| Quantum dots(QDs)have been considered as the most potential light-emitting material for the next-generation flat panel displays and solid-state lightings,due to their high photoluminescence(PL)quantum yield(QY),high color purity,size-controlled turnable emission spectra,high photochemical and thermal stability,and solution-processed synthesis.Since the first demonstration of QD-based light-emitting diodes(QLED)in 1994,their performance(including the luminance,external quantum efficiency,and even operation lifetime)has been steadily improved to meet the requirements for low brightness displays(Indoor displays).However,the short operation lifetime under high brightness still limits the application of QLED in outdoor displays and lightings.So far,the only work that has achieved a long lifetime under high brightness has been based on red QLED.In contrast,there have been no corresponding reports for green or blue ones.It is mainly due to a long-standing problem that compared with red QLED,the larger energy level offset between the emitter and hole transport layers in green and blue devices based on the organic-inorganic hybrid device structures(organic materials used as hole transport layers,and inorganic oxides(especially Zn O)used as electron transport layers)impedes the hole injection,causing the electrons accumulation at this sharp step interface,and eventually resulting in the the efficiency roll-off at high currents and device damage.An obvious route to solve this issue is searching for new hole transport materials,however,it is great challenge to design hole transport materials simultaneously with a high hole mobility and perfectly matched hole energy level.Another strategy is,in turn,to prevent over injected electrons by changing electron transport layers.Unfortunately,while it can improve the carrier-injection balance,it will also weaken the total carrier-injection efficiency and thus require a compromise between high efficiency and high luminance.Recently,considerable efforts have been devoted to improve the energy level matching with the highest occupied molecular orbital(HOMO)of the hole transport layers by optimizing the shell materials of QDs.Based on the above principle,this work focused on improving the hole-injection efficiency by controlling the structure of QDs,and then fabricating green QLED with high efficiency and long lifetime.(1)Synthesis of high-quality CdZnSe/ZnS core/shell structure QDs and applications in QLED.CdZnSe/ZnS core/shell QDs were synthesized via the method of “low-temperature injection and high-temperature growth”.This method enables an alloyed layer to form in the interface between the QD cores and the shell layer,thereby reducing lattice defects between the interfaces,and then increasing the PL QY.The experimental results show that when the ration of Cd/Zn is 1:12 within CdZnSe cores and the thickness of the ZnS shell is 10 monolayers(ML),the PL QY of as-synthesized CdZnSe/ZnS QDs is as high as 95% and the average size is 10.5 nm.As well,its PL QY still remained > 85% after exposure to UV light(365 nm,8 W)for 7 days,indicating of good photochemical stability.Based on CdZnSe and CdZnSe/ZnS core/shell QDs with different shell thickness,devices were fabricated.The maximum external quantum efficiency(EQE)of 18.96% can be achieved at the luminance of 2540 cd/m2 for devices based on CdZnSe/ZnS QDs with 10 ML of ZnS shell.As well,EQE can maintain more than 17% in the range of luminance of 700-12470 cd/m2,which indicates that the efficiency roll-off has been suppressed to some extent.This excellent property is attributed to the appropriate shell material and thickness,which can effectively suppress the non-radiative F?rster resonance energy transfer and Auger recombination.(2)Synthesis of CdZnSe/ZnSe/ZnSeS/ZnS gradient alloy QDs and applications in QLED.Based on CdZnSe/ZnS QDs,ZnSe/ZnSeS as the intermediate shell was introduced into CdZnSe/ZnS to form CdZnSe/ZnSe/ZnSeS/ZnS gradient alloy QDs.The QDs have a PL QY of up to 97% and a single PL decay channel.Correspondingly,the peak EQE is as high as 23.9% and the current efficiency is 100.5 cd/A for CdZnSe/ZnSe/ZnSeS/ZnS-based QLED.More importantly,EQE of this device still maintains 86% of its peak value at the brightness of 5000 cd/m2.These results indicate that the efficiency roll-off under high brightness has been suppressed well through the customization of the QDs shells.Remarkably,CdZnSe/ZnSe/ZnSeS/ZnS-based devices exhibit a long lifetime of T95 > 2500 h at the initial brightness of 1000 cd/m2 and T50 > 1,655,000 h at the initial brightness of 100 cd/m2.This is the best performance for the all solution-processed green QLED to date.Such excellent performance is mainly due to the customized shells,which makes the valence band of the QDs emitting layers matched well with the highest molecular orbital(HOMO)of hole transport layers,and then improves the hole-injection efficiency and the carrierinjection balance at the high current density. |
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