Research On The Work Mechanism Of Quantum Dot Light Emitting Diodes By The Spectroscopy Technologies

Since the inorganic semiconductor colloidal quantum dot was first applied to the light-emitting diode by Colvin in 1994,the quantum dot light-emitting diode(QLED)has been developed rapidly.Due to various excellent advantages such as higher stability,wider color gamut,and better color purity,QLEDs are considered a potential technology for high-quality display and solid-state lighting,which have huge commercial value.Much progress has been achieved for the QLEDs,and the efficiency and stability have partially met the requirements for commercialization.However,the in-depth understanding of the electroluminance(EL)and degradation mechanisms of the QLEDs remains unclear,which is a vital challenge to promote the further commercial application.In this paper,we focus on the investigation of the EL mechanism and temperature-dependent of the QLEDs,and the charge dynamics processes in the white QLEDs by ssing the PL and EL spectroscopy technologies.The main contents of this article are as follows:1.The EL mechanism and carrier dynamic process in the Cu In S₂/ZnS(CIS)based inverted QLED were studied with the device structure consisting of ITO/ZnO/quantum dots(QDs)/4,4?,4?-tris(carbazol-9-yl)triphenylamin(TCTA)/Mo O₃/Al.It was clarified that hole injection can affect the PL process of QDs during the operation of the device,resulting in fluorescence quenching and non-radiative recombination in QDs.In comparison,the injected electron had no effect on the PL process.Further,a simple method was proposed to optimize the device performance in terms of these results.A thin layer of 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene(TPBi)was inserted between CIS/TCTA interface to control the carrier distribution,leading to the excitons formation interface close to the hole transport layer,and reducing the exciton quenching by the ZnO.Moreover,the TPBi can effectively suppress the influence of accumulated holes on excitons,and greatly improve the performance of the device.2.The temperature-dependent EL properties of the QLEDs based on CISQDs were studied,and the influence of temperature on the carrier dynamic processes in the device was analyzed.The structure is ITO/ZnO/QDs/TCTA/MoO₃/Al.With temperature decreasing,the current density decreased obviously,the turn-on voltage also increased significantly from 1.8 V(300K)to 4.8 V(20 K).However,the device performance has been significantly improved,and the maximum brightness increased nearly 292%from 2736 cd/m²(300 K)to 10712 cd/m²(20 K).And the efficiency has increased by 43%(from 2.33 cd/A to 3.33 cd/A),indicating that the EL process is more efficient at low temperature.Combined with the temperature-dependent PL characteristics of QDs,we found that the improvement of the EL performance is due to the reduction of the thermal quenching effect of the QDs and the enhancement of the PL efficiency at low temperature.On the other hand,the exciton formation area gradually expands from the ZnO/CIS to CIS/TCTA interface due to different temperature-dependent properties for the hole and electron injection.3.A model white QLED consisting of a bilayer CdSe/ZnSeSQDs//Cu InS₂/ZnS emissive layer has been used to analyze the white-light emission mechanism.In this design,the CdSe/ZnSeS QDs and CuIn S₂/ZnS QDs contribute to the blue and yellow emissions,respectively,in the dichromatic white QLED.Wavelength-resolved transient EL(TrEL)results demonstrate that the excitons are mainly formed on the CuInS₂/ZnS QDs in the QLED operated at low biases due to the low barrier to hole injection and energy transfer from the CdSe/ZnSeS QDs to the CuIn S₂/ZnS QDs.Further,the TrEL decays of both white and monochromic devices reveal that the emission behavior of the white QLED is closely related to that of the monochromic device,but is minimally affected by the interactions between different emission units.The simulation results performed by the solar cell capacitance simulator model agree well with the experimental data.Our results show an insight into the EL processes in the white QLED and demonstrate a powerful tool to investigate emission behavior of the white QLED.We believe that our results can be extended to the design of other multi-layer devices.