The Controlled Synthesis Of Reverse Type-? QDs And Their Application In Light Emitting Diodes

High efficient quantum dots LED require not only appropriate device structure,but also need high quality quantum dots.At present,although PL quantum yield QY of QDs in solution have utmost to 100%,it would sharply decline after spun into films.It is found that the PL QY of quantum-dot-basded films directly determines the photoelectric performance of the LED device,so it is particularly important to increase the PL QY of QD-based films.What's more,the structure of QDs is also pretty important for carrier transport state in the device.So,it has been one of the research hotspots all the time that,exploring the influence element on device performance based on the design of quantum dot structure.In this paper,Taking the design of internal structure of QDs as starting point,on the one hand,we aim to improve PL QY of QDs through cladding modification,on the other hand,we explain that the carrier transport and photoelectric performance of LED device are affected by the internal structure of QDs.This paper mainly discusses with the following three points:?1?Compare with the type-I QDs,reverse type-I QDs'wavelength can more easily be adjusted,advantaged to full color display.More importantly,reverse type-I QDs due to their special energy levels,and the radiation is recombined in the outer shell layer,which facilitates the direct injection of electrons and holes as well as raises the current density of the device.However,research on the application of reverse type-I QDs to light-emitting diodes has rarely been reported so far.Consequently,We designed and synthesized Cd_0.1Zn_0.9S/CdSe reverse type-I QDs with 61%PL QY,and realized the wavelength adjustable from 450nm-670nm.Then we synthesized type-I CdSe/Cd_0.1Zn_0.9S QDs and fabricated the QLEDs.We find that in spite of a lower PL QY of latter,but the external quantum efficiency and brightness?8.23%?11412 cd/m²?of latter are higher than former.Our experiment has proofed that reverse type-I QDs can improve the performance of device because it needn't to overcome the high potential barrier of the shell layer,it is more beneficial for carriers to inject into emitting cores.?2?It is one of the effective ways to improve the luminous efficiency of devices by adjusting the QDs structure to improve the carrier transport equilibrium state.On the base of reverse type-I QDs,we choice CdS as shell material because it can improve PL QY of QDs so that we can produce Cd_0.1Zn_0.9S/CdSe/CdS QDs with 76%PL QY of QDs on the one hand,and on the other hand,explore the QDs structure under the optimal state of charge transfer balance by adjusting the thickness of CdSe layer and CdS layer.Subsequently,by compare the QLEDs based on Cd_0.1Zn_0.9S/CdSe/CdS QDs and the QLEDs based on CdSe/CdS QDs under the same device structure,we find that although the PL QY of the former still low,but the maximum external quantum efficiency of the former is 12.44%,which is much larger than the latter.It's mainly benefit from the structural optimization of QDs that make the charge injection and transmission of the device more balanced.?3?It is found that the fluorescence quantum yield of the core/shell QDs are related to the lattice defect.Lattice defect can lead to the formation of defect states,which can bring many non-radiative recombination channels.The electrons and holes can be subjected to non-radiation by these channels,and the energy is released in the form of heat without any contribution to photon generation,so the stability of devices is decrease naturally.Recent studies show that the inner lattice stress is one of the main causes of lattice defects during the growth process of multi-shell core/shell structure QDs.The existing multi-shell core/shell QDs are mostly concentrates on?-?binary QDs,which is difficult to control the lattice parameters of each layers effectively and reduce the internal tress or defect states.Hence,we use a ternary-alloy QDs to control the lattice constant of each layer,and we have designed the multi-shell quantum dot with high PL QY to alleviate the pressure of the lattice strain.Due to the relief of the lattice strain,we synthesized thick-shell QDs.Thick shell increasing the point-to-point distance between QDs after the film formation,suppresses the coupling between two quantum dots,so the QY also greatly improve after film formation.Finally,we compare the performance of QLEDs under three different shell thicknesses and conclude that the brightness and external quantum efficiency of QLEDs reach153632.2 Cd/m² and 19.25%when the QLEDs are 7.5nm,which is far better than the performance of devices under thinner shell.It is further shown that the design of QDs structure has great potential in improving the performance of QDs.