Fabrication of organic thin films using slit nozzle with a wide viscosity spectrum

Abstract

A slit nozzle with a broad viscosity spectrum is highly demanded for slit coating of various materials with different viscosities without a compromise on the film quality. To this end, we have fabricated a multi-cavity slit nozzle with the inlet and vent holes for each cavity. Compared with a slit nozzle with a high-volume single cavity, such a multi-cavity slit nozzle further reduces the dead volume and thus material waste. As a design parameter in computational fluid dynamics (CFD) simulations for the multi-cavity slit nozzles, we have calculated the coefficient of variation in the internal pressure (Pcv) of the cavity and investigated the correlation between the simulated Pcv value and the measured thickness uniformity of low-viscosity (tens of cPs) poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), medium-viscosity (5000 cPs) polydimethylsiloxane (PDMS), and high-viscosity (18,000 cPs) hard-PDMS films. It is addressed that the Pcv value that can ensure the thickness non-uniformity of less than 5 % is of the order of 1 %, and the internal pressure of the nozzle should not exceed a specific upper limit to protect a coating system against overload. Based on the design guideline provided, we have found that the upper limit of the viscosity spectrum of the triple-cavity slit nozzle with the 800-μm-thick shim is 79,000 cPs when the flow rate is 50 ml/min. Using the multi-cavity slit nozzle, we have fabricated a highly uniform PDMS film exhibiting the tensile strain as high as 180 %, which can be used as a substrate for stretchable displays. Furthermore, we have successfully fabricated flexible OLED stripes on the slit-coated conductive PEDOT:PSS stripes, showing the average luminance of 72.8 cd/m2 at 5 V and inter-stripe luminance non-uniformity as low as 8.2 %.