dauskardt@stanford.edu 1

Nick Rolston, Brian Watson, Chris Bruner, Stéphanie Dupont, Ryan Brock, 

Veerle Balcaen

(OPV, Perovskite and multi-junction active layers and electrodes)

Jared Tracy, Fernando Novoa, Warren Cai

(encapsulation and optics)

Scott Isaacson, Tissa Mirfakhrai

(ultra barrier films) 

Reinhold H. Dauskardt (dauskardt@stanford.edu)

Thermo-Mechanical Degradation 

Mechanisms Relevant for Field Failures and 

Solar Lifetimes

DoE support through the Bay Area PV Consortium and Predicts CPV 

program (SunShot), additional support from former CAMP-KAUST center.

Degradation and Reliability of PV Devices and Modules

crazing and 

delamination

AR coating

W3 untreated W3 625 kJ W3 1250 kJ 0

1

2

3

4

5

6

7

8

9

10 Adhesion Energy, Gc (J/m

2)

Increasing UV Exposure

accelerated 

testing

metallized grids peel 

after thermal cycling

encapsulation 

delamination

blisters in fielded PV 

modules

P3HTthermal cycling, stress, PC60BM

moisture, chemically active 

environmental species, solar 

UV, ...

coupled and often uncertain 

degradation kinetics and 

reliability models.

operating environments:
dauskardt@stanford.edu 2

Outline

• Inherent Solar Thermo-Mechanical Reliability 

• adhesion and cohesion characterization and properties

• silicon, OPV, CIGS and perovskite devices and modules

• Module Components: Backsheets, Frontsheets, Encapsulation

• adhesion metrologies and challenges

• debonding kinetics and lifetime predictions

• Reliability and Operational Lifetimes for CPV Technologies

• coupled mechanical and photo-chemical mechanisms

• correlation of in-door and out-door exposures 

• Challenges for Emerging OPV and Perovskites

• fundamental challenge for mechanically fragile systems

• prospects for improvements

Device Reliability and Evolution of Defects

damage propagates if mechanical stresses are large enough so that

presence of chemical species and photons, damage propagates even if

environment and 

stress accelerates

defect evolution

H2O

Substrate

hn

Role of coupled “stress” parameters:

• mechanical stress

• temperature

• environmental species

• photons 

(photochemical reactions)

 

2

G  GcJ / m

 

2

G  GcJ / m

Organic/Inorganic Layer

cohesion or 

adhesion

mechanical 

“driving force”
dauskardt@stanford.edu 3

Limitations of Thin-Film Adhesion Tests

Interface Crack

Indenter

Plastic zone

Substrate

Film

Interface Crack

Substrate

 Film

M

P

P

Substrate

Film

Cavity in Substrate

• Indentation/Scratch Test

- complex stress and deformation fields

- principally qualitative results

- (nano) scratch test even less quantitative

• Peel/m-ELT Test

- difficult to apply loads

- plastic deformation of film

- temperature complications in m-ELT 

• Blister Test

- compliant loading system

- environmental effects

- etching/machining of cavity difficult

Major limitations: need detailed film properties, film stress relaxation and film plasticity

 principally qualitative results for all above methods!

Quantitative Adhesion/Cohesion and Debond Kinetics

Adhesion/Cohesion Debonding Kinetics

40 C

30 C 20 C 10 C

0 50 100 150 200 250 300 350

10-8

10-7

10-6

10-5 Debond Growth Rate, da/dt (m/s) Debond-Driving Force, G(J/m2

)

RH = 40%

increasing temperature

Polyvinyl fluoride

Polyester

80% RH

0% RH

20 30 40 50 60 70 80 90

0

200

400

600

800

1000

1200

 Unaged Debond Energy, Gc (J/m

2) Ageing Temperature (°C)

1000 hrs

Aging Treatment Temperature (°C)

backsheet

blisters

P

glass

glass

P
dauskardt@stanford.edu 4

0.1

1

10

100

1,000

Fracture Energy, 

Gc (J/m2

)

Dense SiO2

TEOS 

SiO2

ULK 

dielectrics

Gc ~ 5 J/m2

Gc ~ 10 J/m2

OPV

CIGS

CuInxGa(1-x)Se2 

Mo

CdS

Al doped ZnO

Al foil

Perovskites

Fracture Properties of Device Materials

Silicon 

PV

Polymers for

Packaging

Encapsulation

Structural Materials

Protective 

Coatings

Outline

• Inherent Solar Thermo-Mechanical Reliability 

• adhesion and cohesion characterization and properties

• silicon, OPV, CIGS and perovskite devices and modules

• Module Components: Backsheets, Frontsheets, Encapsulation

• adhesion metrologies and challenges

• debonding kinetics and lifetime predictions

• Reliability and Operational Lifetimes for CPV Technologies

• coupled mechanical and photo-chemical mechanisms

• correlation of in-door and out-door exposures 

• Challenges for Emerging OPV and Perovskites

• fundamental challenge for mechanically fragile systems

• prospects for improvements
dauskardt@stanford.edu 5

• Backsheets

– multilayered TPE

– full-size modules

• Frontsheets

– dyad barrier films

– polysiloxane films

• Encapsulation

– EVA and PVB

– ionomers and polyolefins

Mechanical Reliability of Module Components

encapsulation 

delamination

glass

P

Square and tapered specimens have simpler data analysis...

Single Cantilever Single Square Double Square

P

P

Mechanics-Based Adhesion Metrologies

1

0

10 20 30

0

10

20

30

40 Debond energy, Gc (J/m

2) Debond Length, a (mm)

Data Analysis

Gc ≈

Pc

2

4B

Debond Length, a (mm)

Debond Energy, 

Gc (J/m2

Debond Experiment )

0 500 1000 0

10

20

30

40 Debonding Load, Pc (N)

Actuator Displacement,  m) Debonding Load, Pc (N)

Actuator Displacement, Δ (μm)

P

glass

Modified Tapered

glass

glass
dauskardt@stanford.edu 6

titanium

glass

encapsulation

glass

titanium

Validated with FEA

h

a

P 0 4 8 12 16

0

30

60

90

120

150 Load, P (N)

Displacement, (mm)

Gc ≈

Pc

2

4B′

Pc

glass

fracture

Method Insensitive to Glass Fracture

1

1

■ cracked glass (10×t apart) 

□ full glass

0 10 20 30 40 50

0

10

20

30

40

50

60 Mechanical Compliance, C (m/N) Debond Length (mm)

■ cracked glass

□ full glass

P

P

Effect of Temperature on Debond Energy

1

2

10 20 30 40 50 60 70 80

0.0

0.5

1.0

1.5

2.0

2.5

3.0 Debond Energy, Gc (kJ/m

2) Test Temperature, T (°C)

EVA

PVB
dauskardt@stanford.edu 7

Effect of Substrate Treatment on Debond Energy

1

3

0 100 200 300 400

0

200

400

600

800

1000

laminated at 150 °C

UV-ozone 

trichloroethylene Debond Energy (J/m

2)

Film Thickness (m)

acetone + IPA + UV Ozone 

ionomers

Encapsulation Thickness (mm)

Debond Energy, 

Gc (J/m

2

)

trichloroethylene

30 Year Historic Module Encapsulant Adhesion

Adhesion of EVA after 30 years in service (exposed) and storage (unexposed).

Delamination of EVA/glass – EVA brown 

discoloration. 

Debond of EVA from cell - localized EVA 

deformation between grid lines.
dauskardt@stanford.edu 8

• Backsheets

– multilayered TPE

– full-size modules

• Frontsheets

– dyad barrier films

– polysiloxane films

• Encapsulation

– EVA and PVB

– ionomers and polyolefins

Mechanical Reliability of Module Components

Blisters in fielded 

PV modules

Effect of Aging T and RH on Backsheet Adhesion

1

6

80% RH

0% RH

Adhesive

PVF/PET 20 30 40 50 60 70 80 90

0

200

400

600

800

1000

1200

 Unaged Debond Energy, Gc (J/m

2)

Ageing Temperature (°C)

1000 hrs

Aging Treatment Temperature (°C)

Blisters in fielded 

PV modules

PVF

PET

EVA seed

EVA encapsulant

Glass
dauskardt@stanford.edu 9

30 Year Historic Module Backsheet Adhesion

PVF PET

Displacement (μm)

Load (N)

Portable adhesion test on 30 yr old unexposed backsheet

Higher adhesion at PVF-PET interface in discolored region 

at junction of four silicon cells

Adhesion degrades over an order of 

magnitude after 30 yrs in field.

New Portable Full Panel Adhesion

P

a

Back Side of Full Panel

Delaminator (v8.2) 

Adhesion Test System

1

10

100

1000 Debond Energy (J/m

2)

New Module 10 years in field

Gc= 2.8 J/m2

Debond Energy, 

Gc(J/m2

)

debond energy decreased 

by 97% after 10 years in 

Florida sun
dauskardt@stanford.edu 10

Debond Relaxation Model 

Polyvinyl fluoride

Polyester

dc

rp

Viscoelastic

relaxation

Arrhenius

Williams-Landel-Ferry 

(1955)

Temperature-Activated Backsheet Debond Growth

40 C

30 C 20 C 10 C

0 50 100 150 200 250 300 350

10-8

10-7

10-6

10-5 Debond Growth Rate, da/dt (m/s) Debond-Driving Force, G(J/m2

)

RH = 40%

1

9

increasing temperature

0.1

1

10

100

1000

10°C

40°C

30°C

20°C

10 20 30 40 50 60 Backsheet Lifetime (years)

Initial Defect Size, di(m)

Blister formation in PV modules

p

G =

3

32

1 − υ

2 pd4

Eh

3

Blister mechanics

h

2d

Defect-Tolerant Lifetime Estimation

2

0
dauskardt@stanford.edu 11

Outline

• Inherent Solar Thermo-Mechanical Reliability 

• adhesion and cohesion characterization and properties

• silicon, OPV, CIGS and perovskite devices and modules

• Module Components: Backsheets, Frontsheets, Encapsulation

• adhesion metrologies and challenges

• debonding kinetics and lifetime predictions

• Reliability and Operational Lifetimes for CPV Technologies

• coupled mechanical and photo-chemical mechanisms

• correlation of in-door and out-door exposures 

• Challenges for Emerging OPV and Perovskites

• fundamental challenge for mechanically fragile systems

• prospects for improvements

CPV is ideal PV test vehicle with elevated “stress” parameters for reliability studies

• elevated thermal, moisture, light intensities, mechanical stresses

Substrate

Contact

Junction 1

Junction 2

Junction 3

AR

Glass

Lens

SOE

Light

AR

Cell

Gridline contacts

Focus Area 1

Focus Area 2

Coupled Thermo-Mechanical and Photo-Chemical Degradation Mechanisms 

that Determine the Reliability and Operational Lifetimes for CPV Technologies

Dauskardt / Stanford; Miller and Kurtz / NREL; Hebert and Ermer / Spectrolab

PREDICTS Program

DOE Geoffrey Kinsey (Program Manager), Inna Kozinsky (Senior 

Technical Advisor)
dauskardt@stanford.edu 12

Survey of Materials and Interfaces Relevant to CPV Industry 

• Survey of 11 module and 2 optics manufacturers

• Materials/interfaces most relevant to degradation and 

reliability

• Sample questions posed to CPV industry:

• What are the most important interfaces within a CPV module to be 

studied? 

• What interfaces would you recommend we study?

0

2

4

6

8

10

12

14 Number of Responses

Interface AR/POE

Within POE

POE/POE Substrate

POE/Module Body

SOE/Cell AR

Cell AR/Cell

Cell/Carrier

Carrier/Heat Sink

Interconnects/Cell

Pottant/j-box

Module Body

Ventilator

AR (Optics)

Glass Frontsheet

POE (Lens)

AR 

(Cell)

SOE

(Homogenizer)

Role of Time and Temperature during In-Door UVB Exposure

0 20 40 60 80 100 120 140 160 180 200

0

20

40

60

80

100

120

140 Sylgard UV 110 C

 Sylgard UV 110 C

 No Primer

 NuSil UV 110 C Adhesion Energy, Gc (J/m

2

) Cumulative UV, Wtot (MJ/m2)

200 250 300 350 400

0

10

20

30

40

50

60

70

80 Spectral Irradiance (W cm-2 nm-1

) Wavelength (nm)

80oC/10% RH, 50oC/10% RH, 110 oC/10% RH at 4.5 mW/cm2

from 200 nm to 400 nm wavelength UV 
dauskardt@stanford.edu 13

0 1000 2000 3000 4000

0

50

100

150

200

250

300 Adhesion Energy, Gc (J/m

2) Outdoor Exposure Time (hrs)

• Sylgard adhesive aged in 

outdoor concentrator at 

1100x solar flux 

Role of Out-Door Concentrated Solar Exposure

Sapphire Wafer

Silicone Adhesive

Al Beam

Al Plate

Sapphire Wafer

Silicone Adhesive

Al Beam

Al Plate

1000 800 600 400 200 0 Normalized Counts Binding Energy (eV) 1000 800 600 400 200 0 Normalized Counts Binding Energy (eV)

O

C

Si

• XPS characterization of sapphire surface after mechanical testing

• thickness of residual silicone layer on sapphire grows with increase in aging time

• suggests formation of bonds at interface between sapphire and silicone adhesive

XPS of sapphire 

surface, no aging 

Al

XPS of sapphire 

surface, ~15 MJ/m2

O

C

Si

Sapphire Wafer

Silicone Adhesive

Si

H3C O

CH3

O

Si

CH3

Al

O

Al

O

UV Mediated Chemical Degradation at Interface

A

B

A B
dauskardt@stanford.edu 14

3500 3000 1500 1000 500 Absorbance (a.u.) Wavenumber (cm-1)

0 MJ/m2 42.3 MJ/m2 85.1 MJ/m2 151.3 MJ/m2 221.5 MJ/m2

1080 1050 1020 990 Absorbance (a.u) Wavenumber (cm-1)

0 MJ/m2 42.3 MJ/m2 85.1 MJ/m2 151.3 MJ/m2 221.5 MJ/m2

a b

• transmission FT-IR shows increase in Si-O-Si peak with increased UV exposure

• suggests formation of oxygen bridges and crosslinking between chains, leading 

to embrittlement

increasing 

exposure

UV Mediated Chemical Degradation in Bulk

C-H

Si-O-Si

-CH3

Si-(CH3)2

-CH3

0 50 100 150 200 250 300

0

50

100

150

200

250

50 oC

80 oC

110 oC Adhesion Energy, Gc (J/m

2

) Cumulative UV, Wtot (MJ/m2)

Gadhesion = kNcrosslinks + G0

Gadhesion = k1Wtote

−

Ea

kbT + G0

Sapphire Wafer

Silicone Adhesive

Al Beam

Al Plate

Ncrosslink = kuvWtote

−

Ea

kbT

adhesion energy dependent on 

number of bonds between silicone 

and sapphire substrate

linear relation between number of 

crosslinks and UV dosage

Increase in Adhesion as a Result of Crosslinking
dauskardt@stanford.edu 15

Ncrosslink = kuvWtote

−

Ea

kbT

Gcohesion = k Mw

3

2

=

k2

k3+Ncrosslink 3/2

Gcohesion =

k2

k3 + Wtote

−

Ea

kbT

3/2

adhesion energy dependent on 

molecular weight between crosslinks

linear relation between number of 

crosslinks and UV dosage

Decrease in Cohesion as a Result of Crosslinking

0 50 100 150 200 250 300

0

50

100

150

200

250

50 oC

80 oC

110 oC Adhesion Energy, Gc (J/m

2

) Cumulative UV, Wtot (MJ/m2)

Sapphire Wafer

Silicone Adhesive

Al Beam

Al Plate

Outline

• Inherent Solar Thermo-Mechanical Reliability 

• adhesion and cohesion characterization and properties

• silicon, OPV, CIGS and perovskite devices and modules

• Module Components: Backsheets, Frontsheets, Encapsulation

• adhesion metrologies and challenges

• debonding kinetics and lifetime predictions

• Reliability and Operational Lifetimes for CPV Technologies

• coupled mechanical and photo-chemical mechanisms

• correlation of in-door and out-door exposures 

• Challenges for Emerging OPV and Perovskites

• fundamental challenge for mechanically fragile systems

• prospects for improvements
dauskardt@stanford.edu 16

0 1 2 3 4 5 6 7

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4 Crack Growth Rate, da/dt (m/s) Strain Energy Release Rate, G (J/m2

)

No UV

0 mW/cm2

UV intensity

1.2 mW/cm2

0.6 mW/cm2

0

0.5

1

1.5

2 Cohesion Energy, G (J/m2

)

Solar Cells

Quantitative Adhesion/Cohesion and Debond Kinetics

Degradation Kinetics

(temp/environment /UV effects)

Adhesion/Cohesion

threshold crucial 

for reliability

ITO (150 nm )

PEDOT:PSS (50-100 nm)

Al (100 nm)

Ca (7 nm)

P3HT/PCBM (~150 nm)

• Heterojunction layer thickness

– is cohesion in organic layers sensitive 

to layer thickness?

• Composition of the heterojunction layer

– limited bonding to fullerene

– polymer/PCBM ratio makes stronger layer

• Molecular intercalation

– manipulating the types of intermolecular 

interactions

• Annealing

– morphology of the BHJ layer 

changes with annealing

P3HT PCBM Indene C60

Factors Effecting Cohesion of BHJ Layers

Standard 150 C 0.5h 150 C 2h

organic BHJ

elastic layer

elastic substrate

plastic zone
dauskardt@stanford.edu 17

0

1

2

3

4

5

6

7

8 Fracture Energy, Gc (J/m

2) Effect of Molecular Intercalation on Cohesion

PQT-12

bis-PC71BM

non-intercalated cell

PC71BM

intercalated cell

+

+

P3HT

PC60BM

+

pBTTT-C14

PC71BM

intercalated cell

+

bis-PC71BM

non-intercalated cell

+

Glass Substrate

ITO

Al

Failure in BHJ layer BHJ Layer

1:1 poylmer to fullerene mass ratio

Bruner, Dauskardt et al. Adv. Funct. Mater. 2013,

0 4 8 12 16 20 24

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

 150 oC

 130 oC

 85 oC

 45 oC Adhesion Energy, GC (J/m

2

) Annealing Time, t (h)

150 ̊C

130 ̊C

85 ̊C

45 ̊C

Post-Annealing to Control Interdiffusion and Adhesion

annealing time and temperature 

increases the debonding resistance

P3HT

PEDOT:PSS

PCBM

PCBM

LONG TIME AT 130  ̊C

PEDOT:PSS

P3HT:PCBM

REFERENCE

P3HT:PCBM

PEDOT:PSS

WITH INCREASED TIME

AND/OR TEMPERATURE

Dupont, Voroshazi, Vandewal, Nordlundt, Dauskardt,, Adv. Materials (2014)
dauskardt@stanford.edu 18

0 2 4 6 8 10 12 14 16 18

0.4

0.8

1.2

1.6

2.0

2.4

2.8

3.2 Efficiency,  (%) Cohesion Energy, Gc

 (J/m2)

 100kDa

 79kDa

 53kDa

 28kDa

Efficiency vs. Cohesion

Increasing 

BHJ layer 

thickness

Thin BHJ layer - low efficiency due 

to lower photon harvesting 

Thicker BHJ layer - greater degree 

of charge carrier recombination

x

ε =

VOCJSCFF

Pin

P3HT

Distance from surface

+

+

+

+

-

-

-

-

a)

b)

c)

Photon collection

Bruner, Dauskardt, et al. Macromolecules 2014. 

Perovskite: Not Just Your Average Salt

Material CH3NH3PbI3 NaCl

Density 4.29 g/cm3 2.16 g/cm3

Fracture

Energy, Gc

???? J/m2 0.6-1.8 J/m2

Solar Cell 

Efficiency ~20% 0%
dauskardt@stanford.edu 19

C60-S Increases Device Cohesion

10 12 14 16 18 20

0.0

0.5

1.0

1.5 Fracture Energy, Gc (J/m

2)

Efficiency, PCE (%)

Planar, Large 

Grain

Mesoporous w/ PTAA

Mesoporous w/ Spiro

Planar, Slot-Die

PCBM

PTAA

Spiro-OMeTAD

CH3NH3PbI3

Doped P3HT

PCBM

ZnO C60

Acetate 

(regular)

C60-S, Acetate 

(inverted)

C60-S

ZnO

cohesion 

increases 

500%

improved fullerene-based electron 

transport layer (C60-S)

Factors Influencing Perovskite Films

• Morphology/Processing

- Grain size

- Mesoporous vs. planar 

- Substrate temperature

- Precursor ratio

• Deposition method

- Roll to roll vs. blade coating

• Exposure conditions

- Humidity

- Temperature

• Molecular Additives

How do these 

factors affect 

perovskite film 

cohesion?
dauskardt@stanford.edu 20

0

1

2 Cohesion Energy, GC(J/m

2)

Effect of Morphology/Process Conditions

Planar, Large-Grain Acetate Mesoporous 

>10 μm

300-

1000 nm

<500 nm

Grain Size

cTiO2

CH3NH3PbI3

FTO-Glass

mTiO2/CH3NH3PbI3

PEDOT:PSS

CH3NH3PbI3

ITO-Glass

PEDOT:PSS

CH3NH3PbI3-xClx

ITO-Glass

decreased 

MACl / PbI2

ratio

decreased 

substrate 

temperature

0.0

0.5

1.0

Concentration of Diaminopropane Cohesion Energy, GC(J/m

2)

Effect of Molecular Additives

Ref .01 eq .03 eq .1 eq .2 eq

Increasing concentration of diaminopropane

PEDOT:PSS

CH3NH3PbI3/ DAP

ITO-Glass

Goal: 

Strengthen grain 

boundaries