Interfacial Engineering For Low-toxic Cadmium-free Quantum-dot Light-emitting Diodes

Semiconductor quantum dots(QDs)have attracted extensive attention due to their unique physical properties of high photoluminescence quantum yield(PL QYs),narrow spectral emission bandwidths,broad spectral photoresponse from ultraviolet to infrared and good luminescent stability.Quantum-dot electroluminescent light-emitting diodes(QLEDs)are one of the important applications of QDs in the field of optoelectronic devices.Although the device performance has been significantly improved in recent years,there is still a problem of charge injection imbalance in the device,especially in cadmium-free QLEDs.Therefore,it is very important and urgent to improve the performance of cadmium-free QLEDs.The thesis is focused on the modification of electron transport layer(ETL)and hole injection/transport layer(HIL/HTL)for the optimization of Cu-In-Zn-S/ZnS-based and InP/ZnS-based QLEDs.Its main contents and innovations can be summarized as follows:1.All-solution-processed electroluminescent QLEDs were fabricated by using yellow-emitting CIZS/ZnS nanocrystals(NCs)as the emitting layer,in which ZnO nanoparticles(NPs)with different valence ions(Li⁺,Mg²⁺and In³⁺)and different radius ions(Mg²⁺and Ca²⁺)were acted as ETLs.By testing the photoelectric properties of ZnO NPs and the device performance of QLEDs,the effects of different types of metal ions doped ZnO NPs on the performance of QLEDs were studied.The result showed that QLEDs with ZnO:Mg NPs exhibited a superior device performance over other counterparts because of the same valence state and similar radius of Mg²⁺and Zn²⁺.The doping of Mg²⁺not only could effectively adjust the band structure of ZnO NPs,but also could reduce electron injection/transport in the device,which made the charge injection and transport more balanced,thus improving the device performance.In order to solve the problem of surface defect states of ZnO:Mg NPs,Cl?was used to passivate the surface of ZnO:Mg NPs,and studied the effect of Cl?passivated ZnO:Mg NPs(Cl@ZnO:Mg NPs)on the device performance.The introduction of Cl@ZnO:Mg NPs not only reduced the electron injection barrier between emitting layer and ETL,and decreased the electron injection,but also suppressed the quenching effect of ETL with high work function on the emitting material,which thus improved the performance of CIZS-based QLEDs.The peak external quantum efficiency(?_EQE)of the optimized device reached 4.05%.2.The promotion of hole injection is another effective method to increase the charge injection balance except for the reduction of electron injection.As HTL material,poly[(9,9-dioctyfluorenyl-2,7-diyl)-co-(4,4-(N-(p-butylphenyl))diphenylamine)](TFB)has high hole mobility,but its shallow HOMO level leads to large hole injection barrier.Therefore,in order to reduce the hole injection barrier,a thin N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine(TPD)layer was inserted between TFB and emitting layer,which could not only alleviate the hole injection barrier,but also restrain the quenching effect of TFB on the luminescence of nanocrystals,leading to the further improvement of device performance.The peak?_EQE of the optimized device reached5.61%.Besides,as common HIL,the acidity of Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS)will cause corrosion to ITO anode.In order to solve this problem,NiO was introduced between PEDOT:PSS and ITO anode,and the electrical properties of NiO were regulated by passivation of NiO with ethanolamine and Cu doping.The introduction of Cu:NiO could not only prevent the corrosion of acidity PEDOT:PSS to ITO anode,but also form a stepwise energy level alignment between ITO anode and PEDOT:PSS,which was conducive to hole injection and improves the balance of injected carrier,leading to the significant improvement of device performance.3.The interfacial modification strategies of ETL and HIL/HTL were applied to InP-based QLEDs,in which red InP/ZnS QDs were used as emitting material.The device performance has been significantly improved by the careful-selection of doped metal ions in ZnO NPs and the surface passivation with Cl?.The peak?_EQE of the optimized device was 2 times that of pure ZnO NPs as ETL.The further introduction of Cu:NiO between ITO anode and PEDOT:PSS could not only prevent the corrosion of acidity PEDOT:PSS on ITO anode,but also form a stepped hole injection barrier,which improved the hole injection and transport,and further improved the performance of red InP based QLEDs.The peak?_EQE of the optimized device reached 8.62%.