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Spray coating

Markus Hösel

Spray coating is widely known as an (industrial) method for car body painting and from graffiti artists using spray cans. The functional fluid or ink is atomized at the nozzle of the spray head, which generates a continuos flow of droplets as shown in figure 1. Pneumatic-based systems use a stream of pressurized air or gas (e.g. nitrogen or argon) that breaks up the liquid into droplets at the nozzle.DOI:10.1109/JSTQE.2010.2042282 The main parameters for the atomization process are surface tension, viscosity, fluid density, gas flow properties, and nozzle design. The quality of the coated layer is defined by the wetting behavior, surface properties, working distance, coating speed, droplet sizes, and the amount of sprayed layers. Besides the fluid-surface interaction the kinetic impact of the droplets influence the spreading of the droplets. Surface temperature plays an important role as well. The simplest form of a pneumatic-based system is an airbrush gun. More advanced spray generation methods that are commonly available are ultrasonication with directed carrier gases,DOI:10.1016/j.solmat.2008.10.026 or electro-spraying.DOI:10.1166/rnn.2012.1011

Spray coating
Figure 1. Simplified principle of spray coating

The advantage of spray coating is its high throughput and low material waste for large area depositions without patterning. Patterning through masks naturally involves a lot of ink waste. The combination of multiple spray head enables large area covering and/or multilayer coating. It is fully R2R compatible and the fast drying of the small droplets enable multilayer coatings with single solvent systems.DOI:10.1016/j.solmat.2008.11.052

Spray coating for OPV

The method has seen an increasing impact in the lab-scale fabrication of organic solar cells. Spray coating allows a broad range of solvent systems and a versatile selection of material systems. Therefore, it shows its applicability in intermediate layers, active layers, and electrodes. The material variety includes silver nanowires, nanoparticles, polymers, and nanotubes. The typical research focus is on compatible solvent systems, substrate temperatures, flow rates, and layer thicknesses to achieve the desired film morphology and optimized efficiencies.DOI:10.1002/adfm.201001562 The utilization of spray coating for all layers of the OPV stack has been shown by some groups.DOI:10.1016/j.solmat.2012.04.027DOI:10.1016/j.solmat.2011.05.037 An advantage is the possibility to avoid orthogonal solvents for bilayer coatings.DOI:10.1063/1.4807464

A lot of studies on spray coating have been carried out in detail but the application in large-scale R2R fabrication of OPV has yet to be shown. The problem in the realization can be seen in the patterning requirements for full additive manufacturing. Using masks or stencils in combination with spray coating in a R2R line is very challenging.

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

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