Inkjet Printing PEDOT:PSS without Coffee Ring Effect for QLED Applicaitons

This paper describes a method to inkjet print Poly (ethylenedioxythiophene): polystyrene sulphonate (PEODT: PSS) without coffee ring effect. PEDOT: PSS is a unique long-chain polymer with conductivity and transparency properties that are mainly used in the packaging process of quantum dot light emitting displays and solar cells. Due to the ability to increase the transparency and conductivity of PEDOT: PSS in recent years, flexibility becomes another advantage of PEDOT: PSS. Therefore, the flexible electronic device such as the flexible quantum dot light emitting diode display, and the flexible solar cell is another exciting application of PEDOT: PSS. In order to save materials and reduce the manufacturing cost, inkjet printing is applied to fabricate the PEDOT: PSS thin film. The uniformity of PEDOT: PSS thin film is the crucial point for the flexible quantum dot light emitting diode display by inkjet printing. While the critical factor to have a uniform and flat PEDOT: PSS thin film is to reduce the coffee ring effect. The coffee ring effect caused by the fast evaporation rate at the edge of the droplet, which results in a higher degree of diffusion freedom at the edge of the droplet; then the capillary forms from the center to the edge, to supply the solvent for evaporation loss. However, the supplied solvent also brings solute to the edge and the solute despite to the edge, which causes the ring shape at the edge of the droplet. In this paper, the coffee ring effect was reduced significantly by adding co-solvent with high boiling point and high viscosity to the primary solvent. The high boiling point of the co-solvent reduces the driving force of the capillary flow, while the high viscosity of the co-solvent increase resistance of the capillary flow as well. What's more, the surface tension properties of the substrate was modified to increase the wettability of PEDOT: PSS. Thus, PEDOT: PSS thin film with improved uniformity can be obtained due to the reduction of coffee ring effect, which can apply for the packaging process of flexible quantum dot light emitting diode display.