstrated the same orthorhombic phase crystal structure.2)The photoluminescence quantum efficiency(PLQY)of Cs₃Cu₂I₅ perovskite films obtained by using anti-solvent method was enhanced,increasing from 62%without antisolvent to76.0%.3)The Cs₃Cu₂I₅ perovskite films synthesized by using anti-solvent had a relatively small surface root-mean-square roughness of 17.5 nm.4)Most importantly,Cs₃Cu₂I₅perovskite films obtained by using anti-solvent exhibited long-term air-stability,and the PLQY could still reach 76.3%after storing for several months under an ambient atmosphere.All the above results demonstrated the quality and the optical performance of Cs₃Cu₂I₅ perovskite films could be effectively improved by the anti-solvent assisted crystallization strategy,showing potential applications in the fabrication of photoelectric devices based on lead-free perovskites.(3)The resistive switching performance of lead-free Cs₃Cu₂I₅ perovskite films based memristor was studied,and further this memristor had been applied to neuromorphic computing.We introduced environmentally-friendly and uniform Cs₃Cu₂I₅ perovskite films to act as active layer in Ag/Cs₃Cu₂I₅/ITO memristor.Generally,the Ag ions could react with iodide ions and form Ag I_xcompound easily,so the Ag/PMMA/Cs₃Cu₂I₅/ITO memristor was designed by employing the ultra-thin polymethylmethacrylate(PMMA)films to avoid the direct contact between Ag electrode and Cs₃Cu₂I₅ layer.After optimization,the obtained memristor exhibited bipolar resistive switching with low operating voltage(<±1 V),large on/off ratio(10²),stable endurance(100 cycles),and long retention time(>10⁴ s).Additionally,biological synaptic behaviors including long-term potentiation and long-term depression have been investigated,and simulation based on the above properties showed 94%handwritten recognition accuracy by using the Modified National Institute of Standards and Technology handwritten recognition data set.In conclusion,this thesis provides the opportunity of exploring the novel application for the development of next-generation neuromorphic computing based on lead-free halide perovskites.(4)The effect of internal defects of Cs₃Cu₂I₅ perovskite films on the resistive switching performance of the resistive memory was studied.In this work,a new memory device with the structure of Al/Cs₃Cu₂I₅/ITO was fabricated,in which the functional layer of the memory device is an environmental-friendly Cs₃Cu₂I₅ perovskite films.In addition,the effect of the amounts of hydroiodic acid(HI)added in the precursor solution on the structure and morphology of the Cs₃Cu₂I₅perovskite films as well as the resistive switching(RS)performance of the memory device was studied.The results showed that the morphology of the Cs₃Cu₂I₅perovskite films and the RS performance of the Al/Cs₃Cu₂I₅/ITO device changed with the amount of HI.When the added amount HI was excessive,it could crack the Cs₃Cu₂I₅ perovskite films and increase the defects.Furthermore,the electroforming voltage,the set/reset voltage and the on/off ratio would also be reduced.Therefore,an appropriate amount of HI added in the precursor solution was necessary.Studies found that when the adding amount of HI was 5?L,the Cs₃Cu₂I₅ perovskite films had the fewest lattice defects and flattest surface,with which surface root-mean-square roughness was only 13.3 nm.In addition,the fabricated memory device had the lower electroforming voltage(1.44 V),the largest on/off ratio(65),and long retention time(10⁴ s).It was proved that the presence of the appropriate amount of over-proportioned iodine ions can effectively reduce the defects of the films and improve its RS performance.The RS performance is related to the amount of HI,and it provides clues for explaining the RS mechanism based on halide perovskites.