Boosting The Optoelectronic Properties And Stability Of CsPbBr₃ Nanocrystals Facilicated By Polycarboxylic Acid Ligands

Cesium lead-halide perovskite（CsPbX₃,X=Cl,Br,I）nanocrystals（NCs）have good prospects in the field of lighting and display due to their high color purity,high fluorescence quantum yields（PLQY）,high defect tolerance,and tunable emission.Currently,the maximum external quantum efficiencies（EQEs）of red,near-infrared and green perovskite light-emitting diodes（LEDs）have exceeded 20%,which is comparable to commercial organic light-emitting diodes.However,the poor stability of perovskites limits perovskite LEDs for practical application and promotion.CsPbBr₃ is generally used to fabricate green LEDs.The ligands in common methods（hot-injection or ligand-assisted reprecipitation methods）to synthesize CsPbBr₃ NCs are long-chain oleic acid（OA）and oleylamine（OLA）,which are easy to fall off to form defects of NCs.Furthermore,current methods are inevitable to use toxic solvents to fabricate CsPbBr₃ NCs LED.Based on the above issues,this thesis carried out mainly on the following aspects:（1）Introducing citric acid as a polycarboxylic acid ligand for post-treatment of CsPbBr₃NCs improved their fluorescence（PL）performance,stability and film formation.CsPbBr₃ NCs were synthesized by low toxic solvent method.As compared to several of polycarboxylic acid ligands,using suitable citric acid for post-treatment could dramatically enhanced the PLQY of CsPbBr₃ NCs from 41%to 75%,and had less influence to their PL position,phase and morphology.Then the passivation mechanism was studied by Fourier transform infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）.According to FTIR and XPS results,we confirmed that citric acid removed excess tert-butylamine hydrobromide and less CsBr was generated to passivate NCs.Additionally,the strong affinity between citric acid and perovskite inhibited the PLQY degradation resulting from the falling off ligands in the purification processes.Furthermore,the spin-coated citric acid-passivated NCs exhibited better film-forming ability and stability（50 RH%）.At 50 RH%,citric acid-passivated NCs film only decreased 21%PL intensities,while the unpassivated NCs film decreased 72%.（2）Employing polystyrene（PS） to fabricate citric acid-passivated CsPbBr₃ NCs thin films boosted stability in extreme conditions,such as polar solvents,high temperature,and UV irradiation.Adding polymers to encapsulate perovskite NCs can isolate perovskite from water and oxygen,thereby improve their stability to meet the need of commercial phosphor-converted light-emitting materials.We synthesized CsPbBr₃ NCs via hot-injection method to prepare CsPbBr₃NCs@PS films as the comparison sample.Both two samples were exposed to UV light,high temperature（85°C）,water or ethanol,and we periodically monitored their PL intensities evolution.The results demonstrated that citric acid-passivated CsPbBr₃ NCs thin films had improved UV,thermal and polar solvent stability.In order to deeply understand the mechanism,FTIR was employed.The results demonstrated that the citric acid ligand can greatly reduce ligand loss even after 3 times washing processes.Therefore,citric acid strongly affinitied to surface passivates NCs with high PLQY and reduces the dissolution and fusion of CsPbBr₃ NCs in such extreme conditions（3）Citric acid-passivated CsPbBr₃ NCs were used to fabricate green LED.The citric acid passivated CsPbBr₃ was used as the luminescent layer.In order to decrease surface ligand to improve the layer conductivity,we firstly optimized the purification processes.Additionally,PS was introduced to further improve the film formation properties of CsPbBr₃ NCs.The single-hole device characterization showed that citric acid passivation reduced the defect densities of CsPbBr₃ NCs film.Further,the conductivity and hole transport efficiency of poly（3,4-ethylenedioxythiophene）:poly（styrene sulfonate）（PEDOT:PSS）layer were also improved by adding dimethyl sulfoxide（DMSO）.Finally,a maximum EQE of 1.56%and a maximum current efficiency of 4.7 cd/A were achieved by further adjusting the thickness of luminescent layer.The T₅₀ of the LED was 2.5 minutes at a current density of 1.05 m A/cm².