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Task Group 3: 

Humidity, Temperature and Voltage

John Wohlgemuth for Group 3

NREL PV Module Reliability Workshop

February 26, 2014

Golden, Colorado

NREL/PR-5J00-62599
Introduction

• Group 3 on Humidity, Temperature and 

Voltage is chartered to develop accelerated 

stress tests that can be used as comparative 

predictors of module lifetime versus stresses 

associated with humidity, temperature and 

voltage. 

• The tools we have to utilize are:

– Outdoor test results

– Accelerated stress tests results 

– Modeling 
Where we stand today

• The module qualification test sequence IEC 61215 for cry- Si & IEC 61646 for Thin Films contain:

– a 1000 damp heat test (85 °C at 85% RH) and 

– a humidity-freeze test (10 cycles from 85 °C at 85% RH to -40 °C) 

performed after a short UV exposure and 50 Thermal Cycles.

• These stress test appear to do an excellent job of 

screening out module designs and materials that would 

fail in the field in short time periods.

• So TG 3 is looking to find field failures that are not 

identified in the qualification test sequence, but are 

limiting the lifetime of PV modules.

• TG3 is also using modeling to better understand what the 

accelerated testing represents.
Results from Long Term Field Exposure

• Two failure modes identified

– PID leading to significant power loss.

• We had a whole session on this yesterday so I won’t 

add anything except to remind you that IEC has a draft 

standard under development

– Corrosion Associated with Delamination

• Lets review what we see in the field and how we 

might duplicate it in the laboratory
Approaches to Testing for Corrosion

Prolonged damp heat testing causes 

corrosion and power loss, but no visual 

evidence of metal corrosion. Not the 

same corrosion we see in the field

Corrosion seen in the field is 

associated with delamination.

1000 hrs 2000 hrs 3000 hrs

Conclusion: We should move away from prolonged damp heat testing and 

move toward test sequences that cause delamination.
Delamination

• Delamination of encapsulant 

from glass or cells is a failure 

mode seen regularly in the field. 

See picture.

• Much of the evidence of 

corrosion comes in conjunction 

with delamination.

• Any of the metals (grid lines, 

interconnect ribbons, solder 

bonds) will likely corrode if 

exposed to liquid water.

• Delamination is not adequately 

evaluated in the present 

qualification test sequence nor is 

it caused by extending the time 

for the 85/85 test. 

Corrosion of cell metallization occurring with

delamination of the encapsulant. No adhesion

promoter was used on the cell or in the

encapsulant, but the glass was primed. This

module was exposed in Florida for 23 years.
Why do Modules Suffer Delamination?

• Manufacturing issues:

– Inadequate priming

– Inadequate or incorrect 

glass cleaning

– Use of wrong glass; too 

much sodium

– Selection of wrong 

encapsulation material

– Inadequate processing; 

either lack of crosslinking or primer reaction

Corrosion of cell metallization occurring with

delamination of the encapsulant. Glass/glass

module after 13 years in Tucson, AZ.
What Stresses Cause Delamination?

• UV Exposure Degrading 

the Adhesion

• Thermal Cycling

• Humidity Freeze

• Dynamic Mechanical 

Loading especially for 

glass/glass.

• Damp Heat especially 

after adhesive bonds 

are weakened.

Corrosion of cell metallization occurring

with delamination of the encapsulant. 

Glass/polymer backsheet module after 27

years in California.
Status of Test Development

• TG5 is running a large experiment to measure 

the change of adhesion between the 

encapsulant and the glass as a function of UV 

exposure, temperature and humidity.

• There is a materials task group that is looking at 

the best method for measuring adhesion.

• This may not be simple because of glass 

breakage. The adhesion value a glass/glass 

double cantilever beam wedge sample can 

measure is an order of magnitude less than the 

pristine adhesion value for EVA to glass.
Status of Test Development (cont)

• The qualification test already has a sequence of UV/TC/HF. (Probably why we 

don’t see delamination occurring rapidly in qualified designs.)

– UV irradiation can break the adhesive bonds formed between the encapsulant and glass or 

encapsulant and cells.

– Thermal cycling puts mechanical stress on the interfaces because of different thermal 

expansion coefficients.

– Humidity freeze pumps moisture into the package. 

– During the cold part of the cycle the moisture in any voids that have formed at the interface 

will condense as liquid water and then freeze. 

– This rips apart any weakened interfaces.

• WG2 has a draft standard for Dynamic Mechanical Loading (DML) with a plan to 

propose a UV/DML/TC/HF sequence. DML will add additional mechanical stress 

on the interfaces especially for glass/glass constructions.

• Hopefully the TG5 experiment will provide data on the appropriate UV exposure 

to improve the sequence for incorporation into the Comparative Testing 

Standard.

• Both NREL and JRC are planning experiments with sequential tests to validate 

their ability to duplicate the observed field failures.
Conclusions

• TG3 is looking for a combined set of accelerated stresses that can 

lead to delamination. Likely using the UV/DML/TC/HF sequence 

(with improved test levels especially for UV) followed by an 

appropriate damp heat exposure. 

• TG3 is continuing to investigate field failures and is looking to: 

– Inspect older arrays (>10 years), especially those exposed in 

hot/humid environments.

– Determine the correlation between visual corrosion (with or without 

delamination) and power loss due to lower fill factor.

– Use failure analysis to identify failure mechanisms and then try to 

duplicate them using sequences of accelerated stress tests.

• The goal of this effort is to develop comparative tests for moisture 

driven failure modes in PV modules.