Study On Carrier Regulation And Performance Of ZnSe-Based Blue Quantum Dot Electroluminescent Devices

Quantum dot light-emitting diodes（QLED）have received widespread attention due to their high luminous efficiency,narrow emission peaks,and wide color gamut,and have broad application prospects in the field of next-generation display technology.Most of the current high-performance QLED are based on cadmium-based quantum dots（QDs）,but the inherent toxicity of the heavy metal cadmium limits their further development and application.Therefore,the development of sustainable and environmentally friendly light-emitting QDs materials comparable to cadmium-based QDs is the key to driving the QLED field to a wide range of applications.Although the previously reported high-quality ZnSe based QDs can achieve high quantum yield synthesis,the emission peaks are mainly concentrated in the violet range（400-435 nm）,which cannot meet the requirements of display applications because the luminescent materials for commercial display applications must have relatively high quantum yields.In addition to high fluorescence quantum yield（PL QY）and good stability,it is also necessary to satisfy the emission characteristics that the emission peak is in the blue light range（440 nm<PL<480 nm）.There are two main methods to achieve the luminescence peak of cadmium-free ZnSe quantum dots in the blue luminescence region:one is to introduce narrow bandgap ZnTe to form ternary ZnSe Te alloy cores;the other is by increasing the size of ZnSe quantum dots.Although the formation of ZnSe Te alloy structure can be regulated to blue light emission（PL>450 nm）,the difficulty of controlling the uniform alloying distribution of Te elements leads to a broadening of the emission spectrum,which in turn reduces the color purity of the light-emitting devices.Meanwhile,the preparation process needs to be carried out in a harsh oxygen-free environment because of the instability of Zn-Te covalent bonding which is prone to oxidation defects leading to QDs luminescence quenching.The optimization of the synthesis scheme to regulate the particle size of QDs is an effective way to achieve blue light emission from ZnSe quantum dots while maintaining high stability.In addition,the short lifetime of Cd-free system blue QLED is also a critical bottleneck that restricts the relevant devices to practical applications.The reason for the short lifetime can be attributed to the following two points:One is more surface defects made by the small size of blue QDs,which in turn causes serious non-radiative losses when it is used as a light-emitting layer;the other is that the wide bandgap of blue QDs makes it a large potential barrier between the conventional hole and electron injection/transport layer materials,resulting in low charge injection transfer efficiency.Therefore,optimizing quantum yield and stability of the QDs core simultaneously,combined with device transport layer modulation to enhance the carrier injection balance is an effective idea to improve the efficiency and stability of blue QLED.This dissertation focuses on enhancing ZnSe-based light emitting materials and their electroluminescent device performance.To address the problem of high-performance blue light emission of ZnSe based QDs,a"low-temperature nucleation,high-temperature shell"QDs preparation method was developed to obtain blue ZnSe/ZnS core-shell QDs with a size larger than its Bohr radius,high quantum yield（>90%）,and ultra-narrow emission.In addition,to address the problem of ZnSe-based blue light QLED,the charge transport layer was modified and tuned from the device structure level to finally achieve the construction of high-performance cadmium-free blue QLEDs.The main research works are as follows.（1）Synthesis of bulk-like ZnSe blue quantum dots and application in electroluminescent devicesIn this chapter,we adopt the technique of"low-temperature nucleation and high-temperature shell growth"to obtain bulk-like ZnSe nucleate（10 nm）by continuously growing a certain thickness of ZnSe on the ZnSe nuclei while maintaining the quantum yield and making the size larger than the Bohr radius（～10nm）,achieving ZnSe emission located in the blue region（>440 nm）.After further cladding,the ZnS shell layer,ZnSe/ZnS core/shell QDs with PL QY up to 95%and size～12 nm was obtained,with the PL peak at443 nm and full width at half maximum only～9.6 nm,which are the quantum yields and full width at half maximum that have been reported to be the optimal values for this system.In addition,the upward shift of the valence band of these ZnSe-based QDs lowerd the energy barrier between QDs and the hole transport layer,which promoted the injection and transport of holes.The QLED constructed based on bulk-like ZnSe/ZnS QDs showed bright blue emission with 12.2%external quantum efficiency（EQE）and 237 h device lifetime(T₅₀@100 cd m^-2),which were the highest values for the ZnSe system.（2）Sn-doped ZnO suppresses electron injection to improve cadmium-free blue QLED performanceZnO nanoparticles（NPs）can provide effective electron injection into the QD layer,but the deep valence band of QDs causes a severe shortage of hole injection,resulting in unbalanced carrier injection.The excessive electron injection limits the ratio of charge to exciton conversion and allows electron accumulation at the interface of the emitting layer,which increases the non-radiative composite loss of QDs and thus reduces the device performance.This chapter uses Sn-doped ZnO NPs to regulate the electron transport properties to address this problem.The combination of theory and experiment revealed that Sn doping enhanced the ZnO conduction band and reduced its mobility and surface defect sites,suppressed excess electron injection into the device to achieve charge balance,and reduced exciton quenching in the QDs to improve radiative recombination.Ultimately,this improved the efficiency and operational lifetime of the blue cadmium-free QLED（with an emission peak of 443 nm）.The results showed that the EQE was improved from 5.1%to 13.6%at 4%Sn content,and the device lifetime was improved from 13.5 h to 305 h,with a 21-fold improvement.（3）Modulating the hole transport layer to enhance the stability of ZnSe based blue QLEDTo address the difficult problem of efficient injection of effective holes in wide bandgap blue quantum dots,this chapter achieves hole injection efficiency enhancement through hole transport material screening and optimization to further balance charge injection and improve device stability by reducing intermittent complex loss.Firstly,the effects of three hole transport materials,TFB,PVK,and CBP-V,on the carrier transport performance in ZnSe-based QLED were investigated.The results showed that the mobility of the hole transport material was an important factor governing the device turn-on voltage,while its HOMO energy level was a key factor in affecting the device efficiency.Although TFB with high mobility hadlower turn-on voltage,the device efficiency and brightness were poor.However,high-efficiency and high-brightness QLED construction can be achieved based on PVK and CBP-V as hole transport layers,respectively.Therefore,the hybrid structure of PVK with high efficiency and CBP-V with high brightness was selected to improve the device performance.By optimizing the doping ratio and comparing the process conditions,using a simple liquid phase mixing method,when the doping amount of PVK was 15 wt.%,the brightness and EQE of the device reached 3087 cd m^-2 and 14.41%,respectively,and the device lifetime was as high as 1043 h.