Organic-inorganic Hybrid Bismuth-based Perovskite Variant:Material Synthesis And Optoelectronic Properties Characterization

At present,in the dual context of environmental pollution and energy crisis,the rational development and utilization of green luminescent materials are particularly important.Recent years,halide perovskites have drawn extensive attention due to their excellent photoelectric properties and do good jobs in applications such as solar cells and light emitting diodes.The newly emerged lead halide?APbX₃,A=MA,Cs;X=Cl,Br,I?perovskite quantum dots?QDs?owns the low-cost synthesis process and continuous tunable band gap by modifications of halide composition and crystal size.Comparing to traditional Cd-based QDs,lead halide perovskite QDs show higher photoluminescence quantum yield?PLQY:⁹0%?,narrower full width at half maximum?FHWH:12²5 nm?and wider color gamut?¹50%NTSC?,making them potential candidates for future lighting applications.However,the toxicity of lead represents a potential obstacle to further utilization.In addition,it's sensentive to achohol and other polar solvent and would even decompose in humid environment.Bismuth,with much lower toxicity than lead,is promising to constitute as a lead-free perovskite materials because Bi³⁺is isoelectronic to Pb²⁺and more stable than Sn²⁺.These features make MA₃Bi₂Br₉ excellent candidates as lighting materials.In this thesis,we mainly investigate the fundamental properties of bismuth-based perovskite variant?MA₃Bi₂Br₉,MA₂BiBr₅,MA₃BiBr₆?and the synthesis of MA₃Bi₂Br₉nanosheets and quantum dots.The following is the main content:?1?Different kinds of single crystals with millimeter size were obtained via a facile and low-cost solution evaporation method at room temperature.We investigated the composition-dependent crystal structures of MA_xBi_yBr_z perovskites variant and their influence over optoelectronic properties.In addition,the thermal stability,optical and electrical properties of these materials were also investigated.These laid a foundation for the synthesis and research of nanomaterials,especially quantum dots.?2?Compared to the bulk phase of semiconductor materials,nanosheets have many unique optical and electrical properties.However,there is rarely researchs about lead-free halide-based perovskite nanostructures.Due to the 2D properties of MA₃Bi₂Br₉,regular,single-crystalized,hexagonal nanosheets can be successfully obtained via facile and low-cost solution methods like solution evaporation method and dissolution recrystallization method.In addition,the dependence of morphology and experimental parameters such as precursor solution concentration were carefully studied.Finally,the phase composition,optical properties of nanosheets were also explored.?3?MA₃Bi₂Br₉ perovskite variant QDs were synthesized by a collaborative solvent ligand-assisted re-precipitation?Co-LARP?method.Raman spectra for each step were used to understand the dissolution mechanism.For MA₃Bi₂Br₉ QDs with an average diameter of 3.05 nm,we observed an emission peak at 430 nm and a PLQY as high as12%.XRD,TEM,PL and TRPL are applied to investigate material and optical properties of quantum dots.Also,the surface of as-synthesized QDs have been analyzed theoretically and experimentally.We can know that the surface is rich of Br and passivated by octyl amine.The PL peaks could be tuned from 360 to 540 nm through halide composition control.In addition,it was also found that the alcohol stability of MA₃Bi₂Br₉ QDs was much better than that of MAPbBr₃ analogues.?4?In order to obtain better optical properties,we have tried to prepare bismuth-based perovskites as quantum dots.Because of the incompatible crystal structures between MA₃Bi₂Br₉ and MA₃Bi₂Cl₉ and the careful kinetic control during the synthesis,Cl^-anions are engineered to mainly locate on the surface of QDs acting as passivating ligands,which effectively suppress surface defects and enhance the PLQY.Via simply changing the ratio of BiCl₃ and BiBr₃ in the precursor solution,we successfully get a Cl-passivated MA₃Bi₂Br₉ QDs with an emission peak at 423 nm,FWHM of 42 nm,especially a remarkable high PLQY up to 54.1%.Finally,stability test was applied to Cl-passivated MA₃Bi₂Br₉ QDs.The results indicated that it has better stability than pure MA₃Bi₂Br₉ QDs,which may due to less defects.