Investigation On Ammonium Hexafluorosilicate Passivated CsPbBr₃ Perovskite Nanocrystals

Cesium lead halide(CsPbX₃,X=Cl,Br,I)perovskite nanocrystals have emerged as one of the most promising materials for light-emitting devices owing to the unique characteristics,including tunable emissions,high color purity,and high photoluminescence quantum yields(PLQYs).However,survival of these characteristics in practice is a great challenge due to the instability of CsPbX₃ nanocrystals which often associates with the surface defects,especially the shallow halogen vacancies deriving from the inherent ionic nature and low formation energy.Actually,except their poor stability under environmental stresses,another often-overlooked problem is the serious thermal quenching behavior.Extensive studies have revealed that CsPbX₃ nanocrystals as well as traditional quantum dots suffer from rapid photoluminescence(PL)loss with the increasing temperature.For most luminescent devices,the working temperature is higher than room temperature.Thus,the thermal quenching behavior(temperature dependent luminescence)has severely hindered the practical applications of CsPbX₃nanocrystals.In this thesis,CsPbBr₃ perovskite nanocrystals are studied based on the idea of the bifunctional passivation.After comparing the influencess of hexafluorosilicate acid,potassium hexafluorosilicate and ammonium hexafluorosilicate on the stability of CsPbBr₃ perovskite nanocrystals,we find that a simple surface treatment using ammonium hexafluorosilicate(AHFS,(NH₄)₂Si F₆)can drastically reduce the thermal quenching of CsPbBr₃ nanocrystals while enhance their photostability.The AHFS treated sample sustains 90%of its original emission intensity as the temperature rises to 353 K,which is much better than that(17%)of the pristine sample.Meanwhile,the thermal-stable AHFS treated sample could maintain 93%of its initial PL emission after 450 nm LED illumination of 53 h.The structure,morphology,size distribution,chemical composition,element distribution and chemical state of AHFS treated CsPbBr₃ nanocrystals are analyzed by structural and surface characterizations.We believe that the hydrolysable AHFS absorbed on the surface could lead to a bi-functional passivation for CsPbBr₃ nanocrystals.Finally,we design and prepare the electroluminescent LED device based on the AHFS-treated stable CsPbBr₃nanocrystals,and it has superior performance compared with the LED device based on the pristine CsPbBr₃ nanocrystals.Test results show that the starting voltage decreased from 5 V to 3 V,and the maximum current efficiency,EQE and power efficiency increased by more than 10 times,in contrast to the LED device(0.51 cd/A,0.27%and 0.30 lm/W)based on the pristine CsPbBr₃ nanocrystals,which further verify the optoelectric application value of AHFS treated stable CsPbBr₃ nanocrystals.