Investigation On Optoelectronic Regulation Of Flexible Quantum-dot Light-emitting Diodes

Quantum dots（QDs）as a type of inorganic semiconductor optoelectronic material within nanometer-scale,possess several advantages over other materials,including tunable emission wavelength,high color purity,narrow emission spectrum,and high photoluminescence quantum yield（PLQY）.Therefore,QDs can be widely used in a wide range of applications,such as solar cells,electroluminescent diodes,biomarkers,and so on.Among them,quantum dot light emitting diodes（QLED）have the advantages of high color saturation,high stability,low operating voltage,and low fabrication costs,showing broad application prospects in the fields of lighting and display.Currently,with the progress of QDs synthesis technology and the optimization of QLED device structure,the external quantum efficiency（EQE）of monochromatic red,green,and blue QLED have exceeded 20%.Additionally,the brightness and operating lifetime of QLED are approaching or even exceeding those of organic light emitting diodes（OLED）,meeting the basic requirements for commercial applications.However,these research advances mainly focus on QLED based on rigid glass/indium tin oxide（ITO）substrate,with relatively few reports on flexible QLED.Nevertheless,with the development of society,there is a growing demand for wearable and portable flexible display devices.Given the advantages of QLED technology,such as high color purity and efficiency,it naturally emerges as a strong candidate for the next generation of flexible displays.Therefore,the development of efficient and stable flexible QLED is particularly necessary.In addition to facing similar challenges as rigid QLED（such as the performance gap between blue QLED and red/green QLED）,there are other challenges in the research of flexible QLED.These include the optoelectronic performance of flexible QLED still lags behind that of rigid QLED,the lack of high-quality flexible electrodes,and the need to improve the bending stability of flexible QLED and so on.To address these issues,we primarily optimize the device through electrical and optical modulation to enhance the performance of flexible QLED.The specific research contents are as follows:（1）Flexible QLED based on charge generation layer（CGL）.We constructed the CGL with efficient charge generation efficiency by using p-type material PEDOT:PSS and n-type material Zn O,and applied it in flexible QLED.Unlike conventional charge injection structures,the carriers of CGL mainly originate from its interior rather than the injection of electrode.This mode of charge generation reduces the dependence of charge injection process on the interface between electrode and charge injection layer in flexible QLED,thereby mitigating the issue of restricted charge injection caused by electrode detachment during bending process.At the same time,compared to flexible QLED based on conventional electron injection（ETL device）,flexible QLED based on CGL（CGL device）exhibits higher EQE and power efficiency,attributed to a more balanced carrier distribution within CGL device.Moreover,in the measurement of transient electroluminescence（Tr EL）,we confirmed that CGL possesses charge storage capability,enabling the device to achieve faster EL turn-on speed and more stable EL emission.Additionally,CGL device exhibits stronger bending stability compared to ETL device.Hence,we further investigated the factors affecting the bending stability of flexible QLED.By conducting bending experiments on various functional layers of the flexible QLED and observing the photoelectric performance of these devices after bending,we found that the bending process of the functional layers does not significantly affect the performance of the flexible QLED,as well as the corresponding processes of carrier injection,transportation,and recombination.Subsequent tests on adhesion,surface morphology,and elastic modulus of flexible QLED revealed that the top metal Al electrode and its interface with adjacent function layer are the primary factors limiting the bending stability of flexible QLED.（2）Optical simulation of flexible microcavity QLED based on PEI-Ag bottom electrode.Utilizing optical microcavity is one of the effective strategies to improve the light extracting efficiency of flexible QLED,and the key to forming high-quality optical microcavity lies in the fabrication of semi-transparent flexible electrodes.In this work,we chose a thinner metal Ag electrode as the semi-transparent electrode on the light-emitting side.However,during the deposition of the thinner metal electrode,limited by the Volmer-Weber nucleation mode,metal atoms tend to aggregate on the substrate and form irregular three-dimensional（3D）metal island structure.To avoid this issue,we introduced ultra-thin polyethyleneimine（PEI）as a seed layer on the flexible PET substrate to regulate the growth process of metal Ag film.There are chemical interactions between the amine groups of PEI and Ag atoms,which causes the initially deposited Ag atoms to be uniformly and densely fixed on the surface of PEI,inducing the growth of subsequent film.Compared to the Ag film directly deposited on the PET substrate（bare-Ag）,the Ag film induced by PEI（PEI-Ag）exhibits more uniform surface,superior optoelectronic performance,and enhanced bending stability.Based on the excellent flexible PEI-Ag electrode,we designed the flexible microcavity QLED with optical microcavity effect.And optical simulations of microcavity device based on PEI-Ag electrode were performed to analyze the effect of functional layers thickness on the light extraction efficiency of the device.The optimal microcavity device structure with the maximum optical extraction efficiency in theory was also obtained.（3）Efficient and flexible QLED based on optical microcavity effect.To verify the effect of optical microcavity on the performance of flexible QLED,we experimentally optimized the thickness of Zn O functional layer and bottom PEI-Ag electrode.The experimental results show that when the thickness of Zn O and PEI-Ag electrode are 70 nm and 16 nm respectively,the flexible microcavity QLED based on the PEI-Ag electrode（PEI-Ag device）exhibits the maximum EQE of 25.6%.This is consistent with the results of the previous optical simulation.To further demonstrate the impact of semi-transparent electrode on optical microcavity and the effect of microcavity on device performance,we compared the performance of three devices with different bottom electrodes（ITO device,bare-Ag device,and PEI-Ag device）.Among them,the PEI-Ag device exhibits optimal electrical performance.Benefiting from the enhancement of the microcavity effect on the exciton radiative rate,the PEI-Ag device shows ultralow efficiency roll-off maintaining the EQE greater than 20%over an extremely wide brightness range from 300 to 160,000 cd/m².Furthermore,the flexible PEI-Ag device also demonstrates the strongest operational stability,with a T₅₀ lifetime of 159,843h at an initial brightness of 100 cd/m²,fully meeting the requirements for conventional displays,outdoor displays,and lighting applications.