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Requirements for a Standard Test to Rate the 

Durability of PV Modules at System Voltage

Peter Hacke

Kent Terwilliger

Steve Glick

Michael Kempe

Sarah Kurtz

Ian Bennett*

Mario Kloos*

*ECN, Petten, The Netherlands

This presentation does not contain 

any confidential material

Presented at the 2011 Photovoltaic 

Module Reliability Workshop

February 16-17, 2011

Golden, Colorado

NREL/PR-5200-51099
Motivation

Over the past decade, there have been observations 

of module degradation and power loss because of 

the stress that system voltage bias exerts. This 

results in part from qualification tests and standards 

not adequately evaluating for the durability of 

modules to the long-term effects of high voltage bias 

that they experience in fielded arrays. 

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“Oh no! our modules are down 40%, we think it is 

potential–induced degradation”

-anonymous module manufacturer, 2010
Three degradation modes - overview

1. Ionic motion in the packaging of the active layer 

leads to accumulation of charge or charged ions 

over the semiconductor surface

- Charge influences surface field of semiconductor active layer 

- In severe cases, accumulation of mobile ions such as Na in 

the glass leads to delamination

2. Ionic motion also takes place within the active layer, 

degrading semiconductor junction properties and 

causing shunts

3. Usually in the presence of humidity in the packaging, 

electrolytic corrosion occurs and macroscopic 

transport of ionized conductor metal is observed.

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Mode 1 : Na ions moving to glass TCO interface

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Reported by BP Solar...

• a-Si modules

• - 600V bias

• 12 mo. in the field

• Sodium ions migrating to the 

TCO/glass interface causes 

delamination of the TCO, 

electrochemical corrosion 

• Key drivers are:

• Negative cell polarity vs. 

ground

• Moisture ingress

• Temperature

• Na content in glass

4
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Mode 1 : Na ions moving away from active layer

glass

cell

i

– – – – – – –

h+

SunPower reports 'surface polarization' in modules in 2005

• Positive bias string leads to 

leakage current through glass to 

ground, leaving negative charge 

on cell surface, degrading 

effectiveness of the n+ front 

surface field of the n+/n structure

V

I degradation

• Minority carriers (holes) recombine 

at front surface, leading to degraded 

cell performance

5
Mode 2: Ionic motion into active layer – pn junction shunt

• Electroluminescence of mc-Si module strings indicating shunting 

in the negative portion of a center mounted or floating string *

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* S. Pingel, O. Frank, M. Winkler, S. Daryan, T. Geipel, H. Hoehne and J. Berghold SOLON SE, Presented at 35th IEEE PVSC, 2010

Shunting indicates junction degradation

EL Thermography

Proposed model

ionic motion within active layer of thin film devices also active
Mode 3 corrosion and macro - transport 

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Corrosion of gridlines and Si-grid 

interface leading to higher series 

resistance evident visibly, in 

electroluminescence, and I-V curves

Corrosion and ionic transport at 

bus ribbons toward frame 

(positive string)

Realini (TISO)
Testing methods – low T & RH regime

Near ambient temperature 

tests applying system voltage

- Saline water film, wet towel, or 

carbon paste act as the ground 

electrode to simulate wet surface, 

salt mist, or conductive dirty 

surface

- Degradation generally reversible 

by reversal of ionic motion

- No visible damage

- Degradation modes 1 & 2 active

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1 cell mc-Si mimos, + 1000 V bias applied to shorted leads; the 

module face grounded with C-paste, 120 h stress tests at 45 

°C, 30% RH
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Testing methods – high T & RH regime

• 6 commercial module designs

• 85°C/ 85% RH

• 600 V system voltage

Activates modes 1,2 and 3, including 

corrosion, series resistance

Strongly differentiates modules’ 

durability to system voltage bias 

potential-induced degradation
Summary

• Three modes of system bias potential-induced degradation

• Fielded systems showing system bias potential degradation

• Modern higher system voltages are more stressful on modules

• Increasing need to push down overall degradation rate

• Increasing variety of solar cell technologies sensitive to system bias 

potential (e.g., selective emitter with junction closer to cell surface)

• Long-term degradation by system bias voltage is not presently 

captured in IEC 61215 and IEC 61646

• Tests around ambient temperature succeed reasonably well at 

elucidating 2 of 3 degradation modes, but higher temperature and 

humidity required to show durability to corrosion

• Exact, most appropriate level of stress is yet to be determined, but 

we shouldn’t wait to implement some screening test

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