Research On New-structure Quantum Dot Light-emitting Devices And Carrier Regulation

Colloidal quantum dot(QDs)materials have attracted tremendous attention due to their advantages of size-tunable emission wavelength,high color purity and saturation,chemical stability,and solution processability.Therefore,quantum dot light-emitting devices(QLEDs)have great commercial value and application prospects in display and solid-state lighting fields.The realization of efficient and stable devices is the basis for promoting the industrialization of quantum dot products.However,there are some problems in current QLED generally,such as materials instability,energy levels mismatch,imbalanced carrier transport,and exciton quenching caused by adjacent function layers,which decrease devices performance and also restrict the process of industrialization.In order to solve above problems and improve device efficiency and operation stability,from the perspective of improving device structures,this study with core-shell Cd Se/Zn S QDs as emitting layer(EML),can realize carrier regulation through function layers'optimization,including hole injection layer(HIL),hole transport layer(HTL),electron transport layer(ETL),and emitting layer,and the specific research contents are as follows:(1)In the forward QLED,the commonly used hole injection material is poly(3,4-ethylenedioxythiophene):poly(styrenesurfonate)(PEDOT:PSS),which can promote hole injection by adjusting anode'work function.However,PEDOT:PSS is acidic(p H=1.5),which is not conducive to the performance and stability of the device during long-term operation.In addition,the ITO/PEDOT:PSS interface is very sensitive to air and it can absorb ambient moisture,leading to the indium ions of ITO diffuse into device,potentially resulting in exciton quenching and degrading device performance.To overcome the shortcomings of PEDOT:PSS,copper-based polyelectrolyte Cu-PSS is synthesized.The Cu-PSS and PEDOT:PSS are used into devices as HIL,and their results show that under the same voltage,the current densities and luminances are quite different,but their efficiencies are almost identical.The maximal external quantum efficiency of Cu-PSS-based device is 8.02%,which is comparable to that of PEDOT:PSS-based device(8.17%),meanwhile the T₅₀driven by constant current is improved by 44.69%.Subsequently,the mixture of Cu-PSS and PEDOT:PSS are introduced into device as HIL.The device performance is significantly improved,and the maximal external quantum efficiency of optimized device is 11.84%.Furthermore,according to the optimized device structure,the flexible device is successfully fabricated on poly-(ethylene terephthalate)(PET)substrate.(2)In inverted green-emitting QLEDs,the commonly used electron transport material is Zn O nanoparticles.Because of its high electron mobility,device'electron injection efficiency is high.While for hole transport,although the commonly used materials such as TCTA and TAPC have high hole mobility,there is high hole injection barrier between EML and HTL,resulting in mismatched hole injection energy levels and low hole injection efficiency.The differences in electron and hole injection efficiencies lead to poor device efficiency and stability.To address the issue,a thin layer is inserted at the interface of EML/HTL to form hole transport trilayer based on hole transport bilayer.In the hole transport trilayer device with CBP,the performance enhancement is obvious.According to the energy levels diagrams,single-carrier and capacitance characteristics,the reason for performance enhancement is the formation of cascaded hole injection energy levels,which can realize efficient hole injection from the anode to EML.The number of interfacial accumulative holes can be reduced and the carrier injection is more balanced.Meanwhile,the operation stability can also be significantly improved.The influence of thicknesses of CBP is also be studied in detail.In addition,the performance enhancement phenomenon can also be achieved in the red-emitting QLEDs with the similar structure.(3)The device efficiency and stability are poor in QLEDs with metal oxide SnO₂as ETL,and the problem of imbalanced carrier injection also exist.Besides,the ETL film'quality is related to the annealing temperature,in which the residual hydroxide precursors are detrimental to device stability that need to be removed by high-temperature annealing which will reduce film quality.The imbalanced carrier injection and presence of hydroxyl components are detrimental to device performance.To address the issue,the device structure with three types of HTLs is studied at first.The results show that higher hole mobility and proper energy levels can achieve high efficiency.The maximal external quantum efficiency in hole transport trilayer QLED is 9.89%.On this basis,the annealing temperature of ETL is studied,and the optical temperature is in the range of 120-140?.After that,the ultraviolet-ozone(UVO)treatment is carried out on the spin-coating annealed ETL to decompose the residual hydroxide.With UVO treatment,the device efficiency is slightly improved,but the operation stability can be enhanced significantly.At the initial luminance of 500cd/m²,the T₅₀of device with UVO treatment is 2168 h,which is much higher than that of control device(1015 h).Therefore,the efficient and stable QLEDs can be fabricated successfully.(4)The solvent environment is crucial to QDs'fluorescence efficiency,stability,absorption and emission spectra.When QDs are dissolved in(weak)polar solvent such as toluene,the fluorescence efficiency can be reduced due to the interaction between the polar solvent and the shell of QDs.In addition,QDs with high concentration are closely packed,and there will be non-radiative recombination between them,which will also decrease the luminous efficiency.Therefore,the performance of the currently reported QLEDs based on polar solvents is much lower than that of devices based on non-polar solvents.In order to improve the performance of toluene solvent-based QLEDs,the hybrid EML is proposed,that is,QDs are mixed with other solutions physically.First,the hybrid EML is composed of QDs and polymer PVK,and with the increase of PVK composition,there is a tread of increasing first and then decreasing in devices'performances.The single-carrier and transient electroluminescence characteristics of devices and the photoluminescence decay curves of films indicate that the introduction of a small amount of polymer can promote hole injection and improve device performance.Subsequently,the EML is further adjusted,and the doping material is host-guest dye solution mixed with polymer PVK and phosphorescence FIrpic in a ratio of 9:1.The introduction of a small amount of host-guest dye solution can improve performance obviously,and the maximal external quantum efficiency for optimized device is 15.56%,while more host-guest dye component will decrease performance rapidly,and the blue-emitting from FIrpic appears in the spectrum.From materials'absorption spectrum,photoluminescence spectrum and thin films'photoluminescence decay curves,it can be confirmed that energy transfer occurs between FIrpic and QDs,and the excitons formed on the FIrpic can transfer their energy to QDs through F?rster resonance energy transfer.Therefore,device efficiency can be improved further.In addition,devices based on hybrid EML also exhibit longer operation lifetime.