## Slot die coating

Markus Hösel
Figure 1. Principle of slot-die coating.

Slot-die coating is a non-contact large-area processing method for the deposition of homogeneous wet films with high cross-directional uniformity. It can handle a broad range of viscosities between less than 1 mPa$\cdot$s and several thousand Pa$\cdot$s while the coating speed has a similar wide spectrum between less than 1 m/min and more than 600 m/min. The method belongs to the pre-metered coating processes, whereby all of the supplied liquid is deposited on the substrate. The working principle is shown in the figure above. The wet film thickness is controlled by the flow rate, coating width, and speed. The resulting dry layer thickness $d$ in cm for a given ink can be expressed with the formula $$d=\frac{f}{v\cdot w}\cdot \frac{c}{\rho}$$ where $f$ is the flow rate in cm$^{3}$/min, $v$ is the coating speed in cm/min, $w$ the coating width in cm, $c$ 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

The slot-die coating head as seen above is made from stainless steel and contains an ink distribution chamber, feed slot, and an up- and downstream lip. An internal mask (shim) defines the feed slot width and allows stripe coating.DOI:10.1007/s11998-013-9485-3 The mask thickness depends on the viscosity of the ink and is typical in the range of 25-50 µm for low-viscous inks <20 mPa$\cdot$s. Furthermore, stripe coating at slow speed is supported by a second internal mask with small protrusions (meniscus guide) at the coating lip that prevent the joining of the menisci between two adjacent stripes.DOI:10.1016/j.solmat.2008.12.012 The main purpose of slot-die coating are full layers or 1-dimensional stripes but it also allows intermittent batch coating of high viscous slurries as used in the fabrication of battery electrodes. Slot-die coating operates in certain parameter regimes (coating windows) that can be calculated or evaluated experimentally. The slot-die process has been extensively studied in all its varieties beginning from lip forms, manifold design, coating windows, over flow-simulations to meniscus forming.DOI:10.1002/app.38617DOI:10.1002/pen.21360DOI:10.1002/adv.21271 General and detailed information about the coating method can be found in the booksTracton, Coatings Technology: Fundamentals, Testing, and Processing Techniques.

### Slot-die coating for OPV

Figure 3. Slot-die coating of active layer ink (16 stripes).

The stripe coating capability as seen in figure 3 it is favorable for the fabrication of OPV devices, as it allows easy stacking and cross-directional alignment of the layer stack for modules with serially connected cells. The fully closed system between pump and ink exit is ideal for highly volatile solvents. It has been extensively used for the coating of ZnO, active layers, PEDOT:PSS, and silver.DOI:10.1016/j.orgel.2009.03.009DOI:10.1016/j.solmat.2008.12.012DOI:10.1016/j.solmat.2012.07.004 More or less all of the currently R2R fabricated OPV devices from our group (and showcased on plasticphotovoltaics.org) contain at least slot-die coated ZnO and active layers in its layer stack. Important work on the fluid-dynamic properties of the active layer and PEDOT:PSS with different solvents and additives was carried outDOI:10.1007/s11998-013-9483-5, while the coating speed limitation was determinedDOI:10.1016/j.solmat.2012.10.007. The advantage of the pre-metered processing is the control over flow rate and wet layer thickness, which makes it an ideal tool for gradient studies of functional layers along the web direction. A huge parameter space of different material ratios or layer thickness in OPV devices can be tested very fast under real production conditions.DOI:10.1021/am100505e

Figure 4. Video of slot die coating.

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

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