| Colloidal quantum dots（QDs）are excellent luminescent nanomaterials with high photoluminescence quantum yield（PL QY）,narrow full width at half maximum of fluorescence,solution processability,and good photochemical stability.More importantly,the emission wavelength of QDs can be adjusted by changing the particle size of QDs.As a new generation of electroluminescent devices based on QDs,quantum-dot light-emitting diode（QLED）show great development potential in the field of display and solid-state lighting due to their excellent superiorities of application in lighting and displays,such as good monochromaticity,low power consumption,self-luminous,thinness,wide color range and fast response time,etc.However,compared with red and green QLED,the performances of blue QLED remain poor,and hinder the application of the QLED,which is mainly reflected in high turn-on voltages,poor stability,and short lifetimes,etc.Therefore,it is urgent to improve the performances of blue QLED.To improve the performances of blue QLED,research works mainly focus on developing synthesis strategies of blue QDs,selecting carrier transport materials,regulating the interfaces,and optimizing device structures.Although the PL QY of QDs is nearly 100%,the external quantum efficiency（EQE）of the QLED can only reach around 20%due to most of light trapped in device.In consequence,enhanced light outcoupling efficiency offers a great potential to improve device performance,light outcoupling technology can be divided as external and internal light outcoupling technology.Internal light outcoupling technology,inserting micro/nano structures into device,can extract the trapped light in the waveguide mode.On the other hand,external light outcoupling technology can extract the trapped light in the substrate mode.The commonly used micro/nano structures,including scattering film,moth-eye nanostructures,microlens arrays,glass hemisphere,grating structures,and nanopillars,etc.Periodic micro-nano structures have angle dependence,resulting in uneven light distribution.However,a wrinkle pattern with rich Fourier spectra in k-space is a special quasi-period micro/nano structure,which is a good choice for light outcoupling,however,the application of wrinkle structure that can be precisely regulated in a large scale in optoelectronic devices is rarely mentioned.Until now,when the aspect ratio is constant,the influence of wrinkle structures with different periods from nano-scale to micron-scale on the light outcoupling efficiency of the blue QLED needs in-depth research.In this paper,we fabricate wrinkle structures from nano-scale to micron-scale.The appropriate wrinkle structure is selected as the external structure or internal structure to extract trapped light in substrate mode or waveguide mode,the light outcoupling enhancement mechanism of wrinkle devices is further analyzed,the summary is as follows:（1）Patterned surface can endow material with special optical properties.The adjustable dynamic wrinkle pattern can control the transmittance and haze.In this chapter,self-assembly method is proposed to fabricate wrinkle pattern,the period of the wrinkle pattern can be adjusted by the thickness of the aluminum layer and characterized by atomic force microscope（AFM）.When the thickness of Al layer increases from6 nm to 100 nm,the period of the wrinkle pattern can be adjusted from 375 nm to 6.67μm.The PDMS wrinkle template is obtained by transfer printing,the wrinkle patterns on the PDMS template are evenly distributed and suitable for large-area production,which provide a universal method for the introduction of wrinkle structure into optoelectronic devices.Through the Fast Fourier transform（FFT）,we can get the circular image of the wrinkle structure,which means that there are multiple periods in the wrinkle structure,the periods contained in the wrinkle structure present a normal distribution,including the main period and other periods,which has no directional selectivity and little effect on the distribution of electric field intensity.When the one-dimensional or two-dimensional grating structure is used in the light-emitting device,the device performance has been improved,but it affects the distribution of the electric field intensity,and lead to the deviation from the Lambert distribution,the quasi-periodic wrinkle structure fabricated by self-assembly method can solve the problem.（2）High-efficient blue QLED is still challenging to make an application in display and solid-state lighting.Enhancing light outcoupling is one of the most effective methods to improve performances of the blue QLED.Here,a strategy for an angularly and spectrally independent boost in light outcoupling of the blue QLED is demonstrated by quasi-periodic wrinkles,which is successfully used as a diffraction grating for extracting trapped light at the substrate/air interface.Quasi-periodic wrinkles can be adjusted from nano-scale to micron-scale under the condition of constant aspect ratio,light outcoupling efficiency increases with the periods of the wrinkle patterns,and the optimizing wrinkle device shows a maximum luminance of 11769cd m-2 and a peak EQE of 15.41%,the enhancement of EQE is 49.5%higher compared to that of the reference device.As an external micro/nano structure,quasi-periodic wrinkles can extract the trapped light devices without changing the angle-dependent emission property.Furthermore,simulation also indicates that external wrinkle patterns are attractive option for boosting the performance of blue QLED.（3）In order to study the effect of wrinkle structure on the extraction of red,green and blue（RGB）trapped light,a unique external wrinkle pattern was proposed for the white QLED（W-QLED）with the mixed RGB QDs as the emitting layer.After adding an external wrinkle structure,the device shows an improved EQE,current efficiency and brightness from 9.68%,19.9 cd A-1,and 88181 cd m-2 to 13.41%,26.8 cd A-1,and 136207 cd m-2,respectively.The corresponding enhancement of these indicators are up to 39.5%,34.7%and 54.4%,respectively.High brightness of 10,000 cd m-2 was achieved at EQEs of 13.21%.The brightness of our W-QLED is the highest value reported so far among QD-mixed W-QLED.The ultra-high brightness is the comprehensive effect of high current density（of the mixed W-QLED）and effective extraction of trapped light by external wrinkle pattern.More importantly,benefiting from the wrinkle pattern with a richer Fourier spectrum and a semi-circular section,it can realize the simultaneous extraction of RGB trapped light and the selective extraction of blue light.Moreover,correlated color-temperature can also be realized the transformation from pure white light to cold white light.This approach is highly meaningful for advancing direct white light emission for new generation lighting and display.（4）In order to extract waveguide light in blue QLED devices,this chapter focuses on the application of nano-scale wrinkle pattern as internal light outcoupling structure in blue QLED devices.Firstly,the wrinkle PEDOT:PSS as hole injection layer（HIL）is constructed.On this basis,a blue QLED device is fabricated and the influence of nano-scale wrinkle pattern on the performance of the device was investigated.Compared with the reference device,the maximum EQE of wrinkle QLED device increased from 10.11%to 12.73%,the enhancement of EQE is up to 25.9%,The luminance increased from 6738 cd m-2 to 8467 cd m-2,the corresponding enhancement of luminance is 25.6%,and the performance of the wrinkle device is greatly improved.The wrinkle HIL has a wide Fourier spectrum,which can realize the broadband and effective extraction of trapped light.Compared with the periodic grating structure,the wrinkle structure has angle independence,and the distribution of electric field intensity still conforms to Lambert emission.At the same time,the introduction of wrinkle pattern has no effect on the spectral stability of the device,and the wrinkle pattern extracts light from the waveguide modes in QLED,and improves the emission intensity of EL without affecting the Lambert emission.（5）For further extract the trapped light in the device,the wrinkle patterns of different scales are simultaneously introduced into the device as the internal and external light outcoupling structure.The synergistic effect of internal and external light outcoupling structures greatly improves light outcoupling efficiency.Under the luminance of 1000 cd m-2,the EQE of the device with both internal and external wrinkle structures is up to 17.62%.On the other hand,the QLED with the wrinkle pattern shows excellent spectral stability,and presents wider angular emission range.By introducing the internal and external wrinkle structure,the extraction of trapped light in waveguide and substrate mode is realized,and the performance of the device is significantly enhanced.Compared with widely-used external periodic patterns and optical lens,the wrinkle structure has the advantages of simple transfer,adjustable area and angle independence.It is very suitable for light extraction in the field of lighting and display. |
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