Research On Mixed Halide Blue Perovskite Nanocrystals And Their Light Emitting Diodes

As a new type of semiconductor,halide perovskite is widely applied in optoelectronic devices.In terms of light emission,halide perovskite has a high color purity and narrow-spectrum lighting（the full wavelength half maximum is about 100meV）.It has a wide color gamut,which can cover 140% of the standard of the International Television Display System Luminescence Committee.It has tunable luminescent band gap（1.55-3.03 eV）,which wavelength can cover the entire visible regions with the change of halogen ions.It has good carrier transport properties,flexible fabrication methods,and low cost.Based on those advantages,metal halide perovskites have shown great research potential in luminescence,color display,and fluorescence imaging.Perovskite nanocrystals serve as good light emitters,among which red and green perovskites achieve perfect fluorescence emission and high-performance light-emitting diodes（LEDs）.However,the development of blue-light perovskite is slow,which is due to wide luminous bandgap,low fluorescence performance,poor material stability,insufficient exciton binding energy,many defect sites,and mismatched device energy level structure.However,the research on the effect of defects in mixed halide perovskite nanocrystals on non-radiative recombination is still insufficient.The defects produced by the mixed-halogen blue light-emitting perovskite nanocrystals cause luminescence quenching,causing lattice structure distortion,phonon scattering,mixed-halogen ion migration,and phase separation.Therefore,it is particularly important to understand the defects from the root and develop high-performance mixed-halogen blue perovskite nanocrystals for the development of blue LEDs.Exploring the root of the luminescence loss of blue-light perovskite nanocrystals has become the core issue of blue-light perovskite modification.Based on this,the research is carried out around the repair of local defects in the structure of perovskite nanocrystalline materials,lattice frame regulation and surface reconstruction.The detailed researches are as follows:（1）Monovalent Cu ions weaken halide ion lost and reduce localized defect states in the crystal structure.Metal ion doping method reduces the halogen defects,which cause by localized lead（Pb）excess,improves lattice stability,and reduces non-radiative recombination.By reducing the vacancy defects during the synthesis process.The luminous efficiency of blue-light perovskite nanocrystals is effectively improved,and the PLQY after purification is increased from 21%to 45%.Through steady-state time-resolved spectroscopy and transient absorption spectroscopy analysis,it is proved that reducing the localized state defects of perovskite nanocrystals to reduce non-radiative recombination.The introduction of Cu⁺has a slightly effect on the energy level structure of perovskite nanocrystals,lowering the energy level barrier for hole injection.In this regard,the blue LED device has also obtained good device performance.The maximum external quantum efficiency of the optimal device with Cu⁺doped Cs Pb Cl Br₂ has reached 3.78%,and the maximum brightness of 1578 cd m^-2.In addition,the corresponding blue light LED stability of the electroluminescent spectra of the device is also improved.（2）Formamidine cation（FA⁺）regulates coordination octahedra to improve in-band electron filling and decrease non-radiative recombination to achieve bright blue light nanocrystals.A room temperature synthesis strategy is developed to realize multi-cation regulating light emission and explore the mechanism.Here,we realized formamidine acetate（FAAc）as a precursor to prepare high-performance pure blue-light perovskite nanocrystals with mixed halide ions.Introducing organic cations with a large ionic radius improves the stability of the perovskite lattice framework,strengthens the interaction between the coordination octahedron and the A-site ions,so suppresses the formation of defects.Through the introduction of FA⁺,the blue light emission of perovskite nanocrystals can be manipulated and the photoluminescence quantum yield raised from 12%to 65%.Transient absorption and first-principle calculations are introduced to understand mechanism of the organic A-site doping.The cations can regulate the electronic filling state of perovskite nanocrystals,accelerate the hot carriers cooling,reduce the electron-phonon scattering of perovskite nanocrystals,and also play a role in the energy level structure.For blue LED devices,the blue perovskite nanocrystal LED has excellent electroluminescence performance for the maximum external quantum efficiency of reaching 5.01%,the maximum brightness of 1452 cd m^-2,and the working life of the device prolonging 1050 s.（3）Ligand passivation of surface existent defects suppresses light loss and enhances spectral stability.The light emitting effect of the surface structure defects was studied by ligand post addition.And the intrinsic of surface defects and relation with the nonradiative recombination and photon loss need to be explored.In order to further realize the high-performance luminescence of perovskite blue quantum dots,octylamine bromide（Oct Br）ligands with surface activity and low polarity were synthesized to study the surface of nanocrystals.The ligand compensates the halogen vacancies,which generated during the purification of perovskite nanocrystals.Through the filling of halogen vacancies and increasing the interaction of surface ions o with the ligands,the surface structure of the perovskite nanocrystals was stabilized.The synthesized Oct Br was used to modify the surface defect of the nanocrystals to weaken electron-phonon coupling.The PLQY of the modified perovskite nanocrystals reaches95%.This obvious enhancement is due to the suppression of non-radiative recombination on the surface,and it is also accompanied by the reduction of Urbach energy.In addition,the corresponding deep blue perovskite nanocrystal LED achieves a maximum external quantum efficiency and brightness of 2.42%and 264 cd m^-2,respectively.The pure blue perovskite nanocrystal LED device also achieves a good light emission.Moreover,its corresponding electroluminescence spectra realize excellent stability.The results demonstrate that the photon loss and ion migration induced by the mixed-halide perovskite surface can be effectively suppressed by nanocrystal surface reconstructionTo summarize,from the perspective of non-radiative recombination in mixed-halide perovskite nanocrystals,we explored the origin of non-radiative recombination and achieved bright blue emission by tuning the structure and surface of the nanocrystals.This study can also provide a reference for the further development of high-performance blue perovskite LEDs.