Studies On Photogenerated Carrier Properties Of Lead Halide Perovskite Crystals

Lead halide perovskite crystals are a kind of very important materials with good magnetic,optical and electrochemical properties,and have become one of the hot research materials in the field of optoelectronics.Although lead halide perovskite crystals inevitably have defects,they show the carrier characteristics expected by almost defect-free semiconductors.These remarkable characteristics include long carrier lifetime,long carrier diffusion length and low electron-hole recombination rate constant,which are important reasons for the extensive exploration and successful application of lead halide perovskite materials in high-efficiency photovoltaic devices.While,the complex and changeable synthesis conditions in the practical research and application process may cause the difference of crystals carrier properties,due to the comprehensive influence from defect state trapping and charge transfer between interfaces.Therefore,it is particularly important to conduct a detailed investigation of the carrier properties of perovskite materials.Considering the decreasing dimension of materials,several representative perovskite crystals,including millimeter single crystal,microcrystalline and nanocrystalline,have been studied in this thesis,centering on the main point of photogenerated carrier properties of lead halide perovskite crystals.The photogenerated carrier properties and carrier dynamics of the material were studied from the aspects of halogen doping,size and morphology control,etc.Meanwhile,the application potential of the material for optoelectronic devices,including device performance and optical stability of the material,was also evaluated.?1?In large perovskite single crystals,the influence of grain boundary on photophysics can be neglected.The transport of photogenerated carriers depends mainly on the intrinsic phase structure of perovskite,and the effect of defect state is very important.On this basis,we found that halogen doping can effectively control the electronic structure and defect states density.So,we investigated the influence of different amounts of bromine doping on carrier lifetimes of CH₃NH₃PbI_3-xBr_x single crystals,and the influence of different amounts of chlorine doping on carrier diffusion lengths of CH₃NH₃PbI_3-xCl_x single crystals.We found that halogen doping could improve the carrier lifetimes and diffusion lengths of single crystals,and this is mainly attributed to the fact that Br or Cl doping into CH₃NH₃PbI₃ single crystal simultaneously changes the trap state density and valence band of single crystal,thus affecting the charge recombination and transfer process.Finally,the single crystal with the best performance showed lower trap states density and charge recombination,as well as effective electron injection.?2?When perovskite crystals are on the micrometer scale,the grain boundary and crystal surface structure play a decisive role in the transport of photogenerated carriers.How to control and optimize the defect state density at the interface is the key to improve the efficiency of optoelectronic devices.FAPbBr₃ microcrystalline thin films were fabricated by improved reverse-temperature crystallization method.The defect density of states,charge collection and photogenerated carrier properties of the materials were evaluated,and all of them exhibited excellent performance.Finally,these microcrystals were fabricated into photodetectors for making full use of the advantages of effective charge transfer,realizing the rapid diffusion of photogenerated carrier.The device realized an ultra-high responsivity as high as 4000 A/W under 400 nm light excitation.In addition,it's two-photon response also reached 0.07 A/W,which is four orders of magnitude higher than the reported MAPBr₃ single crystal photodetector.?3?When the size of perovskite crystals shrinks to the nanometer scale,its unique quantum confinement effect occurs,and the difference in morphology of nanocrystals often brings about differences in properties.On the one hand,we investigated and analyzed the influence of morphology control on carrier performance and absorption cross section of nanocrystals by preparing CsPbBr₃ nanocrystals with three different morphologies,including nanocubes,nanosheets and nanowires.The excited state dynamics analysis of these nanocrystals showed that the influence of morphology on the absorption cross section is much smaller than that of size and volume at the same excitation wavelength,and calculation based on density functional theory also confirmed the minor influence of morphology on the electron transition.In addition,compared with the difference in photogenerated carrier dynamics and absorption cross-section values caused by excitation light of different wavelengths,the absorption cross-section value measured by 460 nm light excitation closer to the edge of the absorption band was smaller than that of 400 nm excitation.On the other hand,under the guidance of PbBr₂ precursor solution,zero-dimensional Cs₄PbBr₆ nanocrystals can be converted into CsPbBr₃ nanocrystals.CsPbBr₃ nanocubes or CsPbBr₃ nanoplatelets can be obtained by controlling the amount of PbBr₂ precursor solution,which is another important way to prepare nanocrystals with different morphologies.Based on the time-resolved fluorescence spectra,the phtogenerated carrier dynamics of two transformed nanocrystals with different morphologies were investigated.It was found that the blue-emitting CsPbBr₃ nanoplatelets showed faster PL decay than that of green-emitting CsPbBr₃ nanocubes,and the reason is mainly related to the existence of more defect states.?4?Finally,we find that the optical stability of nano-scale perovskite is different from that of bulk materials due to its huge specific surface area,especially the organic ligands on their surface,which determine the optical stability of nanocrystals.We studied the photostability and analyzed the influence of light on the photogenerated carrier dynamics of CsPbI₃ nanocrystals,which have great potential to be applied in solar cells.The size and lattice structure of nanocrystals did not change much after continuous illumination,but their fluorescence emission and quantum yield decreased significantly.Time-resolved spectroscopy results revealed that nanocrystals exposed to light would cause the separation of surface capping agents and leave defects to increase surface charge trapping.On the other hand,Pb⁰ precipitates to form new trapping centers.These two points are the main reasons for the decrease of nanocrystal fluorescence and quantum yield.