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Flexographic printing

Markus Hösel

Flexography or flexo printing is a commonly used technology in the commercial printing industry, especially for foil substrates. It is a very fast method for a wide range of substrates with web widths beyond 1.5 m. Industrial flexo printing presses can run at hundreds of meters per minute.

The printing method is illustrated in Figure 1 and is based on a soft printing plate whereby the raised areas transfer the ink. The material can be rubber or photopolymer of different hardnesses and material qualities depending on the application and ink. The printing plate is taped onto the printing cylinder or manufactured as a gapless sleeve. The plate making process can be carried out through light exposure, developing and washing, or through direct laser engraving.

Flexo printing
Figure 1. Principle of flexo printing.

The second important part of the printing unit is the anilox roller made from ceramics. It contains an engraved surface with small cavities/cells of a certain ink volume (ml/m$^2$) that can be transferred to the substrate depending on the overall pick-out and transfer ratio. The anilox roller is filled with ink through a fountain roller (3-roller-system) or directly through a closed chambered blade system (2-roller-system). Excess ink is scraped off with a doctor blade. It is then brought into contact with the printing form cylinder that picks out the ink and transfers it to the substrate. Contact pressure between the printing form cylinder and impression cylinder is kept low to avoid typical flexo printing phenomena such as halos (squeezed edges). Viscous fingering due to the ink splitting between printing form and substrate surface is a typical phenomena and the printed layers can have slightly inhomogeneous surfaces. The ink rheology is similar to gravure printing inks with low to medium viscosities below 500 mPa$\cdot$s. Further printing parameters that can be adjusted for achieving optimized print results are anilox cell geometries, ink surface tension, nip pressure, and printing speed. Furthermore, the ink rheology and drying time has an effect on the ink leveling after exiting the printing nip. General and detailed information about the printing method can be found elsewhere Kipphan, Handbook of Print Media - Technologies and Production Methods.

Flexo printing silver nanoparticle ink
Figure 2. Flexo printing silver nanoparticle ink.

Conventional 4-color flexo printing is based on half-tone images but in functional printing full layers or fine line patterns are required. Resolutions below 100 µm can be easily achieved. The flexo printing process is not yet broadly employed for OPV like the other printing methods and limited to a narrow range of functional inks such as silver nanoparticle (AgNP) ink or PEDOT:PSS. The applicability on printed electronics devices is reported with the fabrication of paper-thin loudspeakers,DOI:10.1016/j.orgel.2012.06.048 OTFT electrodes and dielectrics,DOI:10.1016/j.orgel.2007.02.009DOI:10.1016/j.orgel.2012.05.045 and electroluminescent layers.DOI:10.1039/B809450K

Flexo printing for OPV

In OPV devices flexo printing is mainly used for the fabrication of silver electrodes. An exception is the deposition of a transparent PEDOT:PSS anode on top of the active layer using a highly optimized ink mixture. The paper based solar cell achieved an efficiency of >1.3 %.DOI:10.1002/aenm.201100394 Otherwise, AgNP grid front electrodes for the fabrication of transparent conductive substrates are currently the main application for flexography. The FlextrodeDOI:10.1002/ente.201200029 substrates with honeycombs or slanted grid fingers and PEDOT:PSS/ZnO have been heavily used for the fabrication of a variety of OPV devices.DOI:10.1002/ente.201200029DOI:10.1002/adma.201302031 Finger widths below 100 µm and fabrication speeds of >20 m/min are possible.

The challenge of flexo printed silver grid lines and layers can be spikes that may appear after ink transfer to the substrate. Studies for the flexo-based fabrication of fine line silver grid networks show promising results for the employment of this production method.DOI:10.1016/j.tsf.2010.05.125DOI:10.1109/EPTC.2011.6184475 The reports show the importance of printing pressure and ink volume on the achievable line width, height and layer morphology.

Figure 3. Video of flexographic printing.
M. Hösel, Large-scale Roll-to-Roll Fabrication of Organic Solar Cells for Energy Production, PhD thesis

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