| Quantum dot light-emitting diodes（QLED）based on semiconductor quantum dots（QDs）have been regarded as one of the most potential alternatives in the field of next-generation display and lighting due to a series of excellent luminescence characteristics such as continuously adjustable emission spectrum,high color purity,high luminous efficiency and low energy consumption.In particular,white QLED has been attracted more attention because of its wide application in flat panel display and solid-state lighting.However,the low efficiency,short lifetime,poor color stability,and high driving voltage for white emission have become the main bottlenecks restricting the wider application of white QLED.This is mainly due to that the large hole injection barrier between the QD emitting layer and the organic hole transport in the organic-inorganic hybrid device stuctures with Zn O as the electron transport layer,and the lower hole mobility than the electron mobility of Zn O,resulting in low hole injection efficiency and unbalanced carrier injection;F（?）rster resonance energy transfer（FRET）between different color quantum dots leads to an increased chance of non-radiative recombination;and the difference in efficiency roll-off for different color QDs reduces the color stability.Therefore,it is critical to improve the performance of white QLED by improving the hole injection efficiency and carrier injection balance,suppressing the FRET probability and reducing the difference in efficiency roll-off among different color QDs.Therefore,On the basis of high-quality Zn Cd Se-based core-shell quantum dots,high-efficiency and color stable white QLED has been realized in this paper through precisely adjusting the mixing ratio of different color QDs and combining with the optimization of hole transport layer materials.As well,the driving voltage required for white emission has been significantly reduced.The main contents are the following:（1）Fabrication of white QLED by adjusting the ratio of Zn Cd Se-based dichromatic core-shell QDs.The orange-yellow Zn Cd Se/Zn Se/Zn S QDs with an average size of about 11 nm are synthesized by"low-temperature nucleation,high-temperature long shell"technology,and the introduction of the Zn Se intermediate shell relieved the lattice stress at the core-shell interface,so that the fluorescence quantum yield of the QDs reaches～95%.The corresponding orange-yellow QLED based on Zn Cd Se/Zn Se/Zn S QDs exhibits the maximum external quantum efficiency（EQE）of 21.9%,and the maximum luminance of533320 cd m-2.After that,white QLED is constructed used TFB as the hole transport layer,through systematically optimizing the mixing ratio of orange-yellow and blue Zn Cd Se-based QDs.The results show that the maximum EQE of the device reaches 10.3%,the maximum luminance is 363000 cd m-2when the mixing ratio of blue-yellow QDs is 5:0.2.Correspondingly,the device exhibits a standard white emission with CIE coordinates of（0.32,0.34）at the driving voltage of 5.2 V.Furthermore,PVK with deep HOMO energy level are used as the hole transport layer to fabricate white QLED,aimed to reduce the hole injection barrier,and then improve the hole injection efficiency.The device possesses the maximum EQE of 18.9%with a minimal color variation ofΔCIE of（±0.03,±0.03）in the luminance of 100-160000 cd m-2indicating of high color stability.（2）Construction of high-efficiency and stable color white Zn Cd Se-based QLED.On the basis of small-size Zn Cd Se/Zn Se/Zn S yellow QDs,Zn Cd Se/Zn Cd Se/Zn S core-shell structure QDs with an average particle size of 14.37 nm are synthesized through further regulating the shell structure,aimed to suppress the FRET between different color QDs.The introduction of Zn Cd Se shells further reduces lattice mismatch between core-shell and shell-shell interfaces,making the fluorescence quantum yield of up to 95%.The Zn Cd Se/Zn Cd Se/Zn S-based QLED has the maximum EQE of 23.4%due to the suppression of FRET using large-size QDs as emitting layers.Furthermore,white QLED was constructed by precisely optimizing the mixed ratio of the large-size yellow QDs and blue QDs.The corresponding device shows the EQE of21.0%.In particular,the results of transient fluorescence spectroscopy characterization suggest that the FRET probability in the mixed film based on the large-size（14.37 nm）Zn Cd Se/Zn Cd Se/Zn S QDs was only 18%,which was decreased by～70%compared with that the small-size（11 nm）Zn Cd Se/Zn Se/Zn S QDs-based mixed film.However,the drive voltage for white emission is as high as 5 V.Moreover,high-efficiency and color-stable white QLED operating at low driving voltage has been achieved by controlling the hole transport layer.The results show that when PF8Cz is used as the hole transport layer,the maximum EQE white QLED reaches up to 28.8%,which is the recorded value.Remarkably,the device possess an excellent color stability with a minimal color variation ofΔCIE of（±0.03,±0.03）in the luminance of 100-450000cd m-2.As well as,the device exhibits a standard white emission with CIE coordinates of（0.32,0.36）at an extremely low input voltage of 3.6 V.The improvement of device performance is attributed to that PF8Cz as the hole transport layer improves the hole injection/transport,promoting the carrier-injection balance,and reducing the difference in the efficiency roll-off between the dichromatic QDs. |
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