Introduction to OLED Design and Test Method Development
OLED SSL Stakeholder MeetingHollis Beagi – Josh CumminsOctober 2017
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Flexible OLED Overview
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OLED Overview: Configuration
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OLED Overview: Flexible Design Problem
Materials must meet following criteria without failure:
1. 50,000+ cycles of bending at ambient conditions
2. 5, 3, or 1 mm bending radius
3. Unfolding of display at hot and cold temperatures
4. Survive scratch and impact
5. Ball drop / device drop
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OLED Overview: Important Terminology
Terms Important to Understanding OLED and Flexible Design:
1. Stress: force per unit area
2. Strain: amount of elongation or compression for a given load
3. Shear: force causing two surfaces to move parallel to each other
4. Elastic (Young’s) Modulus: determined by slope of initial linear region of stress-strain curve. Indicates relative springiness or rigidity of a material
5. Poisson’s Ratio: ratio of contraction of material in lateral direction to extension of material in stretch direction
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OLED Overview: Important Terminology
Terms Important to Understanding OLED and Flexible Design:
6. Yield: point at which a specified amount of plastic deformation has occurred
7. Ultimate: maximum load or stress a material can withstand before breaking
8. Fatigue: repeated loading and unloading leading to failure
9. Neutral Axis: line or plane through a surface connecting points at which no extension or compression occurs during bending
10.Decoupling: separating or eliminating the interaction between surfaces/layers in a stack up of layers
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OLED Overview: Theoretical Approaches
Design Strategy: Place the fragile layers at the neutral plane
What We Need To Know: For each layer….
• Modulus (E) & Poisson’s ratio
• Thickness (t)
• Yield & Fracture stress/strain
• Fatigue behavior
Assumptions: No delamination or slip between layers and plane sections remain plane
Success Metric: A good design keeps stresses/strain below failure levels in each layer subject to
adequate optical engineering
US20140367644A1
Test Method Development
Dynamic Bend Testing Capabilities3M Dual Hinge Bend TesterYuasa
Dynamic Bend Testing Capabilities3M Dual Hinge Bend TesterYuasa
Strained Bending – Single-hinge and Mandrel Bend Designs(why we do not use these types)
Moving the attachment adhesive out to the ends imparts less strain on the sample. Tom Corrigan, SEMS
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Str
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Fixed Distance "f" [mm]
Max Strain vs Fixed Distance(2mm gap)
Hinge design is important. Single hinge and mandrel bending will increase strain on the test stack, whereas dual
(or multiple) hinge designs can enable bending without added strain.
*Some images shown here taken from paper by Tom Corrigan
Mandrel Bend Motion Profile (f/ Yuasa website)https://www.yuasa-system.jp/pdf/tech/20151202_FS_en.pdf
Dynamic Bend Testing Capabilities3M Dual Hinge Bend TesterYuasa
Yuasa vs 3M Dual-Hinged Bend Tester
Sample bows above bend apex
Sample stays flat against plates during bending
We see different results (cycles to fail) for Multi-layer samples on Yuasa vs 3M DH Bend Tester (see previous slides).
This may be due to differences in stress amplitude because:
1) Yuasa does not control shape of sample (force U-shape) during
bending
2) Yuasa cannot open all the way to 180D flat
or fully close to 90D folded during bending.
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Attachment Location
Natural bent conformation of thin film stacks is elliptical, not semicircular.
This must be considered when bonding / constraining the test stack during testing to
prevent undue strain.
Above: Large deflection shear deformable bending analysis (conducted by Fay Salmon, SEMS) shows that thin elastic films will conform to the shape of an ellipse.Left: Bent thin film stack image analysis and validation of bent shape conformation (Samad Javid, CRPL)
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Crazing (whitening) and Breakage due to film strain and fatigue
dislocations
Local buckling due to adhesive shear and adhesion failure
Inverted Bendingdue to severe global buckling
Mechanical Failure Modes
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Effects of Plate, Fold Axis or Sample Misalignment
8% increase 8% decrease 5% increaseStress contour plot:
9% increase 8% decrease 5% increase
Aligned plates
1° plate rotation around x-axis
1° plate rotation around x-axis, sample off centered
1° plate rotation around z-axis
Strain contour plot:
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Effects of Plate, Fold Axis or Sample Misalignment
General Smile Pattern (root cause = curved plates – confirmed by gap measurement using feeler gauge)This graph shows cycles to break results for 4mil LmPEN (Low Melt PEN) tested at R2. Plates B show smile pattern as compared to two replicates of data collected with plates A.
Wave-form (root cause = bearings too loose, and torque occurring on one end of plate where drive belt is located, allowing “wave-like” motion of plates during folding)
Improved capability with changes from Gen1 to Gen2 3M DH Bend Testers:Each stage of the control chart above represents a change to tester design, which improved variability greatly. At this point, the primary source of variability between 3M DH Bend testers is due to slight differences in gap size as all issues with misalignment and plate curvature have been addressed.
Flexible OLED Light Testing
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OLED Flex Testing
Objective 1: Develop test method for flexible OLED devices
Objective 2: Test flexible OLED devices to failure and characterize failure modes
• Started with optically in-active sample for TM development
• Look for mechanical failures first
• Test working OLED sample looking for change in light emission and/or failure
Sample ID Dimensions (cm) Thickness Contacts Notes
B3.1 14 x 1.5 10.7 Wire Leads
B3.2 14 x 1.5 10.2 Wire Leads Creased
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Flex Testing
*Thousand cycles
400k cycles total on same sample
Room temp (70F, ~40% RH)
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Flex Testing
*Thousand cycles
400k cycles total on same sample
Room temp (70F, ~40% RH)
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Flex Testing
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Flex Testing
200k cycles total on same sample
Room temp (70F, ~40% RH)
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Wavelength (nm)
Intensity vs Wavelength for 6mm Bend Radius Flex Test
0 Cycles 100 Cycles 1,000 Cycles 2,000 Cycles
3,000 Cycles 4,000 Cycles 5,000 Cycles 10,000 Cycles
15,000 Cycles 20,000 Cycles 25,000 Cycles 30,000 Cycles
40,000 Cycles 50,000 Cycles 60,000 Cycles 80,000 Cycles
100,000 Cycles 130,000 Cycles 160,000 Cycles 200,000 Cycles
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Intensity vs Wavelength 3 mm Bend Radius Flex Test
0 Cycles 100 Cycles 1000 Cycles 3000 Cycles
10k additional cycles on same sample
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Flex Testing
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1. Adhesive properties
2. Tunability of Material Properties
3. Geometry
4. Process Parameters
5. Accurate Prediction
OLED Overview Summary: Important Considerations