Assembling And Performance Of High Out-coupling Efficiency Of Quantum-dots Light-emitting Diodes With Nanostructure By Nanoimprint

The CdSe colloidal quantum dots(QDs)were applied to light-emitting diodes for the first time by the team of Alivisatos in 1994,which marked the beginning of research on the development of quantum-dot light-emitting diodes(QLEDs).n the past 20 years,QLED has made great progress.The external quantum efficiency of red,green and blue QLED devices has been increased to 20.5%,20%,and 18%,close to the maximum theoretical value..However,these values are still far from the organic light-emitting diodes(OLEDs)and can not fully meet the requirements of commercialization.One of the problem is the low light extraction efficiency.From the theoretical analysis,a large fraction of light(80%)is lost due to the different mechanisms takes place inside the device including substrate mode,indium-tin-oxide(ITO)/organic waveguide(WG)mode,and surface plasmon-polariton(SPP)mode.In recent years,in relatively mature OLEDs,the introduction of a suitable periodic micro/nano array structure has become an important method to improve the efficiency by applying these structures such as the moth eyes structure,the grating structure,the columnar structure,one-dimensional and two-dimensional micro-nano patterns,the light extraction efficiency.According to the application of micro-nano structure in the different positions,it can be roughly divided into inner structure and outer structure,all of them can contribute to the light extraction efficiency of the device.Compared with the outer structure,the inner structure can get a much better effect by affecting the multi-layer films structure.In this way,the EQE of the device could be greatly improved.In the traditional QLEDs,PEDOT:PSS is used as hole transport layer which has the advantages of high conductivity and high light transmittance in the visible region by spin coatingmethod.What's more,due to PEDOT:PSS can be processed under atmosphere compared with other layers,patterned PEDOT:PSS is used for researching the effect of micro-nano structure on the light out-coupling efficiency of QLED devices..However,the aqueous solution of PEDOT:PSShas poor plasticity,and does not has obvious glass transition temperature which made it can not conform the requirement of traditional nanoimprint technology.In this paper,a small amount of glycerol was doped in PEDOT:PSS as plasticizer.Various patterns of PEDOT:PSS layers were fabricated by secondary nanoimprint methods and then applied on QLED devices for investigating the optoelectronic properties of the devices.Finally,Finite-difference time-domain(FDTD)method was used for analyzing the mechanism of the efficiency improving of the QLED devices by the nanostructures.The specific work of this thesis is divided into the following three aspects:(1)Using single grating Si substrate to fabricate multi-structure PEDOT:PSS patterns:In this paper,Si template with 700 nm period,350 nm line width,and 130 nm trench depth is used for transferring one-dimensional grating structure on IPS polymer.And then one-dimensional grating PDMS stamp was obtained by replicating the IPS template.By means of increasing the pressure and lowering the temperature in the secondary imprinting process,columnar structure could be obtained also.Finally,patterned PDMS soft template used for building one-dimensional,two-dimensional grating structure of PEDOT:PSS and applied as hole injection layer to the QLED device.The morphology of Si template,the IPS polymer template,the PDMS soft template,and the patterned PEDOT:PSS obtained by imprinting was characterized.The results indicated that PEDOT:PSS hole injection layer with one-dimensional grating structure period of 700 nm,height of 36.5 nm,and a duty ratio of 0.5 are obtained.The photoelectric performance of patterned PEDOT:PSS show that the patterned PEDOT:PSS has a transmittance as high as93%and higher conductivity than the standard flat-film PEDOT:PSS substrate.(2)Effects of PEDOT:PSS hole injection layer on the outcoupling of QLEDs with micro-nano structures:The PEDOT:PSS layer with one-dimensional grating structures was used as a hole injection layer to construct a green QLED device which having a multilayer pattern.The experimental results show that the introduction of the structure does not change the peak position of the electroluminescent spectrum of the device,and the maximum luminance of the one-dimensional patterned QLED device can be increased from29010 cd/m~2 to 44150 cd/m~2,which is 52%higher than that of the standard green QLED device.the maximum current efficiency has an increase of 20%,from 43.45 cd/A to 52.23 cd/A,and the EQE can be increased from 11.13%to 13.45%,it is an increase of 21%.At the same time,the devices with one-dimensional grating patterns also have certain advantages in the viewing angle.To further optimize device performance,the two-dimensional micro-nanostructures were introduced into green QLED devices.Compared with standard QLED devices,the maximum luminance,current efficiency,and EQE have been improved from 29010 cd/m~2,43.45 cd/A,and 11.13%to 37770 cd/m~2,49.11 cd/A and 12.21%,respectively.And the performance of two-dimensionally patterned QLED device also has a further improvement in the viewing angle with respect to the one-dimensional patterned device and a better universality of light at different wavelengths.(3)Simulation and analysis of the green QLED device by FDTD,and the analysis for the mechanism of QLED's enhancement with nanostructures:FDTD could be used to analyze the near-field and far-field electromagnetic fields of the PEDOT:PSS layer and the QLED device with the grating structure.The simulation results show that the single-layer PEDOT:PSS with micro-nano structures has a certain coupling effect on light,which can be explained by enhancement of the transmittance.By far-field simulation analysis,the total reflection angle of light in single-layer PEDOT:PSS with one-dimensional and two-dimensional grating structures is significantly larger than that of standard single-layer PEDOT:PSS with flat-film structures,and the overall energy distribution is smaller.So,the introduction of micro-nanostructures improves total reflection angle of the PEDOT:PSS layer.Then,the outcoupling of the patterned QLED device was simulated and analyzed by FDTD.The results show that,in the QLED device,the introduction of the grating structure PEDOT:PSS layer can reduce internal total reflection,thereby inhibiting the waveguide mode to extract a part of the light from the device interior,and ultimately improving the device's EQE.Finally,we compared the lateral simulation of one-dimensional and two-dimensional grating structures,and combined with the actual measured fast Fourier transform(FFT),proved that the two-dimensional grating structure has superior performance in the viewing angle.