The CsPbBr₃-Cs₄PbBr₆ Composite Nanocrystals For Highly Efficient Pure Green Light Emission

The all-inorganic perovskite nanocrystals have recently gained huge attention due to their excellent optical and electronic properties,and it can be applied in various fields such as solar cells,light-emitting diodes?LEDs?,lasers,and photodetectors,etc.In order to assemble high-brightness LEDs,stable quantum dots with high photoluminescence quantum yield?PLQY?still remains challenging.Perovskites can be classified in four crystal structures according to the connectivity of[PbX₆]⁴⁺octahedral,including three-dimensional?3-D?,two-dimensional?2-D?,one-dimensional?1-D?,and zero-dimensional?0-D?lattices.The 0-D perovskites Cs₄PbBr₆-related systems include a complete isolation of individual[PbX₆]⁴⁺octahedra in crystal lattices,resulting in stability superior to those with other dimensionalities.Herein,for a quick comparison,a facile way without inert gas protection has been successfully applied to synthesize pure cubic CsPbBr₃ and hexagonal CsPbBr₃-Cs₄PbBr₆ composite nanocrystals?NCs?with molar ratio of PbBr₂and Cs₂CO₃ modified from 4 to 3.According to size and shape analysis of the NCs observed via transmission electron microscopy,CsPbBr₃ NCs are cube-like shape with an average edge length of 10.2 nm.The CsPbBr₃-Cs₄PbBr₆ composite NCs are hexagon shaped with their average edge length being 14.3 nm and attached small CsPbBr₃nanoparticles of about 4 nm.The optical properties of the synthesized nanoparticles were further characterized by optical testing instruments.The band-edge absorption peak of the perovskite CsPbBr₃ NCs was at 504 nm,and the emission peak was 514 nm with a narrow FWHM of 36 nm,and the band-edge absorption peaks of the perovskite CsPbBr₃-Cs₄PbBr₆composite NCs were at 503nm and 313nm,the emission peak was 519 nm with a narrow FWHM of 33 nm,respectively.At the same time,it is worth noting that the PLQY of synthesized CsPbBr₃-Cs₄PbBr₆ composite nanocrystals was twice as high as the pure phase CsPbBr₃ nanocrystals and up to 74%,and this composite NCs stay photoluminescence properties in solid state.The high PLQY and stability originates from CsPbBr₃ nanocrystals well passivated and protected by the zero-dimensional Cs₄PbBr₆ matrix.The temperature-dependent steady state photoluminescence spectra show that the band gap of synthesized CsPbBr₃ and CsPbBr₃-Cs₄PbBr₆ composite nanocrystals have a blue shift with increasing temperature,and the half width are both increased.Fitting the temperature-dependent FWHM with the mathematical formula quantifies optical-phonon energy affecting PL spectral broadening.In addition to the study of physical properties,we cast two nanocrystals as phosphor powders on a 460nm blue chip to form light-emitting diodes.It was found that the LED fabricated by CsPbBr₃-Cs₄PbBr₆ composite NCs realized pure green emitting,and the Commission Internationale De L'eclairage?CIE?chromaticity coordinates of the fabricated green LEDs was?0.2013,0.7461?,Moreover,the green LEDs displayed luminous efficiency of 35 lm/W under an injection current of 10mA,and it achieve maximum under an injection current of 25mA.The comparison between the green LEDs fabricated with pure and composite perovskites imply that the CsPbBr₃-Cs₄PbBr₆ composites with higher quantum yield could be an effective way to get the brightness and the stability of pure green LEDs.