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Melinex®and Mylar® are registered trademarks of DuPont Teijin Films U.S. Limited Partnership. Teijin® Tetoron® is a registered trademarks of Teijin Limited and are licensed to DuPont Teijin Films US, Limited Partnership.
Teonex® is registered trademark of Teijin DuPont Films Japan Limited and licensed to DuPont Teijin Films U.S. Limited Partnership. Copyright © 2004-2009 DuPont Teijin Films (UK) Ltd. All rights reserved.
Recent Improvements in PET film for
Flexible Electronics and Photovoltaic Applications
Jan LaRiviere, Keith Rollins, Bill MacDonald, and Bob Rustin
AIMCAL 2012
Scope
Polyester film process
Heat stabilized polyester film
Latest film developments for flexible electronics Low Bloom, Planarization, Refractive Index control
Latest film developments for flexible PV
Weatherability, Adhesion
Conclusion
Tm = 255 °C
Tg = 78 °C
O
O
O
On
Melinex®, Mylar® and Teijin® Tetoron® Polyethylene terephthalate (PET)
n
O
O
O
OTm = 263 °C
Tg = 120 °C
Teonex®
Polyethylene naphthalate (PEN)
Forward Draw
Transverse Draw • PET and PEN polyester films
• Biaxially oriented, semi-crystalline
• High stiffness
• Dimensional stability
• Optical transparency
• Solvent resistance
• Thickness = 0.6-500 µm
Polyester Film Technology (1)
Polyester Film Technology (2)
• Off-line Heat Stabilisation
• Allows relaxation in MD
Minimum shrinkage on both directions
• In-line Heat Stabilisation
• Film can relax in TD but not in MD
Leads to shrinkage on subsequent processing
• Oven
Upper temperature for processing
Young’s Modulus at 20 °C
Young’s Modulus at 150 °C
Glass transition temperature
Haze
Shrinkage in MD after 30 min at 150 °C
CTLE
Moisture pick-up at 200 °C, 40% RH
ST506 (PET)
Q65FA (PEN)
0.05%
180-220 °C
5 GPa
3 GPa
120 °C
0.7%
1000 ppm
20 ppm/°C
0.1% 150 °C
4 GPa
1 GPa
78 °C
0.7%
1000 ppm
25 ppm/°C
Heat-Stabilised PEN and PET Films
Minimal shrinkage at
temperatures > Tg
Polyester Films for Flexible Electronics
• Red denotes new • Melinex® -A diverse range of heat stabilised PET films
– Dimensionally stable up to 150C – Thickness 50 micron to 250 micron – UV stabilised – Range of pretreats for enhanced adhesion to functional coatings
• Tetoron®-Low shrink, planarized PET films – Ultrasmooth defect free surface for improved device performance
• Teonex® -Leading range of high performance PEN films – Dimensionally stable up to 180-200C – Thickness 25-200micron – Pretreated for enhanced adhesion to functional coatings – White film at 75 micron
• Teonex®- Low shrink, planarized PEN films – High temperature performance with ultrasmooth defect free surface – 50 and 125 micron film – Protect film (one or two side) available
Polyester Films for Flexible Electronics
• To discuss specific requirements please contact
• Jan LaRiviere (US)
• Thane Gough (Europe)
Films for Touchscreen
• Rapid Touch Screen Market growth
• Requires continued optimization for PET
Source: DisplaySearch 2011 Touch Panel Market Analysis
Touch Screen Revenue Forecast
Films for Touchscreen
• Projected Capacitive Touch is the primary driver for growth
– Many designs use PET film as the transparent conductor substrate
– Manufacturing processes expose PET film to very high temperatures for extended periods
• Heat stabilized film satisfies the shrinkage requirement
• But, PET will “haze-up” under these conditions
Cyclic Oligomers
100 o C for 10 min 120 o C for 10 min 140 o C for 10 min
PEN
PET
Polyhedral or hexagonal platelike oligomer crystals form, a few microns in size Soluble in solvents (e.g. MEK)
Observed that if PET film is held at temperatures above 100C film starts to go hazy
due to migration of cyclic oligomers to the surface
<1% Haze Increased Haze
Cyclic Trimer
C
C O
C
O
OOC
O
CO
O
O
C
O
O
OO
Cyclic trimer Tm 318C present at ca 1.1 to 1.4wt%
Other cyclics present but in lower amounts. Trimer is low strain relative
to other cyclics.
The Cyclic Oligomer Equilibrium
Driven by
-time at temperature
in melt
-amount of work in
extrusion system
Driven by
Process control
CC
O O
O CH2CH2 O H
OC COOCH2CH2O
n=3
PET
PET
Mol Wt =x
Mol Wt =x-3
Cycylic Oligomer Crystals
• Perovic, J Mat Sci, 20, 1985, 1370
• 130C –hexagonal crystals up to 5-6 micron initially but can grow larger with time
Traditional Strategy
• Traditional strategy used is to coat the surface with a coating that acts as a barrier to oligomers migrating to surface
• ITO blocks to an extent but blooming becomes more of an issue with other approaches to conductive films eg printed silver grids etc
Strategies for Control -Block
Non planarized PET : 30mins / 120 C planarized PET : 30mins / 120 C
• Presence of planarizing coatings significantly reduces bloom
• Coatings acting as a barrier
Features of planarized films
-10
0
10
20
30
40
50
0 5 10 15 20 25 30 35
Time / hours
Increase i
n h
aze %
Two side planarised
Q65WAUncoated Q65FWA
Time dependence of oligomer migration (measured by haze increase) of PEN and two side planarized PEN at 200C
planarizer
Uncoated PEN
New Strategy for Control -Process Control
• Through process control at PET polymerisation stage and during film processing it is possible to – Significantly reduce the cyclic content in the PET polymer
– Minimise the reformation of the cyclic oligomers during subsequent filming process
New Developments
• New development grade, 1% haze on ageing at 150oC /30 mins
• Now in qualification with customers
• Able to tailor with respect to surface treatments for specific applications
• DTF is investigating further strategies to minimise the impact of blooming on subsequent processing
• For more information contact Nicole Williamson – [email protected]
Refractive index control
• Controlling refraction and reflection effects in optical stacks
An example: Reduced iridescence in hard coats
• Typically, hard coated PET films exhibit iridescence or rainbow which is objectionable in display and touch applications
• Rainbow results from interference fringes stemming from reflections in the optical stack
• Through optical modelling we can model the effect and design the stack to minimize fringing
– Monitor the effect of changing specific layer parameters, e.g. refractive index and thickness
– Look for ‘fringing’ in visible spectrum as an indication of rainbow effect
• For more information contact Nori Mandokoro – [email protected]
1.5
1.52
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
0 5 10 15 20 25 30 35
Frame No.
Refr
acti
ve I
nd
ex o
f F
ilm
Layer,
n
Substrate refractive index is cycled between 1.67 and 1.51 to show effect on fringing (right), Fringing occurs at higher RI values.
Modeling the transmission spectrum shows the fringing effect
Hard Coat (RI=1.5)
Substrate (RI varied)
Modeled Transmission Spectrum
In the real world, it’s a bit more complex
• Manipulation and control of the adhesion primer can optically bridge the gap from substrate to top layer
Hard Coat (RI=1.5)
PET Film (RI=1.67)
Adhesion Primer (RI = X) Note: most top layers require an adhesion priming layer
DTF primer technology can reduce hard coat fringing
Transmission spectra of new trial films + hardcoat, compared to standard film control
83
84
85
86
87
88
89
90
91
92
93
400 450 500 550 600 650 700
Wavelength (nm)
%T
Optimized Iridescence
Reduced Iridescence
No index matching, high iridescence
Reduced Iridescence
ACTUAL DATA
New Product
Competitor X Competitor Y Standard PET
Visual of the new film’s effect
Each sample is hard coated PET and photographed under a monochromatic light source
Latest Film Developments for PV
Encapsulation Encapsulation / Cell
c-Si
Gen I Gen II Gen III
CdTe CIGS a-Si DSSC OPV
Thin Films
A complex film development agenda!!
Front Sheet
Active Layer Stack
Back Sheet
Superstrate
Substrate
Cell Module
Substrates for PV Cells – Gen. 2 & 3
Functionalities
Dimensional Stability
Surface Quality
Barrier
Weatherability
Adhesion
Light Management
Conductivity
Heat Stability
Front Sheet
Back Sheet
Substrate
Active Layer
Superstrate
Dimensional Stability
Surface Quality
Barrier
Weatherability
Adhesion
Light Management
Conductivity
Heat Stability
Superstrate
Substrate
Active Layer
Front Sheet
Back Sheet
Photo-oxidative Degradation
This pathway
to colour This pathway
to chain breaking
COOCH2CH2OOC
n=ca 100
COOCH2CH2OCO
n=ca 100
COOCHCH2OOC
n=ca 100
OC
hv + O2 hv + O2
hv + O2
COOCH2CH2OCO
n=ca 100
COOCH2CH2OCO
n=ca 100
OH
HO
O
O
hv + O2
OC
P-H
+P
..
.
.
Upon UV light-induced degradation:
• Yellowness increases – formation of new light-absorbing chemical species
• Haze increases (clear films) – bulk + surface scattering
• Gloss decreases – surface roughens
• Light transmittance decreases – more absorption and scattering
• Mechanical properties i.e. %ETB, UTS decrease – chains break down
UV Stabilisers
• UV stabilizers work by: – Energy dissipation ie radiation is absorbed and then dissipated (heat ,
fluorescence)
– Radical deactivation and retardation of propagation of degradation reactions
– Singlet oxygen quenching
– Peroxide decomposition
• Efficacy of a UV stabilizer depends on: – How its optimum absorption wavelength(s) fits with the optimum
degradation wavelength(s) of the polymer
– Its efficiency at converting UV radiation into less harmful energy (heat)
– Its intrinsic UV stability
– Its thermal stability to survive PET processing temperatures
Weatherability – UV Resistance
• Lifetime perception: “Polyester films degrade rapidly under UV light exposure” → In reality, only non-UV stabilised films will! • Polyester films can be modified to have improved resistance to UV light
• Typical results from Weather-Ometer® ageing of a DTF UV stabilised film 1) Mechanical properties:
% Retention of Ultimate Strength
0%
20%
40%
60%
80%
100%
120%
0 2000 4000 6000 8000 10000
Hours in the Weatherometer
UV stabilised PET fi lm
Standard PET fi lm
`
% Retention of Elongation to Break
0%
20%
40%
60%
80%
100%
120%
0 2000 4000 6000 8000 10000
Hours in the Weatherometer
UV stabilised PET fi lm
Standard PET fi lm
`
% Retention of Ultimate Strength
0%
20%
40%
60%
80%
100%
120%
0 2000 4000 6000 8000 10000
Hours in the Weatherometer
UV stabilised PET fi lm
Standard PET fi lm
`
% Retention of Elongation to Break
0%
20%
40%
60%
80%
100%
120%
0 2000 4000 6000 8000 10000
Hours in the Weatherometer
UV stabilised PET fi lm
Standard PET fi lm
`
Method: ASTM 4892-2
Weatherability – UV Resistance
• Typical results from Weather-Ometer® ageing of a DTF UV stabilised film 2) Optical properties:
• 10,000 hours in Weather-Ometer® – Equivalent irradiation = 5 (Florida) to 11 years (Northern Europe) This is not a lifetime guarantee
% Increase of Yellowness Index
0%
50%
100%
150%
200%
250%
300%
0 2000 4000 6000 8000 10000
Hours in the Weatherometer
Standard PET fi lm
UV stabilised PET fi lm
`
% Retention of Light Transmittance
80%
85%
90%
95%
100%
105%
0 2000 4000 6000 8000 10000
Hours in the Weatherometer
Standard PET fi lm
UV stabilised PET fi lm
`
Weatherability – Hydrolysis Resistance
DTF's filled Melinex® 238 at 50 µm reaches 2000 h at 85 °C / 85% RH DTF can also apply this technology to optically clear films
% Elongation at Break (Melinex® 238)
0
50
100
150
200
0 500 1000 1500 2000
Damp Heat Test (hours)
ETB
(%
)
• Lifetime perception: “Polyester films hydrolyse rapidly” → This is very slow under normal atmospheric (T,P) conditions !
• Polyester films can be modified to pass the standard “Damp Heat” test – Retention of 10% ETB after 1000 h at 85 °C / 85% RH
• Some industry interest in higher performance PET films for extended testing times (2000+ hours in Damp Heat test)
Hydrolysis of PET
• Catalysed by COOH end groups in PET
Strategies for Improving Hydrolysis Resistance
• Raising Mol Wt of film
• Chemically modifying end groups
• Affecting film crystallinity
PV Active Layer
Dimensional Stability
Surface Quality
Barrier
Weatherability
Adhesion
Light Management
Conductivity
Heat Stability
Front Sheet
Sun-facing Substrate
Non Sun-facing Substrate
Back Sheet
Adhesion
• Multi-layer adhesion is appearing as one of the most challenging technical areas and is key to optimise device/module lifetime and lab certification
• DTF has designed Melinex® films with enhanced adhesion to EVA (a typical cell backside encapsulant) - Received positive market feedback
• Similarly films with excellent adhesion to DuPont™ Surlyn® have also been developed
Adhesion to Glass
• Industry-standard bonding interlayer eg – polyvinyl butyral (PVB)
– “ionoplast” systems such as DuPont’s SentryGlas® 48
• These do not show good adhesion to many standard PET film pretreatments, but specialist systems are available.
• The key parameter in this respect is the bonding or laminating temperature required to give good adhesion and whether this is in excess of what can be tolerated by the completed o-PV cell.
• This remains an area of active research.
Conclusion
• Recent improvements in PET film support emerging flexible electronics and flexible PV markets
• Active continuing innovation to meet future needs
• Underlying science that affect processing on polyester films being researched
• Knowhow on how to get the most out of the handling and processing on polyester film available to users
• Please contact us with any questions you may have
• THANK YOU !