All-solution Processed Quantum Dots Light-emitting Diodes

Quantum dots（QDs）have attracted a lot of attention due to their unique optoelectronic properties,such as tunable light emission color,narrow emission width and high color purity.In particular,QDs have solution processability,and QD-based light-emitting diodes（QLEDs）prepared by solution processing exhibit long life-time,high brightness,ultra-thin,flexible and other excellent photoelectric performance.Solution processed cathode is of great importance to meet the process adaptability of QLEDs fabrication and realize low-cost and large area manufacturing.At present,there are some drawbacks in all-solution processed devices reported in literatures,such as high turn-on voltage(V_on),low luminous efficiency and large square resistance of electrodes.The research aims to achieve all-solution processed QLEDs with low V_on and high luminous efficiency,and mainly focusing on the cathode interface characteristics of all-solution processed QLEDs by using AgNPs cathode and ITO/PEDOT:PSS/TFB/QDs/ZnO/AgNPs device structure.We solved the problems of non-uniform light emission,the increase of cathode interface barrier and the limitation of electron injection by optimizing drying processes and introducing a buffer layer at cathode interface.All-solution processed red,green and blue QLEDs（R-,G-,B-QLEDs）with uniform light emission and low V_on were realized,and the working mechanism of the devices was also discussed and analyzed.The following is the achievements.Based on the QLEDs using silver（Ag） as the cathode,a gradient annealing process was used to dry ZnO films,and the problem of TFB film cracking and non-uniform light emission of device which caused by direct annealing of ZnO film at high temperature was solved.With gradient annealing process,R-,G-,B-QLEDs with uniform light emission were fabricated.By testing the device layered films’micrograph through polarizing microscope,we found that when ZnO was directly annealed at 150℃,the TFB film was cracking and the QLEDs did not emit light at the cracks.For further research,we found the cracks were caused by the internal stress which was generated from solvent evaporation and volume change,as well as the mismatch stress of different expansion coefficient between TFB film and ZnO film.In order to solve this problem,we proposed a gradient annealing process to dry ZnO films,and the large internal stress which generated from direct annealing ZnO film at high temperature was reduced.The process inhibited cracks of TFB film,and R-,G-,B-QLEDs with uniform light emission were successfully fabricated.By optimizing the drying process of AgNPs,all-solution processed R-,G-,B-QLEDs with low V_on were obtained.In order to solve the problem of the higher V_on of all-solution processed QLEDs,the cathode interface characteristics were studied.By testing the photovoltaic effect of the devices,we found that the increase barrier in cathode interface of all-solution processed QLEDs was the reason which causing the higher V_on.By using SEM and film profilometer to characterize the cross-section morphology and thickness of ITO/ZnO/AgNPs devices,we found that the increase barrier at the cathode interface was due to the blending of ZnO/AgNPs.By using a combination of low vacuum and thermal annealing process to dry the AgNPs cathode,the interface barrier could be effectively inhibited,and this result was further verified by the photovoltaic effect test of the devices.Finally,the V_on of the all-solution processed R-,G-QLEDs reduced from 3.0 V and 4.2 V to 1.9 V and 2.5 V,respectively,and the V_on of the B-QLED was 3.4 V,which were comparable to that of the evaporated cathode devices.By introducing PVP buffer layer into the interface of ZnO/AgNPs,the performance of all-solution processed R-,G-,B-QLEDs were improved.Specifically,the maximum brightness of R-,G-,B-QLEDs increased from 27541 cd/m²,16885 cd/m² and 139 cd/m² to 39917 cd/m²,46591 cd/m²and 348 cd/m²,respectively.The maximum current efficiency of the devices increased from 5.2 cd/A,5.1 cd/A and 0.07 cd/A to 8.3 cd/A,9.4 cd/A and 0.15 cd/A,respectively.The mechanism of device performance enhancement was that the PVP buffer layer suppressed the defects of ZnO effectively,which were caused by the solvent and additives of AgNPs.Furthermore,the carrier balance of the device was improved due to the insulating properties of PVP.