Construction And Performance Of Dual-functional ZnO As Electron Transport Layer In QLED Device

Quantum dots(QDs)are an important zero-dimensional semiconductor material,which is well recognized for its high fluorescent quantum yield,strong photochemical stability,and continuously adjustable emission spectrum.Based on the advantages of quantum dots,quantum dot light emitting diodes(QLEDs)have wide color gamut and high luminous efficiency.QLED devices prepared by solution processed method consist of different functional layers,and the interface barriers and interface states between the functional layers directly affect the performance of QLED devices.Zinc oxide nanoparticles(Zn O NPs)are widely used as electron transport layer(ETL)materials in QLED devices due to their good stability,high electron mobility,and level matching with adjacent emitting layers(EML).As a commonly used electron transport layer material,Zn O NPs mainly affect the performance of QLED devices in two aspects: Firstly,the Zn O surface has abundant surface defect states,and these defect states act as non-radiative recombination centers,leading to exciton quenching during device operation.Secondly,the good level matching and excellent electron mobility of Zn O may cause excess electron injection,resulting in QDs charging phenomenon and increasing the possibility of non-radiative Auger recombination.Researchers mainly take two directions to improve these issues: one is to insert a layer between the emitting layer and the electron transport layer,which mainly avoids direct contact between Zn O and QDs and reduces exciton quenching,while also blocking electron transport.The second approach is to modify Zn O itself,including doping and encapsulation,to change the level position of Zn O and suppress surface defect states,thereby improving the device performance.However,most of the current work only focuses on the QDs/Zn O interface to suppress exciton quenching,reduce ETL electron transport,and improve device efficiency,but the brightness is difficult to be effectively improved.The previous research results of our research group have shown that modifying the Al/Zn O interface can effectively improve the brightness of QLED devices,which is conducive to accelerating the commercialization process of QLEDs.To address the above issues,this paper modifies the interface between the ETL and Al electrode with an active material water,without increasing the complexity of QLED devices.At the same time,Mg-doped Zn O(ZMO)is used to regulate the level structure,surface defect state,and electron transport properties of the ETL,to obtain a dual-functional Zn O electron transport layer.Water,as an interface active material,can induce metalization reaction at the interface,block hole leakage,and interrupt electron capture pathways,thus achieving high-efficiency electron injection and improving the overall performance of the device.This paper will conduct research from the following three aspects:(1)Explore the influence of different active materials regulating the Zn O/Al interface on the device performance.Firstly,four active materials,acrylic acid,acetic acid,isobutyric acid,and water,were selected,and ethanol solutions with different concentrations were prepared to modify the Zn O and Al interfaces to construct green QLED devices.The results of the physical characterization of steady-state fluorescence spectra,atomic force microscopy,X-ray diffraction,and the optoelectronic performance testing of QLED devices show that among the four active materials,water has the best performance in constructing green QLED devices by modifying the Zn O and Al interface.However,the device performance decreases after the water content exceeds 5%,due to its damage to the surface integrity of the Zn O film.When the Zn O/Al interface is modified with trace amounts of water,the maximum brightness of the device increases from 149800 cd/m2 to 200500 cd/m2,an improvement of 33.8%.The results indicate that the water-modified Zn O/Al interface does not alter the crystal structure of Zn O,but increases its surface conductivity.It widens the bandgap of Zn O,and slightly shifts the conduction band upwards and the valence band downwards.This promotes the injection of charge carriers while blocking the leakage of holes.(2)The influence of trace amounts of H2 O modification on the Zn O/Al interface on the device.Mg ions are first doped into Zn O,and then a series of ethanol solutions with different water content are used to modify ZMO as an electron transport layer to realize the dual function of Zn O electron transport layer.Physical properties characterization includes transmission electron microscopy,X-ray diffraction,transient and steady-state photoluminescence spectra,ultraviolet-visible absorption spectra,ultraviolet photoelectron spectroscopy,and X-ray photoelectron spectroscopy.In addition,single carrier devices,electrochemical impedance spectroscopy,device lifetime testing,and photophysical testing of QLED devices were performed.Compared with the device constructed with standard Zn O as the electron transport layer,the modified device had the highest brightness increased from 183000 cd/m2 to 266300 cd/m2,a 45.52% improvement.The lifetime of the device was significantly extended,and the T50 lifetime of the device at an initial brightness of 100 cd/m2 was as high as 19918 hours.(3)Preparation of dual-function ZMO ETL by modifying ZMO/Al interface with trace amounts of H2 O to improve device performance.A series of ethanol solutions with different water contents were used to modify ZMO as an electron transport layer,achieving the dual-functionalization of Zn O electron transport layer.Physical property characterization,including transmission electron microscopy,X-ray diffraction,transient and steady-state photoluminescence spectra,ultraviolet-visible absorption spectra,ultraviolet photoelectron spectroscopy,and X-ray photoelectron spectroscopy,as well as single-carrier devices,electrochemical impedance spectroscopy,device lifetime testing,and photophysical testing of QLED devices were performed.The results indicate that Mg ion doping effectively modifies the QDs/ZMO interface,passivates the surface defect states of Zn O,significantly reduces exciton quenching,and improves device efficiency.H2 O,as an active material,induces the reaction between Al and Zn O to form Al Ox,enhancing the conductivity of the device,achieving efficient electron injection and improving device brightness.The maximum brightness of the standard device was increased from 213700 cd/m2 to 300400 cd/m2,a 40.57% improvement.The maximum external quantum efficiency of the device was increased from 15.51% to 21.92%,a 41.33% improvement.The T50 lifetime of the final device at 100 cd/m2 reached 38438 hours.