Study And Regulation Of The Fluorescence Properties In Organometallic MAPbBr₃ Perovskite Single Crystals

Perovskite material AMX₃（A=organic or inorganic cation,M=metal cation,X=halogen anion）is a new type of semiconductor material.Due to its unique crystal and electronic structure,it shows excellent photoelectric properties（such as high absorption coefficient,high carrier mobility and long diffusion length）and has been applied to various optoelectronic devices（such as solar cells,light emitting diodes,lasers,field effect transistors and photodetectors,etc.）.Up till now,the reported perovskite materials include quantum dots,nanowires,nanorods,platelets,polycrystalline thin films,single crystal thin films and single crystal bulk.Among these morphologies,perovskite single crystals show the best carrier transport properties due to their low defects density and negligible grain boundaries,which are beneficial to applications such as solar cells and photodetectors.However,compared with polycrystalline thin films,the light-emitting property of traditional organic metal trihalide MAPbBr₃（MA=CH₃NH₃⁺）single-crystal is usually weaker,and there are few reports of light-emitting devices based on perovskite single crystal.Therefore,while maintaining the excellent carrier transport of MAPbBr₃single crystals,exploring strategies to improve and regulate the fluorescence characteristics shows significance for the preparation of self-supply light absorbing-emitting devices（such as light-emitting solar cells）and light-emitting transmission devices（such as light-emitting transistors）.Defects（including surface defects and volume defects）of MAPbBr₃single crystal are the main obstacles that affects carrier transport,fluorescence,and limits the performance of optoelectronic devices.Reducing the defect states density will help improving the carrier transport properties,extending the carrier lifetime,enhancing the fluorescence and optimizing the device performance.To control the surface defects,a widely studied method is surface passivation,which passivates and cures the defects by a surface engineering process,i.e.,adding a variety of additives,including ammonium methyl bromide,guanidinium bromide,potassium iodide,phenethyl iodide,poly（3-hexylthiophene-2,5-diyl）,choline iodine,and1-butyl-3-methylimidazolium tetrafluoroborate.However,this method requires precise control of the amount of the additives,the order of addition,and the reaction time,which makes this process complicated and results in a high risk of loss.To tune the volume defects,a known strategy is irradiating perovskite with high-energy ultraviolet light,sunlight,near-infrared light,etc.This strategy requires a long repair time and sometimes results in irreversible damage to the materials,which makes the process complicated.Therefore,it is urgent to explore and develop novel,simple,controllable,and effective strategies to passivate and regulate defects,and then achieve the purpose of regulating the fluorescence.Based on the above research status and research purposes,we have determined three experimental schemes:through femtosecond laser micromachining technology,thickness optimization strategy and voltage regulation engineering to regulate the fluorescence properties of the MAPbBr₃single crystal.This thesis focuses on the fluorescence properties and regulation strategies of organometallic trihalide MAPbBr₃perovskite single crystals.The main research work of this paper is as follows:1.Dramatically enhanced photoluminescence（PL）is achieved from micro/nano-structures on MAPbBr₃single crystal surface induced by femtosecond laser.Various surface topography（such as ripples,nano-protrusions,micro/nano rods,micro/nano wires,and micro/nano-network）are induced on the MAPbBr₃single crystal surface through a single-beam femtosecond laser micromachining system by changing the peak laser intensity and scanning speed.Through steady-state PL characterization,it was found that the fluorescence in laser-processed region achieved two orders of magnitude enhancement under ambient conditions,and three times enhancement under nitrogen environment.The above fluorescence enhancement mechanism is mainly attributed to the two aspects:one is that the laser-induced micro/nano structure promotes photon recycling and light output coupling,and the other is that the micro/nano structure increases the roughness and specific surface area of the MAPbBr₃single crystal surface,which further promotes the passivation of the surface recombination center.2.The carrier lifetime and mobility of MAPbBr₃single crystals show thickness dependent.By measuring the carrier transport property and defects density of MAPbBr₃single crystals,the dependence of perovskite carrier lifetime and mobility on crystal thickness is explored.Firstly,controlled thickness（from 1.47mm to 10.55mm）MAPbBr₃single crystal thin films are synthesized by confining the space between two slides and adjusting precursor solution concentration.Then,through time-resolved photoluminescence（TRPL）and space charge limited current（SCLC）measurements,the carrier average lifetime and carrier mobility of MAPbBr₃single crystals with various thickness are monitored,and the dependence of carrier lifetime and mobility on crystal thickness are extracted.Furthermore,the surface recombination velocity and surface defects（trap）density are calculated,which give a straightforward explanation of the evolution results mentioned above.Finally,a three-layer carrier transfer model of MAPbBr₃single crystals is proposed to describe the thickness-dependent carrier transport property evolution process,which may work not only for MAPbBr₃but also for other perovskites like MAPb Cl₃,FAPb I₃,etc.3.Voltage regulation engineering modulates the optical and electrical properties of MAPbBr₃single crystals and its’application in memristors.We demonstrate that voltage regulation is an efficient method to tune defect density,as well as the optical and electrical properties of perovskite single crystals.A three-step carrier transport model of MAPbBr₃single crystals is proposed to explore the defect regulation mechanism and carrier transport dynamics via an applied bias.Dynamic and steady-state PL measurements subsequently show that the surface defect density,average carrier lifetime and PL intensity can be efficiently tuned by the applied bias.In particular,when the regulation voltage is 20 V(electrical poling intensity is 0.167Vmm^-1),the surface defect density of MAPbBr₃single crystals is reduced by 24.27%,the carrier lifetime is prolonged by 32.04%,and the PL intensity is increased by112.96%.Furthermore,a voltage-regulated MAPbBr₃single crystal memristor device shows an adjustable multiresistance,weak ion migration effect and greatly enhanced device stability.Voltage regulation is a promising engineering technique for developing advanced perovskite optoelectronic devices.