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Group 3:

Humidity, Temperature and Voltage

John Wohlgemuth

February 28, 2013

2013 Thin Film PV Reliability Workshop

Colorado School of Mines: Golden, CO

NREL/PR-5200-59294 
Introduction

• Group 3 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 sequences IEC 

61215 and IEC 61646 contain a 1000 damp heat 

test (85 °C at 85% RH).

• This stress test appears to do an excellent job of 

screening out module designs and materials that 

would fail in the field in short time periods.

• So Group 3 must look to find field failures that 

are not identified in the 1000 hour damp heat 

test, but are limiting the lifetime of PV modules.
Group 3 Efforts 

• Making observations of field failures related 

to humidity

• PID Testing – Adding voltage to H and T

• Modeling to understand conditions within 

module

• Effectiveness of Qualification Test

• Look at results of testing beyond qualification
Thin Film Susceptibility to Humidity 

Tamizhmani 2010 

PVMRW

Observed Field Failures

• If you don’t do something about it, the number one field failure mode 

for CdTe and CIGS appears to be due to moisture ingress.

• For a-Si humidity, temperature and voltage can lead to delamination of 

TCO from glass.

• Many of today’s most successful thin film products are designed to 

keep moisture out.

Observed Qualification 

Test Failures
Crystalline Silicon Findings

• At present time group 3 does not believe that damp 

heat testing beyond 1000 hours is justified.

• Looking for combined sets of stresses that can lead to 

delamination. Possibilities

– UV and temperature

– Dynamic mechanical loading/thermal cycling/humidity 

freeze.

• We are looking for: 

– Older arrays exposed in hot/humid environments to visit.

– Reports on and samples of product returns that appear to 

be humidity and temperature related.
How do the cry-Si findings apply to Thin Films?

• Likely that the damp heat test results and 

field failures due to moisture are more 

closely related to the packaging than the 

technology that is inside.

• Lets look at 3 different cases:

– Packages that keep moisture out.

– Flexible package where device is exposed to 

moisture.

– PID
Package that keeps moisture out

• A module that relies on 

sealing out the moisture 

(glass-glass with an edge seal) 

is not likely to fail an 85/85 test, 

but you are not likely to learn 

much about its long term durability 

from such a test either.

• What we should be developing are tests that stress

the package before performing the 85/85 test.

• Kempe (2012 NREL PVMRW) reported that a

combination of edge pinch and UV exposure resulted in loss 

of adhesion of the edge seal and subsequent ingress of 

moisture into the package.
Flexible PV Packages

• In flexible packages the moisture will reach 

equilibrium within the module in less time 

than in a package with one or more vapor 

barriers (glass, cells, etc.).

• For example modeling by Kempe shows that 

it takes about 1000 hours at 85/85 to mostly 

saturate the encapsulant between a cell and 

the glass.

• On the other hand with a flexible package it 

may only take a few hours for the humidity 

to reach equilibrium in the encapsulant.

• It is likely that for flexible packages 85/85 

for 1000 hours is too much stress for 

evaluating a 25 year lifetime outdoor in 

most terrestrial environments.

• Can use modeling to determine a better 

combination of temperature and humidity 

where maybe using a longer (than 1000 

hours) test will be a much better indicator of 

long term performance in most (if not all) 

terrestrial environments. (See Kempe paper 

from PVMRW 2013)

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.002

0 1 2 3 4 5

Absolute Humidity (g/cm3)

Days

Bangkok Thailand Module Back-EVA Absolute Humidity

Insulated Back,

Glass/Polymer

Close Roof, Glass/Glass

Open Rack, glass/glass

open Rack, Glass/Polymer
PID in Thin Films

• Remember PID was first 

reported in thin film products. 

(See Carlson et. al. PIP, 11, 

2003)

• Changes to barrier layers 

slowed the process down 

considerably – it was no 

longer the squeaky wheel.

• However, even some a-Si 

modules built after the “fix” 

still suffer from some PID.

• PID tests developed for cry-Si 

modules may do a good job of 

identifying PID susceptible 

thin film product.

Electro-Chemical Corrosion of 

TF Module

From Neelkanth Dhere, FSEC 

Later generation a-Si module
Summary

• Accelerated stress tests for moisture related 

failures need to address the package more than 

the package’s contents.

• Adding thin film specialists to work in group 3 

will lead to development of better accelerated 

stress tests for temperature, humidity and 

voltage for both crystalline Silicon and thin film 

modules.

• Expanding the effort to increase focus on 

flexible packages may be useful.