IEC TR 63279:2020

Derisking photovoltaic modules - Sequential and combined accelerated stress testing

IEC TR 63279:2020 reviews research into sequential and combined accelerated stress tests that have been devised to determine the potential for degradation modes in PV modules that occur in the field that single-factor and steady-state tests do not show. This document is intended to provide data and theory-based motivation and help visualize the next steps for improved accelerated stress tests that will derisk PV module materials and designs. Any incremental savings as a result of increased reliability and reduced risk translates into lower levelized cost of electricity for PV. Lower costs will result in faster adoption of PV and the associated benefits of renewable energy.

General Information

• Status: Published
• Publication Date: 20-Aug-2020
• ICS: 27.160 - Solar energy engineering
• Technical Committee: TC 82 - Solar photovoltaic energy systems
• Current Stage: PPUB - Publication issued
• Start Date: 21-Aug-2020
• Completion Date: 04-Sep-2020

Standards Content (Sample)

IEC TR 63279 ®
Edition 1.0 2020-08
TECHNICAL
REPORT
colour
inside
Derisking photovoltaic modules – Sequential and combined accelerated stress
testing
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IEC TR 63279 ®
Edition 1.0 2020-08
TECHNICAL
REPORT
colour
inside
Derisking photovoltaic modules – Sequential and combined accelerated stress

testing
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-8737-8

– 2 – IEC TR 63279:2020 © IEC:2020
CONTENTS
FOREWORD . 5
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 8
4 Framework for sequential and combined stress testing . 8
5 Sequential and cyclic sequential test methods . 9
5.1 Extended damp heat and addition of ultraviolet light . 9
5.2 Sequential/combined testing with damp-heat, thermal cycling and ultraviolet
light . 10
5.3 Consideration of interaction of UV radiation and damp heat . 12
5.4 Test-to-failure—A sequential test protocol . 13
5.5 Sequential test protocol optimized for differentiating backsheets . 16
5.6 Mechanical stress testing in combination with damp-heat, humidity-freeze,
and thermal-cycling tests for examining cell cracking and its effects . 20
6 Mechanism-specific multi-factor stress tests . 22
6.1 General . 22
6.2 Testing for delamination . 22
6.2.1 General . 22
6.2.2 Delamination – UV irradiation with high-temperature stress . 22
6.2.3 Delamination – UV irradiation with thermal-cycling stress and humidity
freeze . 23
6.2.4 Delamination – UV irradiation with cyclic dynamic mechanical loading,
thermal cycling stress, and humidity freeze . 24
6.2.5 Delamination – Temperature, humidity, and electric field associated with
system voltage . 25
6.3 Testing for potential-induced degradation . 28
6.3.1 General . 28
6.3.2 Testing for potential-induced degradation with humidity, voltage, bias,
and light . 28
6.3.3 Factor of salt mist . 29
6.4 Testing in damp heat with current injection and as a function of temperature . 30
6.5 Cell cracking and propagation in cyclic loading at various temperatures. 31
7 Combined-accelerated stress testing . 33
7.1 Combined-accelerated stress testing for tropical environments . 33
7.2 Combined-accelerated stress testing for multiple environments . 36
8 Future directions. 39
Annex A (informative) Overview of degradation modes and causal stress factors . 41
Annex B (informative) Failure modes plotted on a failure tree diagram for selected
clauses in this document . 43
Annex C (informative) Summary table of sequential and combined testing: Samples,
factors, combination, and stress-test results . 44
Bibliography . 49

Figure 1 – Framework for sequential and combined stress testing, showing three axes
of comprehensiveness – testing samples, the number of stress factors of the natural
environment, and their sequence or combination of application. . 9
Figure 2 – Fraction power loss of modules though stress testing . 10

Figure 3 – (a) Combined test sequence, and resulting (b) normalized power loss,
(c) short-circuit current (I ), and (d) fill factor (FF) [1] . 11
SC
Figure 4 – Power degradation of modules in 85 °C and 85 % relative humidity as a

function of extent of preconditioning under Xe light [9] . 13
Figure 5 – (a) Overview of the test-to-failure sequences, and (b) results showing
module power normalized to their post-light-soak values for seven module types. 14
Figure 6 – Examples of field-relevant degradation modes seen in modules tested in
the test-to-failure protocol . 15
Figure 7 – Module accelerated sequential tests (MAST) . 17
Figure 8 – Degradation modes from MAST and fielded modules . 19
Figure 9 – (a) Front-side mini-module exposure in a xenon weathering chamber with
water spray; (b) fielded module with six years of service in North America with 30 %
power loss [21] . 20
Figure 10 – (a) Test-stage description; (b) relative change in standard test condition
(STC) module parameters as a function of stage and maximum  power determined at

STC [23] . 21
Figure 11 – (a) Stress testing at 65 °C combined with UV radiation dose of 180 W/m
in the range of 300–400 nm, 900 h; (b) 75 °C without UV radiation, 1 000 h [28] . 23
Figure 12 – Delamination in sequential test . 25
Figure 13 – Delamination associated with system voltage . 27
Figure 14 – Degradation of three modules with and without UV-A light irradiance in
chamber at 60 °C, 85 % RH, and 1 000 V (positive or negative polarity depending on
the sample) . 29
Figure 15 – Sheet resistance measured on glass surfaces with various soil types, as a
function of relative humidity (RH %), at 60 °C [41] . 30
Figure 16 – Cyclic unidirectional 4-point bending with loading alternating between 0 N
and 500 N at different temperatures as shown, with duration of 4 s at each of the high-
and low-pressure dwells, 10 000 to 30 000 cycles with pressure (“Press”) from the
front-glass side or backsheet side [49] . 32
Figure 17 – Example of 24 h PV module combined accelerated stress-testing protocol
modified from ASTM D7869 . 34
Figure 18 – Shrinkage of polymer C backsheet leading to delamination and cracking . 35
Figure 19 – Multiple-environment C-AST sequence . 37
Figure 20 – Failure of two mini-modules with a polymer B outer-layer backsheet type
undergoing different multiple-environment C-AST sequences . 38

Table 1 – Extended damp heat and ultraviolet light . 10
Table 2 – Sequential/combined testing with damp-heat thermal cycling and ultraviolet
radiation . 12
Table 3 – Ultraviolet light and damp-heat interaction . 13
Table 4 – Test-to-failure – Sequential test protocol . 16
Table 5 – Module accelerated stress test 1 (MAST #1) . 18
Table 6 – Module accelerated stress test 2 (MAST #2) . 18
Table 7 – Module accelerated stress test 3 (MAST #3) . 18
Table 8 – SML-TC-HF sequential test . 21
Table 9 – UV irradiation under high-temperature conditions . 23
Table 10 – UV irradiation with TC stress . 24
Table 11 – UV irradiation with DML-TC-HF sequential test . 25

– 4 – IEC TR 63279:2020 © IEC:2020
Table 12 – DH – Negative system bias stress sequential test . 28
Table 13 – UV irradiation – negative system bias stress combined test . 29
Table 14 – Bending load test at various temperatures . 33
Table 15 – Partial list of observed degradation modes, attributed mechanisms, and
stress factors seen in the first application of the combined accelerated stress-testing

protocol based on ASTM D7869 . 35
Table 16 – Combined-accelerated stress test (Tropical 24 h ASTM D7869-based
sequence) . 36
Table 17 – Multiple-environment combined-accelerated stress test . 38
Table A.1 – Degradation modes and potential stress factors that can lead to their
manifestation . 42
Table C.1 – Table summarizing sequential and combined stress testing . 44

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
DERISKING PHOTOVOLTAIC MODULES – SEQUENTIAL
AND COMBINED ACCELERATED STRESS TESTING

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
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The main task of IEC technical committees is to prepare International Standards. However, a
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example "state of the art".
IEC TR 63279, which is a Technical Report, has been prepared by IEC technical committee 82:
Solar photovoltaic energy systems.
The text of this Technical Report is based on the following documents:
Enquiry draft Report on voting
82/1657/DTR 82/1692B/RVDTR
Full information on the voting for the approval of this technical report can be found in the report
on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

– 6 – IEC TR 63279:2020 © IEC:2020
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DERISKING PHOTOVOLTAIC MODULES – SEQUENTIAL
AND COMBINED ACCELERATED STRESS TESTING

1 Scope
This document reviews research into sequential and combined accelerated stress tests that
have been devised to determine the potential for degradation modes in PV modules that occur
in the field that single-factor and steady-state tests do not show. This document is intended to
provide data and theory-based motivation and help visualize the next steps for improved
accelerated stress tests that will derisk PV module materials and designs. Any incremental
savings as a result of increased reliability and reduced risk translates into lower levelized cost
of electricity for PV. Lower costs will result in faster adoption of PV and the associated benefits
of renewable energy.
2 Normative references
The following documents are referred in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60721-2-1, Classification of environmental conditions – Part 2-1: Environmental conditions
appearing in nature – Temperature and humidity
IEC 61215-1:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type
approval – Part 1: Test requirements
IEC 61215-2:2016, Terrestrial photovoltaic (PV) modules – Design qualification and type
approval – Part 2: Test procedures
IEC 61730-2:2016, Photovoltaic (PV) module safety qualification – Part 2: Requirements for
testing
IEC TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols
IEC TS 62782:2016, Photovoltaic (PV) modules – Cyclic (dynamic) mechanical load testing
IEC 62788 (all parts), Measurement procedures for materials used in photovoltaic modules
IEC TS 62804-1, Photovoltaic (PV) modules – Test methods for the detection of potential-
induced degradation – Part 1: Crystalline silicon
IEC TS 62804-1-1, Photovoltaic (PV) modules – Test methods for the detection of potential-
induced degradation – Part 1-1: Crystalline silicon – Delamination
ASTM D7869-17 Standard Practice for Xenon Arc Exposure Test with Enhanced Light and
Water Exposure for Transportation Coatings

– 8 – IEC TR 63279:2020 © IEC:2020
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 61836 apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
4 Framework for sequential and combined stress testing
A number of researchers, companies and testing laboratories have explored aspects of
sequential and combined stress testing to fill outstanding needs. Such needs include testing
beyond IEC 61215-2, which for the most part does not purport to examine for end-of-life wear-
out and failure mechanisms. In other cases, stresses are sequenced and combined to elicit
failure modes that have been seen in the field that existing IEC tests may not evaluate.
A framework for organization is proposed that implements stress factors of the natural
environment, sequences and combinations of applying them, and sample types that may be
employed for evaluation. To illustrate this, Figure 1 is introduced, which gives a three-
dimensional plot with the axes of sample, factor, and combination, that together indicate the
comprehensiveness of test methods to represent the effects of the natural environment on the
sample in accelerated testing.
First the sample comprehensiveness axis of Figure 1 is discussed. As a new material is
explored, the material itself is studied to achieve a basic understanding of its intrinsic
degradation mechanisms and durability. Thus, material and coupon tests as they are performed
now according to IEC 62788 series material tests will be valuable. However, failures often occur
at the interfaces between materials, and the performance of one component of the module often
depends on the behaviour of another component or material in the assembly. Therefore, to
represent the material interactions, boundary conditions in actual use, and stresses
experienced, it is necessary to examine mini-modules, and most comprehensively, full-size
modules with all their components.
Next the factors comprehensiveness axis is discussed. This is the number of stress factors of
the natural environment applied in testing of the sample. Moving from single stress factor tests
to multi-factor tests increases confidence of capturing the factors relevant to both known and
unknown degradation modes. Using one factor alone may be useful to evaluate an acceleration
factor or an activation energy associated with that stress for a specific degradation mode or
mechanism that is already understood to depend principally on that stress factor independently
of others.
Finally, the combination comprehensiveness axis is discussed. It represents the manner of
integration of the stress factors on the sample. We seek to sequence and combine the stress
factors in a manner that represents how they appear together in nature to increase the
probability of accelerating only the real degradation modes in the module as they would
manifest in nature. As stress factors are considered, individually or in combination, it is
necessary to understand whether stress levels applied are maintained within the levels of the
natural environment, or if they are exceeded. If exceeded, acceleration of the test may be
increased, but there is significantly increased potential of incurring degradation modes that are
artifacts—modes not necessarily representative of those that would be seen in the natural
environment.
Tables are given in this document for various experimental results in the framework of Figure 1
condensed into two dimensions. These serve to explain how the sequential and combined
accelerated stress tests, with consideration of sample type, factors, and their combination, have
served to produce particular failures or degradation modes. In these condensed two-

dimensional plots, the various column-listed stress factors may be an individual stress factor
such as mechanical load, or an existing IEC 61215 stress test, such as damp heat or thermal
cycling, which in itself contains factors of temperature cycling and current through the cell
circuit. Annexes in which the failure modes are collected for reference are as follows: Annex A :
Overview of degradation modes and causal stress factors, Annex B: Failure modes plotted on
a failure tree diagram for selected clauses in this document, and Annex C : Summary table of
sequential and combined testing: samples, factors, combination, and stress test results of the
samples studied. The templates in these Annexes may be useful for classifying other failure or
degradation modes as they become understood in the future.

Points shown are possibilities for testing within this space.
Figure 1 – Framework for sequential and combined stress testing, showing three axes
of comprehensiveness: testing samples, the number of stress factors of the natural
environment, and their sequence or combination of application
5 Sequential and cyclic sequential test methods
5.1 Extended damp heat and addition of ultraviolet light
Extended damp-heat (DH) testing has frequently been used to attempt to differentiate durability
of PV modules. An example of this is shown in Figure 2a). Five modules undergo five iterations
of 1 000 h duration DH tests at 85 °C and 85 % relative humidity (RH). Four module types
exhibit great degradation after 2 000 h that is due to fill factor (FF) loss from metallization to
silicon contact-resistance increase. The degradation comes at test conditions with temperature
in combination with humidity significantly exceeding those found for modules in PV field
installations, and the degradation mechanisms observed with extended DH tests have
frequently been inconsistent with those seen in fielded PV modules [1] . Reviews of
agglomerated field-degradation data for crystalline silicon cell modules have shown degradation
primarily by short-circuit current (I ) loss followed by FF loss and the least degradation
sc
exhibited by open-circuit voltage (V ) [2].
oc
Excessive humidity may lead to unrealistically high levels of acetic acid formation, leading in
turn to unrealistically high FF losses through grid finger to silicon contact corrosion and other
mechanisms. Therefore, excessively long DH stress tests that produce very high acetic acid
__________
Numbers in square brackets refer to the Bibliography.

– 10 – IEC TR 63279:2020 © IEC:2020
levels are believed to have limited use in evaluating the durability of conventional crystalline
silicon PV modules installed in the field.
If, after 2 000 h of DH testing, modules were transferred for ultraviolet (UV) exposure in a DH
environment with an 85 °C target module temperature, then the power losses were more modest
as shown in Figure 2b) and reported to be primarily associated with I degradation [1], which
sc
is representative of what is observed in the field. Including UV radiation is necessary to
represent this stress factor of the natural environment. A summary of the sample type used,
stress factors, and their sequence and combination, along with resulting degradation modes
seen for the modules in the study, is given in Table 1.

a) b)
a) Module M1 with thermoplastic and modules M2–M5 with ethylene vinyl acetate encapsulant through 1 000 h of
85 °C and 85 % relative humidity damp heat cycles;
b) Modules through 2 000 h of the damp heat exposure in (a) followed by placement under UV radiation and damp
heat [1].
Figure 2 – Fraction power loss of modules though stress testing
Table 1 – Extended damp heat and ultraviolet light
Sequential/ Stress factor A Stress factor B Combined stress
combined effect(s)
DH UV+DH
Material
Coupon
Mini-Module
Module 85 °C/85 % RH Degradation of I and FF
200 W/m UV-A,
sc
2 000 h
in better proportion to field,
85 °C module temperature,
toward field-relevant levels
of humidity after UV
2 000 h
exposure.
Sequential / Combined: A  B
5.2 Sequential/combined testing with damp-heat, thermal cycling and ultraviolet light
A sequential test is shown in Figure 3a) developed for the application of additional stress factors
with more balanced levels considering the relative levels seen in outdoor exposure and to
produce degradation modes in reasonable proportion to those seen in the field. In addition to
DH and DH with UV sequences, temperature cycling is included, which adds thermomechanical
stresses. Table 2 summarizes the stress factors applied, the levels, and the results of the
combined stress effects.
Because of acetic acid formation, attention has been given to humidity levels when alternating
between DH in the dark, which drives humidity into the module, and DH with UV radiation, which
drives moisture out of the module, as illumination does in PV modules installed in the field [1].
On the backsheet side of the cell, humidity levels reach correspondence with chamber
equilibrium; on the front side of the cell, simulations show humidity levels stabilizing around
30 % lower in the alternating sequence than in the continuous DH case, reducing unrealistically
high formation of acetic acid that can affect the metallization-silicon contact of some solar cells.
The UV can also degrade the cell-front passivation, reducing I and V [3], and it can cause
sc oc
transmission loss in the encapsulant, contributing to degrading I [4], which is observed in the
sc
results in Figure 3b) to d).
Appropriate humidity levels and durations for accelerated tests in DH depend on whether
moisture-barrier components are required and on the degradation kinetics of the particular solar
cells [5]. Based on simulations, it has been proposed that testing for more than 3 000 h in 85 °C
and 85 % RH is necessary to duplicate the moisture-ingress distance experienced by an edge
seal after 25 years of exposure in the Miami, Florida (USA) use environment [6]. This level,
however, causes hydrolysis of polyethylene terephthalate (PET) layers in backsheets used in
many crystalline silicon cell-based modules. Extensive degradation by hydrolysis of PET has
not been seen in fielded PV modules, so extended DH testing (in this case, 85 °C, 85 % RH,
and 3 000 h) is considered too extreme a level for testing failure modes that could be linked to
PET degradation [7].
a) b)
c) d)
Figure 3 – a) Combined test sequence, and resulting b) normalized power loss, c) short-
circuit current (I ), and d) fill factor (FF) [1]
SC
– 12 – IEC TR 63279:2020 © IEC:2020
Table 2 – Sequential/combined testing with damp-heat
thermal cycling and ultraviolet radiation
Stress factor A Stress factor B Stress factor C
Sequential/ Combined stress
combined effect(s)
DH TC DH/UV
Material
Coupon
Mini-Module
Module 85 °C/ 85 % RH –40 °C/ 85 °C Degradation of I
200 W/m UV-A,
sc
1 000 h or 500 h 100 cycles
85 °C module
and FF in proportion
temperature,
to field, toward field-
500 h
relevant levels of
humidity ingress with
use of UV exposure
Sequential/Combined: A  B  C  [A  B  C] × n
The stress sequence (A–C) is repeated cyclically; however, stress factor A (DH) time is reduced to 500 h after the
first time.
5.3 Consideration of interaction of UV radiation and damp heat
UV radiation affects acetic acid production in susceptible encapsulants [8]. This can be seen in
the results shown in Figure 4 with modules constructed using conventional back-surface-field
cells and poly(ethylene-co-vinyl acetate) (EVA) encapsulant tested with differing durations of
preconditioning with a Xe-full-spectrum arc lamp [9]. The module type that did not degrade at
all in 85 °C and 85 % RH damp heat through 1 500 h showed increasing degradation with
increasing preconditioning with Xe-source illumination of 90 W/m in the range of 300 nm to 400
nm and 65 °C chamber temperature at 30 % RH or less. Under such conditions, the resulting
sample temperature is 90 °C and module surface RH is ≤ 13 %. If exposed to the Xe arc lamp
for 4 000 h beforehand, 42% of the initial power is seen within 1 500 h of the DH exposure.
Higher acetic acid concentration was not found after the UV exposures, but acetic acid levels
were higher after damp heat according to the extent of preconditioning with light before the DH
test; this indicates some other chemical process occurring under light that facilitates the
formation of acetic acid with subsequent DH exposure. Preconditioning in heat alone (90 °C)
also did not promote subsequent degradation in 1 500 h of damp heat. The root cause of the
degradation was assigned to the development of higher series resistance between the grid
fingers and the silicon cell from the acetic acid and humidity. A summary of the sample type
used, stress factors, and their sequence and combination, along with resulting degradation
modes seen for the modules in the study, is given in Table 3.

Figure 4 – Power degradation of modules in 85 °C and 85 % relative
humidity as a function of extent of preconditioning under Xe light [9]
Table 3 – Ultraviolet light and damp-heat interaction
Sequential/ Combined stress
Stress factor A Stress factor B
combined effect(s)
UV DH
Material
Coupon
2 UV radiation activates
90 W/m (300 nm to
appearance of acetic acid
400 nm),
85 °C/ 85 % RH
in subsequent DH, causing
Mini-module
90 °C module temperature increased contact
1 500 h
resistance of grid fingers to
1 500 h to 4 000 h
silicon
Module
Sequential: A  B
5.4 Test-to-failure – A sequential test protocol
The “Terrestrial Photovoltaic Module Accelerated Test-to-Failure (TTF) Protocol” was devised
in 2008 and subsequently demonstrated to fill a gap between qualification testing and
comprehensive accelerated lifetime testing [10-12]. This protocol shown in Figure 5a) also adds
stress sequences in combinations, which manifest in degradation mechanisms not presently
examined in standardized qualification testing. The TTF protocol extends the environmental
chamber testing until failure modes in the module can be seen to a sufficient magnitude to be
studied (e.g. greater than 20 % degradation). The protocol was devised to compare the
reliability of different modules on a quantitative basis and to evaluate the performance of new
module types to incumbents. A key technical and intellectual exercise not to be overlooked in
the analysis of the TTF results is an evaluation of the significance of degradation modes or
failures seen. This is in part because the stress levels applied are far greater than found in
nature, therefore spurious failures may occur. Field testing and further chamber testing to
determine acceleration factors for issues found may need to be performed to evaluate if the
degradation modes will be seen in fielded PV modules.
Examples of module power results through the TTF protocol are shown in Figure 5b).

– 14 – IEC TR 63279:2020 © IEC:2020

a)
b)
Most module types shown were run for five rounds. Rounds consisted of 200 thermal cycles or 1 000 h of damp heat
with either + or – nameplate system-voltage applied. Module type 2 testing was terminated at three rounds of thermal
cycles, and type 7, a latecomer to the program, was tested only through alternating DH (-) / thermal cycle for six
rounds [12].
Figure 5 – a) Overview of the test-to-failure sequences, and b) results showing
module power normalized to their post-light-soak values for seven module types
Module nameplate system-voltage (V ) bias of 600 V, either in (-) or (+) polarity, was applied
sys
to the active cell circuit in the DH environment, [DH (+), DH (-)]. These sequences discern
susceptibility to various potential-induced degradation (PID) modes such as junction shunting,
loss of surface passivation (polarization), and electrochemical reactions on the cell surface that
may lead to delamination and loss of light transmission to the cell due to degradation of the
antireflective coating or the encapsulant.
Modules tested through thermal cycling (TC) with the short-circuit current passing through the
cell strings show mild degradation. In some cases, cracks propagated through cells that led to
partially disconnected cell regions. In some cases, TC can lead to other issues including hot
spots, interconnect fatigue, and solder-bond failure.
The alternating damp heat with bias and thermal cycling are sequences Alt DH (+ or -)/TC. DH
may embrittle some polymers, and the thermal cycling applies some thermomechanical stress
and causes desiccation that can lead to loss of toughness. Examples of degradation modes
found when applying the TTF protocol are shown in Figure 6a) to d). Some of these degradation
modes (backsheet cracking, delamination, and interconnect corrosion) have also been
observed by others in their work on sequential and combined acceleration tests for crystalline
silicon PV modules [13, 14]. The acceleration factors for these degradation modes are still being

studied. The levels of stress applied in the TTF are higher than in the natural environment, but
the resulting degradation modes are frequently seen to be field-relevant considering their
observation in the field [12]. These degradation modes were not seen in any of the single-factor
tests such as DH, thermal cycling, or humidity-freeze tests. However, if modules are run for
longer duration in single-factor tests, some of the degradation modes might eventually be
observed. Because the levels applied in TTF exceed those of the natural environment, the
mechanisms of degradation observed may be different, even if the modes appear the same. A
summary of the sample type used, stress factors, and their sequence and combination, along
with resulting degradation modes seen for the modules in the study, is given in Table 4.

a) b)
c) d)
a) Backsheet embrittled in damp heat and cracked in the following thermal cycling;
b) Degradation of silicon nitride antireflective coating in damp heat with positive system voltage bias applied to the
cell circuit;
c) Delamination of an ionomer-based thermoplastic encapsulant through 3 000 h damp heat and positive system
voltage bias applied; and
d) Corrosion, delamination, and ion migration around bus ribbon in 2 000 h of damp heat with positive system voltage
applied [12].
Figure 6 – Examples of field-relevant degradation modes
seen in modules tested in the test-to-failure protocol

– 16 – IEC TR 63279:2020 © IEC:2020
Table 4 – Test-to-failure – Sequential test protocol
Sequential/ Combined stress
Stress factor A Stress factor B Stress factor C
combined effect(s)
Cyclic application of
DH / V bias (+) or
TC with application
sys
stress factors A and
of current
(-)
B
Material
Coupon
Mini-module
Module 85 °C/85 % RH, Stress Factors A and Backsheet cracking,
−40 °C/ 85 °C,
B corrosion,
+600 V or −600 V,
I , 200 cycles
delamination, PID,
sc
1 000 h
solder-bond failure,
antireflective coating
degradation
Sequential/Combined: C = [A  B] × n

5.5 Sequential test protocol optimized for differentiating backsheets
Based on detailed field studies of module degradation modes, accelerated tests to better the
failure mechanisms in backsheets have been devised after finding that single-factor stress tests
capture some of the observed field failure modes, but that they miss others resulting from
synergistic effects or combinations of stresses applied in a sequence. To address this, three
sequential stress tests referred to as module accelerated stress tests (MASTs) were developed
and applied; these are shown in Figure 7 [15] and summarized in Table 5 through Table 7.
These were developed using mini modules, but some of these tests are also being extended to
full size module.
The first of these tests, MAST #1, incorporated DH of 85 °C and 85 % RH in an initial exposure
of 1 000 h. This duration is based on comparing the mechanical properties of backsheets from
the field and backsheets exposed to the DH. Both modules exposed to this DH level and
modules in the field for over a 25-year period using a polymer A-layered backsheet did not
exhibit impaired mechanical properties, as confirmed by measurements of tensile strength and
mechanical elongation to break tests. Exposure to greater than 1 000 h of DH was found to
cause hydrolysis damage to the polyester core within the backsheet, leading to degradation in
mechanical properties not seen in the field.
UV dose applied in the MAST assumes an albedo exposure of 12 %. The total UV exposure
over 25 years of the back side of the PV module is estimated to be about 276 kWh/m (300 nm
to 400 nm) based on meteorological data in a desert climate. Four 1 000 h UV-A exposure
segments at an intensity of 65 W/m (300 nm to 400 nm) are employed in MAST #1. The test
uses either a 70 °C black-panel temperature (BPT), with sample temperature of about 70 °C;
or, for higher acceleration filtered Xe lamp exposure, 90 °C BPT with sample temperature of
about 75 °C [16].
To apply thermomechanical stress, standard thermal cycling (-40 °C to 85 °C) up to 600 cycles
is applied, which is commonly used in the industry for extended durability testing. Although not
yet incorporated into a MAST, cyclic mechanical loading steps are also being studied for
inclusion because they have been found to assist in replicating an encapsulant/glass interface
delamination mode that has been observed in the field; this is discussed in 6.2.4. A summary
of the sample type used, stress factors, and their sequence and combination, along with
resulting degradation modes seen for the modules in the MAST #1 study, is given in Table 5.
Figure 8a) to d) shows optical photos of module degradation modes resulting from the MAST
#1 testing, compared to images obtained from fielded PV modules. The images reveal cracks
in a polymer
...