Study Of Regulating Carriers Balance Of Quantum Dot Light-emitting Diode

Quantum dot light-emitting diode(QLED)based on the emissive material of colloidal quantum dots(QDs)is the focus of the direction of development in the display field.QLED not only possesses the advantages of organic light-emitting diode(OLED),but also has the characteristics of saturated emission color and excellent solution processability,thus facilitating the application of wide-color-gamut,low-cost,large-area(especially for television)display.However,at present,the imbalance of holes and electrons injection or transport and other problems still exist in QLED,thereby resulting in that the performances of QLED cannot meet the practical requirements.In this paper,we aimed at the problem of carriers imbalance existing in most QLEDs,and proposed research plans based on blocking superfluous electrons transport and enhancing holes transport,respectively.In terms of impeding electrons transport,we,for the first time,used Zn O nanoparticles doped organic small molecular electron transporting materials as the ETL(Electron-transporting layer)of QLED,which improved the luminous efficiency of QLED.For Zn O doped Li Q(Lithium 8-quinolate),by optimizing the doping mass ratio of Li Q,the resulting red QLED with conventional structure exhibited the optimal performances at peak current efficiency(CE)and external quantum efficiency(EQE)up to 8.07 cd/A and 7.74%,which are about 1.93-and 1.94-fold higher than the device with pristine Zn O ETL,respectively.To unveil the origin of the high-performance QLED realized by applying Li Q-doped Zn O as ETL,we carried out in-depth studies in the surface morphology and electronic structure of Li Q-doped Zn O films,the carriers balance of device and the physical effect on the interface between QDs film and Zn O or doped Zn O film.It is found that the dopant of Li Q can indeed suppress the electron transporting property of Zn O effectively and thus balance the carriers.At the same time,doping Li Q into Zn O can partly suppress the exciton quenching effect caused by the direct contact between QDs film and Zn O film,thus resulting in the improvement of QLED’s performances.In addition,we separately introduced the other two familiar organic electron transporting materials of TPBi(1,3,5-tris(Nphenylbenzimidazol-2,yl)benzene)and BPhen(4,7-diphenyl-1,10-phenanthroline)as the dopant to further study.It is found that the behavior of performances enhancement was also reflected in the device based on the dopant of TPBi.As a result,the maximum CE of 6.60 cd/A and the maximum EQE of 6.15%can be achieved by replacing the dopant of Li Q with TPBi,whereas it is not as high as that of the QLED based on the Li Q dopant.The lower performances based on the dopant of TPBi than the dopant of Li Q can be mainly attributed to that TPBi has a weaker effect on the electronic structure of Zn O than Li Q,thus leading to a limitation on the improvement of balance of holes and electrons.Although BPhen can also increase the barrier of electrons injection to block excess electrons injection,it tends to readily crystallize owing to its low glass transition temperature(T_g)and thus makes a negative effect on the quality of Zn O film,which caused the device performances’deterioration but not improvement.Consequently,the T_g of organic small molecular materials should also be paid attention to when we select dopants.Besides,we also conducted a research on the scheme of enhancing holes transport.Be aware of the high hole mobility and deep VBM(Valence band maximum)of Cu SCN,we,for the first time,designed triple HTLs(Hole-transporting layer)consisted of Poly-TPD(Poly(N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)-benzidine))/Cu SCN/PVK(Poly(9-vinlycarbazole))to form a stepwise energy level,thereby facilitating holes injection and transport so that the performances improvement of QLED can be achieved.Based on the introduction of triple HTLs,the green QLED with conventional structure owned the highest CE of 14.89 cd/A and the highest PE(Power efficiency)of 9.04 lm/W,which are 76%and 29.9%higher than the device with single HTL of Poly-TPD and 13.1%and 24.9%higher than the device with single HTL of PVK,respectively.The performances improvement can be mainly attributed to the stepwise energy level formed by the triple HTLs,which effectively reduced the hole injection barrier and thus realized efficient holes injection.Moreover,Cu SCN with high hole mobility also promoted holes transport.In addition,we also studied the effect of the thickness of PVK and Cu SCN in the triple HTLs on the devices’performances.The quenching phenomenon of QDs caused by Cu SCN occurred in our experiments,and this quenching of QDs can be effectively suppressed by inserting a PVK layer with a certain thickness.However,the theoretic quenching mechanism of QDs induced by Cu SCN needs to be further studied as well.