Synthesis Of Efficient And Stable Pure Red CsPbI₃ Quantum Dots And Their LED Application

Recently,all inorganic lead halide perovskite（CsPbX₃,X=Cl,Br,I）nanocrystals（NCs）,as a newly developed direct bandgap semiconductor luminescent material,have many advantages,such as high photoluminescence quantum yield（PLQY）,tunable photoluminescence（PL）colors,high color saturation,high exciton binding energy and facile solution processability.Therefore,CsPbX₃NCs are suitable to fabricate of high performance,large area and low cost light-emitting diodes（LEDs）,which have the potential to promote the development of next-generation display and lighting.Since PL color and color purity depend on halide composition,one can easily synthesize pure blue,green,and red perovskite NCs to meet the standards of Rec.2020 as defined by the International Telecommunication Union（ITU）.Whereas,the mixed halide system also suffers from intense phase segregation resulting in color drifts,especially for red perovskite NCs.All inorganic CsPbI₃ quantum dots（QDs）with their adjustable band gap are regarded as ideal materials for pure red perovskite light-emitting diodes（Pe LEDs）.However,the small size pure red CsPbI₃ QDs（CIE（0.708,0.292））are usually prepared at relative low temperature,which induces a series of problems to affect the performance and stability of the CsPbI₃ QDs and LEDs,such as poor size distribution,unstable cubic phase,large number of ligands and high surface energy.At present,the effective ways to solve such problems are to introduce large number of additives or complex ligand-exchange processes.To address such issues,the main contents of this paper are as follows:（1）Pure red CsPbI₃ QDs were isolated by effective low temperature gradient centrifugation,and the stability and performance of pure red LEDs were investigated.High-quality and stable pure red CsPbI₃ QDs can be successfully prepared and separated by a simple temperature depended hot-injection method combined with low temperature gradient centrifugation,with an average size of 5.60±0.05 nm.Pure red CsPbI₃ QDs exhibited 642 nm PL peak with 37 nm full width at half maximum（FWHM）,and their CIE coordinates located at（0.708,0.292）,which matched well with standard pure red of Rec.2020.The PL half lifetime was 210 days under ambient condition with（0.002,0.002）fluctuation of CIE coordinates.Pure red CsPbI₃ QDs LEDs exhibited4.3%maximum external quantum efficiency（EQE）.Electroluminescence（EL）peak located at 645nm,and their CIE coordinates were（0.707,0.290）.Furthermore,the half-lifetime of LED was 9.0min and its color drifts were only（0.002,0.002）after continuous working 15 min with a constant current density of 3.5 m A/cm².（2）The performance and stability of pure red CsPbI₃ QDs LEDs were boosted by short-chain inorganic ligand（NH₄I）post-treatment.The short chain inorganic ligand NH₄I was used for post-treatment of pure red CsPbI₃ QDs,which effectively replaced part of the long chain organic ligand and passivated QDs without affecting the size of QDs.Moreover,the stability of CsPbI₃ QDs can be significantly improved by NH₄⁺passivation.Hole-only devices showed that NH₄I post-treatment significantly reduced the defect states density and increased carrier mobility.Pure red CsPbI₃ QDs LEDs with CIE coordinates of（0.708,0.292）were prepared by NH₄I post-treatment,which matched well with standard pure red for Rec.2020.LEDs achieved a maximum EQE of 9.4%and a maximum luminance of 523 cd/m².Its half-lifetime was 25.1 min and its color drifts were only（0.002,0.002）after 30.0 min continuous working with a constant current density of 3.5 m A/cm².（3）Ca²⁺doping improved the stability of pure red CsPbI₃ QDs and the performance of LEDs.B-site doping of Ca²⁺with small size could increase the tolerance factor（τ）α-CsPbI₃ to improve the crystal phase stability of pure red CsPbI₃ QDs.A blue shift of PL peak,a decreased lattice distances for high resolution transmission electron microscopy（HRTEM）iamge and a higher angle shift of X-ray diffraction（XRD）patterns certified Ca²⁺doping.The Ca²⁺-doped CsPbI₃ QDs LEDs exhibited a maximum EQE of 10.3%and a maximum luminance of 602 cd/m².