Interface Engineering For Quantum Dot Light-emitting Diodes

Colloidal quantum dots(CQDs)have been widely applied in the field of optoelectronics due to the unique size-dependent optical property,narrow emission peaks,high color purity,high fluorescence quantum yield,and the strong chemical stability.Colloidal quantum dot lightemitting diodes(QLEDs)have become a novel kind of display devices,suitable for the fabrication of the large area,wide color gamut,ultra-thin,and flexible display.The research of this thesis is work for improving the performance of QLEDs,and mainly studied the carrier dynamics at the heterojunction interface of QLEDs.The methods of interfacial modifications and in situ ligand exchanges were utilized to control the carrier transport behaviors of ZnO and QDs,respectively,which regulated the ability of the electron injection to fabricate the high-efficiency and color-purity QLEDs based on red and green CdSe/ZnS QDs and blue CdZnS/ZnS QDs.The research work of this paper is mainly carried out from the following two aspects:In order to address the difficulties of the electron injection in blue wide-bandgap CdZnS/ZnS QLEDs and harmful effects on the spectral quality that ZnO surface defects have,a kind of water/alcohol soluble interface materials,polyethyleneimine(PEI)was employed as the cathode interface layer.PEI layer improved the electroluminescent efficiency,color purity and the stability of blue QLEDs with the inverted structure,leading to the increased maximum current efficiency of 2.5 cd/A and the improved highest external quantum efficiency of 5.5%,which is increased by more than 50%.By characterizing the changes of surface potential with surface kelvin probe microscope,it's found that PEI can effectively reduce the cathode-side electron injection barrier and improve the electron injection.Meanwhile,the spontaneous charge transfer at QD/ZnO interface was found to be weakened by utilizing the time-resolved PL dynamics,maintaining a better electric neutrality of quantum dots and raising the combination efficiency in this inverted device,which can realize the improvement of electroluminescent performance.And more importantly,by studying the effects on optical and morphological properties of ZnO NPs films,PEI was found to be able to passivate the surface oxygen vacancy defects of ZnO,which would improve the color purity by preventing the emissions from ZnO defects of the electrons trapped into the defects states.Compared with the standard devices,the devices with PEI interlayer show the high electroluminescence spectrum stability and color saturation,changing the color coordinates from(0.16,0.06)to(0.14,0.04)which is better than the blue standard of National Television System Committee(NTSC)1953.Based on the studies of ligand modification,the short-chain ethanolamine ligand was used to modify QD films by in situ exchange,which improved the performance of RGB QLEDs with the regular structure.The maximum external quantum efficiencies(EQE)of red,green,blue QLEDs increased from 15.2% to 23%,from 7.8% to 9.74% and from 5.5% to 6.9%,respectively.The EQE of the red one exceeds the highest EQE of 20.5% of monolayer red QLEDs reported in the literature.On the one hand,ethanolamine ligand-modified QDs films show enhanced optical properties since the amine group is an electron donor which is conducive to filling the electron trap states of QDs to confine the electrons in QDs.On the other hand,the charge transfer between the amine groups and the QDs will cause dipole action and reduce the work function of QDs,thereby changing the electron injection barrier at the QD/MO interface,which will promote the carrier balances and result in the enhancement of performance.