Ligand Engineering And Photoelectric Performance Study Of Cesium Lead Halide Blue Perovskite Nanocrystals

All-inorganic cesium lead halide perovskite nanocrystals show good application prospect in the field of light-emitting diode(LED)due to their narrow excitonic emission band,high photoluminescence quantum yields(PLQYs),low trap density and high color purity.Up to the latest report,the maximum external quantum efficiency(EQE)of near-infrared,red and green-emitting perovskite nanocrystals-based LED devices has exceeded 20%.However,the development of blue-emitting perovskite LED is far lag behind those of their counterparts,which seriously restricts the application of perovskite in solid-state lighting and panchromatic displaying.Research shows that the key to improve the performance of LED is to develop high performance blue perovskite nanocrystals and construct balanced carrier transport device structure.Due to the large specific surface area of perovskite nanocrystals,the surface state of perovskite nanocrystals has a significant impact on the quality of nanocrystals,thus the surface ligands play a leading role in its physicochemical,optical and electrical properties.Therefore,passivating the surface defects of perovskite nanocrystals by adjusting the surface ligands is considered to be an effective way to achieve efficient perovskite LED.In this paper,we focus on the topic of developing high-performance blue perovskite nanocrystals-based LED,all-inorganic CsPb(Br/Cl)₃ nanocrystals are selected as the research object,the surface ligand regulation and device structure optimization are taken as the breakthrough points to obtain blue perovskite nanocrystals with highly luminescent and effective charge transfer properties and LED with balanced carrier transport property.Finally,blue perovskite nanocrystals-based LED with high efficiency and stability is realized.The details are as follows:(1)By introducing organic ammonium salts with different carbon chain lengths during the preparation of CsPb(Br/Cl)₃ nanocrystals by room temperature ligand-assisted-reprecipitation method and applying them to LED devices,we investigated the effects of three organic ammonium salts of DDe AB,DDAB and DTAB on the optical properties of CsPb(Br/Cl)₃ nanocrystals and the electrical properties of perovskite LEDs.It is found that the CsPb(Br/Cl)₃ nanocrystals modified by DDAB can achieve a good balance between optical and electrical properties,thus exhibiting the best device performance,which is shown as the maximum EQE of the device is 0.23%,and the maximum luminance is29.31 cd/m².(2)Different concentrations of K⁺were introduced into DDAB-based CsPb(Br/Cl)₃nanocrystals to investigate their effects on the structural,morphology and optical properties of the resulting nanocrystals.The test results show that the introduction of K⁺can effectively improve the crystallinity,enhance PLQY and prolong PL lifetime of perovskite.Through a variety of characterization methods to explore the passivation mechanism of K⁺,we claim that K⁺can effectively passivate the surface defects of perovskite by bonding with halide anions to form a new kind of metal ligands adsorbed on the surface of nanocrystals.(3)Through preparing hole-only and LED devices,we discovered that the introduction of K⁺can effectively improve the charge transport ability of the perovskite nanocrystals.When the K⁺concentration of is 4.0%,nanocrystals with high luminescent and effective charge transfer properties can be obtained.Specifically,the maximum EQE of the resulting LED is increased from 0.23%to 1.19%.With further engineering of the device structure,a stepwise co-hole transport layer of poly-TPD/PVK is introduced.Owing to the reduced hole injection barrier and balanced charge injection,the device performance is further improved.Finally,an efficient and spectrally stable blue perovskite LED with a maximum EQE of 1.96%is obtained,which was second only to the highest value reported at that time.By combining surface ligand regulation and device structure optimization,a series of methods to improve the device performance based on blue perovskite nanocrystals were proposed,which laid the foundation for the development of high-performance blue perovskite LED.