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)

– Jan.S.LaRiviere@usa.dupont.com

• Thane Gough (Europe)

– Thane.C.Gough@gbr.dupont.com

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 – Nicole.M.Williamson@USA.dupont.com

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 – Nori.Mandokoro@USA.dupont.com

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 !