Subcontract Report

NREL/SR-520-45903

June 2009

High Performance Packaging 

Solutions for Low Cost, Reliable 

PV Modules

Final Subcontract Report

26 May 2005 – 30 November 2008

B.M. Ketola and B.J. Marinik

Dow Corning Corporation 

Midland, Michigan
National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393

303-275-3000 • www.nrel.gov

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

Contract No. DE-AC36-08-GO28308

Subcontract Report

NREL/SR-520-45903

June 2009

High Performance Packaging 

Solutions for Low Cost, Reliable 

PV Modules

Final Subcontract Report

26 May 2005 – 30 November 2008

B.M. Ketola and B.J. Marinik

Dow Corning Corporation 

Midland, Michigan

NREL Technical Monitor: Richard L. Mitchell

Prepared under Subcontract No. ZAX-5-33628-02
This publication was reproduced from the best available copy

submitted by the subcontractor and received no editorial review at NREL

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iii

TABLE OF CONTENTS

PROGRAM BACKGROUND .............................................................................................................................. 1

PROGRAM OBJECTIVE ..................................................................................................................................... 1

SCOPE OF WORK ............................................................................................................................................... 1

Technical Approach........................................................................................................................................... 2

Figure 1 – Silicone Encapsulation Concepts ................................................................................................. 3

PHASE III TASK SUMMARY AND RESULTS................................................................................................. 3

Task 13, Production of Prototype Cells & Circuits II........................................................................................ 3

Task 14, Device Protection & Packaging System Evaluation for Thin-Film PV on Glass................................ 5

Figure 2 – Rigid Thin Film Response to Humidity Freeze Cycling .............................................................. 7

Figure 3 - Lap Shear Adhesion Test Sample Configuration.......................................................................... 8

Figure 4 – Lap Shear Adhesion Results of APPLD coated samples.............................................................. 9

Figure 5 - Rigid Thin Film Lap Shear Adhesion Testing Results with Prototype B9 – Room Temperature 

Aging ............................................................................................................................................................. 9

Figure 7 - Prototype B10 Adhesion Testing Results – RT Aging................................................................ 10

Figure 8 - Prototype B10 Adhesion Testing Results – 85 °C/85 %RH Aging............................................. 11

Figure 9 – Rigid Thin Film PV Pmax Response to Damp Heat Aging ....................................................... 12

Figure 10 - Rigid Thin Film Module Prepared with Prototype B9 – Post 1000 hr 85 °C/85 %RH aging. .. 12

Figure 11 - Rigid Thin Film Module Prepared with Prototype B10 – Post 1000 hr 85 °C/85 %RH aging. 13

Task 15, Protection System Scale-Up.............................................................................................................. 13

Figure 12 – Manufacturing Concept for Application of Prototype B Encapsulation................................... 14

Figure 13 – Picture of the Pilot Line for Prototype B Application .............................................................. 14

Figure 14 - Encapsulation Prototypes % Transmission Measured at Dow Corning Corporation................ 16

Figure 15 - Encapsulation Prototypes % Transmission Hemispherical Measurement at NREL ................. 16

Figure 16 - %QE Analysis of Silicone and EVA encapsulated single-crystalline Cells.............................. 17

Figure 17 - Efficiency Gain Pre and Post Encapsulation vs. IV Tester ....................................................... 17

Figure 18 - Cure Rate vs. Cure System........................................................................................................ 18

Figure 19 – Encapsulation Concepts ........................................................................................................... 18

Figure 21 – Back Sheet Delamination after Thirty Three Humidity Freeze Cycles.................................... 19

Figure 22 – Mutlicrystalline cell IV Response to Industry Standard Testing .............................................. 20

Figure 23 - Mutlicrystalline cell IV Response to Extended Industry Standard Testing............................... 20

Figure 24 – 180 Degree Peel Adhesion Schematic Representation............................................................. 21

Figure 25 – Adhesion by 180 Degree Peel Testing of Back Sheet w/ Prototype B9 at 85 °C/85 %RH 

Aging ........................................................................................................................................................... 21

Figure 26 – Back Sheet Comparisons by Response to Humidity Freeze Cycling ....................................... 22

Figure 27 – Back Sheet Comparison by Response to Thermal Cycling ...................................................... 23

Figure 28 – Effect of Moisture Barrier on Humidity Freeze Cycling.......................................................... 23

Figure 29 – Cell Response to Humidity Freeze Cycling With and Without PIB Edge Bead ...................... 24

Figure 30 – Coated Cell Response to Humidity Freeze Cycling ................................................................. 24

Figure 31 – Three Cell Strings Response to Standard Industry Testing ...................................................... 25

Figure 32 – Three Cell String IV Response to Extended Damp Heat Conditions....................................... 25

Figure 33 – Three Cell String IV Response to Extended Humidity Freeze Conditions............................... 26

Figure 34 – Three Cell String IV Response to Extended Thermal Cycling Conditions .............................. 26

Task 16, Evaluation of Protection System with Thin-Film PV on Flexible Substrate..................................... 27

Task 17, Module Fabrication........................................................................................................................... 28

Figure 35– Silicone encapsulated module prior to shipment. ...................................................................... 30

PHASE III SUMMARY...................................................................................................................................... 30

ACKNOWLEDGMENTS................................................................................................................................... 31

REFERENCES.................................................................................................................................................... 31
1

PROGRAM BACKGROUND

The FY2003 procurement, Photo Voltaic (PV) Manufacturing R&D—Large-Scale Module and 

Component Yield, Durability, and Reliability, goals are to improve PV manufacturing processes 

and products while reducing costs, provide a technology foundation that supports significant 

manufacturing scale-up, and position the U.S. industry to meet rapidly emerging large-scale 

deployment and other markets. The primary focus is the enhancement of module, system 

component, and complete system reliability.

During this research effort, Dow Corning Corporation (“Dow Corning”) will address the PV 

manufacturing goals of: (i) improving PV manufacturing processes and equipment; (ii) 

accelerating manufacturing cost reductions of PV modules; (iii) increasing commercial product 

performance and reliability; and (iv) scaling up U.S. manufacturing capacity. To accomplish these 

goals, Dow Corning has focused its efforts on developing a reduced-cost, higher-throughput, 

improved-performance packaging solution for U.S. PV modules manufacturers. 

Dow Corning is working toward the program goals by developing quality assurance (QA) and 

environmental, safety, and health (ES&H) programs in keeping with local, state, and federal 

regulations as applicable.

PROGRAM OBJECTIVE

The overall objective of the research effort over its three-phase duration is to develop a reducedcost, higher-throughput, improved-performance packaging solution for United States Photovoltaic 

(“PV”) module manufacturers. The primary target of the work is to develop a packaging solution 

that will reduce the overall cost contribution of this component by a minimum of 25% relative to 

the conventional Ethyl Vinyl Acetate (“EVA”)/ Poly vinyl fluoride (“PVF”) batch lamination and will 

provide improved reliability for twenty-plus years of performance in the field. During the 

subcontract, Dow Corning shall formulate and optimize adhesive, encapsulant, and barrier 

materials for use with a broad range of current PV technologies. This effort is expected to result in 

improved performance encapsulation materials and the establishment of a pilot-scale, protectionsystem production line capable of processing full-size, single-crystalline Silicon PV modules. 

SCOPE OF WORK

Phase I - Work in this phase targeted the evaluation of multiple approaches to the encapsulation 

solution to crystalline silicon based PV modules. Four specific approaches A-D were developed 

which included sheet based and liquid based options. During this phase options A and D were 

eliminated as possibilities due to the inability to meet the criteria for durability or reduction of 

cost/peak watt. 

Phase II – The work in this phase targeted the optimization, scale up and application of 

encapsulation prototypes to thin film PV technologies. During this phase prototype C was 

eliminated due to the inability to meet cost/peak watt reduction. In addition, it was determined that 

the approach to barrier layer films did not provide any significant improvement in performance. 

Prototype B materials were shown to have the potential for higher manufacturing throughput by 

utilizing a faster cure chemistry. Prototype B materials also were shown to have the potential for 

higher cell efficiency by utilizing the portion of UV light blocked by EVA due to the addition of UV 

absorbers to minimize EVA degradation from UV exposure. The optimization of prototype B for 

durability was continued using IEC and UL testing criteria on mini modules. The optimized 

formulations were the basis to design a pilot line for the application to full size modules. Prototype 

B was also utilized for encapsulation with rigid thin film PV technologies. 
2

Phase III – The work in this phase was targeted on the scaled-up material production and 

application on full size modules. During this phase the pilot line designed in phase II was 

purchased and installed at the Solar Application Development Center in Freeland, Michigan. The 

pilot line was used to demonstrate and optimize the application of the Prototype B encapsulation 

solution on full size crystalline silicon modules. These modules are being submitted to NREL or 

for independent IEC and UL qualification testing by module manufacturers or internally to Dow 

Corning. The optimized Prototype B materials were also applied to thin film PV technologies.

These modules are being evaluated internally to Dow Corning for application development using 

IEC and UL testing criteria. 

Technical Approach

In order to meet the objectives of increased durability and reduction of cost/peak watt as 

compared to EVA encapsulated crystalline silicon modules, literature searches and interviews 

with module suppliers were conducted. This resulted in a technical approach based on the 

following input. A new encapsulant would need to meet or beat the durability of EVA and it must 

deliver at least one of the following criteria: increased module efficiency, increased manufacturing 

throughput, or lower material costs. In addition, the solution would need to be demonstrated at a 

manufacturing scale.

As a result a major technical focus of the subcontract was placed on the durability of the 

encapsulated cells and modules. Outside testing in multiple locations is the best means of 

demonstrating durability but, due to the long time periods needed to demonstrate the durability, 

other means must be employed. Durability can also be demonstrated by evaluating coupons or 

modules using the standardized aging testing developed for IEC and UL qualifications. These 

tests are designed to analyze any changes in efficiency or power and visual defects before and 

after environmental aging in Damp Heat (85 °C/85 %RH), Thermal Cycling (-40 to 90 °C), and UV 

Exposure/Thermal Cycling/Humidity Freeze sequences. 

The other major technical focus was to impact on cost per peak watt by increased cell or module 

efficiency, higher manufacturing throughput and reduced material cost. Increased efficiency can 

be directly influenced by the amount of light that reaches the cell. In order to understand if 

silicone can improve efficiency testing of the % Transmittance of the materials were compared 

against EVA. In addition, the comparison of the % Quantum Efficiency of the cell was completed 

to evaluate the conversion of the light to electricity by the cell. Finally, by direct comparison of 

EVA and Silicone encapsulated cells or module efficiencies or short circuit current (Isc). 

Higher manufacturing throughput could be achieved by reducing lamination cycle times, which is 

significantly influenced by increased cure rates and decreased cure temperatures. Reduced cure 

temperatures and times would also reduce energy costs. Employing process automation and 

reduced material handling are other methods of increased throughput. In addition, higher 

throughput could be achieved if the process could improve process yield through less cell 

breakage or module rework.

Lower material costs can be achieved by developing formulations using high volume 

intermediates, and low cost production equipment. Lower labor rates can be achieved by reduced 

material handling. Further reduction in cost can be achieved by reducing packaging and 

packaging waste, which not only reduce costs but also benefit the environment. Material costs 

can also be minimizing by optimized silicone layer thickness and waste minimization by utilizing 

robotic application methods. Another approach to lowering material costs would be to eliminate or 

replace polyvinylfluoride back sheets. Finally, material costs can be lowered by reducing silicon 

usage through the use of thinner wafers or cells. 

The resulting product concepts and processes taken forward in this program strived to achieve 

these criteria. 
3

As a result of the industry interviews and formulation screening completed in Phase I & II the 

concepts shown in figure 1 were derived for the use of silicone to encapsulate PV modules.

Figure 1 – Silicone Encapsulation Concepts

The first option resulted as a method to overcome the “cut test”. The “cut test” as defined in IEC 

61215 proved to be a major hurdle for the second option since the silicones evaluated the cut 

strength was not sufficient to protect the cells and remain at a thickness that would be cost 

effective. Keeping the back sheet in place allowed for the passing of the “cut test” while 

maintaining a thickness that would allow silicones to be competitive with the cost of EVA.

Even though the “cut test” could not be met by the formulations evaluated, the second option is

still of interest due to the performance of modules encapsulated with silicone only in the late 

1970s and early 1980s by Solarex Corporation (BP Solar), and deployed at their facility in 

Frederick, MD and Georgetown University. These modules would not be a cost effective option 

per the materials and manufacturing methods used at the time. However, the option has the 

possibility to meet program goals of cost effective solution if the proper material and process 

could be developed. Hence, activities around this option are being explored beyond the end of 

this contract. 

PHASE III TASK SUMMARY AND RESULTS

Task 13, Production of Prototype Cells & Circuits II 

During this task functional crystalline Si cells and circuits, and thin film PV circuits that are 

fabricated using standard processes were needed in order to provide representative samples of 

the current PV technology. These circuits were evaluated for the initial performance, and

packaged in appropriate materials to mitigate degradation during shipping. 

Milestone Description

M-3.13.1 Completion of production for functional mono-crystalline Si cells and 

circuits required for fabrication of modules, and functional multicrystalline Si cells and circuits required for fabrication of modules.

M-3.13.2 Completion of 30 thin-film PV devices for use in evaluation of 

encapsulation materials. 

Glass Front Sheet Clear Silicone

Pigmented or 

Clear Silicone

EVA

1st Option 

Silicone 

Encapsulation 

System

Glass Front Sheet

Pre-cured Back Sheet

EVA

Incumbent 

System

Pre-cured Back Sheet

Glass Front Sheet

Pigmented Silicone

2nd Option 

Silicone 

Encapsulation 

System

Clear Silicone 
4

Strategy

The production of crystalline circuits was completed by multiple partners to ensure a 

representative supply from the industry. The mini-modules were supplied first to evaluate and 

optimize the encapsulation system. Then full size circuits or matrices were acquired for module 

production with the optimized encapsulation system. The major issues for shipping matrices were 

the handling, packaging and shipping without damage. To solve this issue a “sandwich” package 

design was developed by Dow Corning to transport the matrices. This design was successful in 

transporting 72 cell matrices without damage from multiple suppliers in multiple locations. 

Mono-crystalline Cells, Strings and Matrices

The first source, supplier 1, sent 125 mm x 125 mm mono-crystalline cells, 3 cell strings, 32 cell 

matrices and 72 cell matrices at no charge to the contract. Because of concerns with potential 

damage when measuring IV the strings and matrices were not evaluated prior to shipment. 

Supplier 2 also sent 125 mm x 125 mm cells to evaluate and optimize the encapsulation system.

Two sets of 100 cells of multi-crystalline were supplied. The cells specifications are as follows:

Cell Type: 125 mm 

Eff. = 14.93% Ipmax = 4.67 Amp

Isc = 5.002 Amp Vpmax = 498 mV

Voc = 608 mV FF = 76.5 %

The single and multi crystalline cells were encapsulated in three cell as well as four cell 

configurations. The four cell configurations were soldered together at Dow Corning for 

encapsulation experiments. 

Evaluation and optimization of the encapsulation system was accomplished by encapsulating the 

cells using option 1 shown in Figure 1. The encapsulated cells were evaluated by visual 

inspection and IV power analysis using a Spire 240 A Sun Simulator at Dow Corning. The 

encapsulated cells were then subjected to environmental aging at Damp Heat (85 °C/85% RH), 

Thermal Cycle (-40 to 90 °C), and UV/Thermal Cycle/Humidity Freeze conditions. Due to the 

concerns over customer confidential information the results from this testing will be reported as % 

change from initial.

The 32 cell and 72 cell matrices were encapsulated using the optimized formulations derived from 

the single cell and multi cell strings. The encapsulation of the matrices was used to optimize the

pilot line encapsulation process developed in Task 12. 

Milestone Description

M-3.13.2 Completion of 30 thin-film PV devices for use in evaluation of 

encapsulation and barrier materials

Strategy

The production of thin film circuits was completed by multiple partners to ensure a representative 

supply from the industry. Mini-modules were supplied first to evaluate and optimize the 

encapsulation system. Then full size circuits were or will be acquired for module production with 

the optimized encapsulation system.
5

Task Results

Supplier 1

Fifty cells (modules) were sent from rigid thin film Supplier 1. These were used in encapsulation 

trials, and environmental aging studies, and compared to incumbent encapsulant materials as

control samples.

A second shipment of twenty cells (modules) was sent using a silicone based insulating tape from 

the rigid thin film supplier. In addition, a laminated module was received as standard reference for 

the Spire 240 A Sun Simulator.

The modules were visually inspected and characterized using the Spire 240 A Sun Simulator 

prior to and after encapsulation as well as after environmental aging studies. These results were

reported as % change in Pmax. 

Supplier 2

Supplier 2 sent 20 – 30 cm x 30 cm modules and back glass for adhesion and encapsulation 

mock ups. These will be used to develop adhesion testing and demonstrate the use of prototype 

B formulations with these types of modules.

In addition, 58 modules were sent from Supplier 2 for DOEs studying electrical connection and 

insulator composition. In addition, a laminated module was received as standard reference for the 

Spire 240 A Sun Simulator.

The modules were visually inspected and characterized using the Spire 240 A Sun Simulator 

prior to and after encapsulation as well as after environmental aging studies. These IV results 

were reported as % change in Pmax. 

Task 14, Device Protection & Packaging System Evaluation for Thin-Film PV on Glass

This task is designed to evaluate the application of the protection system, Developed in Phase I 

and optimized in Phase II, on thin film devices provided by alternative suppliers. The 

encapsulated devices will be evaluated for protection against environmental aging. In order to 

optimize the protection system for thin-film applications, the materials and processes will be 

applied the to additional thin film devices and perform the characterization tests.

Milestone Description

M-3.14.1 Demonstration of the protection system developed for single-crystalline 

Si cells on thin-film PV on glass cells in order to verify applicability of 

system to a broader range of thin-film PV technology

M-3.14.2 Demonstration of an optimized protection system on thin-film PV on 

glass cells in order to verify applicability of system to a broader range of 

PV technology

Strategy

The target for this task is to evaluate the potential benefits of silicone encapsulation versus EVA, 

and the various technologies to prevent moisture ingress using these modules. Research papers 

have recently been questioning EVA as the best encapsulant for thin film technologies1,2,3,4. A 

major issue is corrosion potential due to acetic acid generation. Another issue that is known in the 

industry is “dark loss” or drop in initial efficiency from elevated temperatures during the lamination 

cycle. In addition, there is some suspicion that the high modulus of the EVA can cause high 

stresses during thermal changes which could result in premature failure. The use of perimeter 
6

seals has been proposed as a potential solution for moisture ingress1,2,3 from the edges in a glass 

on glass construct. However the use of perimeter seals do add a processing step which can slow 

production, add cost and can potentially cause issues with stress build up in traditional 

lamination.

It is theorized that silicone can address several of these issues. Based on Dow Corning 

experience in the electronics and automotive industries, it is well known that silicones, when 

properly formulated, can prevent corrosion in electronic devices. From this experience it has been 

learned that moisture ingress is not an issue for corrosion if the encapsulation material has good 

adhesion with no void spaces for condensation of moisture. In addition, there has to be ionic 

contamination, such as metallic ions, acetic acid or acidic peroxide by-products, associated with 

condensed moisture to promote corrosion mechanisms. Silicones have been formulated to excel 

in adhesion, with good rheological properties to prevent void spaces, having low in ionic content, 

and do not form ionic materials due to the chemical make up and cure mechanisms. 

Silicones can be formulated with multiple cure chemistries that can significantly lower the cure 

temperature, some as low as room temperature, and significantly reduce the cure times. This not 

only helps with “dark loss”, but can also significantly increase throughput. In addition, the proper 

cure chemistry and rheology can possibly eliminate traditional lamination techniques.

Finally, silicones can be formulated for much lower modulus and have inherently lower Glass 

Transition Temperature (Tg) to provide stress relief during thermal changes over temperatures 

ranging from -40 to >150 °C.

The strategy employed by this task will attempt to demonstrate the potential benefits of silicone 

over EVA based on these properties. 

Task Results

In Phase II the evaluation of thin film modules from Supplier 1 was started. The crystalline 

encapsulant formulation prototype B was applied to the several modules and tested using 

humidity freeze conditions. Surface defects were noted during this testing. Visual observation 

seemed to indicate that the surface defects were associated with the tape used as the insulator 

under the lead foil. It was theorized that the acrylic adhesive was a possible factor. Experiments 

that were completed to replace the tape also indicated that the surface defects were associated 

with the acrylic adhesive used on the tape. As a result modules were prepared with silicone 

adhesive insulator tape by the supplier. The modules were then prepared with prototype B. The 

modules were then subjected to HF cycling. Modules with acrylic tape adhesive were included in 

the study. The IV response to HF cycling as measured by percent change in Pmax is presented 

in Figure 2. 
7

-25.00

-20.00

-15.00

-10.00

-5.00

0.00

5.00

% Change Pmax

Prototype F - Silicone TapePrototype F - Silicone Tape

EVA - Acrylic TapeEVA - Acrylic Tape

Prototype F - Acrylic TapePrototype F - Acrylic Tape

Pmax after HF Cycling

5 HF Cycles

10 HF Cycles

17 HF Cycles

23 HF Cycles

30 HF Cycles

37 HF Cycles

43 HF Cycles

50 HF Cycles

Figure 2 – Rigid Thin Film Response to Humidity Freeze Cycling

The data indicated that percent change Pmax performance was acceptable for all modules up to 

thirty cycles. To understand if there were any major differences in IV performance after further 

cycling the modules continued to be aged. Significant Pmax degradation took place for at least 1 

of the samples from each configuration at each of the next intervals. The Pmax degradation 

correlated with a drop in FF or Voc which could be attributed to increases in resistance and 

degradation of the PV device respectively. 

It should be noted that the above data should be used for qualitative purposes only due to the fact 

that the unencapsulated modules with acrylic adhesive tape had been stored for an extended 

period of time. 

Visual observation of the two Prototype B modules with silicone tape had minimal surface 

defects, and essentially no defects in the silicone. Surface defects developed along the edge of 

the modules when observed at 50 cycles, and the silicone showed minor adhesion loss at the 

perimeter. 

Visual observation of the Prototype B modules with acrylic tape was showing surface defects

under the insulating tape and along one edge of the module after 22 cycles. The surface defects 

out to 50 cycles did not change significantly after being first observed. The silicone defects again 

showed minor defects along the edge. 

Visual observations of the EVA modules revealed significant surface defects on the edges of the 

modules after 22 cycles. At 30 cycles one module was showing significant delamination of the 

EVA from the metallization layer. In addition, the EVA was very hazey along the perimeter. The 

delamination continued to grow in size out to 50 cycles, and the corners began to delaminate. In 

addition, the surface defects had continued to increase. In the areas along the long edge of the 
8

module and near the junction box hole the PV material appeared to be blistered or delaminating 

from the front glass.

The results of the experiment lead to the search for the root cause of the surface defects. It was 

theorized that the surface defects could be a function of loss of adhesion which allowed moisture 

to migrate to the PV material. Experiments were performed by varying Prototype F formulations 

with different adhesion promoter packages on small samples cut from a module. These samples 

were subjected to 85 °C and 85% RH conditions and inspected for surface defect formation. 

These experimental results revealed that optimizing the adhesion package can have a significant 

impact on surface defects. 

. 

Lap shear adhesion was utilized and the samples were prepared as shown in the Figure 3. 

Figure 3 - Lap Shear Adhesion Test Sample Configuration

Samples from supplier 1 prepared in Task 7 were tested to determine if the coatings applied 

using the APPLD technology will have a benefit for this application. Samples were prepared for 

adhesion testing from coated, coated with heat treatment, and uncoated substrates using the 

Prototype B formulation used in the module testing above. The samples were subjected to 85 

°C/85 %RH conditions to compare the adhesion and surface defect formation. The results of the 

adhesion testing are shown in Figure 4. Visual inspection of the samples after three weeks at 85 

°C/85 %RH showed that the defect formation in the coated samples was much improved over the 

uncoated samples. However after seven weeks in 85 °C/85 %RH conditions the only samples 

that showed no defects were the samples that had been heat treated. Unfortunately the module 

supplier noted that the heat treatment at this stage of the processing would most likely degrade 

module performance and would not be an acceptable processing step. Since this method of 

adhesion promotion did not meet the customer criteria, and it would have an extra processing 

step this approach was abandoned. Instead, adhesion improvement was pursued by evaluating 

adhesion promoter packages.

Thin Film PV Cell (1” x 3”) Silicone (1”x 1”) Back Glass (1” x 3”)

Top View

Side View

Pull 

Force

Pull 

Force
9

Lap Shear Adhesion

0

5

10

15

20

25

30

21 49

Days at 85 C/85%RH

lbf

0

20

40

60

80

100

120

% Cohesive Failure

Uncoated

Precursor 2, Anneal

Precursor 1, Anneal

Uncoated % Cohesive

Failure

Precursor 2, % Cohesive

Failure

Precursor 1, % Cohesive

Failure

Figure 4 – Lap Shear Adhesion Results of APPLD coated samples

Prototype B formulations using various adhesion promoter packages were prepared using 

materials provided by supplier 2, and tested by lap shear adhesion testing before, and after aging 

at room temperature and in 85 °C/85 %RH conditions. These were tested ~1 week intervals to 

monitor the changes adhesion in both conditions. The results of the adhesion testing are shown 

graphically in Figures 5 and 6. 

Figure 5 - Rigid Thin Film Lap Shear Adhesion Testing Results with Prototype B9 – Room 

Temperature Aging 

Prototype F9 - Room Temp Ageing

0

20

40

60

80

100

120

140

160

180

200

control AP1 AP2 AP3 AP4

% cohesive failure

0

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8

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Lap shear Strength

24hr RT % cohesive failure

Week 1 RT % cohesive failure

Week 2 RT % cohesive failure

Week 3 RT % failure

Week 4 RT % cohesive failure

Week 5 RT % cohesive failure

24hr RT

Week 1 RT

Week 2 RT

Week 3 RT

Week 4 RT

Week 5 RT
10

Prototype F9 - 85/85 Ageing

0

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control AP1 AP2 AP3 AP4

%cohesive failure

0

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Lap shear stress

24hr 85/85 % cohesive failure

Week 1 85/85 % cohesive failure

Week 2 85/85 % cohesive failure

Week 3 85/85 % failure

Week 4 85/85 % cohesive failure

Week 5 85/85 % cohesive failure

24hr 85/85

Week 1 85/85

Week 2 85/85

Week 3 85/85

Week 4 85/85

Week 5 85/85

Figure 6 - Rigid Thin Film Lap Shear Adhesion Testing Results with Prototype B9 – 85 

°C/85 %RH Aging 

Based on this data Prototype B9 with AP2 showed the best performance in RT and 85 °C/85 

%RH. This adhesion package would be brought forward in further testing.

A second Prototype B formulation, B10, was also prepared tested for adhesion at RT and 85 

°C/85 %RH aging conditions for at least 6 weeks with tests at 1 week intervals. The results of the 

testing are shown in Figures 7 and 8. 

Prototype F10 RT Ageing

0

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Control AP1 AP2 AP3 AP4

% Cohesive Failure

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Lap Shear Strength (ppi)

24hr RT %CF

1wk RT %CF

2wk RT %CF

3wk RT %CF

4wk RT %CF

5wk RT %CF

6wk RT %CF

8wk RT %CF

10wk RT %CF

24hr RT

1wk RT 

2wk RT

3wk RT

4wk RT

5wk RT 

6wk RT

8 wk RT

10 wk RT

Figure 7 - Prototype B10 Adhesion Testing Results – RT Aging
11

Prototype F10 85/85 Ageing

0

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Control AP1 AP2 AP3 AP4

% Cohesive Failure

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24hr 85/85 %CF

1wk 85/85 %CF

2wk 85/85 %CF

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5wk 85/85 %CF

6wk 85/85 %CF

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24hr 85/85

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4wk 85/85

5wk 85/85

6wk 85/85

8 wk RT

10 wk RT

Figure 8 - Prototype B10 Adhesion Testing Results – 85 °C/85 %RH Aging 

Based on the fact that RT adhesion develops over time with AP3 seems to be best suited for this 

formulation. This adhesion package will be used in future encapsulation of rigid thin film PV 

modules.

Several samples were encapsulated using prototype B9 and B10 formulations to develop the 

encapsulation method. The successfully encapsulated modules were subjected to Humidity 

Freeze Cycling to understand the response of the modules to the aging conditions. Samples 

prepared with Prototype B9 showed some surface defects. Samples prepared with Prototype B10 

showed no surface defects. After discussing the results with the supplier it was determined that 

conclusions from this study could not be drawn since the modules were not of acceptable quality. 

As a result another set of modules were supplied for use in a DOE to study adhesion promoters 

and module assembly techniques. These samples were encapsulated using the B9 and B10 

formulations. Samples were then characterized visually and by IV and put into 85 °C/85 %RH

aging for 1000 hours. Intermediate testing results show no degradation of the modules at ~400 

and 700 and 1000 hr testing intervals. The data is shown graphically in Figure 9. 
12

Thin Film Module Pmax Response To Damp Heat Aging

0.500

0.600

0.700

0.800

0.900

1.000

1.100

1.200

1.300

1.400

1.500

Module 1Module 2Module 3Module 4Module 5Module 6Module 7Module 8Module 9

Module 10

Module 11

Module 12

Module 13

Module 14Module 15Module 16

Reference 

PMax (W) (Normalized)

418 hr 85/85

727 hours of 85/85

1064 hours of 85/85

Figure 9 – Rigid Thin Film PV Pmax Response to Damp Heat Aging

Visual inspection of the modules has shown no major defects after aging. The pictures below are 

representative of two samples prepared for Rigid Thin Film Testing. These have been prepared 

with two versions Prototype F 9 and Prototype F10.

Figure 10 - Rigid Thin Film Module Prepared with Prototype B9 – Post 1000 hr 85 °C/85 

%RH aging. 
13

Figure 11 - Rigid Thin Film Module Prepared with Prototype B10 – Post 1000 hr 85 °C/85 

%RH aging. 

The modules from Supplier 2 have shown very good IV response to the 85 °C/85 %RH conditions 

with essentially no degradation in Pmax. Based on these results, plans are being made for full 

size panels to be tested on the pilot line described in Tasks 12, 15 and 17.

Task 15, Protection System Scale-Up 

A major goal of this task will be the scale-up activities for the pilot-scale, protection-system 

production line through production-run trials of the collective system. During this task the tools for 

the application of the adhesive and encapsulant will be designed, installed and started up. 

Following start up and calibration, protective system trials will be completed and the protectionsystem performance tested as compared to previous evaluation results.

Milestone Description 

M-3.15.1 Completion of scale-up and calibration of pilot-scale, protection-system 

production line, as needed to demonstrate benefits, including the barrier, 

adhesive, and encapsulant tools

Strategy

The encapsulation system development during Phase I and II resulted in the concepts depicted in 

Figure 1. The materials selected for these concepts are based on prototype B formulations, which 

are liquids. Because of the use of liquid materials, a completely new system of material 

application needed to be developed as compared to EVA sheet applications. To demonstrate the 

liquid application system a pilot line has been designed based on a conceptual production line 

shown schematically in Figure 12.
14

Figure 12 – Manufacturing Concept for Application of Prototype B Encapsulation

In order to handle multiple types and sizes of modules the pilot line was designed with a high 

degree of flexibility in mind. In addition, the line was designed to be a semi continuous operation 

with electronic communication capability, where capable, between pieces of equipment. The 

resulting pilot line consists of glass staging, silicone dispensing, array placement, back sheet 

placement and curing. Details of the unit operations equipment operation is provided below.

The picture in figure 13 shows the actual pilot line that has been installed at the Dow Corning 

Solar Application Center in Freeland, Michigan.

Figure 13 – Picture of the Pilot Line for Prototype B Application

Glass Staging, Silicone Application and Cure Equipment

In Phase II of the contract the design and specifications for the glass staging, dispensing and 

curing equipment were completed and sent to prospective suppliers. The glass staging 

Glass 

Washer

Silicone 

Application

Array 

Placement

Bus Bar

Lay Up 

Station

Stringer

Silicone 

Application

Back Sheet 

Application/Cure
15

equipment was designed as an inspection station to simulate the outlet of a glass washing 

operation. The two dispensing tools were designed to dispense a layer up to 0.5 mm thick over 

an area as large as 1.2mm x 1.8mm in less than 2 minutes. Cure equipment was designed for 

autonomous conveying and curing of laminates at temperatures up to 150 °C in less than 2 

minutes. Quotes for staging, dispensing and curing equipment for the adhesive and encapsulant 

layers, and curing were then acquired, and the equipment was then ordered based on the cost 

and timeliness of delivery. Equipment demonstrations completed at the selected equipment 

supplier that revealed some necessary changes to meet desired process throughput targets. 

Final quotes were issued and purchase orders were then issued for this equipment with expected 

delivery in Q4 2007. Due to delays at the manufacturer and modifications to approve the 

equipment design and operation delivery was accomplished in Q1 2008. The equipment was 

installed in Q1 2008 and started up in Q2 2008. Optimization of dispense methods were

conducted in preparation for crystalline matrices being sent from suppliers in Q2 and Q3 2008. 

This dispensing optimization focused on the dispense speed, coating thickness variation control

and pattern accuracy. Curing optimization focused on optimizing cure conditions at less than 150 

°C resulting in a bubble free product.

Cell/Array Placement Apparatus

Cell placement was determined to be a potential bottle neck for the increased throughput desired. 

A proprietary and potentially patentable prototype was designed to demonstrate a faster 

application method. The prototype was developed and used successfully in single cell and 

multiple cell coupons. A provisional patent was submitted on the design. A prototype capable of 

handling multiple strings or matrices was then designed. As with the dispensing equipment this 

apparatus was developed with the capability to handle multiple sizes of module designs as well 

as keeping unit operation times below a 2 minute maximum. In addition, the equipment was 

designed to simulate matrices supplied by standard stringing and lay up operations. Particular 

attention was paid to transporting without cell damage and precise locating and aligning the glass 

and cells. A quote for the apparatus was acquired, and a purchase order was initiated with 

anticipated delivery in Q4 2007. During a trip to the manufacturer to approve the design 

equipment changes were identified. The completion of these modifications changed the 

equipment delivery schedule to Q1 2008. The equipment was delivered, installed and started up 

in Q1 2008. The apparatus was used successfully with 32 cells by Q2 2008 and 72 cell matrices 

by Q3 2008. The equipment start up focused on the accurate placement of matrices without cell 

damage. Due to the novelty of the cell placement design, the prototype was highly manual in 

operation and depended heavily on mechanical alignment. Although this design met the 

specification of less than 2 minute unit operation, the equipment needs further development to 

automate the design, making it capable for manufacturing processes. 

Back Sheet Placement Apparatus

In order to maintain the desired throughput, it was desired to have a continuous application of the 

back sheet. Research in this area revealed a device that could cut and apply the back sheet in 

line. A unit was found to be in storage at Spire Corporation and available for purchase. The back 

sheet placement apparatus was purchased from Spire. Since this had been in storage and needs 

some modification for use with the proposed encapsulation system it was sent to the 

manufacturer to be started up and modified. A trip to the manufacturer to approve the changes 

was completed in Q4 2007. Minor modifications were identified, and the equipment was 

scheduled to be shipped in Q1 2008. The equipment was delivered in Q1 2008 and installed and 

started up in Q2 2008. The equipment is being used when roll quantities of back sheet are being 

used for encapsulation trials of matrices. Optimization focuses on the alignment of the back sheet 

to be laminated, and hole size and location for the junction box leads.
16

Milestone Description

M-3.15.2 Completion of protective-system trials, comparing the protection-system 

performance to previous evaluation results

Strategy

Adhesive Layer

Phase I results had shown that Prototype B (Liquid Silicone) was the best candidate to take 

forward for the adhesive layer primarily based on optical properties as shown in Figures 14 and

15. In addition, %QE analysis of silicone versus EVA was completed at NREL (Figure 16) which 

showed the significant gains in % QE in the Ultra Violet (UV) region.

UV-vis

 1 mm Quartz Glass

70

75

80

85

90

95

200 300 400 500 600 700 800

Wavelength (nm)

% Trans.

Prototype A 2.6 mm Thick

Prototype C 2.6 mm Thick

EVA 2.6 mm Thick 

Protoype B 2.6 mm Thick

Figure 14 - Encapsulation Prototypes % Transmission Measured at Dow Corning 

Corporation 

 UV Vis Data - 3 mm Krystal Klear Glass

70

75

80

85

90

95

300 400 500 600 700 800 900 1000 1100

Wavelength (nm)

% Transmission

EVA-1.59 mm

Thickness

Prototype B 1.59

mm Thickness

Courtesy Mike Kempe @ NREL

Figure 15 - Encapsulation Prototypes % Transmission Hemispherical Measurement at 

NREL
17

 % QE of Prototypes and EVA 

0

20

40

60

80

100

120

282303

341

361

381

421460499539579619658697751801824850873

903

951

10061054

1104

11511202

Wavelength (nm)

% QE Unscaled

EVA Protoype C

Prototype B

Courtesy of Tom Moriarty @ NREL

Figure 16 - %QE Analysis of Silicone and EVA encapsulated single-crystalline Cells

In Phase II the efficiency improvement of cells encapsulated with silicone over EVA was 

demonstrated using Supplier 3 cells as shown in Figure 17. 

Figure 17 - Efficiency Gain Pre and Post Encapsulation vs. IV Tester

As shown in Figure 17 the demonstration of the improved Isc response required the proper IV 

testing equipment. This is due to the fact that most AM1.5 cell testers are designed to filter out 

the wavelengths below 400 nm. As can be noted in Figures 1, 2 and 3 a significant portion of the 

efficiency benefit for silicones are realized at wavelengths below 400 nm. This is a challenge to 

showing the efficiency improvements with silicone at the module level. Until a Sun Simulator is 

developed to include a comparison of EVA and silicone encapsulated modules in a multi module 

array will need to be done in direct sunlight to quantify any gains in efficiency or power output. 

Encapsulant Layer

Prototypes B & C (Thermoplastic Copolymer) were chosen for the encapsulant layer due the 

options for application methods and cure system/speed (Figure 18) that allow for the potential of 

increased throughput and the elimination of traditional laminator processing. Jsc change pre and post lam

-2

-1

0

1

2

3

4

Glass/EVA

Glass/silicone

Glass/EVA

Glass/silicone

Class A tester Class C tester

Configuration within Tester

Variability Chart for Jsc change pre and post lam
18

Moving Die Rheometer

 @150 deg C; 100 RPM

0

0.2

0.4

0.6

0.8

1

1.2

0 1 2 3 4 5 6 7 8 9 10

Time (min)

Torque (Normalized)

Silicone Cure

System B

EVA

Silicone Cure

System A 

Figure 18 - Cure Rate vs. Cure System

As a result of the choices for the adhesive and encapsulant layers two encapsulation concepts, 

shown in Figure 19, were then developed.

Figure 19 – Encapsulation Concepts

The first option primarily targeted liquid silicone (Prototype B) as the encapsulant and adhesive 

layers. The second option primarily targeted Prototype B for the adhesive layer and a copolymer 

(Prototype C) in sheet form as the encapsulant layer. The potential to use Prototype C depended 

heavily on the economic evaluation. It was theorized that the use of prototype C would require the 

elimination of the back sheet to allow for economic benefit. Attempts to eliminate the back sheet 

did not result favorably due to the lack of protection during cut test analysis without a prohibitively 

thick encapsulant layer. This discovery eliminated prototype C a viable material for option 2.

These options were evaluated in Phase II by evaluating encapsulating cells then exposing the 

cells to Humidity Freeze (HF) Cycling as a screening test. Options that passed the HF screen 

then would be then subjected to industry standard testing of 1000 hrs 85 °C/85 %RH, 200 

Thermal Cycles (TC) and 420 hrs UV/50 TC/10 HF Cycles for final selection. The samples were 

evaluated for visual defects and characterized at the supplier for changes in efficiency or using a 

Glass Front Sheet Clear Silicone

Pigmented or 

Clear Silicone

EVA

1st Option 

Silicone 

Encapsulation 

System

Glass Front Sheet

Pre-cured Back Sheet

EVA

Incumbent 

System

Pre-cured Back Sheet

Glass Front Sheet

Pigmented Silicone

2nd Option 

Silicone 

Encapsulation 

System

Clear Silicone 
19

Spire SPI 240 AL Sun Simulator at Dow Corning for change in performance before and after 

aging conditions testing. 

Another key factor used for the prototype selection was based on economic evaluation of 

formulation options to ensure the cost of the material would achieve the goal of 25% reduction of 

$/Wp. This analysis not only focused on material usage and cost, but utilized a Cost of Ownership 

Model that emphasized gains from increased cell efficiency, and production throughput. 

Task Results

The encapsulation system development during Phase I and II resulted in the concepts depicted in 

Figure 20. 

1st Option - Encapsulation 

A two layer model (1st Option) using silicone adhesive and encapsulant layers and pre-cured 

back sheet are the primary focus. The two layers are based on a curable liquid silicone two part 

formulations

These formulations were used to assemble 4 cell strings and subjected to Humidity Freeze 

Cycling as a screening exercise. Visual observation of the screening samples revealed some 

defects in the adhesive layer under the bus bars and where the tabbing is interconnected at the 

edge of the cells as well as some delamination of the back sheet was observed around the 

perimeter after 33 cycles of Humidity Freeze. This is shown in the picture in Figure 21. 

Figure 21 – Back Sheet Delamination after Thirty Three Humidity Freeze Cycles

Formulation optimization was pursued to improve these issues. This exercise focused primarily 

on the adhesion of the encapsulant to the glass and back sheets.

When the four-cell string had passed Humidity Freeze Cycling, six more strings were constructed 

and subjected to industry standard testing of 1,000 hours 85 °C and 85 %RH, 200 TC and 420 

hrs UV/50 TC/10 HF. The results of the IV testing are shown in the Figure 22. 
20

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

% Change Isc

420 hr UV 420 hr

UV/50 TC

420 hr

UV/50

TC/10 HF

100 TC 200 TC 500 hr 85/85 1000 hr

85/85

4 Cell Multi-crystalline Response to Industry Standard Type Testing 

%Isc measurement error +/- 2%

Prototype B - Sample 1

Prototype B - Sample 2

Figure 22 – Mutlicrystalline cell IV Response to Industry Standard Testing

As with the four-cell string humidity freeze screening, the IV results were satisfactory with all three 

types of the testing. Similarly, visual observation of the samples revealed defects in the adhesive 

layer under the bus bars and where the tabbing is interconnected at the edge of the cells. And 

the 85 °C/85 %RH sample showed some adhesion issues with the silicone and the back sheet 

was around the perimeter.

To understand the impact of the visual defects the four-cell string samples were put back into the 

environmental chambers and continued to be aged well beyond IEC or UL standards. The results 

of which are quite positive with minimal degradation of Pmax as shown graphically in Figure 23. 

 Multicrystalline 4 Cell Strings - Prototype B9 

Accelerated Environmental Aging Impact 

0

1

2

3

4

5

6

7

8

9

10

DH - smpl 1DH - smpl 2

UV/TC/HF - smpl 2UV/TC/HF - smpl 2

TC - smpl 1TC - smpl 2

REFERENCE CELL 

Pmax (Watts)

Pmax: Post Encapsulation 

Pmax: After Aging 3500

hr, 34 HF, 500 TC

Pmax: After Aging 5470 hr,

89 HF, 722 TC 

Pmax: After Aging 5990 hr,

111 HF, 830 TC 

Figure 23 - Mutlicrystalline cell IV Response to Extended Industry Standard Testing
21

To improve the visual observation defect issues formulation optimization was pursued. These 

experiments were completed with single cell constructs. These constructs were placed in 

Humidity Freeze Cycling, and were observed at regular intervals for changes in appearance. The 

physical observations indicated that visual defects were resolved at a single cell level. 

In parallel to the above experiments the adhesion promoter packages were screened in the 

encapsulant layer formulations. Samples have been tested by preparing samples for 180 degree 

(angle) peel testing. The test method schematic is shown in Figure 24.

Figure 24 – 180 Degree Peel Adhesion Schematic Representation

These samples were tested for initial adhesion and tested at regular intervals after aging at room 

temperature and 85 °C/85 %RH conditions. The result of this testing is shown graphically in 

Figure 25. 

Prototype F9 Adhesion Testing 85/85 Age

0

50

100

150

200

BS1 No AP BS1 AP1 BS1 AP2 BS1 AP3 BS2 No AP BS2 AP4

% Cohesive Failure

0

0.5

1

1.5

2

2.5

3

3.5

Peel Strength (ppi)

% Cohesive 3day

% Cohesive 1 wk

% Cohesive 2 wk

% Cohesive 3 wk

% Cohesive 4 wk

% Cohesive 5 wk

% Cohesive 6 wk

% Cohesive 7 wk

Strength 3day

Strength 1 wk

Strength 2 wk

Strength 3 wk

Strength 4 wk

Strength 5 wk

Strength 6 wk

Strength 7 wk

Figure 25 – Adhesion by 180 Degree Peel Testing of Back Sheet w/ Prototype B9 at 85 

°C/85 %RH Aging 

Several samples performed well in this testing. AP4 was chosen to be used in future formulation 

development and scale up. Cells and four-cell strings were then encapsulated using prototype B9 

Glass

Silicone

Back Sheet

Pull Force

180 Degree Peel Force Testing Schematic
22

and AP4. These were then submitted for the Damp Heat, Thermal Cycling, and UV/Thermal 

Cycling /Humidity Freeze aging. IV performance, discussed below, and visual observations of the 

samples confirmed the improvement in adhesion performance with no delamination of back 

sheets in the samples tested in environmental aging. As a result of the adhesion the testing of the 

cells and mini strings the B9 formulation with AP4 was targeted to be used on the 72 cell arrays 

being shipped from supplier 1 and 2 as discussed under Task 17.

 

Supplier 1

In phase II an issue with low temperature cycling was discovered while applying the option 1 

encapsulation solution to supplier 1 cells. A root cause analysis of the issue was pursued for the 

first several months of Phase III.

The root cause analysis continued with an evaluation of the back sheets. The results are shown

in Figure 26. 

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

% Change Isc

No BackSheet - 1

No BackSheet - 2

No BackSheet - 3

Type A -1Type A -2Type A -3

Type B - 1Type B - 2Type B - 3

Type C - 0Type C - 1Type C - 2Type C - 3

Isc Change After HF Cycling

Isc measurement = +/-2%

 5 Cycles HF

 10 Cycles HF

17 Cycles HF

Figure 26 – Back Sheet Comparisons by Response to Humidity Freeze Cycling

This data seemed to indicate the best results are achieved by not using a back sheet. This was 

seen as a clue to the root cause of the issue. The issue could be due to higher stress when the 

back sheet is present or trapping of moisture. Experiments to further investigate these theories as 

potential root causes were completed. 

An experiment was completed to test the stress induced from different back sheets exposed to 

thermal cycling to minimize the effects of moisture. The data shown in the Figure 27 indicated

that the stress induced from temperature cycling without significant moisture present did not 

cause degradation at up to 100 cycles. 
23

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

% Change Isc

Type C -

1

Type C -

2

Type C -

3

Type A -

1

Type A -

2

Type A -

3

No Back

Sheet

No Back

Sheet

No Back

Sheet

% Change Isc vs. Thermal Cycling

27 TC

100 TC

Figure 27 – Back Sheet Comparison by Response to Thermal Cycling

Another set of experiments was started to evaluate the effects of moisture by preparing cells with 

a moisture impermeable edge seal with a glass back sheet. These cells were characterized at the 

supplier and were exposed to Humidity Freeze conditions. The results from the first 10 cycles are 

shown in Figure 28. 

-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

% Change Eff

001I 002H 004H

Glass on Glass w/ Edge Seal Response to HF Cycling

10 HF

20 HF

30 HF

Figure 28 – Effect of Moisture Barrier on Humidity Freeze Cycling

The results were positive with good response out to 30 cycles. In addition, repeats of glass on 

glass with and without the edge seal were prepared to confirm the results above. The data in the 

in Figure 29 showed that the samples without edge bead fail within 10 cycles, and those with 
24

edge bead do not. Sample Edge Bead 3 seemed as though it may be failing. Inspection of the cell 

indicated a potential path for moisture through the edge bead. 

-15.00%

-13.00%

-11.00%

-9.00%

-7.00%

-5.00%

-3.00%

-1.00%

1.00%

% Change Eff

Edge Bead

1

Edge Bead

2

Edge Bead

3

No Edge

Bead 1

No Edge

Bead 2

No Edge

Bead 3

Glass on Glass w/ PIB response to HF Cycling

10 HF

Figure 29 – Cell Response to Humidity Freeze Cycling With and Without PIB Edge Bead

Another experiment to confirm the results of the edge bead test using cells prepared with a 

special layer was completed. This was conducted with 3 cell strings characterized at the supplier 

before and after HF cycling. The results of the testing are shown in Figure 30. 

-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

% Change Eff

1813 1814 1815

3 Cell Strings w/ Special Coating Eff Change from HF 

10 HF

20 HF

30 HF

Figure 30 – Coated Cell Response to Humidity Freeze Cycling

The results out to 30 cycles showed a strong indication that the coating works effectively with 

Prototype B. 
25

With the positive results from the above experiments a new set of 3 cell strings with special 

coating was prepared and subjected to the 420 hr UV/50 TC/10 HF, 200 TC & 1000 hr 85%RH/85 

°C testing sequences. Testing of the cells has shown no significant degradation of cell 

performance as shown graphically Figure 31. 

-10

-8

-6

-4

-2

0

2

% Change Efficiency

110 TC Eff

Chng

200 TC Eff

Chang

420 hr UV Eff

Change

50 TC Eff Chng 10 HF Eff Chng 568 hr DH

(85/85) Eff

Chng

1065 hr DH Eff

Chng

Supplier 1 - 3 cell string Aging

Silicone

Silicone

Silicone

Control EVA

Figure 31 – Three Cell Strings Response to Standard Industry Testing

All aging conditions showed acceptable levels of degradation. These samples continued to be 

aged, and monitored periodically. The results of the continued aging can be seen in the graphs in 

Figures 32, 33 and 34. 

Figure 32 – Three Cell String IV Response to Extended Damp Heat Conditions

Silicone

Silicone

Silicone

EVA
26

Figure 33 – Three Cell String IV Response to Extended Humidity Freeze Conditions

Figure 34 – Three Cell String IV Response to Extended Thermal Cycling Conditions

According to this data, the Damp Heat condition at 2,500 hours, and at 3,600 hours indicate a 

more significant loss of power for the EVA encapsulated module where as the silicone

encapsulated modules seem to be maintaining. The power degradation appears to be starting in 

the EVA encapsulated module at 78 Humidity Freeze Cycles as well. However no degradation 

can be seen in either encapsulation material out to 550 cycles. These modules continue to be 

aged. 

2nd Option – Encapsulation 

In order to achieve a lower cost encapsulation method the elimination of the back sheet is a 

potential route to reduce cost. In addition, work done with silicones and no back sheet has shown 

promise in protecting amorphous silicon mini-modules during damp heat conditions. This is 

believed to be a result of the inherent ability for silicone to “breath” coupled with low moisture 

solubility, low ionic content and good adhesion. These properties have proven successful in 

protecting electronic devices and should be applicable to PV modules.

Previous attempts to eliminate the back sheet have met with failure for various reasons. The lack 

of thickness control and cut test resistance have been two major reasons observed when trying to 

use this approach with silicones for this project. Study of these issues has continued to develop a 

Silicone

Silicone

Silicone

EVA

Silicone

Silicone

EVA
27

method of back sheet elimination. A proprietary and potentially patentable approach has been 

discovered and will be pursued for the rest of this contract.

To maximize the potential of this approach a design of experiment was performed to optimize the 

formulation based on material rheology, cured properties and cut test response. The optimized 

formula will be applied to cells for encapsulation method optimization and response to aging 

studies.

Two formulations from the optimization were used to encapsulate single cells. The appearance of 

these cells was not acceptable due to uneven coating which is believed to be unacceptable for 

cut resistance. The formulation and application techniques are being reevaluated to improve upon 

this issue. Formulations from this reevaluation have been prepared and were used in 

encapsulation trials. The test showed what is believed to be acceptable cut resistance, but a

current leakage test needs to be established to evaluate the cut strength. Equipment for this test 

is being purchased and methods for completing single cell testing are being developed.

Task 16, Evaluation of Protection System with Thin-Film PV on Flexible 

Substrate

This task shall evaluate the application of encapsulation and barrier materials previously 

developed for application on thin film PV devices on flexible substrates. Thin-film PV on flexible 

substrates will be purchased. The protection system developed to date shall be applied and be 

evaluated using the test methods established under Task 5 as well as performing characterization 

tests. The expected result of this task includes the materials and processes for a protection 

system application to a broader range of PV technology, including additional flexible, thin-film 

technologies.

Milestone Description

M-3.16.1 Demonstration of the protection system developed in Task 14 on thinfilm PV on flexible substrate cells in order to verify applicability of system 

to a broader range of thin-film PV technology

Strategy

The plan for this task was to use the protection system for the crystalline application directly on 

flexible substrates. In phase II several suppliers were targeted and were approached for this 

activity. Unfortunately, an agreeable method of testing was not able to be reached. This is mainly 

due to customers that were either not interested in working with a silicone solution, not willing to 

ship samples outside their facility, or not willing to change their design to accommodate a liquid 

silicone solution. Discussion of this issue with NREL has resulted in an option to seek out flexible 

thin film cells from an NREL subcontractor. 

The team pursued contacts at flexible thin film suppliers in an effort to gain access to at least a 

small supply of flexible, thin-film PV for evaluation of the encapsulation system on flexible cells. 

Contact was made at each organization. NDAs were established and a Statement of Work was

submitted to both potential suppliers. One supplier agreed to the work as long as all samples 

were sent back to their facility for testing. 

Task Results

Twenty cells were received from the supplier. The first ten were used for development and 

practice of the application techniques. The second ten were used for the actual test samples. 

Both sets of cells were characterized by IV analysis at the supplier prior to shipment. All of the 
28

samples were encapsulated using the adhesive and encapsulant layers using two configurations. 

The first configuration used a clear fluoropolymer front sheet and an opaque white 

polyvinylfluoride laminate back sheet. The second configuration used a glass front sheet and an 

opaque white polyvinylfluoride laminate back sheet. As of this writing, the encapsulated cells 

were shipped back to the supplier for visual inspection and IV analysis after encapsulation, and 

subsequent environmental aging in Damp Heat, Thermal Cycle and UV/Thermal Cycle/Humidity 

Freeze conditions. The data from these tests will be sent by the supplier at the intervals of 

intermediate testing.

Task 17, Module Fabrication

The Subcontractor shall apply protective coating systems to full-size modules fabricated using 

crystalline Si cells on glass. The Subcontractor shall demonstrate the scaled-up material 

production and application process for the newly developed protection system for use in 

fabricating full-size, crystalline Si modules. Through a lower-tier subcontract, the Subcontractor 

shall ensure that a lower-tier subcontractor fabricate crystalline Si cells for use in the modules. 

The Subcontractor, in collaboration with a lower-tier subcontractor, shall assemble and test the 

complete modules prior to delivery to NREL. The Subcontractor shall characterize the 

performance of the protection system utilizing standard evaluation techniques. The expected 

result of this task is the fabrication of ten, crystalline Si PV modules.

Milestone Description

M-3.17.1 Completion of full-size, single-crystalline Si modules and multicrystalline Si mini-modules utilizing the protection system developed

Strategy

The pilot line was installed and started up in Q1 2008. Optimization and encapsulation trials 

began in Q2 and continue to be run through Q4. 

The pilot line was designed to handle matrices up to 125 mm x 72 cell or 156 mm x 60 cell

modules. It was designed without a stringing operation specifically to allow the testing of the 

encapsulation system with the standard stringing operations presently in use at the suppliers.

This required shipment of strings or matrices prepared on the suppliers stringing and lay up 

operations. Handling, packaging and shipping of the matrices without damage was a hurdle to 

overcome. Specialized packaging was designed and constructed to allow damage free shipment 

and compatibility with the cell placement apparatus operation. Once this packaging was 

successfully developed matrices were shipped from the suppliers for testing on the pilot line.

 

In addition to the matrices used to make the deliverable goal, extra matrices for optimization of 

the handling and alignment operations in the cell placement apparatus were included in the 

planning.

In addition, glass and back sheet materials were also sent from the matrix suppliers. Both 

materials were shipped in standard containers. The back sheet was supplied in roll form long 

enough in length for use on the continuous back sheet placement tool. Glass would be cleaned 

by washing with water and soap and allowed to dry by air. A light wipe with IPA to increase drying 

rates would be employed prior to placing into the pilot line. Back sheet materials were typically 

used as is from the roll.

Finished laminates of acceptable quality would be characterized by IV testing at Dow Corning 

using a Spire 4600 Sun Simulator. These would be sent to the respective suppliers for IV 

characterization and testing.
29

Task Results

Supplier 1

As of this report both 125 mm x 32 cell and 125 mm x 72 cell matrices have been received from 

Supplier 1. 

Pilot Line trials began in Q2 2008 using 32 cell arrays sent from Supplier 1. Most of these 

modules were used to optimize the cell placement handling and alignment accuracy. In addition, 

the optimization of silicone thickness and dispense patterning of the adhesive and encapsulant 

layers was performed to ensure complete bubble free encapsulation. Several modules were 

successfully completed with minimal defects such as bubbles. Because of damage sustained 

during handling and the poor alignment most of the modules were not characterized. Modules of 

sufficient visual quality were sent to NREL as a part of this task deliverable.

Pilot line trials for 72 cell matrices from supplier 1 were encapsulated in Q4 2008. As with the 32 

cell modules matrices were used for practice in processing for alignment and handling. Due to the 

optimization of the handling of the 32 cell modules fewer matrices were damaged. The remaining 

matrices were successfully laminated. These laminates were IV tested at Dow Corning. The data 

showed normal functionality. The laminates were sent to the supplier for characterization and 

testing compared to EVA encapsulated modules. Sun Simulator IV characterization data at the 

supplier confirmed the data on the Dow Corning Sun Simulator. The results will not be reported 

here as it is proprietary information to the supplier.

Supplier 2

Plans for the delivery of 72 matrices from the supplier 2 were made in Q2 2008. Packaging for 72 

cell matrices was prepared and shipped to the supplier in early Q3 2008. The matrices were 

received in late Q3 2008. Encapsulation trials on the pilot line were run in Q3 and Q4 2008. Early 

trials encountered issues with cell handling and alignment. This was primarily due to the transfer 

of modules from the shipping package to the cell placement apparatus, and the alignment of the 

cell strings after transfer. This was overcome by better techniques and attention to realignment of 

the strings prior to placing the matrix into the cell placement apparatus. 

Similar to supplier 1 trials, several matrices were used for practice in processing for matrix 

alignment and handling. Two matrices were then successfully encapsulated utilizing the prototype 

B formulations described in Task 15. A picture of a module is presented in figure 35. The resulting 

laminates were shipped to supplier 2 to evaluate compatibility with framing and junction box 

application. Once this testing was completed the remaining matrices were encapsulated and 

shipped to supplier 2 for characterization and environmental aging testing. In addition, several 

single cell coupons were sent to supplier 2 for accelerated UV aging testing. These will be aged 

to simulate 20 years of UV exposure and tested at the supplier. Due to the extended length of this 

aging the results from this testing will not be available for this report.
30

Figure 35– Silicone encapsulated module prior to shipment.

PHASE III SUMMARY

The work in Phase III was based on the development and optimization of prototype formulations

in Phases I & II using single cell and multi cell strings that were subjected to IEC and UL testing 

standards for simulated environmental aging. In Phase III the prototype formulations were further 

optimized by improving adhesion to substrates. In addition to the formulation optimization, 

application methods were developed and optimized for successful encapsulation using liquidbased materials. The application methods developed included methods to maximize process 

throughput and minimize silicone usage to help reduce the cost of modules on per peak watt 

basis. In addition, evaluation of the percent transmittance of silicone and EVA showed a potential 

benefit for solar cell efficiency. This was supported by the analysis of cell quantum efficiencies 

after encapsulation with EVA and silicone. Furthermore the efficiency improvement was 

successfully measured on cells encapsulated with silicone and EVA. This work will be further 

investigated by comparing full size module power output in side by side outdoor testing.

The optimized prototype B formulations and application method were also applied to rigid thin film 

PV technologies. Extended aging testing in Damp Heat or 85 °C/85 %RH has shown minimal 

power degradation of 30 x 30 cm modules from Supplier 2. Further testing is planned for full size 

modules beyond this contract. When encapsulating rigid thin film modules from supplier 1, 

variable results have been seen when subjected to environmental aging testing. Further 

evaluation of these modules will continue beyond this contract.

The optimized prototype B formulations and application methods have been applied to flexible 

thin film PV cells. These cells are under evaluation at the cell supplier. A decision to pursue this 
31

encapsulation system with the flexible will be made by the supplier depending on the results of 

the evaluation.

As a result of successful encapsulation testing a pilot line to demonstrate the encapsulation 

method on modules up to 72 x 125 mm cells was designed in Phase II and installed and started 

up in Phase III. The pilot line has been used to successfully encapsulate crystalline matrices and 

rigid thin film modules with prototype B formulations. The full sized modules are being evaluated 

against testing standards at the suppliers or internally.

ACKNOWLEDGMENTS

We wish to acknowledge the contributions of the following people for the successful completion of 

this subcontract: 

Dan Friedman, Mike Kempe, Richard Mitchell, and Tom Moriarty

Nick Shephard, Mustafa Mohamed, James Tonge, Steven Altum, Tom Peitz, Aaron Seitz, Steve 

Jenkins, Kevin Houle, David Johnson, Steve Crofoot, Paul Geisthardt, Scott Fleming, Mary Kay 

Tomalia, Craig Liberaki, Ann Norris, and Joe Wardlow,

REFERENCES

1. Moisture Ingress Protection by Combined Backsheet and Encapsulant 

Constructions; K. Terwilliger, M. Kempe, G. Jorgensen, C. Kennedy 

2. Testing of Packaging Materials for Improved PV Module Reliability; G.J. 

Jorgensen, K.M. Terwilliger, M.D. Kempe, and T.J. McMahon; Prepared for the 

31st IEEE Photovoltaics Specialists Conference and Exhibition Lake Buena Vista, 

Florida January 3–7, 2005 February 2005 • NREL/CP-520-37366 

3. Materials Testing for PV Module Encapsulation; G. Jorgensen, K. Terwilliger, S. 

Glick, and T. McMahon; National Center for Photovoltaics, National Renewable 

Energy Laboratory 1617 Cole Blvd., Golden, CO 80401 USA; NCPV and Solar 

Program Review Meeting 2003; NREL/CD-520-33586 Page 191. 

4. Development of New Low-Cost, High-Performance, PV Module 

Encapsulant/Packaging Materials; Annual Technical Report, Phase 1, 22 October 

2002–30 September 2003; S.C. Agro and R.T. Tucker; Specialized Technology 

Resources, Inc., Enfield, Connecticut; March 2004 • NREL/SR-520-35683. 
F1146-E(10/2008) 

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1. REPORT DATE (DD-MM-YYYY)

05/21/2009

2. REPORT TYPE

Subcontract Report

3. DATES COVERED (From - To)

26 May 2005 - 30 November 2008

4. TITLE AND SUBTITLE

High Performance Packaging Solutions for Low Cost, Reliable PV 

Modules: Final Subcontract Report, 26 May 2005 - 30 November 

2008

5a. CONTRACT NUMBER

DE-AC36-08-GO28308

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

B.M. Ketola and B.J. Marinik

5d. PROJECT NUMBER

NREL/SR-520-45903

5e. TASK NUMBER

PVB75301

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Dow Corning Corporation

2200 W. Salzburg Road 

Midland, Michigan 48686

8. PERFORMING ORGANIZATION

REPORT NUMBER

ZAX-5-33628-02

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory

1617 Cole Blvd.

Golden, CO 80401-3393

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL

11. SPONSORING/MONITORING 

AGENCY REPORT NUMBER

NREL/SR-520-45903

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: R.L. Mitchell

14. ABSTRACT (Maximum 200 Words)

The overall objective of the research effort over its three-phase duration is to develop a reduced-cost, higherthroughput, improved-performance packaging solution for United States Photovoltaic (“PV”) module manufacturers. 

The primary target of the work is to develop a packaging solution that will reduce the overall cost contribution of this 

component by a minimum of 25% relative to the conventional Ethyl Vinyl Acetate (“EVA”)/ Poly vinyl fluoride (“PVF”) 

batch lamination and will provide improved reliability for twenty-plus years of performance in the field. During the 

subcontract, Dow Corning shall formulate and optimize adhesive, encapsulant, and barrier materials for use with a 

broad range of current PV technologies. This effort is expected to result in improved performance encapsulation 

materials and the establishment of a pilot-scale, protection-system production line capable of processing full-size, 

single-crystalline Silicon PV modules.

15. SUBJECT TERMS

PV; high performance; packaging solution; low-cost; reliability, modules; manufacturer; crystalline silicon; ethyl vinyl 

acetate; poly vinyl fluoride; 

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OF ABSTRACT

UL

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