Study On The Formation And Spreading Of OLED Ink Droplet Based On Inkjet Printing

Organic light emitting device(OLED)is widely used in display,lighting,and medical fields due to its advantages of fast response,wide color gamut and flexible preparation.Therefore,low-cost and high-efficiency manufacturing technologies for OLED is emerging.Compared with the current mainstream vacuum evaporation method,inkjet printing technology has gradually become one of the advanced technologies for OLED preparation due to its advantages of non-contact,high material utilization rate and simple process.The preparation of OLED by inkjet printing technology covers the processes of ink droplet generation,ink droplet flight and ink droplet spreading during substrate impacting,which synergistically affects the quality of the printed product.However,the generated satellite ink droplets during the droplet formation process and the nonuniform spreading of the droplets jointly affect the properties of the organic layer film,which in turn hinder the performance of the device.Therefore,it is of great significance to explore the characteristics from ink droplet generation to spreading process on substrate,under the influences of ink rheological characteristics and substrate characteristics,etc.,to improve printing quality and printing efficiency.In this paper,numerical simulation model of ink droplet generation and droplet impact on the substrate are established by COMSOL through the Level Set Method to investigate the influence of nozzle structure,ink properties and substrate characteristics on the ink droplet generation and droplet impact on the substrate process,which provides a theoretical basis for reducing the loss of raw materials and saving production costs.The main conclusions are as follows:For the ink droplet generation process,the influence of nozzle structure on the ink droplet generation process was studied first.The results show that the converging nozzle is more liable to generate stable ink droplet,sustained with the smaller the initial velocity required to generate ink droplet.Then,the influence of the initial velocity,ink viscosity,surface tension and density on the droplet generation process was studied by the converging nozzle.The simulation results show that a moderate inlet driving velocity is conducive to the droplet generation.Based on the model established in this paper,the droplet could be stably generated with the inlet velocity of 1.3m/s～2.6m/s.The droplet formation quality was evaluated quantitatively by taking the reciprocal Z value of the Ohnesorge number Oh into consideration.When the dynamic viscosity of ink ranges from 3.8mPa·s to 12.32mPa·s(that is,Z value ranges from 2.524 to 8.186),the quality of ink droplets is higher.When the ink surface tension ranges from 33.5mN/m to 76.38mN/m(that is,the Z value ranges from 8.375 to 12.647),the ink droplet can be stably generated,and the beam breaking time and the ink droplet ejecting velocity decrease with the increase of the surface tension,and the generated ink droplet diameter decreases with the increase of the surface tension.And the increase of the ink density from 736kg/m3 to 1410kg/m3(that is,the Z value range is 6.995 to 9.682)leads to the decrease of the ejecting velocity,and the increase of beam breaking time and the generated droplet quality.By combining the numerical simulation results with the beam breaking time,beam length,jet velocity and mass of the droplet,the droplet can be stably generated when the Z value is between 1.671 and 12.647,and the minimum inlet speed is greater than or equal to 1.3m/s.For the process of droplet impacting on substrate,the influence of droplet falling velocity on droplet spreading was studied first,which showed the droplet spreading process was divided into two modes of continuous spreading and discontinuous spreading according to the different falling velocity.Subsequently,the effects of substrate contact angle,bank contact angle,droplet diameter,surface tension and dynamic viscosity on the droplet spreading process were studied using a continuous spreading model.The results show that when the contact angles of substrate and bank change,a favorable spreading of droplets should exclude too large or too small contact angle.When the contact angle is between 30° and 90°,the spreading shape can be obtained.When the substrate contact angle is constant,the bank contact angle θ2 is less than 30°,droplet spreading final shape differs;while θ2 is greater than 30°,the differences in droplets spreading final shape are limited.When the bank sloping angle increases from 41.36° to 53.7°,the droplet expansion coefficient increases and the droplet center height decreases.When the droplet diameter increases,the droplet expansion coefficient decreases and the spreading time increases 60%.Furthermore,the time required for droplet spreading is inversely proportional to the ink density.And when the dynamic viscosity of the droplet increases from 3.8mPa·s to 18.8mPa·s,the final expansion coefficient of the droplet decreases,and the time required for the droplet to reach the stable spreading state also decreases.On the other hand,as the surface tension increases,the time required for the droplet to reach the final stable state decreases.Finally,the fitting empirical relationship between the spread time,Weber number and Reynolds number is obtained,and the phase diagram of Weber number-Reynolds numberexpansion coefficient is getted.