High-performance Red Quantum Dot Light-emitting Diodes Based On Organic Materials Regulating Carrier Balance

Colloidal quantum dots(QDs) have prominent advantages such as high color purity,tunable emission spectra,high photoluminescence quantum yield(PLQY)and excellent intrinsic stability.It has attracted wide attention in the field of flat panel display.In recent years,quantum dot light-emitting diodes(QLEDs)have made some progress in luminescence efficiency and lifetime,but there is the problem that the electron injection is more intensive than the hole injection in QLED,which restricts the commercialization of QLEDs.Compared with inorganic materials,organic materials have a wide variety and are prepared by solution processed mostly,which has better film-forming property.Therefore,aiming at the carrier injection imbalance,we introduced 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine(26DCz PPy)with deep highest occupied molecular orbital(HOMO)level and the amphiphilic poly[(9,9-bis(3'-(N,Ndimethyl)-N-(ethylammonium)-propyl)-2,7-fluorene)-Alt-2,7-(9,9-dioctylfluorene)]dibromide(PFN-Br),and utilized the two organic materials to regulate the carrier balance from the two aspects of enhancing hole transport and blocking excessive electron transport.Firstly,we introduced a small molecule hole-transport material with deep HOMO level26DCz PPy in the common polymer hole transport material poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(p-butylphenyl))-diphenylamine)](TFB)with different mixing ratios to fabricate red QLEDs based on Cd Se QDs.The device structure is ITO/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)/TFB:26DCz PPy/QD/Zn O/Al.It was found that when the blend mass ratio of TFB:26DCz PPy was 7:3,the maximum luminance(L_max)of the device reached 153605 cd/m²,which was 21%higher than that of the standard device(TFB was used as the hole transport layer(HTL)).Moreover,the maximum current efficiency(CE_max)of the device was 19.3 cd/A,which was 25%higher than that of the standard device(15.4 cd/A).The turn-on voltage of the device decreased from 1.8V to 1.6V,making the maximum power efficiency(PE_max)of the device increase from 18.8 lm/W to 26.8lm/W,which was increased by 42%.More importantly,the T₅₀lifetime of the device was increased from 14.5 h to 56.4 h,which was four times longer than that of the standard device.After analyzing the characterization of the film morphology,energy level and single-carrier device,we found that the improvement of device performance was mainly attributed to the reduction of film roughness,which was conducive to interface contact and enhanced current injection.At the same time,the deep-HOMO-level material 26DCz PPy combined with high-mobility material TFB as HTL can improve the hole injection efficiency greatly and make the carrier injection more balanced in the device.Thus,the impacts of non-radiative recombination caused by excessive electron injection and high operating voltage of the device can be alleviatedSecondly,water/alcohol-soluble organic material PFN-Br is an amphiphilic interface material,which can not only form interface dipole layer,but also make the film surface smooth er and more uniform.Therefore,aiming at the electron transport layer(ETL)of red QLEDs,we introduced the organic material PFN-Br in Zn O to study the effect of this material on the device performance.The device structure was ITO/PEDOT:PSS/PVK/QD/ETL/Al in QLEDs based on four different ETLs,including Zn O,Zn O/PFN-Br,PFN-Br and PFN-Br/Zn O respectively.It was found that when PFN-Br/Zn O acted as ETL,the CE_max of the optimized device reached26.7 cd/A,which was nearly 50%higher than that of the standard device(ETL was used as Zn O).In addition,the luminance and PE of the device were also improved,while the device performance employing the other ETLs decreased compared with the standard device.The performances improvement can be mainly attributed to the PFN-Br thin layer,it can not only act as a physical buffer layer to protect the QDs layer from the impacts of the Zn O layer,but also act as an interface dipole layer to enhance the energy band of the QDs layer,so as to facilitate hole injection and block excess electron injection better.Moreover,the insertion of PFN-Br can make the QDs film surface smoother.Furthermore,we studied the device performance based on Zn O:PFN-Br ETL.When the mass ratio of Zn O:PFN-Br was 100:2,the CE_max of the device was 19.7 cd/A,which was slightly larger than that of the standard device(18.3 cd/A).Moreover,the luminance and PE of the device based on Zn O:PFN-Br were not improved.This indicated that the contribution of PFN-Br as a dopant to the device performance improvement may not be obvious,which may be related to the poor electron mobility,the energy level alignment of PFN-Br and the inability to forming interface dipole.Therefore,the work mechanism about PFN-Br still needs to be studied further.