Module Design, Materials, & Packaging Research• T. J. McMahon (1.0)• Steve Glick (1.0)• Gary Jorgensen (0.85)• Mike Kempe (1.0)• John Pern (0.5)• Kent Terwilliger (0.5)

FY 06 Total 4.85 FTEs

FY 07 Total 4.00 FTEs

Purpose of this Task

• Confirm reliability of new photovoltaic packaging materials and strategies to insure a 30-year module life. Usually Industrial collaborations.

• Give special attention to module reliability problems with developing technologies.

• Measurement techniques developed/acquired for module failure diagnostics: usually one technique developed each year.

Collaborations and Inquiries

• AKT• Bucyk Glass• AVA• GE• Plextronics• Sol Focus• TruSeal• Madico• Miasolé• STR

• Fraunhofer Institute• InnoSense LLC• Solar Roofing Systems• Planar Systems, Inc. • Deerfield Urethane • Pilkington Glass • Dow Chemical • Silicon Valley Solar• Applied Films• Saint Gobain

• Dow Corning• Gen 3 Solar• Sealed Air• First Solar• Siemens• Prime Star• DuPont• PPG• BRP• Global Solar

Team Capabilities • Characterization

– Adhesion, cohesion, and toughness; peel, butt and lap shear strength, and torque vs angle

– Electrical conductivity; surface and bulk– WVTR; water transmission, solubility, diffusion– Rheology; modulus

• Accelerated tests– UV, temperature, damp heat, acceleration factors

• Module and cell diagnostics– IR imaging, individual cell shunt, coring , transient

currents, internal resistance• SiONC barrier coatings

– Sputtering and PECVD– characterization

• Modeling– Moisture ingress and egress– Cell-to-frame leakage current– Device(AMPS) and Module(PSpice)

Task Activities and Direction:

• Past and continuing problems: adhesion, cohesion, formation of T-F weak diodes, shunt and series resistance problems w/ aging, water ingress.

• SAI Accelerated Aging WKSHPS (w/ DOE and Sandia) now to replace Thin-film partnership reliability teams.

• Module level packaging issues addressed by our task– Alternatives to double glass for thin films(soft backsheets

and hard coat barrier films).– EVA substitutes cheaper/better perhaps w/o transparency.– Hot/Humid survival: Field and 85%/85°C stress survival

depend on adhesion, cohesion and water diffusion barriers.

– Moisture; relevant properties of polymers and coatings measured. Modeling of moisture ingress and egress into module structures.

PV Packaging:

• 90% of the field returns * ^• 50% of the PV module cost

* Includes cell interconnects.^ Failure rate and cause depend on how mature the

technology is, e.g. BP Silicon is 1/4200 module year; Newbee modules are 1/10 - 1/100.

Budget / FTEs

$1,413,000 4.85 FTE FY 06

$1,218,000 4.00 FTE FY 07

Talk Outline

Standard adhesion results

Coring/Torque vs Angle Procedure for Adhesion, Cohesion, and Toughness(see Gary Jorgensen’s poster)

DOE outdoor to Accelerated Aging correlation study.

Infrared (3-5 micron) cell and module diagnostics.

Standard AdhesionTesting

AlignmentPlate

Substrate

Grip

Encapsulant /Backsheet

Grip

Peel Adhesion Testing“Adhesion And Thin-Film Module Reliability,” T.J. McMahon and G.J. Jorgensen, IEEE

Photovoltaic Specialist Conference, May, 2006

• Interface peel strength values of various T-F module technologies. Damp heat stress reduces starting values of 7 N/mm to as little as 0.6 N/mm. Extended UV exposure can also reduce strength.

• Adhesion at higher T’s is greatly reduced: 7 N/mm @ 25° C > 1.1 N/mm @ 60°C >> 0.05 N/mm @ 80° C

– Require a minimum adhesion strength perhaps at higher T and RH.

– The softening of EVA near 85 °C can lead to failure.

• Achieve highest adhesion possible for corrosion and water ingress reduction.

Scratch Adhesion Testing“Adhesion And Thin-Film Module Reliability,” T.J. McMahon and G.J. Jorgensen, IEEE

Photovoltaic Specialist Conference, May, 2006

• For the T-F technology the ASTM D 3359-02 “scratch test” can be useful as a screening test.

Shear Strength and Toughness

DOE sponsored meeting on “Accelerated Aging Testing in Photovoltaics” Feb 2006

A key recommendation by the attendees was to study a major failure mode and demonstrate that meaningful correlations between field data and accelerated test results could be derived.

Interfaces and layers of weathered PV modules

System for Measuring Torque as a Function of Twist Angle of Cored Modules

Close-up Image Showing Details of Torque Sensor and Coupling Hardware

Shear Strength Measurement at Front Cell/EVA Interface

Tempered Front Glass

EVA

Si Cell

EVA

TPE Backsheet

Tempered Front Glass

EVA

Si Cell

Tempered Front Glass

EVA

Si Cell

EVA

TPE Backsheet

Tempered Front Glass

EVA

Si Cell

EVA

TPE Backsheet

Shear Strength Measurement at EVA/Back Cell Interface

Details of the Cell Coring Process

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Torque vs. Angle; Large Mono-Crystalline Si Modules; Control; Frontside of Si Cell/EVA

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45-55 °C1750 MJ/m2

45-70 °CUnexposedExposure

Conditions:

Toughness of Small Mono-Crystalline Si Modules

(see Gary Jorgensen’s poster for more details)

Talk Outline

Standard adhesion results

Coring/Torque vs Angle Procedure for Adhesion, Cohesion, and Toughness(see Gary Jorgensen’s poster)

DOE outdoor to Accelerated Aging correlation study.

Infrared (3-5 micron) cell and module diagnostics.

I-R Image of Cells and Modules

CdTe cell weak diodes(WD).

CIGS module weak diodes, shunts, and series resistance.

CdTe cell WD: IR and IVs

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4.5 - 6 % after stress.

Hot in forward bias.

Not in reverse bias

NEDT 25 mK

CdTe cell WD removal

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T.J. McMahon, T.J. Berniard, and D.S. Albin, " Non-linear Shunt Paths in Thin-Film CdTe Solar Cells," T.J. McMahon, et al., J. Appl. Phys. 97, (2005) 054503

9 %4.5 - 6 %

IR images; forward bias

#3044 Control #2994 5 y @ NREL

Uniform recombination heat.

Localized WD heating

Two-Terminal, Non-destructive Technique

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30FS90mA(2mA)@+(-)9.6V20s-zeroDry 85°C, 2280 h

B:Weak-Diodes A:Shunts C:Recom. Area

Forward Bias Reverse Bias

32FS@+/-9.6V20s-zero85%/85°C, 497h

+81 mA -0.24 mA

32FS 81mA@9.6V5s-zero85%/85°C, 497h

Summary

• Provide relevant performance measures for new and existing packaging materials.

• Measurement techniques developed/acquired for module failure diagnostics: This year’s is twist strength and toughness.

• Give special attention to emerging module reliability issues; water and heat stress to CdTe and CIGS.

• Research and collaborations on barrier coatings.• IR images used identify specific failure mechanisms.• Support of DOE’s SAI reliability teams, PV industry and

suppliers.