Digital thin film encapsulation
The ink design will always be a compromise between optimal system performance and optimal fit for use. To reach the highest performance quality in the most efficient way, ChemStream is using a modular inkjet printing (and curing) device to test different ink prototypes by successive iteration cycles. The MPU is very flexible towards:
- Printhead selection and replacement
- Print strategies
- Substratum selection and substratum height
- Ink replacement (small amounts)
In an ever changing world where focus is lying on sustainability and customized requirements, inkjet printing becomes a dominant manufacturing process for applying functional inks. Consequently, the related ink development process evolves towards a process that on its own has to be customized and based on sustainable processes and chemistry.
Within an integrated system approach, smart and fast iterations create the opportunity to develop dedicated low volume functional inkjet inks in a cost efficient way.
The best guarantee to reach the requirements of the customized functional inks is to start the ink development and formulation process together with the evaluation of the other system components (including printhead, drying /curing, supply system, …) during the feasibility phase.
Digital thin film encapsulation technology
Inkjet technology is one of the promising digital technologies to reduce the manufacturing cost of OLED displays (Organic Light Emitting Diodes) by patterning both the active layers and the TFE (thin film encapsulation) layers.
It is well known that the active layers must be protected from the environment (protected against oxygen and water penetration) to increase their lifetime.This can be accomplished by staggering several SiN (silicon nitride) layers (applied f.i. by atomic vapor deposition) in combination with on organic layer (applied by DOD (drop on demand) inkjet ink). A stagger of at least three SiN/organic double layers is necessary to minimize the negative effect of the always presented pinholes in the SiN-layer.Therefore the main functional requirements of the digital organic TFE layer can be summarized as follow:
- Layer thickness: 2-4 μm
- Water and oxygen barrier properties
- Good adhesion on SiN
- Fast spreading and leveling properties on SiN
- High transmission in visible region
- No yellowing after aging
Due to the industrial process, some additional system requirements must be added:
- Zero defect performance on Konica-Minolta 1024iS gray scale Printhead.
- Printspeed of 20 m/min
- UV Led curable (395 nm, 8W/cm2)
Figure 1: A quick view of 2 MPU’s (Modular Printing Units) in ChemStream printing facilities
Figure 2: Flexibility towards printhead selection: adding additional printhead module on MPU
Figure 3: Typical structure of OLED-cell with focus on the thin film encapsulation layer
Figure 4: Optimizing jetting performance by drop visualisation
Development of TFE Ink
The ink design is based on a combination of low viscous monomers and functional acrylic oligomers considering the rheological constraints for KM1024 printhead (viscosity < 10 mPas, surface tension: 30 mN/m).The fast spreading and leveling properties were realized at level 4 of the gray scale (full coverage without dewetting) by tuning the concentration of the surfactants taking inti account the possible interactions with the high surface energy substrate (SiN).The high UV-LED cure speed and full cure at 395 nm without yellowing, is based on an optimized photo initiation system in combination with well selected multi-functional monomers.The waveform / voltage optimization was performed by drop visualization with a drop watcher. The zero defect performance was evaluated by printing tests on the MPU (open time evaluation, nozzle failure, side shooters, ink/media interaction…).
ChemStream's System Integrated Approach