## Gravure printing

Markus Hösel

Gravure printing is a traditional printing method for high-volume applications such as magazines, catalogs or packaging with printing speeds beyond 15 m/s. It found the way into printed electronics due to its potential of high resolution prints, smooth layers, and the variety of processable materials.

Figure 1. Principle of gravure printing.

The working principle is illustrated above. The gravure cylinder is made from steel and a thin copper layer that holds the printing image consisting of engraved cells. A final chromium layer acts as a wear resisting layer. The cells of the printing form can be fabricated through etching, electro-mechanical engraving, or laser engraving. Depending on the application and the desired printing results the cells have different depths, sizes, screen ruling, or shape that define the print volume in ml/m$^{2}$. The printing cylinder is immersed in an ink bath that fills the cells with ink while rotating. The doctor blade scrapes off excess ink leaving the unpatterned chrome surface blank. Another way of inking the gravure cylinder is through a chambered blade system that keeps the ink in an enclosed system without exposing it to the environment. The ink is transferred to substrate through surface interactions the nip zone between impression roller and gravure cylinder. The nip force between the hard gravure cylinder and the soft impression roller is generally quite high compared to flexo printing. The ink transfer is based on complex interactions between the cell characteristics, printing parameters and ink rheology. The transferred ink of low viscosity with tens of mPa$\cdot$s requires good leveling after deposition to form a homogenous layer due to the structured printing image based on small cells. A variety of direct gravure printing is gravure offset, whereby the image is first transferred to a soft blanket roller and then onto the substrate.

Figure 2. Gravure cylinder with engraved stripe pattern.

Gravure has the ability to print a variety of functional materials and fine lines with resolutions below 30 µm. It can therefore be used for printing transistor structures DOI:10.1016/j.orgel.2010.09.003DOI:10.1109/LED.2011.2165695 or functional traces for printed electronics.DOI:10.1109/TCAPT.2009.2021464 For example a high resolution grid pattern with line widths down to 20 µm can be printed through a gravure offset process.DOI:10.1039/c2jm30198a

### Gravure printing for OPV

Gravure printing has the ability for very homogenous and thin layers, and was therefore used in the fabrication of OPVs and similar structured OLEDs. The typical materials printed are PEDOT:PSS as electrodes and P3HT:PCBM as active layer with studies on wetting behavior, solvent compositions for best printability, printing parameters and morphology. Exemples of publications on gravure printed OPVs are DOI:10.1016/j.solmat.2010.12.020DOI:10.1016/j.solmat.2010.05.027DOI:10.1016/j.solmat.2012.12.044DOI:10.1016/j.solmat.2010.10.013. Characteristic for the majority of the gravure experiments is the usage of active layers with high solid loads beyond 10 wt%. Although some R2R processes were carried out most of the studies were performed on flatbed gravure test systems. Gravure printing is often used just for printing full layers or continuous stripes where the cell was later on built up, often with evaporated electrodes. The 2-dimensional pattern generation that is important for material-efficient cell structuring and module fabrication is limited. The achieved efficiencies are below 3 %.

Kipphan, Handbook of Print Media - Technologies and Production Methods M. Hösel, Large-scale Roll-to-Roll Fabrication of Organic Solar Cells for Energy Production, PhD thesis

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