NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by 

Accelerated Stress Testing

Testing, HAST, Field Expe

do they all mea

John Wohlgemu

February 26, 20

2013 Photovoltaic M

Reliability Workshop 

NREL/PR‐5200‐58371
Introduction• The commercial success of PV is based on 

reliability and safety of the deployed PV m

• Today most PV modules are warranted for 

with a maximum allowable degradation ra0.8%/year.

• These modules are typically qualified/cert

 IEC 61215 for Crystalline Silicon Modules IEC 61646 for Thin Film Modules IEC 62108 for CPV Modules

• These qualification tests do an excellent jo

identifying design, materials and process f

could lead to premature field failures.
Introduction (Continued)• What we would really like is to have 

tests that we could perform on the that would predict their long term f

performance.

• Such a set of tests does not exist tod

• That was a major reason for the form

International PV QA Task Fo
Goals of Talk• Try to describe the relationships betw

o Field test resultso Accelerated stress testso Qualification tests

• Will try to do this in the logical manne

developed in PV.

• Define HAST Tests and explain why PV 

uses this approach.

• Summarize the International PV Modu

Force
What is our overall goal• To evaluate the long term performance of PV 

variety of terrestrial climates.

• Really should use outdoor performance data 

• However, none of us wants to wait 25 years t

if a particular module type is going to have a 2

lifetime.

• Therefore, we use accelerated stress tests to t

what is going to happen outdoors.

• These accelerated stress tests are based on du

failure modes observed in the field.

• The first step in this process is to identify the v

failures that have been observed for different 

modules.
HISTORY OF FIELD FAILURES for Cry‐Si

• Broken interconnects

• Broken cells• Corrosion of cells, metals and connectors

• Delamination/loss of adhesion between layers

• Loss of elastomeric properties of encapsulant or backsheet

• Encapsulant discoloration

• Solder bond failures

• Broken glass• Glass corrosion• Hot Spots• Ground faults due to breakdown of insulation package

• Junction box and module connection failures

• Structural failures

• Bypass Diode failures

• Open circuiting leading to arcing• Potential Induced Degradation
Examples of Field Failures

Broken Interconnects

Bro

Figure 2. Solar-Cell Electrochemical Corrosion

Delamination

Corrosion

From JPL 

From P
Additional Failure Modes for Thin Film Mod• Electro‐chemical corrosion of TCO.

• Light Induced Degradation

• Inadequate Edge Deletion

• Shunts at laser scribes

• Shunts at impurities in films

• Diffusion of metals from contacts throug

junction
Additional Field Failures for Thin F

Electro-Chemical Corrosion of TF Module

From Neelkanth Dhere, FSEC Broken Glass Lea
Additional Failure Modes for CPV Mod

– Tracker misalignment– Tracker failures– High current densities leading to overh

– Rapid and numerous thermal cycles st

cell to substrate bond

– UV degradation of optics– Moisture condensing with optical pack

– Overheating of the encapsulant due to 

darkening
Additional Failures for CPV

From Matt Muller, NREL

initial 500 cycles 1000 cycles 15

Progression of IR images illustrating die‐attach cracking

through thermal cycling from Nick Bosco, NREL

From Dav
Developing Accelerated Stress Tests• Need to look at each of the failure modes and try to dete

stress or stresses in terrestrial environment caused the fa

• Was it?

o Operation at high temperature

o Changes in temperature due to diurnal variations or clouds

o High humidity

o Wind or snow loading

o UV exposure

o Or maybe a combination of several or all of the above or some

• Once the driving force for the failure mode has been iden

then try to accelerate that stress to cause the failure to o

time period.

• Some examples

o Operate at higher temperature

o Cycle temperature quickly

o Use higher humidity and temperature than seen in the field
Some Rules Governing our ASTs• In developing accelerated stress tests 

must cause degradation.

• The degradation occurring in the AST m

due to the same failure mechanism we 

outdoors.

• Because the AST is causing the same fa

there is a chance that we can extrapol

test date to provide a lifetime predicti

one failure mode.

• The 35 years of PV history with ASTs h

us a good background to build on.
Accelerated Stress TestsAccelerated StressTest

Failure Mode 

Thermal Cycles Broken interconnect

Broken cellsElectrical bond failure

Junction box adhesionModule open circuit – potential for arcing

Damp Heat Corrosion

DelaminationEncapsulant loss of adhesion & elasticity

Junction box adhesion

Electrochemical corrosion of TCO

Inadequate edge deletion

Potential Induced Degradation

Humidity Freeze Delamination

Junction box adhesionInadequate edge deletion

Insufficiently cured encapsulant
Accelerated Stress Tests for PV (coAccelerated Stress

Test

Failure Mode 

UV Test Delamination

Encapsulant loss of adhesion & elasticity

Encapsulant & backsheet discoloration

Ground fault due to backsheet degradation

Degradation of Optics

Static MechanicalLoad(Simulation of wind

and snow load)

Structural failures

Broken glassBroken interconnect ribbons

Broken CellsElectrical bond failures

Dynamic MechanicalLoad

Broken glassBroken interconnect ribbons

Broken CellsElectrical bond failures
Accelerated Stress Tests for PV (coAccelerated Stress Test Failure Mode

Hot spot test Hot spots

Shunts in cells or at scribe lines

Inadequate by‐pass diode protection

Hail Test Broken glass

Broken cellsBroken Optics

By‐pass Diode Thermal Test By‐pass diode failures

Overheating of diode causing degradat

of encapsulant, backsheet or junction b

Salt Spray Corrosion due to salt water & salt mist

Corrosion due to salt used for snow an

removal
Qualification tests• Qualification tests are a set of well defined ac

stress tests developed out of a reliability prog

• They utilize accelerated stress tests to duplica

modes observed in the field.

• They incorporate strict pass/fail criteria.

• The stress levels and durations are limited so 

can be completed within a reasonable amoun

cost.

• The goal for Qualification testing is that a sign

number of commercial modules will pass.

(If not there will be no commercial market.)• Qualifies the design and helps to eliminate in

mortality
Passing IEC 61215, IEC 61646 or IE• So what does it mean if a module type is q

IEC 61215, IEC 61646 or IEC 62108?

• Passing the qualification test means the p

met a specific set of requirements.

• Those that have passed the qualification t

much more likely to survive in the field an

design flaws that lead to infant mortality.

• Most of today’s commercial modules pass 

qualification sequence with minimum cha

meaning the qualification tests do not pro

means of rankings within the group that hthe requirements.
How Successful are the Qualification Tests?• They must be fairly successful because the PV industry h

rapidly.

• Reports of Field Failures/ Warranty Returns:

 Whipple reported on 10 years of field results in 1993 

from Rosenthal, Thomas and Durand) that

• Pre‐Block V modules suffered from 45% field failure rate

• Post‐ Block V modules suffered from < 0.1% field failure ra

 Hibberd from 2011 PVMRW – 125,000 modules from 11 

module manufacturers deployed for up to 5 years with o

failures. (0.005%)

 Wohlgemuth et. al. from 20th EU PVSEC – Solarex/BP Sol

crystalline Si modules deployed from 1994‐2005 with 0.

return rate (1 failure every 4200 module years of operat

 Wohlgemuth et. al. from 23rd EU PVSEC – Solarex/BP Sol

crystalline Si modules from 2005 onward with an annua

0.01%
Limitations of Qualification TestsBy design the qualification tests have limita

They were designed to identify early infant 

problems, but not to:

• Identify and quantify wear‐out mechanisms

• Address failure mechanisms for all climates and sys

configurations

• Differentiate between products that may have long 

lifetimes

• Address all failure mechanisms in all module design

• Quantify lifetime for different applications or climat
HAST Tests• What are HAST Tests?

– Highly Accelerated Stress Tests

• How is HAST used?

– To identify design and component weaknesse

the product to increasing stress until failure o

– To increase margin of strength of design, not 

qualitative lifetime or reliability of product

• Examples of HAST.

– Temperatures > 100 °C with > 1 atmosphere o

100% RH

– Rapid thermal cycling (to > 85 °C) plus high vi
So what tests do we use in PV?• Field results are used to guide AST but 

long to be PV’s main reliability tool.

• Accelerated stress tests are the main r

tests used in PV.

o Trying to duplicate field failures

• Qualification tests are the main comm

tests used in PV.

o Looking for design/infant mortality issue

been observed in the field.

• For PV modules HAST is seldom used:

o In PV we are trying to reduce the cost no

product robust to failures not observed in 
International PV Module QA Task • ~ 150 of us met in July, 2011 in San Francis

• Prepared Goals on next page• Chartered first 6 Task Groups

o Group 1 – Guideline for PV Module Manufact

modules are made correctly

o Groups 2 to 5 – Selected 4 sets of stresses tha

judged to cause the most field failures in Cry‐

2. Thermal cycling and mechanical fatigue3. Humidity, temperature and voltage4. Diodes, shading and reverse bias

5. UV (light), temperature and humidity

o Group 6 – How to organize and communicat

proposed QA rating system
International PV Module QA Task ForcGoals of International PV QA Task For1. To develop a QA rating system that provid

comparative information about the relative of PV modules to a variety of stresses as a tool to PV customers and as a starting poiimproving the accuracy of quantitative PV predictions.1) Compare module designs2) Provide a basis for manufacturers’ warranti3) Provide investors with confidence in their i4) Provide data for setting insurance rates

2. Create a guideline for factory inspections 

system used during manufacturing.
PV QA Task Force Task Group 1: Guideline for Manufacturing 

Consistency

Task Group 2: Thermal and mechanical fatig

including vibration

Task Group 3: Humidity, temperature, and voTask Group 4: Diodes, shading and reverse Task Group 5: UV, temperature and humidity Task Group 6: Communication of PV QA rati

community

Task Group 7: Wind and Snow Loading (New Task Group 8: Thin Film PV (New group)Task Group 9: CPV (New group)
Testing for Wear‐out. What groups 2‐5 are • Determine which accelerated stress test or combina

accelerated stress tests best duplicates a failure see

• Study each failure mode to determine what parame

parameters in the field exposure are most responsib

phenomena – Is it temperature, humidity, light expo

in temperature, vibration or combinations of the ab

• Perform experiments or use published data to deter

reaction rate of the failure mechanism.

• Model the system to determine the equivalence bet

accelerated stress test(s) and field performance.

• Use model to predict results at some different stress 

• Perform experiments to validate model.

• Propose test for wear‐out based on selected climate

world.
Summary• Accelerated stress testing beyond th

qualification test levels is necessary 

predict PV module wear‐out.

• Development of such tests requires

understanding the science behind th

observed failure modes.

• This effort is now underway as part 

Module QA Task Force, involving hu

people around the world.