| Cesium lead halide perovskite materials have attracted intensive interest in recent years,due to their unique photoelectric properties.Their properties,such as large light absorption coefficient,long electron hole diffusion length,and tunable band gaps make them possible to realize high-performance solar cells,light-ernitting diode(LED),lasers,and photodetectors.Up to date,the power conversion efficiencies of perovskite-type solar cell is now over 22%,and perovskite quantum dots show desirable optical properties,including widely tunable absorption and emission in visible ranges,narrow emission line width,high photoluminescence(PL)quantum efficiencies.From lead halide perovskite materials,choice of elements,the synthesis of materials,to design the structure of crystal and different crystalline phases,great progress has been made in the study of the properties and applications of perovskite materials.However,applications of perovskites are hindered by their instability,which is mainly due to a breakdown of their crystal structures by moisture,UV light,temperature,and oxygen.Therefore,improving their stability is a key issue necessary for promoting their applications.This dissertation is aimed to improve the stability of lead-halide perovskite materials.Firstly,the X-type ligand-protected perovskite crystals were obtained by replacing sulfhydryl with lead salts.Then,in order to expand production,I adopt the method of direct synthesis to get the stable X-type ligand-protected tetragonal perovskite crystals,which have the adjustable emission peak in the visible range.The crystal phase and morphology were also studied.After that,for the purpose of making perovskite crystals more stable,tetragonal perovskite doped by Mn2+ crystals were directly synthesized to keep the crystal phase and structure intact.Finally,the phase transition mechanism of the reversible phase transition between orthorhombic phase CsPbBr3 and tetragonal phase CsPb2Br5 nanosheets was studied.Main research work of this thesis:(1)A novel method to advance the stability of perovskites with X-type ligands was developed.Multi-steps of ligand exchange and ion filling processes were necessary to make sure the X-type ligand layer is compact enough to sustain their stability in a polar solvent.It is worthwhile emphasizing that after surface treatment,surface defects can be further fixed through a photoactivation process,which is opposite to a photodamaging process.Such process can be monitored by timedepended PL spectra.The PL intensity increased to 108%after 30 minutes,and even after 7 hours of illumination the PL intensity can be as high as 136%.Most importantly,the X-type ligand-protected perovskites can balance their stability and conductivity.The current in a device fabricated by such materials was stable even after washing with ethanol.However,the current decreased quickly to 1/500 after the same treatment with normal perovskites.Therefore,X-type ligand-protected perovskites can greatly improve their stability and maintain their conductivity simultaneously,and such methods can promote wide applications of high performance perovskites in electronic and optoelectronics devices.(2)We demonstrate a new chemical strategy to synthesize shape-and crystalline phase-controlled perovskiterelated CsPb2Br5 NWs and NSs with tunable morphology and a crystalline phase at room temperature.By changing the ratio of alkyl-thiols to alkyl-amines or alkyl-thiols to alkyl-acids,uniform NSs and NWs with high yield can be obtained.Simultaneously,the crystalline phase can be rationally transformed from orthorhombic-phase CsPbBr3 to tetragonal-phase CsPb2Br5.The innovation points:(?)This is the first report or shape-and crystalline phase-controlled synthesis of perovskite-related CsPb2Br5 nanocrystals.(?)To the best of our knowledge,this is the first report in the literature of the synthesis of tetragonal-phase CsPb2Br5 NWs.(?)This is the first work to directly synthesize perovskite nanocrystals using thiol ligands as the capping ligands.Importantly,our alkyl-thiol-protected CsPb2Br5 NWs and NSs have significantly improved stability in high-temperature and high-humidity environments.(3)We report a facile room-temperature method to direct synthesis of stable tetragonal CsPb2X5 NWs.By changing the ratio of Cl/Br or I/Br,the uniform-morphology compositionally-related CsPb2X5 NWs can be obtained,and their color can be tuned from blue to red.Furthermore,we found that the directly synthesized CsPb2(Cl0.7/Br0.3)5 and CsPb2(I0.7/Br0.3)5 NWs are more stable as compared to the materials obtained by anion-exchange method.Such results may offer more options for the application of stable and color tunable compositionally-related perovskite materials.(4)The nanocrystals of Mn2+ doped tetragonal phase CsPb2Cl5(CsPb2Cl5@Mn)were obtained by direct synthesis at room temperature.By controlling the ratio of lead to manganese ions,different compositions of CsPb2Cl5@Mn can be obtained.The absorption peaks and fluorescence emission peaks were 407 nm and 610 nm,respectively,and remained constant,among which the highest PLQY of CsPb2Cl5@Mn was 13.6%.In a high-hunmidity environment for over 1000 hours,CsPb2Cl5@Mn perfectly maintains optical stability and stability of the crystal phase,and this high stability can promote its wide application in the field of optoelectronics and optical devices.And through ferromagnetic research,it was found that CsPb1.73Mn0.27Cl5 had obvious paramagnetism,but no obvious magnetic change.The anti-ferromagnetic fitting temperature is-2k and-5k,which indicates that the anti-ferromagnetic coupling is strong.(5)We have demonstrated a novel reversible lightmediated reversible compositional and structural transition process from orthorhombic CsPbBr3 to tetragonal CsPb2Br5 nanosheets or vice versa.This study on reversible compositional and structural transitions of cesium lead halide perovskites will further deepen our understanding of their controlled synthesis,post-processing,and decomposition pathway. |
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