Low-Cost Fabrication of Organic Photovoltaics and Polymer LEDs - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 225

Table 2 The printing parameters and features in conventional printing technologies

Inkjet Screen Gravure Flexo. Offset

Resolution (lm) 16-50 30 20-75 75 30-50

Thickness (nm) \100 [1000 \50 nm \50 nm \1000

Viscosity (mPa s) \20 500-50000 50-200 50-500 20-100 k

Coating speed (m 2 /s) 0.1 \10 60 10 20

Polymer ink s s s s 9

limited by the requirement of high ink viscosity. Inkjet printing is useful for fine
patterning and thinner film using less viscous solution, but due to its serial process
nature it is not suitable for large area film coating. In terms of large area coating,
roll-to-roll coating has more flexibility to be adapted for the various coating area
and to obtain varied film thickness.

1.2 General Coating Characteristics in Coating Methods:
Thickness Control

This section introduces and compares general coating processes such as spin
coating, knife coating, and slot-die coating. Since film thickness and uniformity
are more important factors for electronics devices than traditional coating indus-
tries, we will examine some equations to get a clear picture of the key parameters
affecting the film thickness. We aim at understanding the general coating char-
acteristics for the practical coating process.

1.2.1 Spin Coating

Spin coating is one of the most popular coating methods as it is simple and easy to
realize the intended film thickness. The solution dispensed on the substrate can be
spread by a centrifugal force and the wet film is thinned down during the spin
process. The film thickness depends on the viscosity and concentration of the
solution. However, the final thickness of the film is mainly controlled by the spin
speed, and thinner film can be achieved at higher spin speed.
In the following equations h w is the wet film thickness, x: rate of the solids
contacting on the substrate, x: angular speed, q: solution concentration, C: con-
stant for the coating gas in the coating chamber, l: viscosity of the solution. The
wet film thickness can be expressed as follows:

1 = 3

e
21 x

h w ¼ x ð 1 Þ

ð Þ K