| As a class of devices that directly convert light into electrical signals,photodetectors play an important role in the fields of military,national defense,medical imaging,environmental monitoring,security inspection,and basic scientific research.Up to now,photodetectors based on traditional inorganic semiconductor materials still dominate the market.The photodetectors made of inorganic materials including GaN,Si,Ge,InGaAs and HgCdTe can detect the range from ultraviolet to infrared.Since the 21st century,due to the advantages of light weight,low cost,and large area production in solution-processable organic semiconductor materials,the performance of devices made of organic materials is comparable to that of inorganic semiconductor detectors,which has attracted widespread attention.In the past 10 years,the lead halide-based perovskite material with the general formula APbX3(A=Cs+,Rb+,[HC(NH2)2]+(FA),[CH3NH3]+(MA)),X=Cl,Br,I)has become a new semiconductor material because of its excellent photoelectric properties.Polycrystalline thin film and single crystal structure are two common forms of perovskite materials in photovoltaic applications.The initial research was based on perovskite polycrystalline films,using different film forming methods to control the crystallization process,thereby effectively controlling the grain size,surface roughness and film thickness.However,these factors have a greater impact on the absorption of photons and the transfer of charges.Moreover,the polycrystalline film has a large number of defects,which seriously affects the performance of the device.In 2015,researchers conducted in-depth research on the optical and electrical properties of perovskite single crystals(SCs)for the first time.Due to the elimination of grain boundaries,SCs exhibit better characteristics than polycrystalline thin films.For example,the trap density of states(t DOS)of a perovskite SC is approximately in the range of 109-1011 cm-3,which is about 5 orders of magnitude lower than that of a polycrystalline thin film.This ultra-low t DOS enables the diffusion length of carriers to be tens to hundreds of microns.In addition,the lead halide-based perovskite SC also has the characteristics of high X-ray absorption coefficient,high mobility,long carrier lifetime and low-temperature solution growth.It is expected to overcome the shortcomings of the existing radiation-type photodetectors in terms of cost and sensitivity,and promote the development of detectors that can work at room temperature,low-cost,and high-sensitivity imaging.Therefore,lead halide-based perovskite single crystal is considered to be the most promising material for visible light and X-ray detectors.The thesis focuses on the preparation of lead halide-based perovskite SCs and devices.Through the passivation treatment of the crystal surface,changing the ion composition of single crystal materials and doping large-size cations at the A site to improve the performance of the device,the following research work has been mainly carried out.First,a dual-function detector made of high-quality and high-temperature stable FAPbBr3 perovskite SC is introduced.Large-size,crack-free FAPbBr3 SCs are grown by low-temperature crystallization with gradient heating,and the crystal exhibits excellent thermal stability at a high temperature of 460 K.Surface defects of FAPbBr3 SCs were effectively passivated by UV ozone treatment,facilitating the crystal to exhibit a high carrier mobility-lifetime product(??)and an ultra-low surface charge recombination velocity.FAPbBr3 SC-based detectors directly converted high-energy X-ray flux into collectible charges,possessing a high sensitivity and a low detection limit.Furthermore,it also contributes a specific detectivity as high as 3.5×1012 cm Hz1/2 W-1,keeping stable at high-temperature of 460 K as a photodetector.A prototype of an imaging system with diffuse reflection mode is constructed using FAPbBr3 SC photodetectors as receivers,enabling defined scanning images in both room and high temperature environment.It fully demonstrates the potential of the device in visible light and X-ray detection,and imaging applications.In Chapter 3,a series of researches are carried out on the Cs0.1FA0.9Pb(I0.9Br0.1)3 perovskite SC detectors.In order to solve the problem that the FAPbI3 SC is prone to phase transformation in the air,the ionic composition of the perovskite SC material is tuned.By replacing FA cations with Cs,and I anions with Br.The compositionally dependent stability of perovskites can be semi-quantitatively rationalized using the Goldsmith tolerance factor(GTF)concept.Thus,Cs0.1FA0.9Pb(I0.9Br0.1)3 SCs with outstanding semiconductor characteristics and long-term stability was obtained.After 109 days of storage in air,the crystal maintained excellent phase stability,and the device could still work normally.Besides,the detector based on high-quality Cs0.1FA0.9Pb(I0.9Br0.1)3 SC shows high sensitivity to both visible light and X-ray.In particular,for high-energy X-ray particles,the device has a high sensitivity of 12230?C Gy-1air cm-2 under an electric field of 50 V mm-1,and the lowest detectable dose rate is as low as 31.5 n Gyair s-1.This means that the detector can sensitively capture the“shadowless killer”.In Chapter 4,the Cs0.1FA0.9Pb(I0.9Br0.1)3 perovskite SC detector is systematically studiedbased on the A site cation engineering.In order to further improve the performance of the perovskite SC detectors,a strategy of alloying large-size cations guanidine(CH6N3+,GA+)at A site was adopted to significantly increase the materials defect formation energy and decrease the electron-phonon coupling strength.The optical and electrical characteristics confirmed that the GA+cations were incorporated into the crystal by directly substitution of FA,which gradually expanded the volume of the unit-cell,forming a mixed Cs0.1(FA1-xGAx)0.9Pb(I0.9Br0.1)3(x=0.05,0.15,0.25)SC.Comparative experiments show that the proper mixing ratio of GA+can effectively improve the quality of the single crystal.When x=0.15,Cs0.1(FA0.85GA0.15)0.9Pb(I0.9Br0.1)3 SC displayed notably improved??product,demonstrating increased charge collection efficiency.More importantly,dark current also significantly reduced for the alloyed SC due to the suppressed shallower defect density.With judiciously designed device architecture,a high detector sensitivity is 2.2×104?C Gy-1aircm-2,was achieved for Cs0.1(FA0.85GA0.15)0.9Pb(I0.9Br0.1)3 SC.In the meantime,the lowest detectable dose rate is 11.3 n Gyair s-1.The overall detector performance confirms Cs0.1(FA0.85GA0.15)0.9Pb(I0.9Br0.1)3 SCs to be one of the most sensitive perovskite X-ray detectors to date.In addition,the device successfully realized the X-ray imaging application of the annular metal gasket.In this thesis,SC detectors based on lead halide-based perovskite have been investigated.By passivating the crystal surface defects,tuning the ion composition of the SC,and alloying large-size cations at the A site,the detection performance of the device for visible light and X-ray is effectively improved.These strategies promote the optimization of important performance parameters such as dark current,responsivity,sensitivity and stability of the detectors.In short,the thesis provides a valuable reference for material selection,crystal growth,structure design,device working mechanism analysis,and imaging applications of perovskite SC photodetectors. |
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