Research On Quantum Dots Ink Fomulation For Inkjet Printing

Quantum dot light-emitting diodes(QLEDs)have been paid much attention in recent years because of their excellent photoelectric properties.Inkjet printing technology is a solution processing technology that does not require high precision mask and has high material utilization rate.It may become the preferred way of QLED display mass production technology.However,there are still some problems in the reported inkjet printing quantum dot devices,such as unsuitable printing ink,poor printing film morphology and low device performance.Aiming at the pixel structure device,this paper designs a new type of composite quantum dot ink,which is composed of cyclohexylbenzene(CHB)with low boiling point and high surface tension and 1-octadecene(ODE)with high boiling point and low surface tension.The ink,while ensuring stable printing,enhances the Marangoni flow.The Marangoni flow is adjusted by adjusting the ratio of the two components to balance the capillary flow in the solution,eliminate the coffee ring effect,and obtain a smooth and uniform QD film.Due to the addition of high boiling point ODE in the ink,the evaporation rate of the solution is slowed down,and the printing time can be extended to 15 minutes,which improves the long-term printing ability of the ink and the uniformity of the film.On this basis,the 240 PPI resolution of the printed inverted top emitting green QLED device was successfully prepared,the turn-on voltage of 2.7 V,the maximum brightness of 132510 cd/m~2,the maximum EQE of 14.0%.In view of the large performance difference between inkjet printing devices and spin coating devices,we studied the effect of each step of inkjet printing on the performance of quantum dot devices,and improved the performance of the devices by adding surface ligands.The analysis of each step of inkjet printing shows that the vacuum operation will cause the surface ligands to fall off,the surface defect states to increase,and the fluorescence quantum yield(PLQY)to decrease,thus reducing the device performance.By adding an appropriate amount of octyl mercaptan into the ink,the loss of ligands on the surface of quantum dots caused by ink replacement was supplemented and the surface defects were passivated.As a surfactant,octyl mercaptan can enhance the Marangoni flow from the outside to the inside,improve the flatness of the QD films and the uniformity of the electroluminescence of the devices.By screening the content of octyl mercaptan,it was found that 5wt%octyl mercaptan could keep the morphology of the printed QD films in the flattest shape,and also keep the PLQY of the printed QD films in the highest level.On this basis,we finally prepared 120 PPI printing inverted red QLED device,the turn-on voltage of 2.7 V,the maximum current efficiency of 16.2 cd/A,the performance of the printing device is 82%higher than that of the printing device without octyl mercaptan.