Low-Cost Fabrication of Organic Photovoltaics and Polymer LEDs - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 238

Table 3 The sheet resistance change of conducting polymer anode and ITO-PET following the
change in bending angle
0 30 60 90 120 150 180
ITO PET (X/sq) 13.9 17.6 26.1 40.3 52.6 68.2 309.3
Polymer anode (X/sq) 68.8 69.0 69.0 69.1 69.2 69.5 69.7
The sheet resistance of ITO is significantly increased when the bending angle is increased. In
contrast, the conducting polymer shows almost the same sheet resistance. Reproduced from [ 15 ]
with permission of Wiley-VCH Verlag GmbH & Co. KGaA 2013
2.3.3 Coating Evaluations of Functional Layers in the PLEDs
For large-scale devices, the thickness of the blade-only coating is nonuniform in
the coating direction. Accordingly, the layer thickness in the initial state is usually
greater than that in the final state for a large-scale device. Thickness variation is a
serious problem associated with PLEDs, because difference in the functional layer
thickness, in particular the very thin electron injection layer causes variation in the
luminance in different areas. The final thinner area will be brighter than that of
initial thicker area as shown in Fig. 15 . Furthermore, an excessively thin area can
even cause electrical shorts in the PLED layers.
Coating of the active layers in PLEDs using the blade-slit coating system can
effectively address the above issues. The coating mechanism is similar to the slot-
die coating. The main difference between the two is the ink supply. Organic
electronic devices such as OPVs, OTFTs, and OLEDs have thin layers (from a few
nanometers to a few hundreds of nanometers). Thus, it requires smaller feeding
capacity of the solution than what typical slot-die coater delivers. Especially in the
PLEDs illustrated above, the ZnO NP layer/Ionic complex layer as the electron
transport/electron injection layer is even thinner: the total thickness of the two
layers is only from 15 to 30 nm. Therefore, it requires less and homogeneous
amount of ink supply. In this effort, we aimed at fabricating not only the hole
injection layer, emissive layer, and electron transport layer but also the much
thinner electron injection layer using the new blade-slit coating method. The
blade-slit coating system we developed does not employ external pumping system,
but utilizes only natural gravity and surface tension of the solution to flow out from
the capillary to the surface of the substrate, which can effectively reduce the flow
rate and the wet film thickness.
The following is a summary of the fabrication process. Each layer was fabri-
cated at a temperature of 45 C on the hot plate under ambient air conditions. The
blade-slit speed was 15 mm/s. Sputtered ITO glass (15 X/sq) was cleaned
beforehand by ultrasonic treatment in pure water, acetone, and IPA. It was then
subjected to a UV-ozone treatment for approximately 1 h. A PEDOT:PSS layer
(40 nm) was blade-slit coated onto the ITO glass, where the slit gap between the
blade and the slide glass was 70 lm. The yellow light-emitting polymer (S-Y)
dissolved in toluene at 0.6 wt% was then blade-slit coated (approximately 75 nm),
with the slit gap of 210 lm. The ZnO NP layer (approximately 30 nm) was