## Screen printing

Markus Hösel

Screen printing as illustrated in the figure 1 was probably the first and therefore the oldest method for the fabrication of "printed electronics". It was already used during the 1940s for the mass production of electronic circuits.DOI:10.1108/eb044350 Nowadays it is common in the graphics art industry, manufacturing of printed circuit boards, and metallization of silicon solar cells.

Figure 1. Principle of flatbed screen printing.

The mesh is placed in a certain snap-off distance on top of the substrate and a floodbar distributes the ink and fills the mesh. The printing process is initiated by the squeegee that presses against the screen mesh onto the substrate. The moving squeegee forces the ink through the open areas while the rebound of the screen induces shear to the columns of materials. Typically, high-viscous thixotropic ink pastes with shear thinning properties are used for screen printing. This prevents the instantaneous run through the mesh and enables the ink flow once a force is applied. The imprint of the mesh on the deposited ink is minimized throughout the subsequent leveling that depends strongly on the ink characteristics and drying procedures.

Screen printing is a characteristic thick layer deposition method with wet layer thicknesses from less than 10 µm up to more than 500 µm. It is given by the screen volume $V_\text{screen}$ measured in cm$^3$/m$^2$. The final pickout ratio $k_p$ depends on the process parameters such as squeegee force, printing speed, snap-off distance, snap-off angle, and ink rheology. The dry layer thickness $d$ can be empirically calculated with $$d=V_\text{screen}\cdot k_p\cdot \frac{c}{\rho}$$ where $c$ is the concentration of the solids in the ink in g/cm$^3$, and $\rho$ the density of the material in the final film in g/cm$^3$.DOI:10.1016/j.solmat.2008.10.004 Printing resolution of less than 100 µm are possible when using special screens. Detailed studies on the fundamentals of screen printing can be found elsewhereDOI:10.1108/eb044350Kipphan, Handbook of Print Media - Technologies and Production Methods.

Figure 2. Principle of rotary screen printing.

Flatbed screen printing is basically limited to a semi-continuous process workflow due to the up and down movement of the screen, although specialized machine designs allow a full continuous R2R process. True continuous prints with the possibility of gapless infinitely long repeating patterns are achieved through rotary screen printing as illustrated in figure 2. The basic functionality is similar to flatbed screen printing, whereby the squeegee is fixed inside a cylindrical screen that rotates relatively to the squeegee with the same speed as the substrate. The screen is typically an electroformed nickel mesh tube, either seamless or wrapped containing a seam. End rings stabilize the printing form and enable the mounting in the printing unit. A variety of mesh parameters are available to achieve different wet layer thicknesses and print resolutions. The continuous process can easily reach very high speeds of 180 m/min.

### Screen printing for OPV

Figure 3. Rotary screen printing of PEDOT:PSS back electrodes for OPV modules

The required ink properties (e.g. viscosity, low volatility) for screen printing can be limiting factors in the broad usage in the field of OPV. Although commercial silver and PEDOT:PSS inks are available, ink adjustments are sometimes necessary. Nevertheless, flatbed screen printing has been used for the deposition of active layers.DOI:10.1143/JJAP.48.020208 The main usage is the printing of PEDOT:PSS and silver electrodes, either full layer or grids. An important study was made to evaluate the influence of different silver paste solvents on the OPV behavior.DOI:10.1016/j.solmat.2010.11.007 The full upscaling potential has been shown in several reports where rotary screen printing was used for the deposition of front and back electrode PEDOT:PSS and silver grids. All processes were printed in register to enable virtually infinitely long modules with thousands of serially connected solar cells. DOI:10.1002/ente.201200055 DOI:10.1002/adma.201302031 Screen printed silver grid can also be embedded in the substrate to fabricate electrodes for OPV devices.DOI:10.1016/j.solmat.2010.08.011

Figure 4. Video of rotary screen printing.

M. Hösel, Large-scale Roll-to-Roll Fabrication of Organic Solar Cells for Energy Production, PhD thesis

## Current weather

Temperature: 21.33 °C
Sample temp: 32.79 °C