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Pathway to Low Cost Electrodes
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Printed Electrodes for OLED Panels
January 30th, DOE SSL R&D workshop
Hongmei Zhang
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• Overview of current OLED fabrication process-critical area for cost reduction
• How solution process can lead to reduction in
Photolithography processes • Material and process technique requirement for printing
anodes • Summary and conclusion
Outline
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Hurdle to OLED Lighting Adoption
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Implication of Cost Reduction Plan from a OLED panel Manufacturer
• Target for cost reduction in Photolithography process is from 100% today down to 3% by Gen8.
• All photolithography processes are in anode structure fabrication
• Direct patterning process technology are needed
* From LG Chem presentation at 2012 OLED summit in San Francisco
HIL
Anode
EML
ETL
Cathode
HTL
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Current Anode fabrication Process
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TC and Metal Contacts(without metal grids) Process
Substrate cleaning
Sputtered ITO
7 step process ($$$$) to obtain anode structure
Photolith Etch Metal Film Deposition
Photolith Etch to
form contacts
If Metal Grids are required
Insulator coating
HIL Film Formation
3 extra process steps are added to insulate metal grids
Photolith etch Substrate Cleaning
Total of 10 process steps are required to form anode structure, 6 of which are photolithography steps.
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• Slot-die coated HIL can potentially eliminated the need for insulator
• Increase thru-put (removed 3 process steps) • Reduction in material cost
Thick Solution Processed HIL Reduces Photolithography Steps
Glass Substrate
Metal Insulator is required in vapor HIL to prevent shorts
Vapor HIL
ITO
Glass Substrate
Metal Solution HIL
ITO
Standard Process
• Metal grids are necessary for panel (uniformity and power efficiency) since best TC > 5 Ω / sq
• Serial connections require even more organic layer patterning steps
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Further Reduction in Photolithography Processes Demands All Solution Processed Anode Structure
ITO still requires patterning
Glass
ITO
Vapor HIL Sputtered/patterned Metal Grids
Glass
Printed HIL ILE
Current Approach
Future Trend
Printed Grids All solution Processed Anode structure will eliminate the need for lithography and etch
Glass ITO
Print HIL Printed Grids
Current OLED Lighting panel manufacture are using ITO, some do not use metal grids
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Grids Requirement and Material Options Grids Requirment: •Sheet resistance < 1 Ohm/sq •>90% optical transparency •< 1 um is prefered •<150 um line width
High Conductivity Metals: •Ag is currently the material of choice for printing, other high conductivity metals such as Cu and Al are challenging for solution process due to oxidation issue. Au is too expensive
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Ag Ink and Printing Techniques
Printing Technique PROS CONS
Ink Jet Printing • High resolution • Good thickness control
• Multi-pass for thicker films • Lower throughput • Yield issue
Screen printing • High throughput • Mature technology •Simple tooling
• Lower resolution • Poor thickness control
Ag ink Options: • Current commercial available IJP or screen printable Ag inks are nanoparticle
based with typical resistivity value around 5 -10x bulk Ag • Thick grids are needed in order to achieve low sheet resistance while maintain
optical transparency. This post challenges for planarization. Example: with 5% area coverage and ρ = 5 * ρ (Ag bulk), the needed line thickness is 3.2 μm for 1 Ω / sq )
• New metal ink technology with resistivity close to Bulk Ag will enable thinner grid lines
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Requirement for HIL as Current Spread Media
Anode type Thickness * (µm)
Grid spacing (cm)
Rs (Ω/sq)
HIL resistivity (ohm-cm)
IJP Ag Grids + HIL 0.2-1 0.15-0.5 1-10 < 0.03
Integration of metal grids with HIL: • HIL wth high Conductivity
and high optical transparency • Compatible solvent with Ag
to prevent potential metal migration issues
Ag inks ρ <5µΩ-cm, grid area <10%, pixel uniformity >90%, line width~75-150 um
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Printed Metal Grids / Printed HIL Process
Roadmap for Low Cost Electrodes
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Current Metal Grids or Bus Bar / Vapor HIL Process Tri layer
metal deposition
Evaporate HIL film
7 step process ($$$$) to obtain substrate with Electrodes
Photolith
Etching to form metal grids
Insulator coating
Photolith Substrate Cleaning
Metal Grid / Printed (Thick) HIL Process Tri layer
metal deposition
Slot-die coating HIL
5 step process ($$$) to obtain substrate with Electrodes
Photolith
Etching to form metal grids
Insulator coating
Photolith Substrate Cleaning
Print Metal Grids
Slot-die Coating Photolith
Etching to form metal grids
Insulator coating
Photolith Substrate Cleaning
3 step process ($$) to obtain substrates with Electrodes
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Large Area OLED Panel on Printed Grids
• Large area single ‘pixel’ OLED made with HIL slot die coated on top of ink jet printed Ag grids on ITO electrode.
• No insulation layer on grids • No shorts observed
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OLED Test Device Made with HIL + Printed Metal Grids as Transparent Conductor
200nm HIL
HIL
Ag
• OLED device fabricated on Slot-die coated HIL on Ag metal grids as transparent conductor are shown above with varying HIL sheet resistance
• Solution HIL is used as both a Hole Injection layer as well as current spread media
Higher Sheet Resistance HIL Lower Sheet Resistance HIL HIL Planarize printed Ag metal grids
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Conclusion
• Research and Development effort in new anode material and process integration are needed for low cost electrodes.
• Material and process development should keep panel
manufacturing process flow in mind while developing new technology
• Cost reduction in OLED lighting manufacturing can not
simply rely on volume production and scale up. Material, process and equipment development will need to work together.
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OLED Adoption Pathway
OLED Display manufacture believes that VTE manufacturing process will not enable the price point for mass market adoption of large area TV. Beyond Gen5, printing process is necessary. This can pave the path for OLED lighting