Low-Cost Fabrication of Organic Photovoltaics and Polymer LEDs - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 229

Fig. 4 The image of the
irregular coating streaks
(yellow patterns) with the
thermally deformed
aluminum cathode (metallic
bubble-like patterns) due to
the leakage current. The
aluminum cathode was
peeled out from the polymer
light emissive layer after the
device test

leads to higher capillary number and instability. Reducing the solution concen-
tration, the film quality can be improved as shown in Fig. 5 . To improve the film
coating quality, the capillary number should be controlled by, e.g., reduction of
coating speed or viscosity. However, the desired film thickness may not be
achievable. Therefore, the contact coatings causing the extensional viscous flow of
the polymer solution between the roll and substrate have more serious instability
issues. The instability analysis can also apply to the coating processes such as
blade coating where the blade does not have direct physical contact with the
substrate (and hence will be referred to as non-contact coating). Because the blade
or the slot-die coatings do not cause extensional viscose flow, they are frequently
more suitable choices.
In the case of low-band gap polymer semiconductor (iI-T3) [ 9 ] dissolved into
dichlorobenzene, because the solution has very low surface tension, the capillary
number depends highly on the coating speed. Coating meniscus can be broken due
to low cohesive energy of the solution under fast coating condition, showing as
irregular patterns due to instability. On the other hand, the coating quality of the
low-band gap polymer will be improved by reducing the coating speed as shown in
Fig. 6 .
There is another important parameter to determine the coating instability. It is
the leveling time for the solvent in the wet film to be evaporated after the coating
process. If the solvent evaporates relatively slowly, the film can be self-leveled
under the surface tension, and the irregular coating pattern resulted from the
instability will disappear by the flow itself.
The wet film thickness containing the instability can be considered as a simple
wave with amplitude d and characteristics wavelength (2p/q) as follow (Fig. 7 ):

e ¼ e 0 þ de cos ð qx Þ ð 9 Þ