NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Ensuring Quality of PV Modules

Preprint

Sarah Kurtz, John Wohlgemuth, Peter Hacke, 

and Michael Kempe 

National Renewable Energy Laboratory

Tony Sample 

European Commission, Joint Research Centre, 

Institute for Energy

Masaaki Yamamichi, Michio Kondo, Takuya Doi, 

and Kenji Otani 

National Institute of Advanced Industrial Science and 

Technology (AIST)

James Amano 

Semiconductor Equipment and Materials International

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-50651

July 2011
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1 

ENSURING QUALITY OF PV MODULES

Sarah Kurtz1, John Wohlgemuth1, Tony Sample2, Masaaki Yamamichi3, James Amano4, Peter Hacke1, Michael Kempe1, 

Michio Kondo3

, Takuya Doi3, and Kenji Otani3 

1

National Renewable Energy Laboratory, Golden, CO, USA 2

European Commission, Joint Research Centre, Institute for Energy, Renewable Energy Unit, I-21027, Ispra (VA), Italy 3

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan 4

Semiconductor Equipment and Materials International, San Jose, CA, USA

ABSTRACT

Photovoltaic (PV) customers need to have confidence in

the PV modules they purchase. Currently, no test can

quantify a module’s lifetime with confidence, but stress 

tests are routinely used to differentiate PV product 

designs. We suggest that the industry would be 

strengthened by using the wisdom of the community to 

develop a single set of tests that will help customers 

quantify confidence in PV products. This paper evaluates 

the need for quality assurance (QA) standards and 

suggests a path for creating these. Two types of standards 

are needed: 1) QA of the module design and 2) QA of the 

manufacturing process.

INTRODUCTION — MOTIVATION

As the PV industry has grown exponentially in recent 

years, the worldwide investment in PV installations is 

approaching $100 billion/yr. Those financing this market 

growth want to be able to predict the risk of failure of PV 

products and are asking for more quantitative tests. 

Quantitative or comparative tests can: 1) compare module 

designs, 2) provide a basis for manufacturers’ warranties, 

3) provide investors with confidence in their investments, 

and 4) provide data for setting insurance rates.

The definitions of failure and lifetime vary. The warranty 

can provide a very specific definition of these aspects, but 

a module that has degraded more than specified by the 

warranty may still have value for some PV customers. 

Here, we discuss the durability of a module as its longterm ability to survive environmental stresses. The 

reliability (often defined as the probability that an item will 

perform a required function) of a PV module includes the 

durability as well as soiling, incorrect installation, and 

many other issues. This analysis focuses primarily on 

assuring adequate durability within the scope of 

accelerated testing and quality assurance programs while 

recognizing that, ultimately, all aspects of the reliability are 

important.

Ideally, the lifetime of a module would be predicted by a 

relatively short set of accelerated stress tests. However, a

technical basis for predicting lifetimes has not been 

established. We, therefore, propose comparative 

standards with efforts to estimate the expected lifetimes as 

a function of the deployment conditions to guide the range 

of stress that is applied. Some may argue that this effort is 

premature. We emphasize that these standards will not be 

perfect, but are a first step toward providing the desired 

confidence.

A growing number of test labs, PV manufacturers, and 

others have been developing new tests [1–4] to meet the

need for more quantitative tests, but the community would 

be best served by a coordinated effort toward a single 

international standard. This study explores the current 

status, needs, and challenges, then proposes creation of 

1) a QA rating system to communicate the relative 

durability of module designs and 2) a guideline for factory 

inspections of the QA system used during manufacturing. 

DEFINITION AND CURRENT STATUS OF QA

The assurance of quality of a product has two parts: 1) 

assurance that the product design suits the needs of the 

customer and 2) assurance of consistency of the product 

quality during the manufacturing, shipping, and installation 

processes. Currently, the former is addressed through 

testing to International Electrotechnical Commission (IEC) 

qualification standards 61215, 61646, and 62108 and to 

safety standards IEC 61730 and UL 1703. These tests 

were developed largely in response to field experience [5]. 

For example, in the late 1970s and early 1980s, the Jet 

Propulsion Laboratory (JPL) purchased multiple sets of PV 

modules, analyzed the field failures, and developed 

accelerated tests to quickly duplicate those failures.

Today’s qualification tests are mainly based on the tests 

developed by JPL. PV modules currently demonstrate 

excellent reliability in the field mostly because of such

tests.

Currently, each company defines their own QA program. 

Because no formal oversight is required by the IEC 

standards, there is no simple way for PV customers to 

confidently evaluate a company’s manufacturing QA 

process. Test laboratories are accredited to complete the 

certification process and issue a certificate. Most test 

laboratories then require periodic factory inspections to 

ensure that the company has not changed the design of 

the module, the manufacturing processes, or the raw 

materials. However, there is no standard guideline for the 

factory inspections and some test laboratories do not 

conduct factory inspections at all. 
2 

QA OF PRODUCT DESIGN — NEEDS

Customer Needs

The results of product testing need to be communicated 

directly and clearly to the customer. The test results 

should be included on the product datasheet and on the 

nameplate, listing the different tests performed and an 

indication of the level of test that was passed. Some 

customers wish to have detailed test results that show the 

evolution of the product degradation rather than a simple 

pass-fail result. Such detailed results should be available 

from the manufacturer, but cannot be recorded on the 

nameplate and may not be easily included on a datasheet.

Customers would like to see the test result information 

presented as the expected lifetime of a module. This is 

not practical for two primary reasons: 1) the lifetime is 

dependent on the local weather, the mounting 

configuration, and the quality of the installation, so a single 

number is not meaningful; 2) a technical basis for 

predicting the module lifetime has not yet been

established. There are dozens of potential failure 

mechanisms. Quantifying each of these to determine the 

lifetime for a module for a given application is beyond our 

current capabilities. Nevertheless, data from modules that 

have been deployed 20–30 years coupled with modeling 

of stresses can be used to estimate expected lifetimes as 

a function of location for specific failure mechanisms. 

When possible, existing information should be collected 

and provided to PV customers to help them interpret the 

QA rating given on the nameplate and datasheet.

When possible, the rating should have a clear meaning. 

For example, modules are currently given a maximum

system voltage rating, which is used by system designers. 

Similarly, a hail rating specifies the size of the hail ball and 

its terminal velocity. 

It should not be overlooked that some customers may 

value low price over long lifetime, as in the case of a PV 

installation on a building that may be torn down in ten 

years. QA information should be presented in such a way 

that customers can make choices to match their 

application rather than providing a pass/fail rating that may 

not be relevant to their needs. 

Finally, the customer needs to have information about the 

uncertainty associated with the rating system. If studies 

show conclusively that failure of a test gives a high 

probability of failure in the field, then it is reasonable for a 

customer to require that the test be passed. On the other 

hand, it would be counterproductive for a customer to 

require passing a test that does not correlate with field 

experience. In such a case, requiring the test could 

needlessly increase cost. When there is non-conclusive 

evidence of a test’s value, it could make sense for an 

incentive program to give somewhat higher rebates for the 

products that pass the test, but still allow other products to 

participate in the rebate program. Thus, to empower 

insurance companies and incentive programs to most 

wisely use the QA rating system, information must be 

available about the certainty and uncertainty associated 

with the predictive value of each test.

Manufacturer Needs

Manufacturers benefit from having a single set of tests that 

are administered internationally under the International 

Laboratory Accreditation Cooperation (ILAC). When 

customers ask that manufacturers certify products in their 

local area, the duplicate testing and associated factory 

inspections are unnecessary costs for the company and, 

ultimately, for the customers themselves. ILAC defines an 

international cooperation to standardize the accreditation 

of the test laboratories so that manufacturers maintain a

single certification with a single test laboratory.

Manufacturers benefit from shorter tests that are less 

costly and that cause minimal delay in the launch of new 

products. Similarly, manufacturers benefit from stable 

standards (revision of a standard may improve the 

standard, but may also force all manufacturers to recertify)

and reasonable retest guidelines (retest guidelines that 

require a complete new certification for small changes in a 

product can increase testing costs prohibitively).

While manufacturers usually prefer simpler test standards, 

new manufacturers benefit from having a robust test 

standard that confidently demonstrates new products to 

customers. Similarly, experienced manufacturers benefit 

from having tough test standards that can differentiate 

their very durable products from less durable products that 

their competitors sell at lower prices. However, it is 

important for the customer that this differentiation is 

relevant to field performance of the modules, because

tests that are more difficult to pass, but don’t indicate 

longer service life, will needlessly increase costs. 

Scientific Needs

While customers would like a test sequence that tells how 

many years a product will last, the scientific knowledge 

does not exist yet to create such a test sequence. 

Nevertheless, the community has collected a wealth of 

information about failure mechanisms of PV modules and 

how to test for these. The QA standards can use the 

existing knowledge while identifying the most important 

gaps to prioritize for near-term research.

A very important opportunity is to design the QA test

standards to facilitate learning about lifetime prediction. 

Decades from now, the QA standard we design now can

facilitate correlation between lifetime and the QA rating. 

The challenge of designing a QA test standard is 

complicated by many scientific challenges, as described in 

Table 1. Table 1 raises the question of whether a single

set of tests can be used to predict service life or even to 

equitably compare expected durability of different designs. 

For example, the acceleration factors between a damp-
3 

heat test and real-time exposure differ dramatically for

modules with glass-glass or glass-PET construction.

Table 1. Factors that affect the correlation of

accelerated tests with service life prediction

Concern Acceleration factors (AF)

Multiple failure 

mechanisms

AFs vary by orders of magnitude for 

dozens of failure mechanisms

Variable 

weather

AFs differ for each location 

Mounting 

configuration

AFs can be a factor of ten lower for 

roof mount than rack mount 

Cross terms Combined stresses can have AFs that 

are orders of magnitude different than 

for each stress alone

Module 

geometry

AFs can vary by orders of magnitude

Degradation 

versus failure

AF for degradation may differ from the 

AF for the final failure (degradation rate 

may change with time)

Module 

variation

A failure mechanism that affects a 

small fraction of the sample population 

may not be identified by testing a 

handful of modules

While the complexities in Table 1 highlight the 

monumental challenge of creating a test that accurately 

predicts long-term PV module performance, useful 

progress can be made if the scope is limited by prioritizing 

the most problematic failure mechanisms and if only a 

handful of weather conditions are quantified.

QA OF PRODUCT DESIGN — PROPOSAL

Our proposal is designed to optimally address the points 

raised above. We propose a QA rating system that defines 

comparative tests to differentiate products with respect to 

their durability, and then propose scientific studies to help 

the community interpret the meaning of the tests. Thus, 

we do not propose that the standard be used for service 

life prediction, but that the standard be designed to help 

collect information to enable lifetime predictions in the 

future. However, even if the standard is not intended for 

lifetime predictions, it is useful to define the severity of the 

tests to span the range of durability that is of interest to the 

customer. This requires estimation of the lifetimes 

associated with the tests for a range of climates.

Stresses

A customer would prefer to be told the expected lifetime of 

a module, but this is not possible without knowledge of the 

intended application. From a scientific perspective, 

evaluation of the durability of a module requires analysis 

of the stresses that will be encountered. A list of stresses 

is shown in Table 2 along with a proposal for how a rating

system might communicate to the customer the durability 

associated with each stress. Columns are also included to 

describe existing or proposed tests and how these relate 

Table 2. Stresses encountered by PV modules and proposed system for rating PV modules

Stress Rating system Accelerated test Environmental definition

Voltage Numeric value for 

maximum system 

voltage

As per IEC 61215 [6], or 

revised to be applied during 

damp heat

System voltage

Temperature Class Hottest, Hot, 

Warm, Cool

Damp heat, possibly with 

voltage bias applied

Use Arrhenius behavior and create maps for 

rack and roof mounting

Thermal 

cycling

Class A, B Thermal cycling as per IEC 

61215 [6], but two levels of 

200 or 500 cycles

Thermal cycling is greater for partly cloudy 

environments, because variable irradiance 

causes frequent changes in module 

temperature

Humidity Class Humid, Dry Damp heat, possibly with 

voltage applied

Average humidity; make map

Snow Numeric rating for kg 

of static load

IEC 61215 [6] static load 

test

Snow load from local building code

Salt spray Numeric severity 

rating

Existing IEC test (edition 2)

[7]

Distance from ocean

Hail Numeric rating for size 

of hail ball

Current default is 25 mm; 

method as in 61215 [6]

Size of hail balls experienced locally

UV Class A, B Use UV component test 

being drafted by WG2

Class A indicates high-altitude or highirradiance site

Wind Numeric rating for 

maximum wind gust

Combination of dynamic 

load test and vibration

Maximum wind speed seen during gusts

Transportation Rough/Smooth Vibration Truck on paved and unpaved roads, train, etc.

Farmland Pass/Fail Use ammonia test drafted 

by WG2

Ammonia in agricultural area
4 

Table 3. Partial list of failure or degradation mechanisms and the related stresses

Type of failure/degradation Related stresses Priority

Broken interconnects, solder bond failures Thermal cycling, mechanical First

Broken glass; structural failures Mechanical First

Corrosion, including electrochemical corrosion; corrosion 

leading to loss of grounding

Humidity, heat, bias V, dry/wet high pot First

Hot spots Shading First or 

second

Broken cells Thermal cycling, vibration, mechanical Second

Delamination and/or loss of elastomeric properties of 

encapsulant

Humidity, heat, humidity-freeze, UV, 

dry/wet high pot

Second

Encapsulant discoloration UV, heat Second

Junction box and module connection failures Thermal cycling, humidity, heat, 

humidity-freeze

Second

Bypass diode failure Heat (applied to diodes) Second

Open circuiting leading to arcing Thermal cycling Second

Ground fault from backsheet degradation UV Second

Ground faults Dry/wet high pot Second

to environmental conditions. We propose that a QA test 

standard differentiate levels of durability to each of 

these stresses as described by the rating system. To 

identify the appropriate severity of each test, it is useful 

to estimate the effects of that stress around the world as 

related to at least one failure mechanism.

Failure Mechanisms

It is not practical to test a module for all possible failure 

mechanisms, and any quantitative prediction must be 

made for a specific failure mechanism. We propose to 

identify the failure/degradation mechanisms that are 

most likely to cause field failures by each of the major 

stresses if not adequately mitigated and prioritize these. 

If accelerated tests are quantified for the highest priority

failure mechanisms, there is a reasonable chance that 

the same test will identify other problems as well (see 

Table 3). We propose to first estimate acceleration 

factors for the prioritized mechanisms listed in Table 3 

as “First.” Other mechanisms may be addressed in the 

future.

Test Standards Under Development

In response to the interest in testing “beyond 61215 or 

61646,” many PV manufacturers and test labs have 

begun to develop more quantitative tests [1–4]. These 

will provide a starting point for creation of a single 

international standard. Tests already exist for each of 

the stress categories. The test definition for 

implementing the QA rating system primarily needs to 

identify how many classes will be quantified for each 

stress and how severe each test should be. A detailed 

discussion of these is beyond the scope of this paper, 

but is discussed elsewhere [8]. 

QA DURING MANUFACTURING

A robust, upfront certification process can give 

confidence in a design, but the quality of the product 

may vary greatly if the manufacturer does not maintain 

an adequate quality assurance program. 

Customer Needs

PV customers would benefit from being able to see a 

certificate demonstrating that the manufacturer’s QA 

program meets a reasonable standard. Currently, some 

investors/customers require the manufacturer to 

document their QA program as part of the investors’ due 

diligence. Although this may continue to be justified for 

the largest investments, such a review adds cost and 

could be avoided if a meaningful review is completed as 

part of the certification process.

Manufacturer Needs

As described above, a manufacturer benefits from 

needing to certify each product only once. Thus, a 

certification process that can be implemented under 

ILAC will allow manufacturers to keep prices low. The 

factory inspection should require only the 

documentation that is useful. Onerous requirements that 

do not translate to added confidence will needlessly 

increase costs. Similarly, if all factories are operated 

under a single QA program, inspecting all factories 

needlessly increases cost.

Scientific Needs

PV products today span a wide range of geometries; the 

processes used to build the modules span an even 

wider range of conditions. Thus, it is not possible to 

define specifics in a QA guideline. A QA guideline 

should direct the test laboratory on what to look for (e.g.

control of encapsulant curing process) and the 

manufacturer on how to present the information to the 

inspectors (e.g. data showing that modules made within 

a given process window pass the certification test) 

without stating the exact conditions. It could be useful to 

define a guideline that differentiates manufacturing QA 
5 

ratings rather than an inspection process that ends up 

with a pass-fail conclusion. For example, a company 

with very few warranty returns would be rated to have 

better QA than a company with many manufacturing 

excursions. 

As with the design of a QA rating system, a QA 

guideline should facilitate learning from the community 

to improve the guideline.

QA OF MANUFACTURING PROCESS — PROPOSAL

Ideally, after certifying the QA rating for a PV module 

design, a test laboratory inspects the associated 

factories to ensure that a strong QA program is in place 

and is carefully followed. We propose, therefore, that 

IEC is the most logical standards organization to 

implement a manufacturing QA guideline. This 

implementation may be simplified by the creation of 

standards for material purity by Semiconductor 

Equipment and Materials International (SEMI) or by 

standards defined for other elements in Table 4.

Table 4. Elements of a QA guideline

Guideline

element

Parts of review

Materials 

qualification

List of materials used in module 

fabrication; criteria and properties 

tracked for each of these materials

Process control Calibration of sensors, acceptable 

process windows for each part of 

process, log of data collected

In-line testing List of measurements completed, 

frequency of these measurements, 

log of data collected

Traceability Documentation from ingot to module 

shipment; maintenance of records to 

trace future failures, ID marking of 

modules

Retest 

schedule

Frequency of qualification or other 

module-level testing, log of data

Warranty return 

program

Documentation of number of returns, 

identified failures, and corrective 

actions

Factory 

inspection 

procedure

Frequency of inspection, fraction of 

manufacturing lines inspected, and 

pass/fail criteria

It is also necessary/useful to ensure quality of the 

installation design and implementation, but this is 

outside of the scope of this paper.

CREATION OF QA STANDARDS

Key Steps for Creating QA Standard for Designs

It often takes years, if not decades, to obtain a 

consensus on a new standard. To expedite this 

process, we provide a step-by-step plan. Key steps 

include:

1. Develop a consensus that a single international 

approach is needed. 

2. Identify what information is to be communicated

(e.g. Table 2). 

3. Use existing knowledge to suggest appropriate 

tests to provide that information. 

4. If existing knowledge does not give confidence, 

then “test the test” by applying proposed tests to 

module designs with known field success and 

failure [8]. 

5. Create maps or other guidance to communicate

test implications to customers. 

6.Apply a similar procedure to thin-film, CPV, and 

balance-of-system technologies. 

The International PV Module Quality Assurance Forum 

is scheduled for July 15–16, 2011, in San Francisco, 

Calif. At this meeting, we hope to accomplish steps #1 

and #2 and form committees to develop tests for each of 

the stresses listed in Table 2. At the Forum, the 

community will be given an opportunity to discuss their 

preference for how to best create QA standards. Tables 

2 and 3 will be used as a starting point for discussion, 

but the participants will determine the outcome of the 

Forum, forming committees to implement the chosen 

plan. These committees will be asked to present 

updates at each IEC TC82 WG2 meeting, with a target 

of proposing a Part 2 to IEC 61215 in spring of 2012 

(step #3). If WG2 approves the proposed Part 2, work 

on steps #4–6 will commence. If the community is 

willing to work together, testing to the international QA 

standard can begin in spring of 2012.

Key Steps for Creating Manufacturing QA Standard

A guideline for QA during manufacturing will also be 

discussed at the International PV Module QA Forum in 

July. A committee will be formed with a similar timeline 

as described above. 

1. Work with IEC to define factory inspection and 

retest guidelines that would become part of the IEC 

61215 certification process. 

2. Work with SEMI and other standards organizations 

to develop standards for material, component, and 

equipment qualification and in-line testing. 

CONCLUSIONS

The time has come for the PV community to move 

toward more quantitative standards that can differentiate 

PV modules according to their durability and provide the 

basis for improved risk analysis. Many groups are 

currently working to create such tests, but the 

community will be better served if these groups can 

work together to create a single set of QA standards 

and guidelines. This paper proposes the creation of a 

QA rating system for adoption as Part 2 of IEC 61215. 

The proposal suggests that the rating system should 

document on the nameplate and datasheet the relative 

durability of the module to each type of stress and that a
6 

full test report would include additional information for 

interested customers. A manufacturing QA guideline for 

defining factory controls and guiding inspections is 

proposed to become a part of the certification process.

Further work will be required to extend this to thin-film 

and CPV testing and to quantify the meaning of the test 

results.

ACKNOWLEDGMENTS

This work was supported by the U.S. Department of 

Energy under Contract No. DE-AC36-08-GO28308 with 

the National Renewable Energy Laboratory.

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