Page 1 May 26th, 2011 Oerlikon
Stress tests and failure modes of thin film silicon photovoltaic modules
Ivan Sinicco – Head of Module Technology
Welcome to Oerlikon Solar
Durability of Thin Film Solar Cells - EMPA Academy
Dübendorf ZH, 4th of April 2012
Agenda
1 About Oerlikon Solar
2 How a PV module is designed?
3 What should be the module durability?
4 Accelerated stress tests
5 Failure Modes
6 Manufacturing Consistency and Reliability/Durability Implications
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Oerlikon Solar at a glance
Oerlikon Solar strives to
make solar power
economically viable.
� A leading supplier of
manufacturing solutions
for thin film silicon
modules
� More than 870 MW
contracted to date
� Approx. 650 employees
including 300 scientists
and engineers as well as
200 global customer
personnel
� R&D investments of
more than MCHF 200 in
last 4 years
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Serving 13 locations in 9 countries
Durability of Thin Film Solar Cells - 2012
Milestones in Oerlikon Solar’s history
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1st equipment sale (Schott Solar)
1st Micromorph® turnkey production line contract
TUV certification for Micromorph®
Micromorph®
technology patent
PV lab at IMT
1st functional 1.4 m2 a-Si module
Oerlikon founds own R&D lab
Launch of ThinFab™ € 0.50/Wp
Champion cell 11.9% efficiency10% efficiency average
1986 1989 1993 2003 2004 2005 2008 2009 2010 2011
1st ThinFab™ sold
Record module with 154
peak W and 10.8%
efficiency
Technology roadmap to
12% efficiency
Concept &
productCommercialization &
mass production
Opportunity
identification
VHF PECVD deposition
1st customer in commercial production
2012
Launch of 2nd Gen. ThinFab™ € 0.35/Wp
Record Cell with 12.5%
Durability of Thin Film Solar Cells - 2012
How a PV module is designed?
• Design proposal
• DFMEA
• IP
• Sourcing
• Compliance to standards
• Test & qualification
procedures
• EHS analysis
• Develop initial
prototipe
• Start material
and interface
testing
• Finalize
feasibility
study
• Market
feedback
• Conclude tests
at module level
• Define EQ
requirements
• Supplier
evaluation
• Prepare
modules for
certification
• Process range
definition
• Certification
obtained
• Engineering lot
in mass
production
• Re-test
engineering lot
• Release BoM &
design freeze
• Design process
specs
• Design material
specs
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Module Reliability and Durability
Durability (t)Durability (t)Durability (t)Durability (t)
(once the «kill ing parameters» are defined)(once the «kill ing parameters» are defined)(once the «kill ing parameters» are defined)(once the «kill ing parameters» are defined)
Bad Module DesignBad Module DesignBad Module DesignBad Module Design
Bad ProcessBad ProcessBad ProcessBad Process
Somehow covered by Somehow covered by Somehow covered by Somehow covered by
IEC (NOT 100%) IEC (NOT 100%) IEC (NOT 100%) IEC (NOT 100%) ���� a a a a
design to failure design to failure design to failure design to failure
model is missing…model is missing…model is missing…model is missing…OffOffOffOff----spec materialsspec materialsspec materialsspec materials
Faulty process controlFaulty process controlFaulty process controlFaulty process control
Module Design onlyModule Design onlyModule Design onlyModule Design only
Page Page Page Page 6666 Polymers 2012, ColognePolymers 2012, ColognePolymers 2012, ColognePolymers 2012, Cologne
What should be the PV Module Durability?
The energy produced is not depending ONLY on the efficiency:
E α η x t
Where t is the time that an usable efficiency is delivered (durability).
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E(t, φ, λ, SV, MS) α η(t, φ, λ, SV, MS) x t
A bit more realistic is the equation below
E(t) α η(t) x t
and further
All topics above were still too academical…
At the end, what really matter is price!
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What should be the PV Module Durability?
Durability of Thin Film Solar Cells - 2012
Source : The drivers of Levelized Cost Of Electricity for utility scale pv (Sunpower)
LCoE as basic
reference point:LCoE = f(financial costs, taxes, inflation rate, BOS price,
Module price, Location, EY, Module durability, System
depreciation, System degradation rate)
What should be the PV Module Durability?
Cost structure installed system
34%
33%
33%
BOS
MODULE
FINANCIAL
Durability of Thin Film Solar Cells - 2012
ca 35% ca 35%
ca 30%
Back to RealityPossible environmental conditions on the Planet Earth
Max. ambient temperature: 58°C
Min. ambient temperature: -67°C
Max. ∆T/24h = 103°C, ∆T/1min = 20°CHighest average ∆T/24h ca 30°C
Max. yearly average UV (280nm – 400nm)
on horizontal surface ca 185KWh/m2
Pressures up to: 10kPa static
: 4kPa dynamic
Hail up to 200gr
rH up to 95% @ 35°C
Basic ambient � PH 11 (near salted
roads, farms, sea)
Acid ambient � PH 3 (industry,
highway..)
System Voltage: 1000V, 1500V…+
Several possible mounting configurationshttp://www.ncdc.noaa.gov
In those conditions the PV
Module must deliver electric
energy with minimal
performance losses!
Durability of Thin Film Solar Cells - 2012
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Accelerated Stress Tests
Durability of Thin Film Solar Cells - 2012
Typical field failure modes:
Broken interconnectsCorrosionDelamination or loss of elastomeric properties of lamination foilSolder failuresBroken GlassHot SpotsGround faultsJunction box and connection faultsStructural failuresBy-pass diode failureArcingElectrochemical corrosion / or delamination of TCOElectro migration of chemical speciesFaulty edge deletionShunts at laser scribesShunts at absorber impurities
� Developing accelerated stress tests that replicates the failure mode observed in the field is the only chance to determine module durability without waiting 20+ years
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From: History of Qualification Standards; J. Wolgemuth
Accelerated Stress Tests
History of Accelerated Testing (or better; Qualification Testing)
• JPL Block Buys I-V (1975 – 1981) � Crystalline Si
• European Community Specifications 501 to 503 (1981 – 1991)
• SERI IQT � Modifications to a-Si (Thin Film) (1990)
• IEEE 1262 � All technologies included (1995 – 2000)
• IEC 61215 � Crystalline Si (ed1 – 1993)
(ed2 – 2005)
• IEC 61646 � Thin Film (ed1 – 2005)
(ed2 – 2008)
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JPL Block buys
Accelerated Stress Tests
From: History of Qualification Standards; J. Wolgemuth
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Accelerated Stress Tests
From: History of Qualification Standards; J. Wolgemuth
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Test Sequences for IEC 61646 Qualification
Durability of Thin Film Solar Cells - 2012
Related to Performance
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Test Sequences for IEC 61730 Qualification
Durability of Thin Film Solar Cells - 2012
Related to Safety
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Failure Modes and IEC Stress Tests
Durability of Thin Film Solar Cells - 2012
IEC Accelerated Stress Test Failure Mode Issue
10.11 Thermal Cycles Broken Interconnect Performance / SafetySolder Bond failures PerformanceJunction Box Adhesion SafetyModule Connection Open Circuits Performance / SafetyOpen Circuits leading to Arching Performance / Safety
10.13 Damp Heat Corrosion SafetyDelamination of Laminate Performance / SafetyLower Adhesion and Elasticity of Polymeric Materials Performance / SafetyJunction Box Adhesion SafetyInadequate Edge Deletion SafetyGlass Corrosion Performance
10.12 Humidity Freeze Delamination of Laminate Performance / SafetyLower Adhesion and Elasticity of Polymeric Materials Performance / SafetyJunction Box Adhesion SafetyInadequate Edge Deletion Safety
10.1 UV Test Delamination of Laminate Performance / SafetyLower Adhesion and Elasticity of Polymeric Materials Performance / SafetyCell Performance PerformanceJunction Box Adhesion SafetyGround fault due to Backsheet SafetyPolymer discoloration (more substrate configuration) Performance
10.16 Mechanical Load Broken Interconnect Performance / SafetySolder Bond failures PerformanceBroken Glass SafetyStructural Failures Safety
10.9 Hot Spot Test Hot Spots PerformanceWeak Cell / Region PerformanceShunting PerformanceBroken Glass Safety
10.17 Hail Test Broken Glass SafetySolder Bond failures Performance
10.18 Bypass Diode Thermal Test Bypass Diode Failures Safety
MST 26 Reverse Current Test Weak Cell / Region PerformanceShunts PerformanceBroken Glass Safety
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Key to model the lifetime of a PV Module (once the kind of stresses that the device will suffer are well defined) is the knowledge of the acceleration factors (AF) caused by each stress factor.Several models can be used to determine the acceleration factors. One of those is the so called Arrhenius factor which deals with temperature factors:
Rate α e [-Ea/kT]
With this model is possible to model degradation rates by using characteristic temperature Teq associated with a time averaged degradation rate
The Peck Model is an extension of the Arrhenius model where the humidity is added and can be used for modeling modules under Damp Heat conditions (even Biased conditions).
Rate α [rH]n x e [-Ea/kT]
Accelerated Stress Tests (modeling example)
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Accelerated Stress Tests
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Accelerated Stress Tests
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IEC Qualification Tests into Reliability Tests?
Durability of Thin Film Solar Cells - 2012
IEC Qualification Tests are a pass/fail criteria only!
• Increase test duration (test to failure approach)
• Use higher stress levels (Note: Higher stress levels could generate
failure modes not observed under real circumstances
• Combined Stresses (i.e. add voltage to Damp Heat)
• Dynamical mechanical load (not present in IEC qualification and present
in real life)
• Introduce Material Designed Tests to focus on material performance
excluding complex degradation modes due to interfaces and cross
reactions
• Change pass / fail criteria into trend determination
• Introduce pure thermal stress tests to identify potential failure modes
associated to diffusion of elements and or chemical reactions
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Relevant reaction / process that may impact module performance
Durability of Thin Film Solar Cells - 2012
Glass or Polymer Superstrate
TCO
Absorber
Encapsulant
Back Glass or Backsheet
TCO
Soiling / Corrosion / Na E field Induced Migration
Weak Cell / Shunt / Hot Spot
Moisture / O2 (from edge), Cations / Anions E field induced Migration (from glass)
Oxidation /corrosion / E field induced Migration / Ion catalyzed reactions /acidic attack from Polymer / bonding defect
Moisture / O2 inward diffusion, Na E field induced Migration / Photochem. Oxidative degrad. Reactions, Acidic Diffusion
Cations / Anions Interdiffusion into Polymer / Adhesion / Delamination
Main drivers: T, rH, V, PH, UV, O2, H2O (vapor or liquid)
Module Test and Reliability Center @ Oerlikon Solar (TBB)
Stressing Method
Light Soaking Visible (B,B,B)
Light Soaking UV (UVA & UVB)
Dynamical Mechanical Load (up to
8000Pa)
Reverse Current Test
Thermal Cycling (-60°C/+165°C)
Humidity – Freeze (-60°C /
165°C/20%..95%)
Damp Heat (30°C / 95°C/20%..95%)
Biased Damp Heat (±1000V; ±4000V)
Bake Test / Boiling test
Module Breakage Test
Chemical compatibility Test
Outdoor
Evaluation Method
Wet Leakage Current
IV scanning (High & Low)
Dark IV, EQE
Spectral matching
Electroluminescence
IR
FTIR; UVis
SIMS; SEM; AFM; HSGC
DSC; TGA
Elemental chemical analysis
Compressive Shear Test
Pull Test; Ring-on-Ring Test
TLM
Visual Inspection (key!!)
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0.5
0.6
0.7
0.8
0.9
1
1.1
0 2000 4000 6000 8000 10000
DH hours
Pow
er r
elat
ive
to in
itial
sampleA
sampleb
IEC limit
Beyond IEC (increase test duration + trend line)
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0
0.2
0.4
0.6
0.8
1
1.2
0 10 20 30 40 50
Number of HF cycles
Pow
er r
elat
ive
to in
itial
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
IEC
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Beyond IEC (increase test duration + trend line)
TC vs Power
0
0.2
0.4
0.6
0.8
1
1.2
0 200 400 600 800 1000 1200 1400 1600
number of cycles
Pow
er r
elat
ive
to in
itial
TC - A0008-04
TC - A0008-05
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IEC
Durability of Thin Film Solar Cells - 2012
Beyond IEC (increase test duration + trend line)
0
1
2
3
4
5
6
7
8
9
Damp Heat Humitity freeze Thermal Cycles
Principal Tests
num
ber
of ti
mes
exc
eedi
ng I
EC
Gen1
Gen2
Gen3
IEC
> 4 > 4
> 7
Beyond IEC (Example of OS modules design evolution)
Average survival (all technologies) according to IEC criteriaPage Page Page Page 27272727 Durability of Thin Film Solar Cells - 2012
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TLM Method - contacting resistance loss (initial)
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TLM Method - contacting resistance loss (after 320 TC)
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Beyond IEC (Biased Damp Heat)
G1G1G1G1
G2 G2 G2 G2 ((((----4000V)4000V)4000V)4000V)
G2G2G2G2
AAAA
BBBB
CCCC
DDDD
EEEE
FFFF
0000 500500500500 1000100010001000 1500150015001500 2000200020002000 2500250025002500 3000300030003000 3500350035003500
91.1% from initial power
95.4% from initial power
TF Si
BDH hoursBDH hoursBDH hoursBDH hours
TF Si
TF Si
4000….4000….4000….4000….
GGGG
HHHH
Thin
Film
T
hin
Film
T
hin
Film
T
hin
Film
–– ––S
eve
ral m
anufa
ctu
rers
Seve
ral m
anufa
ctu
rers
Seve
ral m
anufa
ctu
rers
Seve
ral m
anufa
ctu
rers
Oerlik
on D
esig
nG3G3G3G3 (90(90(90(90°C/95%rH)C/95%rH)C/95%rH)C/95%rH)
5000500050005000 6000600060006000
TF Si
Future IEC Standard?
Benchmark CIGSBenchmark CIGSBenchmark CIGSBenchmark CIGS
Benchmark CdTeBenchmark CdTeBenchmark CdTeBenchmark CdTe
Benchmark Mono & PolycrystallineBenchmark Mono & PolycrystallineBenchmark Mono & PolycrystallineBenchmark Mono & Polycrystalline
http://www.nrel.gov/ce/ipvmqa_forum/index.cfm
100% from initial power AF ≥ 2.3 (extended Peck Model)
See also:
Gossla, M., Hälker, T.,
Krull, S., Rakusa, F.,
Roth, F., & Sinicco, I.
Leakage Current and Performance loss of Thin Film Solar Modules. SPIE, San Diego (2010)
The red indication corresponds the time range where the module had less than 80% of the initial power
�failure. Green color represents at least 80% from initial power. All @ -1000V if not marked.
Beyond IEC (Biased Damp Heat)
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EL example: Module F
928hrs BDH -1000V (51.1% Pini) 800hrs BDH -1000V (60% Pini)
EL example: Module H
Beyond IEC (Biased Damp Heat)
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Moisture Diffusion
& TCO Corrosion
E Field Induced
migration
Durability of Thin Film Solar Cells - 2012
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Example of glass chemistry on reliability
• The leakage current of a PV Module is related to the module durability.• The main contributor to leakage current on PV Modules is glass• Glass chemistry is key on glass resistivity (the better the insulation the lower the
leakage current)
Modules in the field are subjected to voltages between cell and glass reaching the system voltage value in the worse case (and half of that in the best case) and this voltages are related to the way that the system is connected to the inverter.The biased damp heat test simulate such a situation
FROM:
Durability of Thin Film Solar Cells - 2012
Experimental survey of the chemical durability of commercial soda-lime-silicate glasses Christopher W.
Sinton*, William C. LaCourse, Materials Research Bulletin 36 (2001) 2471–2479
Chemical composition variation from float line to float line
Glass Chemistry
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Glass / Foil interface after 1000 h DH 85/85 for a glass with low
magnesia content
Page Page Page Page 36363636 Durability of Thin Film Solar Cells - 2012
From: Analysis of the Glass- Clear PVB Lamination foil interface of Thin Film LaminatesJens Günstera,b, Stefan Krull, Fabia Rakusaa, Florian Roth and Ivan Siniccoa
Safety Integrity: Wet Leakage Currents
Wet Leakage Measurement
05
1015202530354045505560
0 500 1000 1500 2000
DH hours
Wet
Lea
kage
Cur
rent
[uA
]
18.0
19.0
20.0
21.0
22.0
23.0
24.0
25.0
tem
pera
ture
[°C
]
IEC Limit
Glass with low MgO
Glass A
Glass C
Roth, F., Krull, S., Günster, J. and Sinicco, I., “Is The IEC 61646 – 10.15 Test Reliable?“,24th EUPVSEC, 2009, pp. 3553-3556
Generally, for Glass type C,the IEC limit is reached after 7000hrs.
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Care must be taken when using accelerated stress te sts
Durability of Thin Film Solar Cells - 2012
From:
Durability of Thin Film Solar Cells - 2012Page Page Page Page 39393939
From:
Care must be taken when using accelerated stress te sts
Durability of Thin Film Solar Cells - 2012Page Page Page Page 40404040
From:
Care must be taken when using accelerated stress te sts
Durability of Thin Film Solar Cells - 2012Page Page Page Page 41414141
From:
Care must be taken when using accelerated stress te sts
Quality
• Quality of a PV module is a balance between module design , process control and material compliance
• Once the module design is fixed process control and incoming material
inspection/approval are the key element
� Only Fabs with an outstanding Process Control and clear material incoming inspection can afford the risk of warrantees and reduce insurance costs (bankability)
How to insure manufacturing consistency (or is the module successfully tested identical to the approved one)?
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How the designed module is produced?
• Design specs
• Material specs available
• Material specs check
via incoming monitoring
(fingerprint on sampling
plan)
• Supplier agreements
• Storage and logistics
clear
• Are machines
updated?
• How
machines are
monitored?
• Down time
impact clear?
• Shelf life
check
• Process
specs
available
• Process
monitoring in
place?
• Sampling testing
plan according to
sigma in place?
• Labeling according
to design?
• Warranties
according to new
design?
• Packaging well
considered?
• Faulty product
return plan in
place?
• After sales policy in
place?
• WPI are
updated?
• Are operators
well trained?
• Are machine
settings well
adjusted?
• Engineeering
lot
successful?
• Run process
Material control
Process control
Operation control
Product control
Page Page Page Page 43434343 Polymers 2012, ColognePolymers 2012, ColognePolymers 2012, ColognePolymers 2012, Cologne
Implications on Module Reliability and Durability of non consistent manufacturing processses
Durability (t)Durability (t)Durability (t)Durability (t)
(once the measurement (once the measurement (once the measurement (once the measurement
criteria is defined)criteria is defined)criteria is defined)criteria is defined)
Due to offDue to offDue to offDue to off----spec materialsspec materialsspec materialsspec materials
Faulty process controlFaulty process controlFaulty process controlFaulty process control
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How to insure manufacturing consistency? (or is the module produced identical to the approved one?)
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Summary
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� Passing Certification Testing does not insure reliability and even less,durability
� Failure modes are related to:
Module Design (cell included)(~ durability)
Where the modules are located Quality of the used material (gases inclusive)How the modules are installed Effective process and environment control(~durability and reliability) (~reliability)
� To increase market confidence towards PV it is necessary to have a durable moduledesign and adequate QA procedures at supplier and production sites
� To increase product bankability and reduce insurance costs (reducing so the financialrisk) the points above MUST be correctly addressed and proactively informed tothepolicy makers
Durability of Thin Film Solar Cells - 2012
Competitive
Clean
Sustainable
Three reasons to invest into THINFAB™
Thank you for your attention !