Fabrication Of Light-emitting Diodes With Low Efficiency Roll-off Via Controlling The Composition And Surface Lignds Of Quantum Dots

The solution-processed colloidal quantum dots?QDs?are characterized by high color purity,size-controlled tunable emission spectra,high photoluminescence quantum yield?PL QY?,high thermal stability and photochemical stability,which open up the huge potential applications biomedicine,photodetection,new energy and information display.In particular,quantum dots based light-emitting diodes?QLED?have gained more attention.Recently,the performance of the device has been greatly improved,especially the device performance of red and green QLED has been comparable to state-of-the-art OLED.This is due to the development of synthesis technology of quantum dots,the optimization of device structure,and the deep understanding of physical mechanisms.However,so far,the reported high-performance QLED devices have a maximum external quantum efficiency?EQE?that decreases rapidly with increasing current density?J?or luminance?L?,which is commonly so-called efficiency roll-off.The roll-off effect limits practically achievable brightness levels of the device and leads to excessive heat generation,which increases power consumption and reduces device longevity.Thus,it has become a key factor that restricts the application of QLED in practice.The corresponding studies have shown that the main reason for the efficiency roll-off is non-radiative Auger recombination,which is resulting from the following two factors:One is the interface defect caused by the excessive lattice stress accumulated between the core and the shell;the other is the electron and hole injection imbalance.Therefore,in order to suppress Auger recombination,the alloyed quantum dots with a smoother interface barrier can be used,and electron and hole injection efficiency and injection balance can be improved by surface ligand regulation of the quantum dot light-emitting layer.As a result,the work in this paper is given as follows:?1?Fabrication of low efficiency roll-off QLED based on the composition regulation of CdSe_1-xS_xQDs.Because the gradient alloyed QDs have gentle interface barrier,the interface defects caused by lattice mismatch can be effectively suppressed.Therefore,the Auger recombination inside QDs can be suppressed by controlling the compositional.The experimental results show that the PL QY of CdSeS QDs with S:Se=8:1 is as high as 91%,with high color purity?full width of half maximum?FWHM?=29 nm?,good stability and single-channel PL decay dynamics.More importantly,when S:Se is 8:1,the Auger lifetime reaches 150 ps,which is 3-5 times of that of S:Se=3:1 and 15:1.Then,we studied the effect of different components of CdSeS QDs on the efficiency roll-off of the device.The results show that the maximum EQE and the maximum brightness of QLED devices can be up to 14.3%and 91,540 cd/m² when S:Se is 8:1.As well,the efficiency can still be maintained above 70%of the peak efficiency in the range of 845-38,900cd/m²,and the efficiency roll-off occurs at the current density of 220 mA/cm².However,QLED with other S:Se ratios show an efficiency roll-off at the lower current density and brightness.Therefore,when S:Se is8:1,the roll-off effect of the device can be suppressed to some extent.This excellent performance of QLED based on alloyed QDs is due to the elimination of lattice mismatch and lattice compensation caused by defect compensation by proper regulation of the composition of the alloyed QDs,which inhibits the non-radiative Auger recombination,improves the efficiency of exciton radiation recombination,and thus suppresses the efficiency roll-off of the device to some extent.?2?Fabrication of high brightness and low efficiency roll-off blue QLED based on ZnCdSeS QDsmodified with n-propyl mercaptan ligands.Since the insulation of the long-chain oleic acid ligand of the QDs increases the transport resistance of the carrier in the light-emitting layer,the alkyl chain of the n-propyl mercaptan ligand is much shorter than the oleic acid ligand that can improve carrier injection efficiency.Moreover,the thiol as an electron-donating group could shift up the valence band energy level of the QDs,thereby reducing the hole injection barrier and improving the hole injection efficiency.The experimental results show the significantly improved electron and hole injection efficiency of the device after the exchange of n-propanol ligand.and the hole injection efficiency increased to a greater extent.Especially,at a voltage of 5-7 V,the electron-and hole-injection balance increased by 1.7-4.9 times.Because of the improvement of the charge-injection balance,the maximum brightness of the device was52,360 cd/m²,and the maximum of EQE of 9.9%and the current efficiency of 11.4 cd/A can be obtained at the luminance of 1,040 cd/m².It is worth noting that the efficiency can still be maintained 75%of the peak efficiency in the range of 100-10,000 cd/m².In addition,the efficiency roll-off of the device was increased to 540 mA/cm² after ligand exchange,compared to the QLED before ligand exchange?efficiency roll-off occurred at a current density of 155 mA/cm²?.Therefore,the highly bright and low efficiency roll-off blue QLED is ascribed to the short-chain n-propyl mercaptan as the ligands enhance the charge-injection efficiency and the charge-injection balance at the high voltage.