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

Research Opportunities in Reliability of

Photovoltaic Modules

National Center for Photovoltaics

Peter Hacke

Catalyzing Innovation in 

PV Manufacturing -

An NSF Workshop 

May 6-7th, 2010 

Golden, CO 

NREL/PR-520-49001
Overview

Research Opportunities in Reliability of Photovoltaic Modules

• Motivation

• Where we are today and how we got here

• Present needs – general and specific

• Jobs

National Renewable Energy Laboratory 2 Innovation for Our Energy Future
Motivation for Reliability Research in PV Modules

• Reducing the levelized cost of energy through:

• Increased performance, availability/uptime

• Lower materials, operations, and maintenance costs

• Reduced investment and customer risk, reduced cost of 

capital

• Forecasting for performance and reliability

• Choose the relevant tests for materials and components to 

benchmark and improve reliability

• Better confidence for module manufacturers in making 

projections for warrantee returns (standard is 25 years)

National Renewable Energy Laboratory 3 Innovation for Our Energy Future
Value of Reliability

• Reliable products reduce Levelized Cost Of Energy 

(total lifecycle cost/total energy produced) over the long term

• LCOE increases as system lifetime decreases (power law fit)

• LCOE increases 10% per 1% increase in degradation rate

• LCOE increases 2¢/kWh for each additional 1% per year of the 

initial capital expenditure for O&M

• Reliable products over the long term enhance the the amount 

of energy produced and delivered at reduced cost

National Renewable Energy Laboratory 4 Innovation for Our Energy Future

Calculated by the Solar Advisor Model (SAM): www.nrel.gov/analysis/sam/

Based on Granata et. al. Sandia, IPRW(2009) 
Historical Degradation Rates of Fielded Modules

National Renewable Energy Laboratory 5 Innovation for Our Energy Future

D. Jordan 2010 NREL PV Module Reliability Workshop (PVMRW)
The Reliability and Performance of PV Modules in 

the Field is Insufficiently Studied 

National Renewable Energy Laboratory 6 Innovation for Our Energy Future

D. Jordan 2010 NREL PV Module Reliability Workshop (PVMRW)
Source: http://www.rechargenews.com/energy/solar/article182054.ece Source: IEC, Swisswafer, 

electrosuisse

7

Reliability Problems Can be Catastrophic

National Renewable Energy Laboratory 7 Innovation for Our Energy Future
Qualification Testing: First Threshold for Reliability

National Renewable Energy Laboratory 8 Innovation for Our Energy Future

Sarah Kurtz, Jennifer Granata, and Michael Quintana, SPIE, (2009)

• PV Module reliability and durability approach to date, while very helpful, has been phenomenological, 

empirical, and iterative as opposed to systems and model based

Insulation test
National Renewable Energy Laboratory 9 Innovation for Our Energy Future

Field Reliability Experience

Qualification 

Test

Lifetime 

Prediction Test-to-Failure

Steps to Improved PV Module Reliability
Present Research Opportunities

Module Materials

-Polymer encapsulants, how they fail as function of the temperature, UV light 

exposure, humidity, and physical stress

-Degradation in connectors, wires, and junction boxes – how they degrade when 

exposed to the elements

Reliability Engineering

-Mapping of failure mechanisms observed in the field to those obtained under 

accelerated lifetime testing

-Clarifying the failure mechanisms, assigning and validating their rate models

Quantitative reliability models for PV need further development

-Presently underrepresented stresses in accelerated testing: UV, electrical (system) 

bias

-Determining to what extent one can accelerate the degradation meaningfully

-Accounting for combined stress effects (delamination  moisture ingress  corrosion 

 current concentrations/ short circuits  power loss)

National Renewable Energy Laboratory 10 Innovation for Our Energy Future
Corrosion in Modules

One of the key degradation mechanisms in modules 

responsible for power loss and failure in the field

–What are the mechanisms?

• Galvanic

• Electrolytic

–How are they activated?

• Acidity (from ethylene vinyl acetate encapsulant)

• Moisture ingress

• Electrical bias

• Galvanic potentials

• Catalytically induced

–What are the reactants, products?

–How does it lead to failure?

• Increased series resistance

• Shorts/shunts

• Current concentrations, hot spots (fire potential)

–How to characterize it?

• Identification of the reaction equations associated with observed 

corrosion via identification of the species present

• Identification of encapsulant discoloration (ionic migration, 

reactions) via characterization of the species present

• Imaging for series resistance and shorts/shunts

–How to model the degradation?

• Chemical kinetics; Eyring Equation

• Diffusion (O2, H2O)

• Coulombs leaked under bias (electrolytic corrosion)

• Differences in galvanic potential

• Validate model in various field environments

National Renewable Energy Laboratory 11 Innovation for Our Energy Future
Metastability in CdTe, CIGS, and CIS

Thin film modules display metastability associated 

with their exposure (thermal, optical, electrical) 

history

– What are the mechanisms?

• Diffusion of ionic species across the device, 

especially in the depletion region

• Trap states capturing and releasing carriers

– How is it activated?

• Heat

• Light

• Electric field

– What are the species involved?

• Typically Cu, especially in CdTe

• Other, Na? ...TBD

– What is the reliability problem?

• Power measurement variations on the order of +10%

• Shows up predominantly in FF (junction related), Voc (lifetime related), and Isc as well

– How to characterize it?

• Electron microscopy (SIMS, SEM, TEM)

• Auger profiling

• Deep level analysis (Drive level capacitance 

profiling)

– How to model it?

• Ionic drift under electric field

• Arrhenius equation

• Could lead to better fundamental understanding and 

efficiency gains

baseline, never exposed

after 27 kW-h/m2 light-soak

after dark thermal anneal

after 24 h biased dark-soak

after 120 h biased dark-soak

Vb (Volts/cell)

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

Wd (

μm)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1

baseline, never exposed

after 27 kW-h/m2 light-soak

after dark thermal anneal

after 24 h biased dark-soak

after 120 h biased dark-soak

N(Wd) (1/cm3) 1015

1016

1017

Wd (μm)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Measured changes in depletion width (top) and carrier profiles 

(bottom) as a function of exposure (Capacitance measurements)

National Renewable Energy Laboratory 12 Innovation for Our Energy Future
Delamination & Loss of Elastic Properties

Loss of adhesion, elastic properties, and 

cracking of encapsulant, backsheet, and edge 

seal material lead to rapid degradation of 

module performance because of accelerated 

moisture ingress

– What are the mechanisms?

• Hydrolytic reactions

• Oxidation

– How is it activated?

• Heat

• Light

• Electrical bias (tentatively)

• Activity of water

– What are the species involved?

• O2, H2O, reacting with the polymeric materials 

in the module package

– How does it lead to failure?

• Moisture ingress through delaminated or 

cracked polymeric packaging leads to 

corrosion, and in turn series resistance, short 

and leakage currents, then current 

concentrations (fire potential)

– How to characterize it?

• Infrared spectroscopy techniques

• Imaging for series resistance, shorts/shunts

• Mechanical testing

– How to model it?

• Hydrolytic reactions – Arrhenius equation

• Delamination -??

National Renewable Energy Laboratory 13 Innovation for Our Energy Future
Inverter Failure

Of the system components, inverters 

have one of the shortest lifetimes

What are the mechanisms?

• Excess heat from losses in the 

semiconductor switches: long 

switching time, conductor power 

losses, putting the inverter in the sun, 

inadequate heat removal.

• The capacitor bank is very sensitive 

to heat and charge surges (rule of 

thumb: 10°C increase leads to 

halving of the capacitor life), 

degrading dielectrics.

• How is it activated?

• Heat

• High voltage, current

• How does it lead to failure?

• Reduced DC/AC conversion 

efficiency

• Characterization

• Thermography

• FET channel resistance/breakdown 

voltage

– How to model it?

National Renewable Energy Laboratory 14 Innovation for Our Energy Future
Time to Move

Reactive  Proactive

National Renewable Energy Laboratory 15 Innovation for Our Energy Future
Jobs: Who is Hiring Reliability Scientists and Engineers?

PV materials makers

Dow, DuPont ...

PV module manufacturers

Calisolar, Evergreen Solar ... (all of them)

Installers

SunEdison ...

Vertically integrated PV companies

REC, Emcore, Amonix, SunPower ... (all of them)

Testing companies

Underwriters Laboratories

Hiring at many levels of the value chain

National Renewable Energy Laboratory 16 Innovation for Our Energy Future
Thank you for your attention!

National Renewable Energy Laboratory 17 Innovation for Our Energy Future

The Alliance for Sustainable Energy, LLC (Alliance), is the manager and operator of the National Renewable Energy 

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