Interface Materials And Ink Engineering For High-efficiency Inkjet-printed Quantum Dot Light-emitting Diodes

Quantum dots（QDs）have attracted a great deal of attention because of their promising applications in the field of lighting and display.In terms of small-area devices,the external quantum efficiency（EQE）of red/green cadmium-and perovskite-based quantum dot light-emitting diodes（QLEDs）fabricated by spin-coating process has exceeded 20%.At present,the efficiency improvement of blue QLED,the operational stability,and the preparation technology of color pixel patterns are the biggest obstacles to the industrial application of quantum dot light-emitting technology.Inkjet printing technology is considered to be one of the most promising,low-cost,mass-produced patterned QLEDs for next-generation display technologies.However,very few works have focused on inkjet printing of perovskite QD inks and their QLED devices.Therefore,understanding the interaction between the ink and different polymeric underlayers and clarifing why inkjet-printed QLED devices are less efficient than spin-coated devices are key steps in advancing this technology toward practical applications.Given the problems in inkjet-printed QLED devices,this thesis intends to construct high-performance inkjet-printed QLED devices by interface and defect engineering using customer-synthesized cadmium-and perovskite-based QLDs and selecting appropriate hole transport layers（HTLs）.The research of this dissertation consists of the following three main parts:1)A cadmium-based inkjet-printed QLED device with high efficiency was constructed by suppressing the interfacial erosion effect during the inkjet printing process.Three commonly used hole transport materials（HTMs）,namely poly[（9,9-di-n-octylfluorenyl-2,7-diyl）-alt-（4,4’-（N-（4-n-butyl）phenyl）-diphenylamine）]（TFB）,poly(N,N’-bis(4-butylphenyl-N,N’-bis（phenyl）benzidine（poly-TPD）,and polyvinylcarbazole（PVK）,were used as the underlying layer and their corrosion problems associated with common printable solvents and the optimized ink formula（cyclohexylbenzene/indane）were systematically investigated.Among them,PVK exhibited excellent solvent resistance,and the current efficiency of PVK-based printed red QLED device was 28.80 cd·A^-1,and the corresponding maximum EQE exceeded 17.00%.Furthermore,the maximum EQE of inkjet-printed QLED devices with TFB and Poly-TPD as HTLs was 0.20%and 1.90%,respectively,due to their poor solvent resistance.The results of this study indicated that polymeric transport layer materials with different structures exhibited different solvent resistance in inkjet-printed QLED devices and had a significant impact on the device performance.2)A high-performance inkjet-printed red Cd-based QLED was achieved by controlling the molecular engineering of the underlying layer and defect regulation of the inkjet-printed QD films.Three PVK HTMs with different molecular weight（Mw）were systematically investigated,among which m-PVK with an average Mw of 5 M to 10 M g·mol^-1yielded the best QLED device performance,due to its suitable HOMO energy level and the highest hole mobility.This result indicated that the Mw of the polymeric HTMs had a significant impact on the QLED devices performance,and thus m-PVK was selected as the HTM for the next step’s fabrication of inkjet-printed QD thin films and QLED devices.Moreover,a high-quality QD thin film with low surface roughness,orderly packing,high photoluminescence,and good electrical transport properties was obtained by defect regulation of the printed QD thin film with a pressure-assisted thermal annealing process.As a result,inkjet-printed red and green QLEDs achieved a record EQE of 23.08%and 22.43%,and the corresponding operating lifetimes were determined to be 343,342 h@100 cd·m^-2 and 1,500,463h@100 cd·m^-2,respectively.The above results pave the way for efficient and stable inkjet printing of QLEDs,and thus accelerate the commercialization of QLED technology.3)A ternary ink formulation was developed to improve the surface morphology of printed films and reduce surface defects to construct efficient inkjet-printed perovskite QLEDs.By adjusting the ratio of ternary ink to suppress the coffee ring effect,we proposed the corresponding solvent volatilization and film formation for inkjet printing of perovskite QD thin films.Compared with binary-solvent-QD-ink（naphthane and n-tridecane）,the tailor-made ternary-solvent-QD-ink（naphthane,n-tridecane,and n-nonane）exhibitted better printability and film-forming ability,resulting in a much better qualified perovskite QD films with fewer surface defects.The introduction of the low-boiling-point solvent n-nonane could prolong the Marangoni flow during the volatilization process,which accelerated the evaporation flow of the whole system（volatilization time was reduced by half）,and also greatly inhibited the agglomeration of QDs.Consequently,a record peak EQE of 8.54%was achieved in inkjet-printed green perovskite QLED,which was much higher than that of the binary-solvent-system-based device（EQE=2.26%）.Additionally,the ternary-solvent-system exhibited general applicability applicability in inkjet-printed red and blue perovskite QLEDs as well as red and green cadmium（Cd）-based QLEDs.This work demonstrates a new strategy for tailor-making a universal QDs ink formulation for efficient inkjet-printed QLEDs,as well as other solution-processed electronics in the future.