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Blade coating

Markus Hösel

Blade coating, also known as knife coating or doctor blading, is a processing method for the fabrication of large area films on rigid or flexible substrates. The well-defined thickness is mainly controlled by the gap size of the blade to the surface. For lab-scale processing the blade is moved over a flat surface. For large-scale R2R processes the blade is fixed over the moving substrate. The ink is placed in front of the fixed blade whereby the substrate moves relatively to the blade as shown in in figure 1. Adjustable gap widths allow the deposition of variable wet layer thicknesses. The final wet layer thickness is roughly half of the gap width depending on the coating speed and flow behavior.DOI:10.1016/j.cep.2012.03.004 Further coating parameters that influence the film formation are surface energy of the substrate, surface tension of the fluid, coating speed, viscosity, and surface temperatures. The dry layer thickness $d$ can be calculated from the empirical relationship \begin{equation} d=\frac{1}{2}\cdot g\cdot \frac{c}{\rho} \end{equation} where $g$ is the gap width, $c$ 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

R2R knife coating or doctor blading
Figure 1. Principle of R2R knife coating or doctor blading.

Patterning during coating is virtually not possible and needs to be kept in mind for a potential upscaling to R2R fabrication of module based OPV devices. The advantage of blade coating is minimal ink waste during coating and almost all ink will be applied. This is a large advantage compared to spin coating where only a fraction of the ink is utilized.

Blade coating for OPV

The blade coating method has seen an increasing attraction for the fabrication of small lab-scale devices, especially for active layers and electrodes.DOI:10.1016/j.orgel.2009.03.001DOI:10.1002/adfm.201100457DOI:10.1063/1.2402890 Some reports use doctor blading only for one layer of the device but several devices were successfully fabricated in an all blade coated approach.DOI:10.1002/aenm.201200317DOI:10.1002/aenm.201300100 The resulting efficiencies are comparable with spin coating. Doctor blading is a very simple technology and has therefore also been used for studying drying kinetics and morphologies of the active layer film.DOI:10.1021/nn2036279DOI:10.1063/1.3270402

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



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