Structural Design Of CsPbX₃ （X=Cl,Br,I） Perovskite Nanocrystals And Their Electroluminescence Properties

Cesium-lead halide perovskite has excellent photophysical properties and is widely used in light-emitting diodes（LEDs）,solar cells,lasers and detectors.By reducing the crystal size and introducing quantum confinement effects,the exciton binding energy of perovskite can be significantly increased and its fluorescence quantum yield can be improved.At the same time,perovskite nanocrystals（NCs）have the advantages of pure color,wide color gamut,and low cost,making them a potential display material.Their research is in line with the national"14th Five-Year Plan"on"New Display and Strategic Electronic Materials"needs.At present,the external quantum efficiency of perovskite LEDs is close to its theoretical limit,and its stability and lead toxicity have become the main factors restricting its commercial application.In response to the above problems,this thesis focuses on the key scientific issue of regulation of the structure of perovskite NCs.Through crystallization regulation,ligand engineering,defect passivation and other means,a series of perovskite NCswith strong fluorescence emission and stability are obtained.Corresponding LEDs have made breakthroughs in external quantum efficiency and device lifetime.This thesis was completed with funding from the National Natural Science Foundation of China（52072141）and the National Key Research and Development Program of China（2022YFE0200200）.The innovative research results obtained are as follows:（1）NC synthesis involves a series of processes such as nucleation,growth,and maturation.Through precursor engineering,we have preliminarily determined the key factors for whether NCscan achieve doping/alloying,that is,the intermediate phase state before explosive nucleation:if it is an ionic state,it will facilitate doping,if it is an ion cluster,it will induce impurity phases.With the help of soft and hard acid-base theory,we used the ratio of ion charge number to ion radius and ion electronegativity as the main parameters to redefine the acidity of cations commonly used in perovskite nanocrystal synthesis.Taking stable and efficient CsPbBr₃ NCsas the research object,we selected three typical cations:soft acid Cd²⁺,boundary acid Zn²⁺and hard acid Ca²⁺,and verified the doping/alloying mechanism through scientific control experiments and system data analysis.In addition,with the assistance of different acidity and alkalinity ligands,we achieved controllable adjustment of the doping or non-doping of the boundary acid Zn²⁺in perovskite NCs.The mechanism proposed in this part of the work contributes to perovskite nanocrystal composition and structure control.（2）Utilizing the main conclusions of part（1）,we constructed a halogen-rich environment in the precursor,controlled the number of crystal nucleations,and achieved precise control of the size of perovskite NCswith thermodynamics,developing pure red-emitting perovskite NCswith a strong quantum confinement effect.Using ZnI₂ in the CsPbI₃ NC system,a strong quantum confinement effect is achieved by changing the balance of halogen ions in the perovskite NCs.Therefore,the photoluminescent spectrum of perovskite NCsgradually shifts from deep red light（～690 nm）to pure red light（～640 nm）.The NCssynthesized by this method can avoid the spectral instability caused by phase separation in mixed halogen（Br/I）compounds.Through ligand engineering,KI passivates the iodine vacancy defect on the surface of CsPbI₃ NCs,and by exchanging tetrabutylammonium iodide with long-chain oleamine,fluorescence resonance energy transfer between NCsis inhibited,resulting in pure red perovskite NCswith stable structures and spectra.A corresponding LED demonstrated excellent spectrum stability,a high brightness of 5934 cd/m²,and an EQE exceeding25%.The device operating stability was significantly improved:at an initial brightness of 107 cd/m²,the LED had an operating life of 1.5 hours.（3）Using the strongly quantum confined NCssynthesized in Part（2）,high efficiency pure red perovskite NCswith low lead content,high efficiency,and stable structure are synthesized.Based on the configuration entropy and degradation enthalpy,the problem of self-purification of NCsis overcome by an entropy driven strategy,and the A and B position co-alloying of perovskite NCsis realized.However,the self-purification effect of perovskite NC excludes doped ions,which makes it difficult to synthesize perovskite NC with low lead content.Theoretical calculation and experimental data show that the multi-alloyed perovskite NCsdriven by entropy not only reduce the lead content by 60%,but also have 100%photoluminescence quantum yield and single exponential decay lifetime without changing the crystal morphology and structure.Using entropy-driven NCsas emitters,an LED is prepared with a stable electroluminescent spectrum that meets Rec.2020 ultra HD display,with peak brightness of～5000 cd/m²,peak EQE exceeds 20%,and the operating lifetime reaches2.5 hours.（4）The core-shell perovskite NCsand their electroluminescent devices with high phase stability were prepared by referring to doping mechanisms and combining crystal structure design with device structure design.Orthorhombic CaI_x/γ-CsPbI₃/CaI₂ NC core-shell structure was synthesized by a seed-assisted epitaxy growth strategy.A first step in the process involves using ultra-small CaI_x nanoparticles as seeds,which both attract the halogen octahedrons centered on Pb,as well as inducing their arrangement to formγ-CsPbI₃ NCs.Lead halogen octahedrons can be prevented from twisting,and lattice strain can be inhibited to enhance their structural stability.Further,the CaI₂ shell isolates the NCsfrom the outside environment,making them more stable under harsh conditions.Optical and electrical characterizations show that the seed/core/shell structure ofγ-CsPbI₃ NC has a shallower defect distribution than the cubic phaseα-CsPbI₃ NC.Through the device structure design,the use of F（?）rster resonance energy transfer（FRET）reduces the electrochemical reaction on the surface of the nanocrystal,so that the performance of the LED device showed a record EQE at that time.Using the seed/core/shell CaI_x/γ-CsPbI₃/CaI₂ NCsas LED emitters,we observed the FRET resulting in a record EQE.The peak EQE is up to 25.3%,the peak brightness exceeds13,600 cd/m²,and the operating lifetime is about 14 hours.The LEDs can be repeatedly illuminated more than 650 times at a brightness of 500 cd/m² without a decrease in brightness,which indicates their significant potential for commercial applications.To sum up,this thesis reveals the crystal growth and doping mechanism in perovskite NCs,and takes into account the material stability and optical properties through the study of ligand engineering,defect passivation,and structure design strategies.A significant improvement in the optical properties of perovskite NCshas led to their use as emission layers for LEDs,resulting in satisfactory results in terms of brightness,efficiency,and operating stability.