Size And Surface Design Of Perovskite Nanocrystals For Improving The Performance Of Light-Emitting Diodes

As an emerging type of ionic direct bandgap optoelectronic material,metal halide perovskites exhibit excellent optoelectronic properties,such as high carrier mobility,wide colour gamut and high colour purity.Moreover,they possess the advantages of inorganic and organic semiconductor materials in terms of solubility,large-area fabrication,and low cost.Metal halide perovskites are considered as ideal materials for achieving low-cost,high-brightness,large-area,and flexible next-generation lighting and display technologies,with significant application potential.In recent years,significant progress has been made in the performance of perovskite light-emitting diodes(PeLED)through the tremendous efforts of researchers.One of the critical indicators,external quantum efficiency(EQE),has been significantly improved.The EQE of PeLED emitting in red,green,and nearinfrared wavelengths has already exceeded 20%.However,despite the great improvement in EQE of PeLED in a short time,many problems remain to be solved for these devices.For example,most nanocrystal based PeLED face emission line broadening due to strong quantum confinement effects and have a large amount of surface-insulating ligands,resulting in emission spectra with low colour purity and low device efficiency.Meanwhile,the development of blue PeLED,one of the three primary colours,has been slow,with low brightness,efficiency,poor spectral stability,and short lifetimes,severely limiting their applications in high-performance,widecolour-gamut displays and high-colour-rendering-index white lighting.To address these problems,this paper reported a reliable strategy for constructing highly efficient and stable green and pure blue PeLED by regulating the nanocrystal size through ligand molecule design and repairing the grain surface defects.The main contents of the study are as follows:1.We successfully synthesized weak quantum confinement effect CsPbBr3 nanoparticles with an average size of 18 nm and used tributylphosphine oxide and a calcium ion complex(TBPO-Ca)as ligands to passivate the surface defects of the nanoparticles,resulting in CsPbBr3 nanoparticle solutions with excellent colloidal stability and high PLQY.Benefiting from this passivation strategy,we obtained dense and thermally stable CsPbBr3 nanocrystal thin films.The PeLED based on this film emitting at the 520 nm wavelength has a half-maximum full width(FWHM)of 16.4 nm and an EQE of 17.25%,demonstrating high efficiency and colour purity.2.We prepared Ca(BF4)2 and dissolved it in a nonpolar solvent with the assistance of tributylphosphine oxide to form a complex.As a pseudo-halogen salt,Ca(BF4)2 has a tetrafluoroborate ion that can replace the halide ion and passivate the surface defects of CsPbBr3Cl3-x nanoparticles.The use of this complex replaces the long alkyl chain insulating ligand on the nanocrystal surface which increases the colloidal stability of the solution and blocks the migration path of halide ions.Benefiting from this passivation strategy,we obtained dense,thermally stable,and spectrally stable CsPbBr3Cl3-x nanocrystal thin films.The PeLED based on this film emitting at a peak wavelength of 467 nm had an EQE of 3.2%.Due to the pseudo-halogen passivation strategy,this PeLED achieved high-efficiency pure blue light emission and remained spectrally stable even under long-term or high-voltage operation.