High Performance Quantum Dot Light-emitting Diodes With Balanced Charge Injection And Transport

Colloidal quantum dot light-emitting diodes(QLEDs)have attracted great attention owing to their attractive properties,such as tunable colors,saturated emission,narrow full-width at half-maximum(FWHM)and a simple fabrication process,which highlight their potential use in next generation flatpanel display and solid-state lighting technologies.High efficiency,stability red,green and blue QLEDs play a key role in QLEDs technology commercialization.Efficient electron-injection into the emitting layer(EML)is very important in the fabrication of high performance QLEDs.To enhance the charge balance for the high device performance,the selection of functional layer materials and the design optimization of the device structure are investigated respectively in the normal and inverted devices.We controlled the concentration,average size and stability of zinc oxide(Zn O)by optimizing the synthetic method and characterizing the photo-physical/chemical properties of quantum dots(QDs),such as the UV-absorption,photoluminescence(PL),time-resolved photoluminance decay and so on.In addition,the solvent of QDs and the thickness of Zn O layer were also investigated.It is found that the maximum external quantum efficiency(EQE)of the fabricated red QLED device reached as high as 8.9%when the solvent of QDs is n-hexane and the thickness of Zn O is 70 nm.On this basis,the conductivity of the hole transporting layer(HTL)in QLEDs was further enhanced by employing the p-doping strategy.The Li-TSFI-doped HTLs enhance the hole mobility,and then balance the carrier injection,which can contribute to the lower turn-on voltage,higher brightness,and higher efficiency of our fabricated devices when compared with the reference device.Impressively,with an optimal doping ratio of 3.0 wt%,the current efficiency of the QLEDs device can reach as highas 15.5 cd/A.Correspondingly,the EQE(11.46%)was enhanced by nearly two times when compared with that found for the reference QLEDs device(6.58%).Besides,we developed the inverted blue QLEDs devices with modified electron transporting layer(ETL)by employing polyethylenimine ethoxylated(PEIE)to decrease the electron-transporting barrier.Therefore,the efficient electron-transport improves the exciton recombination into the emitting layer and leads to higher device efficiency.Impressively,with the doping ratio of 0.1 wt%,the current efficiency of the optimal QLEDs device can reach as high as 3.1 cd/A.Correspondingly,the EQE(7.85%)was enhanced by 1.4-fold when compared with that of the reference QLEDs device(5.68%).