5101-280 ,. DOE/JPL-1012-113

Flat-Plate Distribution Category UC-63b

Solar Array Project

Environmental Tests of

Metallization Systems for

Terrestrial Photovoltaic Cells

Paul Alexander^ Jr.

(JPL-Pui)l-85-€6) ENVIEGNflENTA l TESTS OF N86-25045

METALLIZATION SYSTEKS FOE fIBBBSlfilAL

PHOTOVOLTAIC ICELLS. (Jet Propulsion Lab.}

79 p BC A05/MF ACT CSCL 10A Unclas

G3/4U 43381 .

December 31, 1985

Prepared for

U.S. Department of Energy

Through an Agreement with

National Aeronautics and Space Administration

by

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

JPL Publication 85-86
5101-280 DOE/JPL-1012-113

Flat-Plate Distribution Category UC-63b

Solar Array Project

Environmental Tests of

Metallization Systems for

Terrestrial Photovoltaic Cells

Paul Alexander, Jr.

December 31, 1985

Prepared for

U.S. Department of Energy

Through an Agreement with

National Aeronautics and Space Administration

by

Jet Propulsion Laboratory

California Institute of Technology

Pasadena, California

JPL Publication 85-86
Prepared by the Jet Propulsion Laboratory, California Institute of Technology,

for the U.S. Department of Energy through an agreement with the National

Aeronautics and Space Administration.

The JPL Flat-Plate Solar Array Project is sponsored by the U.S. Department of

Energy and is part of the Photovoltaic Energy Systems Program to initiate a

major effort toward the development of cost-competitive solar arrays.

This report was prepared as an account of work sponsored by an agency of the

United States Government. Neither the United States Government nor any

agency thereof, nor any of their employees, makes any warranty, express or

implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process

disclosed, or represents that its use would not infringe privately owned rights.

Reference herein to any specific commercial product, process, or service by trade

name, trademark, manufacturer, or otherwise, does not necessarily constitute or

imply its endorsement, recommendation, or favoring by the United States

Government or any agency thereof. The views and opinions of authors

expressed herein do not necessarily state or reflect those of the United States

Government or any agency thereof.

This publication reports on work done under NASA Task RE-152, Amendment

66, DOE / NASA IAA No. DE-AI01-76ET20356.
ABSTRACT

Seven different solar cell metallization systems were subjected to

temperature cycling tests and humidity tests. Temperature cycling excursions

were -50°C to 150°C per cycle. Humidity conditions were 70°C at 98%

relative humidity. The seven metallization systems were: (1) Ti/Ag,

(2) Ti/Pd/Ag, (3) Ti/Pd/Cu, (4) Ni/Cu, (5) Pd/Ni/Solder, (6) Cr/Pd/Ag, and

(7) Thick Film Ag.

All of the seven metallization systems showed slight to moderate

decreases in cell efficiencies after subjection to 1000 temperature cycles.

Six of the seven metallization systems also evidenced slight increases in cell

efficiencies after moderate numbers of cycles, generally less than 100

cycles. The copper-based systems showed the largest decrease in cell

efficiencies after temperature cycling.

All of the seven, metallization systems showed moderate to large

decreases in cell efficiencies after 123 days of humidity exposure. The

copper-based systems again showed the largest decrease in cell efficiencies

after humidity exposure.

Graphs of the environmental exposures versus cell efficiencies are

presented for each of the metallization systems, as well as environmental

exposures versus fill factors or series resistance.

111
ACKNOWLEDGMENTS

Acknowledgments are extended to ASEC Corp. for fabricating the test

cells, and to Jerry Stebbins, Lee Midling, and Ken Gray of the JPL Process

Research Laboratory who conducted the environmental tests and generated the

cell I-V data. Acknowledgments are also extended to Ron Williams for his help

in editing this document.

.INTENTIONALLY BLANK

V
CONTENTS

I. INTRODUCTION 1-1

II. PROCESSING OF SPECIMEN CELLS 2-1

III. DISCUSSION OF METALLIZATION SYSTEMS 3-1

IV. TEST EQUIPMENT 4-1

V. TEST PROGRAM 5-1

VI. TEST RESULTS 6-1

A. I-V CHARACTERISTICS AFTER ENVIRONMENTAL EXPOSURE 6-1

B. SECONDARY ION MASS SPECTROMETRY EVALUATION 6-27

C. I-V CURVES OF TEMPERATURE CYCLING AND HUMIDITY

EXPOSURE TESTS 6-29

VII. DISCUSSION OF RESULTS 7-1

A. OVERVIEW 7-1

B. TEMPERATURE CYCLING TESTS 7-1

C. DISCUSSION OF HUMIDITY TESTS 7-3

VIII. REFERENCES 8-1

IX. SELECTED BIBLIOGRAPHY 9-1

APPENDIX A-l

Figures

4-1. Temperature and Humidity Chamber 4-2

4-2. Temperature Cycling Chamber 4-3

4-3. I-V Test Equipment, Light Source 4-4

4-4. I-V Test Equipment, Data Printout 4-5

6-1. Control Cells (No Environmental Exposures) for

Ti/Ag, Ti/Pd/Ag, Ti/Pd/Cu, Ni/Cu, Pd/Ni/Solder,

Cr/Pd/Ag and Thick Film Ag Paste 6-3

6-2. Control Cells, After Temperature Cycling, and

After Humidity Exposure, Ti/Ag (a, b and c) 6-4

VII

PAQE BLANK NOT
6-3. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Ti/Pd/Ag (a, b and c) 6-6

6-4. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Ti/Pd/Cu (a, b and c) 6-8

6-5. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Ni/Cu (a, b and c) 6-10

6-6. Control Cells, 16x, After Temperature Cycling, and

After Humidity Exposure, Pd/Ni/Solder (a, b and c) ... . 6-12

6-7. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Cr/Pd/Ag (a, b and c) 6-14

6-8. Control Cells, 40x, After Temperature Cycling,

and After Humidity Exposure, Thick Film Ag Paste

(a, b and c) 6-16

6-9. Temperature Cycling: Efficiency Versus Number of

Cycles for Seven Metallization Systems 6-18

6-10. Temperature Cycling: Percent Change in Efficiency

Versus Number of Cycles for Seven Metallization

Systems 6-18

6-11. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ti/Ag 6-19

6-12. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ti/Pd/Ag 6-19

6-13. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ti/Pd/Cu 6-20

6-14. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ni/Cu 6-20

6-15. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Pd/Ni/Solder 6-21

6-16. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Cr/Pd/Ag 6-21

6-17. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Thick Film Ag Paste 6-22

6-18. Humidity Tests: Efficiency Versus Number of Days

Exposure For All Seven Metallization Systems 6-22

6-19. Humidity Tests: Percent Change in Efficiency Versus

Number of Days Exposure For All Seven Metallization

Systems 6-23

Vlll
6-20. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ti/Ag 6-23

6-21. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ti/Pd/Ag 6-24

6-22. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ti/Pd/Cu 6-24

6-23. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ni/Cu 6-25

6-24. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Pd/Ni/Solder 6-25

6-25. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Cr/Pd/Ag 6-26

6-26. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Thick Film Ag Paste ... . 6-26

6-27. Sample of the SIMS Profile Data 6-28

6-28. I-V Curves of Temperature Cycling Test, Selected

Sample, Ti/Pd/Cu 6-31

6-29. I-V Curves of Temperature Cycling Test, Selected

Sample, Ti/Pd/Ag 6-33

6-30. I-V Curves of Humidity Exposure Test, Selected

Sample, Ti/Pd/Cu 6-35

6-31. I-V Curves of Humidity Exposure Test, Selected

Sample, Ti/Pd/Ag 6-37

Tables

2-1. Metallization Processing for Each Metallization

System 2-2

3-1. Block IV Metallization Systems 3-2

5-1. Temperature Cycling Tests 5-1

5-2. Humidity Exposure Tests 5-1

6-1. Summary of the Results for Temperature and

Humidity Exposure Tests 6-2

6-2. Data for I-V Curves of Temperature Cycling Test,

Selected Sample, Ti/Pd/Cu 6-30

IX
6-3. Data for I-V Curves of Temperature Cycling Test,

Selected Sample, Ti/Pd/Ag 6-32

6-4. Data for I-V Curves of Humidity Exposure Test,

Selected Sample, Ti/Pd/Cu 6-34

6-5. Data for I-V Curves of Humidity Exposure Test,

Selected Sample, Ti/Pd/Ag 6-36
SECTION I

INTRODUCTION

The Flat-Plate Solar Array (FSA) Project was formed at the Jet

Propulsion Laboratory (JPL) in 1975 under the sponsorship of the U.S.

Department of Energy (DOE). The objective of the project was to reduce the

cost of making solar cells to the point at which photovoltaic power would be

cost-competitive with electrical power generated by fossil fuels. The cost

goal for making terrestrial solar cells was established at $0.70/watt (1980

dollars), a drastic reduction from over $100/watt for space cell fabrication.

This very severe cost reduction goal generated a re-thinking of the entire

fabrication process, starting with the manufacture of solar grade silicon

material, through silicon sheet formation, through cell processing, and

through solar module fabrication. The terrestrial solar cell evolved from

this re-thinking and is a slightly different species than the space solar

cell. The silicon material, the silicon sheet, the solar cell processing and

module fabrication for terrestrial application evolve from different

fabrication techniques than space cell fabrication.

In the very important area of contact metallization of terrestrial solar

cells, cheaper metals and new ways for applying them were pursued. The

expensive vacuum evaporated titanium-palladium-silver metallization system,

long (and still) the dominant, almost exclusive metallization system in space

solar cells because of its very excellent performance properties, has given

way in terrestrial applications to solder-based metallization systems, thick

film metallization systems and copper based metallization systems, mostly

because of their cheaper costs. Metal deposition by vacuum evaporation has

given way to solder dipping, thick film printing and plating. Again, this is

because of cheaper costs.

The literature on testing of 'solar cell metallization systems deals

almost entirely with titanium-silver and titanium-palladium-silver

metallization systems. This is because the early space cells, going back to

the early 1960s, received considerable attention and funding to evaluate these

two systems. The Ti/Ag system gave way to the Ti/Pd/Ag when it was determined

that the addition of palladium helped the system against moisture ingression

and improved cell performance degradation under humidity conditions. The

Ti/Pd/Ag metallization system is, and has been for over 20 years, the dominant

metallization system for space solar cells, and is also the "standard of

comparison" in terrestrial cell work.

In this work seven metallization systems were selected and tested for

terrestrial application: (1) Ti/Ag, (2) Ti/Pd/Ag, (3) Ti/Pd/Cu, (4) Ni/Cu,

(5) Pd/Ni/Solder, (6) Cr/Pd/Ag, and (7) a Thick Film Ag paste. The tests

conducted were very straightforward, similar to those found in the literature

on testing. It was the intent of this program to evaluate these different

metallization systems (as contrasted with qualifying such systems) and to

assess the relative sensitivities of these metallization systems to the two

most commonly tested environments, temperature cycling and humidity exposure.

It is hoped that the data collected and described in this test program will

add to the body of literature on environmental testing of solar cell

metallization systems.

1-1
SECTION II

PROCESSING OF SPECIMEN CELLS

All cells used in this test program were from the same lot and processed

as one lot through front and back junction formation. The starting material

was 3 in. diameter, chemically polished, 12 mils thick, 2 ohm-cm, p-type,

Czochralski (Cz) grown silicon with (100) orientation. The wafers were masked

with Si02 using a low temperature, chemical vapor deposition process. The

back P+ layer was first formed by boron nitride diffusion followed by the

front N layer (0.3 urn deep) which was diffused using a PGCi3 source. The

sheet resistance of the N layer surface was approximately 30 ohms/square. The

silicon oxide layer, formed during diffusion, was removed by an HF dip. This

procedure eliminated process variations in the front and back junctions since

the junctions were all formed together for all seven metallization groups.

After the front and back junctions were formed, all of the 3 in.

diameter cells were cut into 2 x 2 cm size blanks and equally divided into

seven different metallization groups. Again, to eliminate as many process

variables as possible, six of the seven metallization groups were metallized

by vacuum evaporation through a shadow mask. The seventh group was metallized

by screen printing. After completion of each metallization process, all of

the cells were coated with Multilayer Antireflective (MLAR) coating and tested

at Air Mass 1 (AMI), 28°C.

Table 2-1 presents the metallization process information on the seven

metallization groups.

2-1
Table 2-1. Metallization Processing for Each Metallization System

Metallization System Process

Ti/Ag

Ti/Pd/Ag

Ti/Pd/Cu

Ni/Cu

Pd/Ni/Solder

Cr/Pd/Ag

Thick Film Ag

Evaporate the following: Ti (1000 A) and Ag

(3 urn), on both front and back sides, followed by

400°C, 10 rain sintering in N2

Evaporate the following: Ti (1000 A), Pd (500 A)

and Ag (3 vim), on both front and back sides,

followed by 400°C, 10 min sintering in N2

Evaporate the following: Ti (1000 A), Pd (500 A)

and Cu (2 jam), on both front and back sides,

followed by 325°C, 10 min sintering in forming gas

(10% H2 and 90% N2)

Evaporate the following: Ni (5000 A) and

Cu (2 um), on both front and back sides, followed

by 325°C, 10 min sintering in forming gas (10% H2

and 90% N2)

Evaporate the following: Pd (500 A) and Ni

(5,000 A), on both front and back side, followed by

325°C, 10 min sintering in forming gas (10% H2

and 90% N2), solder dip

Evaporate the following: Cr (500 A), Pd (500 A)

and Ag (3 um), on both front and back side,

followed by 400°C, 10 min sintering in N2

Screen print back side with silver-aluminum

conductor (Thick Film Inc., No. 3398 Ink) and fired

at 750°C for 1 min in N2, followed by printing

silver conductor (Thick Film Inc., No. 3347 Ink) on

front side and sintering at 650°C for 1 min in N2

2-2
SECTION III

DISCUSSION OF METALLIZATION SYSTEMS

Selection of the metallization systems for this study was determined,

for the most part, by consideration of commonly used systems for terrestrial

cells. Titanium-palladium-silver is the most familiar metallization system,

having been in use for over 20 years, and always used in space cells.

Titanium-palladium-silver is also extensively used on terrestrial cells,

especially in the earlier designs. Many of the environmental test results

found in the cell testing literature is on titanium-palladium-silver.

Titanium-silver was a precursor metallization system to titanium-palladiumsilver and is also found in the literature on metallization testing.

Palladium was added to the titanium-silver system when it was determined that

this addition was beneficial against moisture penetration, especially in

humidity testing. The test results, herein, verify this fact. Most of the

earlier terrestrial solar cells (1978 to 1982), which were purchased under the

FSA-JPL Block buy programs (Block I, Block II, Block III, and Block IV), used

solder overlay as the main conducting metal. The metallization systems using

solder were typically palladium-nickel-solder, gold-nickel-solder, or nickelsolder. The underlying metals were typically plated by palladium, gold,>or

nickel solutions and then solder dipped. A palladium-nickel-solder system was

tested in this program. The palladium and nickel under-layers were vacuum

evaporated rather than plated. In fact, all of the metals used in this

program were vacuum evaporated excepting the solder dip operations and the

thick film silver metallization system.

Solder based metallization systems have been slowly giving way to copper

based systems within the last 3 to 4 years in terrestrial metallization

systems. Copper has a much higher electrical conductivity than solder, and on

a conductivity per pound basis, is much cheaper than solder. Also copper can

be ultrasonically bonded to form a superior cell-to-cell interconnection bond

compared to soldered interconnections. Westinghouse, for example, uses a

copper based metallization system fabricated by first vacuum evaporating

titanium and palladium, and then followed by copper plate-up. The cells are

interconnected by ultrasonic bonding. Another terrestrial metallization

design is plated nickel followed by copper plate-up. Two copper based

metallization systems were tested in this program. One system was nickelcopper (both metals were vacuum evaporated); and the other, titaniumpalladium-copper (again, all three metals were vacuum evaporated).

One thick film system, a commercial silver paste, was tested in this

program. Several solar cell manufacturers (Arco Solar, for example) make

cells with thick film silver paste metallization systems and they apply the

paste by screen printing, the same as done in this test program.

A chromium-palladium-silver system was tested in this program. This

metallization system, to this author's knowledge, is not presently being used

on any solar cells being commercially manufactured. However, this system has

been used for various lab samples for years. The results show that the

chromium-palladium-silver system performs as well or better than the

titanium-palladium-silver system after humidity tests.

3-1
Table 3-1 presents data on metallization systems used on the FSA

Block IV Solar Module procurement (Reference 1).

The Block IV module, built in the 1980 to 1982 time frame, three

generations removed from Block I, II, and III, still shows the use of the

Ti/Pd/Ag metallization system, as.shown in Table 3-1. Most of the contractors

were working on more cost effective metallization systems. However, when cell

performance was on the line, there was a tendency to fall back on a proven

metallization system such as Ti/Pd/Ag, even though more cost-effective systems

were being worked on. /

Table 3-1. Block IV Metallization Systems

Manufacturer ,

Metallization System

Front Metallization Back Metallization

Arco Solar

Applied Solar Energy.

Corporation (ASEC)

Printed Ag Paste

Vacuum: Evaporated

Ti/Pd/Ag

General Electric (GE) . Printed Ag Paste

Motorola

Photowatt

Solarex

Spire

Plated Pd/Ni/Solder Dip

Plated Ni/Solder Dip

Vacuum Evaporated

Ti/Pd/Ag

Vacuum Evaporated

Ti/Pd/Ag

Printed Al/Printed Ag

Paste

/

Vacuum Evaporated

Ti/Pd/Ag

Printed Al/Printed Ag

Paste

Plated Pd/Ni/Solder Dip

Plated Ni/Solder Dip

Vacuum Evaporated

. Ti/Pd/Ag

Vacuum Evaporated

Ti/Pd/Ag

3-2
SECTION IV

TEST EQUIPMENT

Figure 4-1 is a picture of the humidity chamber used for humidity

exposure in this test program. The chamber was manufactured by Blue M

Electric Company, Blue Island, Illinois, and is Model No. AC-7502HA-TDA-1(Y).

The chamber was developed for long term high temperature/humidity testing that

is capable of meeting steady-state requirements such as found in Mil-Std-202C,

Method 106B, "continuous operation for 56 days, with low water consumption

(less than 1 gallon/24 hours)." Temperature range is 12°C above ambient to

+93°C (*1/2°C). The relative humidity range is from 40% to 98% saturation.

Figure 4-2 is a picture of the temperature cycling chamber used for

temperature cycling in this test program. The chamber was manufactured by

Blue M Electric Company, Blue Island, Illinois, and is Model No. LN-270B-1MP,

temperature range -200°C to +300°C. The chamber has a 3kW heater for

above ambient heating and is fitted for liquid nitrogen for below ambient

cooling. The unit is microprocessor controlled and can be programmed to run

various temperature cycling programs.

Figures 4-3 and 4-4 are pictures of the current/voltage (I-V) test

equipment. In Figure 4-3, the light source (Solar Simulator, Model No. XT-10)

was manufactured by Spectrolab, Sylmar, California. The cell holder equipment

beneath the light source is water cooled and set to maintain the cell sample

at 28°C. The I-V Plotter and computer equipment are shown in Figure 4-4.

The I-V Plotter was manufactured by Tektronix Inc., Model No. 4662. The

computer equipment was manufactured by Tektronix Inc., Model No. 4052.

4-1
ORIGINAL PAGE IS

OF POOR QUALITY

Figure 4-1. Temperature and Humidity Chamber

4-2
ORIGINAL PAGE IS

OF POOR QUALITY

Figure 4-2. Temperature Cycling Chamber

4-3
Figure 4-3. I-V Test Equipment, Light Source

4-4
ORIGINAL PAGE IS

OF POOR QUALITY

e

•H

(Xi

fl

id

Q

a

e

a

•H

3

cr

en

0)

H

i

•a0)

>j

3

•H

u.

4-5
SECTION V

TEST PROGRAM

Approximately 129 cells were used in the test program. Seven sets of

six cells each (43 total) of seven different metallization systems were

subjected to temperature cycling tests. Similarly, seven sets of six cells

each (43 total) of the seven different metallization systems were subjected to

humidity tests. Seven sets of six cells each (43 total) of the seven

different metallization systems were used as controls. Tables 5-1 and 5-2

outline the temperature cycling and humidity tests.

Table 5-1. Temperature Cycling Testsa»b

No. of Cells,

No. of

Cell Types

6 cells ea

of 7 metal

types, 43

cells total

I-V

Test

43

cells

total

No. of

Temp

Cycles

10

cycles

I-V

Test

43

cells

total

No. of

Temp

Cycles

30

cycles

(40

cycles

total)

I-V

Test

43

cells

total

No. of

Temp

Cycles

100

cycles

(140

eye les

total)

I-V

Test

43

cells

total

No. of

Temp

Cycles

300

cycles

(440

cycles

total)

I-V

Test

43

cells

total

No. of

Temp

Cycles

560

cycles

(1,000

cycles

total)

I-V

Test

43

cells

total

6 ea x

7 sets

aTemperature Excursions were:

From -65°C for approximately 6 min dwell at -65°C to +150°C for approximately 6 rain dwell at 150°C.

Approximately 14 min ramp time between temperatures. Total cycle time was approximately 40 min/cycle.

bCell Metal Types: Bl - Ti/Ag B3 - Ti/Pd/Cu

B2 - Ti/Pd/Ag B4 - Ni/Cu

B5 - Pd/Ni/Solder

B6 - Cr/Pd/Ag

B7 - Thick Film Ag Paste

Table 5-2. Humidity Exposure Testsa»

b

No. of Cells,

No. of

Cell Types

6 cells ea

of 7 metal

type

(43 cells

total)

I-V

Test

43

cells

total

Humidity

(No. Days

Exposure )

3 days

I-V

Test

43

cells

total

Humidity

(No. Days

Exposure)

10 days

(13 days

total)

I-V

Test

43

cells

total

Humidity

(No. Days

Exposure)

20 days

(33 days

total)

I-V

Test

43

cells

total

Humidity

(No. Days

Exposure)

30 days

(63 days

total)

I-V

Test

43

cells

total

Humidity

(No. Days

Exposure)

60 days

(123 days

total)

I-V

Test

43

cells

total

6 ea x

7 sets

aHumidity conditions were 60°C at 100% saturation for the 3-days and 10 days exposures. Conditions were

70°C at 98% relative humidity for the 20, 30 and 60-day exposures. Differences in humidity conditions were

due to testing piggyback with other items which had priority on humidity conditions.

bCell Metal Types: Bl - Ti/Ag B3 - Ti/Pd/Cu

B2 - Ti/Pd/Ag B4 - Ni/Cu

B5 - Pd/Ni/Solder

B6 - Cr/Pd/Ag

B7 - Thick Film Ag Paste

5-1
SECTION VI

TEST RESULTS

A. I-V CHARACTERISTICS AFTER ENVIRONMENTAL EXPOSURE

Solar cells from each of the seven metallization systems were tested for

I-V characteristics after each environmental exposure (see the description of

the Test Program). Nine parameters were measured on each I-V test which

included: short circuit current, mA (Isc); open circuit voltage, mV (Voc);

maximum power, mW (Pmp); current at maximum power, mA (Imp); voltage at

maximum power, mV (Vmp); cell efficiency (n); fill factor (FF); cell series

resistance, ohms (Rs); and cell shunt resistance, ohms (Rsh)-* A very

condensed summary of results is shown in Table 6-1. Extensive light I-V test

data are tabulated and presented in the Appendix.

I-V data for each of the seven metallization cell types were generated. The

data include: efficiency and fill factor versus temperature cycling; and

efficiency and series resistance versus number of days of humidity exposure.

The I-V curves are presented in the figures herein.

Pictures at 40 times magnification were taken of selected cell specimens

before and after testing. The pictures are presented in the figures herein.

*The shunt resistance values in this work are to be taken as a range or trend

as opposed to the absolute values. This is because the algorithms used in

the computer program to calculate shunt resistance values, although measured

in accordance with one of the ASTM standards, tends to generate numbers that

are overly responsive to normal test variations.

6-1
Table 6-1. Summary of the Results for Temperature

and Humidity Exposure Tests

Metal

System

(2x2 cm

Cells)

Ti/Ag

Ti/Pd/Ag

Pd/Ni/Cu

Ni/Cu

Pd/Ni/

Solder

Cr/Pd/Ag

Thick

Film Ag

% Change

in Cell

Efficiency

After 1,000

Temperature

Cycles^- » ^

-4.19

-3.69

-20.7

-17.10

-6.99

-4.6

-14.32

Ranking

(Least

Degradation)

2

1

7

6

4

3

5

% Change in

Cell

Efficiency

After 123

Days Humidity

Exposure^ »•*

-30.15

-11.35

-54.43

-71.14

-12.97

-9.33

-44.40

Ranking

(Least

Degradation)

4

2

6

7

3

1

5

1Temperature excursions were: from -65°C for approximately 6 min

dwell at -65°C to -)-1500C for approximately 6-min dwell at 150°C.

Approximately 14 min ramp time between temperatures. Total cycle time

was approximately 40 rain/cycle.

^Humidity conditions were: 60°C at 100% saturation for first 13 days

and 70°C at 98% relative humidity for 14 through 123 days.

3I-V test conditions for all tests were: AMI at 28°C.

6-2
ORIGINAL PAGE IS

OF POOR QUALITY

(a) TilAg (b) Ti/PdIAg

(c) Ti/Pd/Cu (d) Ni.'Cu (e) Pd/Ni/SOLDER

(f) Cr/Pd/Ag (g) THICK FILM Ag PASTE

Figure 6-1. Control Cells (No Environmental Exposures) for

Ti/Ag, Ti/Pd/Ag, Ti/Pd/Cu, Ni/Cu, Pd/Ni/Solder,

Cr/Pd/Ag, and Thick Film Ag Paste

6-3
Figure 6-2 shows a grid line on each of three cells of the Ti/Ag

metallization system as follows:

(a) Cell 19, control cell, 40x magnification.

(b) Cell 34, after 1000 temperature cycles (-65°C to 150°C),

40x magnification.

(c) Cell 18, after 123 days humidity exposure (70°C at 98%

relative humidity), 16x magnification.

(a) Control Cell

Figure 6-2. Control Cells, After Temperature Cycling, and

After Humidity Exposure, Ti/Ag (a, b and c)

6-4
ORIGINAL PAGE ft

OF POOR QUALfTY

(b) After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-2. (Cont'd)

6-5
Figure 6-3 shows a grid line on each of the three cells of the Ti/Pd/Ag

metallization system as follows:

(a) Cell 42, control cell, 40x magnification.

(b) Cell 37, after 1000 temperature cycles (-65°C to 150°C),

AOx magnification.

(c) Cell 19, after 123 days humidity exposure (70°C at 98%

relative humidity), 40x magnification.

(a) Control Cell

Figure 6-3. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Ti/Pd/Ag (a, b and c)

6-6
ORIGINAL PAGE IS

OF POOR QUALITY

(b) After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-3. (Cont'd)

6-7
Figure 6-4 shows a grid line on each of the three cells of the Ti/Pd/Cu

metallization system as follows:

(a) Cell 25, control cell, 40x magnification.

(b) Cell 45, after 1000 temperature cycles (-65°C to 150°C),

40x magnification.

(c) Cell 28, after 123 days humidity exposure (70°C to 98%

relative humidity), 40x magnification.

(a) Control Cell

Figure 6-4. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Ti/Pd/Cu (a, b and c)

6-8
ORIGINAL PAGE f3

OF POOR QUALITY

(b) After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-4. (Cont'd)

6-9
Figure 6-5 shows a grid line on each of the three cells of the Ni/Cu

metallization system as follows:

(a) Cell 30, control cell, 40x magnification.

(b) Cell 30, after 1000 temperature cycles (-65°C to 150°C),

40x magnification.

(c) Cell 37, after 123 days humidity exposure (70°C at 98%

relative humidity), 40x magnification.

(a) Control Cell

Figure 6-5. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Ni/Cu (a, b and c)

6-10
ORIGINAL PAGE IS

OF POOR QUALITY

(b) After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-5. (Cont'd)

6-11
••

Figure 6-6 shows a grid line on each of the three cells of the

Pd/Ni/Solder metallization system as follows:

(a) Cell 22, control cell, 16x magnification.

(b) Cell 30, after 1000 temperature cycles (-65°C to 150°C),

16x magnification.

(c) Cell 24, after 123 days humidity exposure (70°C at 98%

relative humidity), 16x magnification.

(a) Control Cell

Figure 6-6. Control Cells, 16x, After Temperature Cycling, and

After Humidity Exposure, Pd/Ni/Solder (a, b and c)

6-12
ORIGINAL PAGE IS

OF POOR QUALITY

• '

(b) After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-6. (Cont'd)

6-13
Figure 6-7 shows a grid line on each of the three cells of the Cr/Pd/Ag

metallization system as follows:

(a) Cell 26, control cell, 40x magnification.

(b) Cell 21, after 1000 temperature cycles (-65°C to 150°C),

40x magnification.

(c) Cell 29, after 123 days humidity exposure (70°C at 98%

relative humidity), 40x magnification.

(a) Control Cell

Figure 6-7. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Cr/Pd/Ag (a, b and c)

6-14
ORIGINAL PAGE IS

OF POOR QUALITY

After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-7. (Cont'd)

6-15
Figure 6-8 shows a grid Line on each of the three cells of the Thick

Film Ag Paste metallization system as follows:

(a) Cell 24, control cell, 40x magnification.

(b) Cell 44, after 1000 temperature cycles (-65°C to 150°C),

40x magnification.

(c) Cell 61, after 123 days humidity exposure (70°C at 98%

relative humidity), 40x magnification.

(a) Control Cell

Figure 6-8. Control Cells, 40x, After Temperature Cycling, and

After Humidity Exposure, Thick Film Ag Paste

(a, b and c)

6-16
(b) After 1000 Temperature Cycles

(c) After 123 Days Humidity Exposure

Figure 6-8. (Cont'd)

6-17
12h

11

10

9

8

i

6

A Ti/Ag METAL

A Ti/Pd/Ag METAL

D Ti/Pd/Cu METAL

• Ni/Cu METAL

O Pd/Ni SOLDER

® Cr/Pd/Ag METAL

§ THICK FILM Ag PASTE

I I I I I I

100 200 300 400 500 600

NUMBER OF CYCLES

700 800 900 1000

Figure 6-9. Temperature Cycling: Efficiency Versus Number

of Cycles for Seven Metallization Systems

A Ti/Ag METAL

A Ti/Pd/Ag METAL

D Ti/Pd/Cu METAL

• Ni/Cu METAL

O Pd/Ni SOLDER

® Cr/Pd/Ag METAL

0 THICK FILM Ag PASTE

0 100 200 300 400 500 600 700 800 900 1000

NUMBER OF CYCLES

Figure 6-10. Temperature Cycling: Percent Change in Efficiency

Versus Number of Cycles for Seven Metallization

Systems

6-18
Ib

15

14,

13

^

>- [

z

L1J

H 12

u_

LU

0

 11

10

9

8

— "^----^ -

ZSA £f"'~ ~~~ ~~~ — — ^

~~ — t

D • , C

—

_ A EFFICIENCY %

D FILL FACTOR

1 1 1 1 1 I 1 1 1

U.Bb

0.8

0.75

0.7

0.65

0.6

0.55

0 100 200 300 400 500 600 700 800 900 1000

NUMBER OF CYCLES

Figure 6-11. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ti/Ag

15

H'

13

12

O

UJ

y 10 u_

g 9

8

7

6

5

p^ A _A-

- - - A

—

LJIK^^^ , • C

- —

-

_

—

— A EFFICIENCY %

~ D FILL FACTOR

1 1 1 1 1 1 1 1 1

U.OJ

0.8

0.75

0.7

0.65

0.6

0.55

0 100 200' 300 400 500 600 700 800 900 1000

NUMBER OF CYCLES

Figure 6-12. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ti/Pd/Ag

6-19
S 10

I

A EFFICIENCY %

D FILL FACTOR

I I I

100 200 300 400 500 600

NUMBER OF CYCLES

700

I I

0.8

0.75

0.7

0.65

0.6

0.55

800 900 1000

Figure 6-13. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ti/Pd/Cu

15

14

13

12

11

10

9

8

7

6

5

*

A EFFICIENCY %

D FILL FACTOR

I I I I I j I

0.8

0.75

0.7

0.65

0.6

0.55

100 200 300 400 500 600 700 800 900 1000

NUMBER OF CYCLES

Figure 6-14. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Ni/Cu

6-20
12

11 A

10 -

A EFFICIENCY %

D FILL FACTOR

I I I I I I I I

100 200 300

0.7

0.65

0.6 £

0.55

0.5

400 500 600 700 800 900 1000

NUMBER OF CYCLES

Figure 6-15. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Pd/Ni/Solder

ID

14

L

13

12

11

10

9

8

7

6

5

_ """""""""•'

-

-

-

_

-

A EFFICIENCY %

0 FILL FACTOR

I .I 1 1 1 1 1 1 1

] 100 200 300 400 500 600 700 800 900 10

U.B

0.75

i

J O£

h—CJ

0.7 2

1

0.65

0.6

00

NUMBER OF CYCLES

Figure 6-16. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Cr/Pd/Ag

6-21
7.5

^ 6.5

>-

CJ

zu

E

 6

LU

—LLIJ

" 5.5

4.5

A EFFICIENCY %

D FILL FACTOR

I I I I I

0.5

0.45

0.35

100 200 300 400 500 600

NUMBER OF CYCLES

700 800 900 1000

Figure 6-17. Temperature Cycling: Efficiency and Fill Factor

Versus Number of Cycles, Thick Film Ag Paste

A TilAg METAL

A TilPdIAg METAL

D Ti/PdICu METAL

• IWCu METAL

O Pd/Ni SOLDER

® CrIPdIAg METAL

0 THK FILM Ag PASTE

I I I I I I

10 20 30 40 50 60 70 80

NUMBER OF DAYS EXPOSURE

90 100 110 125

Figure 6-18. Humidity Tests: Efficiency Versus Number of Days

Exposure For All Seven Metallization Systems

6-22
A Ti/Ag METAL

A TilPdIAg METAL

D Ti/Pd/Cu METAL

• Ni/Cu METAL

O Pd/Ni SOLDER

® Cr/Pd/Ag METAL

0 THK FILM Ag PASTE

10 20 30 40 50 60 70 80

NUMBER OF DAYS EXPOSURE

Figure 6-19. Humidity Tests: Percent Change in Efficiency Versus

Number of Days Exposure For All Seven Metallization

Systems

15

14

i

13

12

11

10

9

8

7

6

5

4

3,

2

1

A-

—-A

A EFFICIENCY (PERCENTAGEI

D SERIES RESISTANCE (OHMS)

I | I I

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

10 20 30 40 50 60 70 80 90 100 110 125

NUMBER OF DAYS EXPOSURE

Figure 6-20. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ti/Ag

6-23
15

13

12

11

10

g

8

7

6

5

4

3

2

1

0

A EFFICIENCY (PERCENTAGE)

D SERIES RESISTANCE IOHMS)

I I I

10 20 30 40 50 60

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

0.45

0.4

0.35

0.3

70 80 90 100 110 125

NUMBER OF DAYS EXPOSURE

Figure 6-21. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ti/Pd/Ag

A EFFICIENCY (PERCENTAGE)

D SERIES RESISTANCE (OHMS)

0.3

125

NUMBER OF DAYS EXPOSURE

Figure 6-22. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ti/Pd/Cu

6-24
A EFFICIENCY (PERCENTAGE)

D SERIES RESISTANCE (OHMS)

40 50 60 70 80

NUMBER OF DAYS EXPOSURE

1)0 125

Figure 6-23. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Ni/Cu

15

14

13

12

11

10

t 9 CJ

£ 8

CJ

i. 7

UJ '

L1J c

CJ D

5

4

3

2

1

A EFFICIENCY (PERCENTAGE)

D SERIES RESISTANCE (OHMS)

I I I

10 20 30 40 50 60 70 80

NUMBER OF DAYS EXPOSURE

90 100 110

2.1

1.8

1.5

1.2

0.9

0.6

0.3

125

Figure 6-24. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Pd/Ni/Solder

6-25
15

14,

13

12

11

10

9

8

7

6

5

4

3

2

CK>

-0-

A EFFICIENCY (PERCENTAGE)

O SERIES RESISTANCE (OHMS)

I I I

10 20 30 40 50 60 70 80

NUMBER OF DAYS EXPOSURE

90 100 110

-a1.2

1.1

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

125

Figure 6-25. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Cr/Pd/Ag

A EFFICIENCY (PERCENTAGE)

D SERIES RESISTANCE (OHMS)

10 20 30 40 50 60 70 80 90 100 110 125

NUMBER OF DAYS EXPOSURE

Figure 6-26. Humidity Tests: Efficiency and Series Resistance

Versus Number of Days Exposure, Thick Film Ag Paste

6-26
B. SECONDARY ION MASS SPECTROMETRY EVALUATION

Three samples, one a control, one after 1000 temperature cycles, and one

after 123 days humidity exposure, were taken from three metallization systems

which were Ti/Ag, Ti/Pd/Ag, and Ti/Pd/Cu for a total of nine samples for

oxygen content evaluation. The thinking was that if the moisture (water) from

the humidity tests were absorbed into the metallization system, and if they

reacted to form metal oxides, then an increase in oxygen content of the

metallization system should be noted after exposure to humidity tests. A

Secondary Ion Mass Spectrometer (SIMS) was used to determine oxygen content.

The SIMS operates by boring small holes, 3 to 500 jam in diameter, down to a

depth of 1 mil or less with a depth resolution of 100 to 200 A. The holes are

bored by a beam of high-energy ions (5 to 20 keV) which erodes the material

away, a small fraction of the eroded material being in the ionic form. The

ionized material is then accelerated and passed through a mass spectrometer

where it is analyzed for element identification and count.

Analysis of the SIMS depth profiles showed significant counts of H and 0

in the AR coatings on all three cell types, in all the environments, and in

roughly the same concentrations. The H and 0 counts were assumed to be

water. Similarly, no evidence of elevated oxygen concentration in the

metallic layers were found after either temperature cycling or humidity

exposures. The oxygen count was roughly the same for the control cells, the

teuiperature-cycled cells, and the humidity-exposed cells. Figure 6-27 is a

sample of the SIMS profile data that were generated.

The SIMS instrument cannot discern between bound oxygen (as in the case

of metal oxides) or unbound oxygen (as in the case of trapped oxygen or oxygen

agglomerates). It was surmised, however, that the unbound oxygen count in the

metal layers were orders of magnitude larger and more variable than any bound

oxygen, and swamped (or masked) any subtle changes in oxygen count due to

metal oxide formation. It was concluded that the SIMS was not the proper

instrument to determine metal oxide formation due to environmental exposures.

No more measurements of the oxygen content of the metallization systems were

pursued, because this type of effort was not a major thrust of the program.

6-27
CO

CM

•— __ oo

CD .

CC • • CO

' s

i °= CM

T-* o TCO

cj c

52o m S| ilggl 92 cj o ^ 3s cc

00

CM

a. eg g;

a i— o- i

i-s <si5g§

!E5 i^^ctS^ . i— a a LU ^ t- a

; co cj —i h— S z —"

! <t iJ uj Hj < o ty

: cc oo u. CD co cj LL.

Illll l I mill

o

ro

O

O

CM

o ~

ID CJ

CO

4J

cd

o

o

1-1

Cu

03

2

Cu

CO

CO

CM

I

60

•H

6-28
C. I-V CURVES OF TEMPERATURE CYCLING AND HUMIDITY EXPOSURE TESTS

Tables 6-2 through 6-5 show the temperature cycling and humidity

exposure test data that correspond to each of the three curves in each of the

following figures. For example, the data in Table 6-2, under "A/Before

Testing," correspond to Curve "A" in Figure 6-28; "B/After 440 Cycles," to

Curve "B"; "C/After 1000 Cycles," to Curve "C."

Figures 6-28 through 6-31 show typical I-V curves of Ti/Pd/Ag and

Ti/Pd/Cu metallization systems after subjection to temperature cycling tests

and humidity exposure tests. As the I-V curves demonstrate, the Ti/Pd/Ag

system held up well under the temperature cycling and humidity tests; the

Ti/Pd/Cu did not perform well on either test.

6-29
Table 6-2. Data for I-V Curves of Temperature Cycling Test,

Selected Sample, Ti/Pd/Cu

Ti/Pd/Cu

Cell

ID

Isc

P

Imp

Efficiency

Cell Area

Fill Factor

R

s

*sh

Before

Testing

B3-32

127.9 mA

585.0 mV

46.6 mW

110.8 mA

420.4 mV

11.6%

4 sq cm

0.622

0.551 ohms

486.9 ohms

After

440 Cycles

B3-32

128.1 mA

585.7 mV

47.1 mW

106.8 mA

441.4 mV

11.8%

4 sq cm

0.627

0.469 ohms

13-15.5 ohms

After

1000 Cycles

B3-32

123.4 mA

587.0 mV

36.5 mW

94.0 mA

389.0 mV

9.1%

4 sq cm

0.504

1.318 ohms

315.3 ohms

•mp

Short Circuit Current

Open Circuit Voltage

Maximum Power

Current at Maximum Power

Vmp = Voltage at Maximum Power

Rs = Series Resistance

R = Shunt Resistance

Table 6-2 corresponds with Figure 6-28 on the following page.

6-30
0.20

0.18 -

0.16 —

0.14 —

0.12 —

0.10 -

0.08 -

0.06 —

0.04 -

0.02 -

VOLTS

Figure 6-28. I-V Curves of Temperature Cycling Test, Selected

Sample, Ti/Pd/Cu

6-31
Table 6-3. Data for I-V Curves of Temperature Cycling Test,

Selected Sample, Ti/Pd/Ag

Ti/Pd/Ag

Cell

ID

Isc

Voc

Pmp

I

vmD

Efficiency

Cell Area

Fill Factor

Rs

Rsh

Isc = Short

Voc = °Pen

Before

Testing

B2-22

130.2 mA

593.8 mV

56.8 mW

118.3 mA

480.0 mV

14.2%

4 sq cm

0.734

0.413 ohms

275.1 ohms

Circuit Current

Circuit Voltage

Prap = Maximum Power

After

440 Cycles

B2-22

128.5 mA

583.0 mV

50.7 mW

108.1 mA

469.1 mV

12.7%

4 sq cm

0.677

0.422 ohms

498.4 ohms

After

1000 Cycles

B2-22

124.8 mA

587.0 mV

50.0 mW

105.5 mA

473.6 mV

12.5%

4 sq cm

0.682

0.416 ohms

967.2 ohms

Vmp = Voltage at Maximum Power

Rs = Series

Rsh = Shunt

Resistance

Resistance

Imn = Current at Maximum Power

Table 6-3 corresponds with Figure 6-29 on the following page.

6-32
Q-

•X.

0.04 —

0.02

Figure 6-29. I-V Curves of Temperature Cycling Test, Selected

Sample, Ti/Pd/Ag

6-33
Table 6-4. Data for I-V Curves of Humidity Exposure Test,

Selected Sample, Ti/Pd/Cu

Ti/Pd/Cu

Cell

ID

Isc

Voc

°mp

Imp

vmo

Efficiency

Cell Area

Fill Factor

R

*sh

Isc = Short

Voc = °Pen

Before

Testing

B3-21

125.3 mA

584.3 mV

52.4 mW

116.2 mA

451.0 mV

13.1%

4 sq cm

0.715

0.471 ohms

229.0 ohms

Circuit Current

Circuit Voltage

Pmp = Maximum Power

After

13 Days

B3-21

123.5 mA

577.4 mV

48.9 mW

110.5 mA

442.7 mV

12.2%

4 sq cm

0.685

0.573 ohms

183.4 ohms

After

123 Days

B3-21

109.5 mA

576.6 mV

26.2 mW

73.4 mA

356.4 mV

6.5%

4 sq cm

0.414

1.742 ohms

20.3 ohms

Vmp = Voltage at Maximum Power

Rs = Series

Rsh = Shunt

Resistance .

Resistance

Imp = Current at Maximum Power

Table 6-4 corresponds with Figure 6-30 on the following page.

6-34
CO

UJ

DC

0.20

0.18

0.16

0.14

0.12

0.10

0.08

0.06

0.04

0.02

1 I I T

AFTER 123 DAYS

BEFORE TESTING

AFTER 13 DAYS

I 1 I I I M I

0.1 0.2 0.3 0.4 0.5 0.6 0.7

VOLTS

Figure 6-30. I-V Curves of Humidity Exposure Test, Selected

Sample, Ti/Pd/Cu

6-35
Table 6-5. Data for I-V Curves of Humidity Exposure Test,

Selected Sample, Ti/Pd/Ag

Ti/Pd/Ag

Cell

ID

I

voc

I

vmo

Efficiency

Cell Area

Fill Factor

Rg

*sh

Before

Testing

B2-5

128.6 mA

590.4 mV

55.7 mW

116.0 mA

480.0 mV

13.9%

4 sq cm

0.733

0.431 ohms

202.5 ohms

Isc = Short Circuit Current

Voc = Open Circuit Voltage

Pmp = Maximum Power

Imn = Current at Maximum Power

After

13 Days

B2-5

126.0 mA

565.6 mV

51.8 mW

110.3 mA

469.7 mV

13.9%

4 sq cm

0.691

0.396 ohms

106.9 ohms

After

123 Days

B2-5

122.5 mA

584.4 mV

49.2 mW

103.8 mA

474.0 mV

12.3%

4 sq cm

0.686

0.412 ohms

242.5 ohms

Vmp = Voltage at Maximum Power

Rs = Series Resistance

Rsh = Shunt Resistance

Table 6-5 corresponds with Figure 6-31 on the following page.

6-36
0.20

0.18 —

0.16 —

0.14 —

0.12 —

0.10 —

BEFORE TESTING

AFTER 13 DAYS

0.08 —

0.06 —

0.04 —

0.02 -

Figure 6-31. I-V Curves of Humidity Exposure Test, Selected

Sample, Ti/Pd/Ag

6-37
SECTION VII

DISCUSSION OF RESULTS

A. OVERVIEW

There were no big surprises in the test results of the seven

metallization systems after temperature cycling and humidity tests. A small

surprise was that the Cr/Pd/Ag cell performed the best, i.e., showed the least

degradation in cell efficiency after humidity tests, outperforming the highly

touted Ti/Pd/Ag metallization system. The two systems, Cr/Pd/Ag and Ti/Pd/Ag,

both performed considerably better than the other five systems. The

Pd/Ni/Solder system performed respectably also. Not totally unexpected, the

copper based systems performed the worst, in that they showed the most

degradation in cell efficiency in both the temperature cycling and the

humidity tests.

It should be pointed out that these test results do not necessarily rank

metallization systems as to their ultimate use. Terrestrial cells are

encapsulated and largely protected from the environment by an encapsulation

system. There are studies in the FSA Project which investigated the

environmental impact on encapsulated cells. The works of S. Shalaby

(Reference 2) and J. Lathrop, et al. (Reference 3) address these areas. What

this test program did was to present unencapsulated cell test data that show

the relative vulnerabilities of different metallization systems to temperature

cycling and humidity tests. The test program also isolated or separated

unencapsulated cell data from encapsulated cell data, thereby enabling the

researcher to identify specific variables which effect cell performance.

Regarding the poor performance of the copper based metal systems after

humidity exposure, the question could be raised as to whether or not copper

based metallization systems are suitable for terrestrial solar cells. The

answer would be a qualified "yes." The test results suggest that copper based

contact systems may need additional surface protection during shelf periods

between cell completion and module fabrication. In large volume production,

this shelf period could be several weeks or even months. A nickel or tin dip

after copper plateup would suffice in giving surface protection to copper

based systems.

B. TEMPERATURE CYCLING TESTS

On the temperature cycling tests, another one of the small, but

pleasant, surprises was how reasonably well the temperature cycling tests

matched with published data (References 3, 4, and 5). Six of the seven

metallization systems showed a slight increase in cell efficiency, generally

around 10 cycles, and then tapered off to a lower efficiency at the end

7-1
of 1,000 cycles. The Ti/Ag system showed the largest increase in efficiency

(6,22%) and this occurred after 440 cycles. The one metallization system that

showed no increase in cell efficiency, but a slow steady slight decline in

efficiency at the end of 1,000 cycles, was Ti/Pd/Ag (3.69% decrease).

Firor and Hogan (Reference 6) show data on their thick film silver

paste, solder dipped, that agree reasonably well with the Pd/Ni/Solder

temperature cycling data of this program. Both curves of Reference 6 and this

work show a slight increase in cell efficiency at 10 cycles followed by a slow

decline in efficiency to a lower level than the starting efficiency after 30

cycles.

Discussions with space cell investigators indicate that they temperature

cycle space modules for 20,000 cycles and more with little or no degradation

in module efficiency. Because these space cells use Ti/Pd/Ag, a comparison

with the Ti/Pd/Ag data might be made. The 3.69% decrease in cell efficiency

of the Ti/Pd/Ag system in 1,000 cycles is statistically significant and seems

to be at variance with space cell data which were reported verbally. The

temperature excursions on space module testing can typically range from

100°C to -180°C for a delta T of 280°C. This compares with our 150°C

to -65°C for a delta T of 215°C. There may be an alleviating effect on

metallization stresses when cells are encapsulated.

A review of terrestrial module performance data (Reference 7) shows that

terrestrial modules exhibit performance changes after temperature cycling.

Twelve different mini-module types from different vendors were temperature

cycled between 25 to 200 cycles at temperature ranges of +90°C to -40°C at

6 h/cycle. All of the mini-modules, after disallowing some mini-modules for

obvious degradation reasons such as cracked cells, etc., still exhibited

either a slight increase in power output (1 to 3%) or, in most cases, a slight

decrease in power output (1 to 3%) after being subjected to 25 to 200 cycles

(Reference 7 did not identify which mini-modules were subjected to the exact

number of cycles). However, the mini-module data collectively showed similar

behavior to the temperature cycling data of this test program, even though the

temperature extremes and delta Ts were different, this program being the more

severe with a delta T of 215°C versus their 130°C. A review of the

temperature cycling curves in our tests show that within the first 200 cycles

there is an overall slight decrease (1 to 3%) in cell efficiency with a slight

increase occurring somewhere during the first 20 to 30 cycles, very much in

agreement with the mini-module temperature cycling data (see Reference 7).

A review of the temperature cycling data in the Appendix shows that the

Voc remains (essentially) constant for all of the metallization systems

during all of the 1,000 cycles. The significant changes occurred in the Isc

values and Rs (series resistance) values. This would infer that changes in

the metallization system are occurring. Firor and Hogan (see Reference 4)

indicate that the mechanical stress in metallization systems may be relieved

by temperature excursions, some more than others. It is assumed that Firor

7-2
and Hogan meant mechanical stress at the metal-silicon interface. This seems

to be the general assumption of most of the solar cell researchers and would

explain the slight increases in cell efficiency after initial cycling tests.

However, the steady fall-off in cell efficiency after continued temperature

cycling may indicate a rebuilding of mechanical stress in the metal contact

interfaces. Solar Cell Array Handbook (Reference 8) has a good section on

fatigue of solar module components including cell interconnects, metal

contacts, cell adhesion, etc. ,

J. Lathrop and others (see Reference 3) describe the results of their

temperature cycling/temperature shock tests. Their bias-temperature data show

remarkable similarity to the JPL temperature cycling data. Their bias

temperature tests consisted of passing current through cells equal to

approximately three times their use condition while subjecting the cells to

temperatures at 75°, 135°C and 150°C. Their curves of solar cell power

output versus exposure time showed a slight increase in power output initially

(within the first 300 to 400 hours of testing), followed by a continuing

decline in power output through 8000 hours of testing. This was true for

three of their four metallization systems tested. Their Ti/Pd/Ag and a solder

coated system both showed initial power output increase of 2 to 3% followed by

a slight decrease of 2 to 3%t very similar in trend to the metallization

systems in the temperature cycling tests of the JPL program. Lathrop and

others (see Reference 3) also conducted temperature cycling/temperature shock

tests. Their temperature extremes were the same as JPL's, from -65°C to

+150°C. However, their cells were transferred (immediately) between two

separate chambers each at one of the two temperature extremes, producing

temperature failure modes, one involving lead-loss/cell fracture and the other

mode involving gradual power loss. The JPL temperature cycling test, being

much more gradual in reaching the temperature extremes (14 min ramp time

between temperatures), also experienced several cell fractures during

temperature cycling and exhibited overall decrease in power output after

1000 cycles.

C. DISCUSSION OF HUMIDITY TESTS

The results of the humidity tests were fairly predictable. As expected,

because of the faster oxidation rate of copper, the copper based systems,

Pd/Ni/Cu and Ni/Cu, performed the worst in that they exhibited the largest

degradation in cell efficiencies of the seven metallization systems after

123 days of humidity exposure. All of the cells on all of the seven

metallization systems showed evidence of blistering during humidity testing.

Pictures of cells at 40x magnification after the total 123 days humidity

exposure are shown herein. Bishop (Reference 9) presents a description of the

mechanisms by which Ti/Ag contacts degrade. The model that he postulates to

explain the degradation mechanism involves capillary condensation of water in

the silver layer followed by electrochemical corrosion of the titanium. The

reaction is assumed to be: Ti + H20 *-Ti02 + 2H£. It is believed by

most researchers (including the author) that the tearing and blistering of the

metal contacts is due to the release of hydrogen (or gaseous products) upon

7-3
oxidation of the metal under the metal surface. Bishop further develops the

argument of oxidation under the metal surface by demonstrating that water can

reach a silicon-silver interface through the silver under humidity

conditions. After subjection to humidity conditions, water presence at the

interface of silver coated silicon samples was determined by using internal

reflection spectroscopy techniques involving reflection of infrared radiation

through single crystal silicon. The presence of water was indicated at the

interface by a decrease in transmission in the wave number region of

3400 cm~l. The presence of water at the silicon-silver interface would

provide the necessary catalyst for the degradation mechanism suggested by

Bishop and others.

Becker and others (see Reference 10) looked at the silicon-titanium

contact structure by the use of transmission electron microscopy techniques.

They determined the presence of significant amounts of titanium hydride

(TiH2) at the contact interface and suggested the oxidation of the TiH£ in

the presence of water as a contributing factor in the formation of H2 which

leads to contact blisters and ultimate contact degradation. This is entirely

plausible because many contact metals are normally sintered at 500 to 600°C

in a forming gas of N2/H2 mixture. The Ti contacts used in this program

were sintered in 100% N2 and it is doubtful if any hydride was formed.

Becker and others (see Reference 10) also indicated that the TiH2

distribution in the Ti layers were highly uneven with some samples showing

high TiH2 concentrations while other samples showed little or no

concentrations.

Becker and others (see Reference 10) indicated that they could not get a

clear understanding of the oxygen behaviour of the Ti/Ag and Ti/Pd/Ag

systems. This was due to the complex nature of oxide formations. Although

the main thrust of this program was not to investigate degradation mechanisms,

one of the interesting observations made was to determine the oxygen content

of some of the metallization systems before and after environmental exposure.

The thinking was that if the moisture (water) from the humidity tests was

absorbed into the metallization system and reacted to form metal oxides, then

an increase should be seen in oxygen content of the metallization system after

exposure to humidity tests. A SIMS was used to determine oxygen content. The

control sample, a 1000 temperature cycled sample, and a 123 day humidity

exposed sample, were each tested for oxygen content from the Ti/Pd/Ag

metallization system, the Ti/Ag system and the Pd/Ni/Cu metallization system

(a total of nine samples tested). No significant differences could be found

in oxygen content (count) in any of the samples. Because the SIMS cannot

discern between bound oxygen (as in the case of metal oxides) and unbound

oxygen (as in the case of trapped oxygen or oxygen agglomerates), it was

surmised that the unbound oxygen count, being so much larger and variable,

swamped (or masked) the subtle changes in oxygen count due to metal oxide

formation. It was concluded that the SIMS was not the proper instrument to

determine metal oxide formation in solar cells and that achieving a clear

understanding about the oxygen behaviour of the metallization system was,

indeed, as difficult a task as Becker (see Reference 10) alluded to. There

were no further measurements of the oxygen content of the metallization

systems, because this type of effort was not a major thrust of this program.

7-4
The summary remarks about this environmental test program are that:

(1) it showed decided differences in the performances of different

metallization systems after being subjected to environmental exposures;

(2) these performance differences, however, did not rank the metallization

system, as stated earlier, but did (and does) allow researchers to compare the

relative sensitivities of different metallization systems to environmental

exposures; (3) the test results showed remarkable agreement with other

published results on environmental testing; and (4) the tabulated data in the

Appendix should provide detailed information on cell performances after

environmental exposures.

7-5
SECTION VIII

REFERENCES

1. Hoffman, A.R., Griffith, J.S., and Ross, R.G., "Qualification Testing of

Flat-Plate Photovoltaic Modules," IEEE Transactions on Reliability,

Vol. R-31, pp. 252-257, August 1982.

2. Shalaby, H., "The Degradation Mechanism of Ti-Pd-Ag Solar Cell Contacts

by an Accelerated Electrochemical Testing Technique," Solar Cells,

Vol. 11, pp. 189-193, 1984.

3. Lathrop, J.W., Hawkins, D.C., Prince, J.L., and Walker, H.A.,

"Accelerated Stress Testing of Terrestrial Solar Cells," IEEE

Transactions on Reliability, Vol. R-31, pp. 258-265, August 1982.

4. Firor, K., and Hogan, S., "Environmental Testing of Single-Crystal

Silicon Solar Cells with Screen-Printed Silver Contacts," IEEE

Transactions on Reliability, Vol. R-31, pp. 271-275, August 1982.

5. Prince, J.L., Lathrop, J.L., and Whitter, G.W., "Contact Integrity

Testing of Stress-Tested Silicon Terrestrial Solar Cells," Paper

presented at 14th IEEE Photovoltaic Specialists Conference, San Diego,

California, January 7-10, 1980. Conference Record (A81-27076 11-44),

pp. 952-957, 1980.

6. Firor, K., and Hogan, B., "Effects of Processing Parameters- on Thick

Film Inks Used for Solar Cell Front Metallization," Solar Cells, Vol.- 5,

pp. 87-100, December 1981.

7. Maxwell, H.G., Grimmett, C.A., Repar, J., Frickland, P.O., and Amy, J.A.

FSA Field Test Report 1980-1982, JPL Publication 83-29, Jet Propulsion

Laboratory, Pasadena, California, April 15, 1983.

8. Rauschenbach, H.S., Solar Cell Array Design Handbook, Vols. 1 and 2, JPL

Publication ST 43-38, Jet Propulsion Laboratory, Pasadena, California,

October 1978.

9. Bishop, C.J., The Fundamental Mechanisms of Humidity Degradation in

Silver Titanium Contacts, Paper presented at 8th IEEE Photovoltaic

Specialists Conference, Seattle, Washington, August 4-6, 1970,

pp. 51-61, AIAA 71A16061, August, 1970.

10. Becker, W.H., and Pollack, S.R., The Formation and Degradation of Ti-Ag

and Ti-Pd-Ag Solar Cell Contacts, Paper presented at 8th IEEE Photovoltaic Specialists Conference, Seattle, Washington, August 4-6, 1970,

pp. 40-50, AIAA 71A16060, August, 1970.

8-1
SECTION IX

SELECTED BIBLIOGRAPHY

Aroian, L.A., Luft, W., and McCraven, C.C., "Temperature and Humidity Effects

on Silicon Solar Cells," Paper presented at 7th Photovoltaic Specialists

Conference, Pasadena, California, November 19-21, 1968. Conference Record

(A69-35678 19-03), pp. 214-225, 1968.

Berman, P., Mueller, R., and Solana, M., "Results of Accelerated Thermal Cycle

Tests of Solar Cell Modules," Paper presented at 12th Photovoltaic

Specialists Conference, Baton Rouge, Louisiana, November 15-18, 1976.

Conference Record (A78-1090201-44), pp. 379-387, 1976.

Boiler, H.W., and Koch, J., "Accelerated Fatigue Tests of Solar Cell Interconnectors for Simulation of Thermal Cycles," Paper presented at llth

Photovoltaic Specialists Conference, Scottsdale, Arizona, May 6-8, 1975.

Conference Record (A76-14727 04-44), pp. 153-161, 1975.

Hoffman, A.R., and Ross, R.G., "Environmental Qualification Testing of

Terrestrial Solar Cell Modules," Paper presented at 13th Photovoltaic

Specialists Conference, Washington, D.C., June 5-8, 1978. Conference

Record (A79-40881 17-44), pp. 835-842, 1978.

Luft, W., "Silicon Solar Cells at Low Temperature," IEEE Transactions on

Aerospace and Electronic Systems. Vol. AES-7, pp. 332-339, August, 1971.

Luft, W., and Maiden, E., "Temperature Cycling Effects on Solar Panels," Paper

presented at 4th Intersociety Energy Conversion Engineering Conference,

Washington, D.C., September 22-26, 1969. Conference Record (A69-42236

23-03), pp. 582-589, September, 1969.

Peterson, R.C., and Muled, A., "Silicon Solar Cells with Nickel/Solder

Metallization," Proceedings of the Third International Photovoltaic Solar

Energy Conference, Cannes, France, October 27-31, 1980. Conference Record

(A82-24101 10-44), pp. 684-690, 1981.

Smokier, M.I., and Runkle, L.O., "Experience in Design and Test of Terrestrial

Solar-Cell Modules," Paper presented at AS/ISES 1982 Annual Meeting,

Houston, Texas, June 1-5, 1982.

9-1
APPENDIX

Tables A-l through A-14 present the JPL test data after temperature

cycling and humidity exposure.

A-l
,Q

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A-15
i\crv_/M

1. Report No. 85-86 ^" ^ov*

mm*n

*

Accession No.

4. Title and Subtitle

Environmental Tests of Metallization Systems

for Terrestrial Photovoltaic Cells

7. Author(0

P. Alexander, Jr.

9. Performing Organization Name and Address

JET PROPULSION LABORATORY

California Institute of Technology

4800 Oak Grove Drive , ; / /f\

Pasadena, California 91109 \r\6/KW' ^ ^

12. Sponsoring Agency Nome and Address

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION

Washington, D.C. 20546

3. Recipient's Catalog No.

5. Report Date

December 31, 1985

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

11. Contract or Grant No.

NAS7-918

13. Type of Report and Period Covered

JPL Publication

14. Sponsoring Agency Code

15. Supplementary Notes Sponsored by the u<s < Department of Energy through Interagency

Agreement DE-AI01-76ET20356 with NASA; also identified as DOE/ JPL 1012-113 and as

JPL Project 5101-280 (RTOP or Customer Code 776-52-61).

16. Abstract

Seven different solar cell metalliz

cycling tests and humidity tests. Tempe

ISd'PC per cycle. Humidity conditions we

metallization systems were: J9^Ti/Ag, -£

©^ Pd/Ni/Solder, ,B^Cr/Pd/Ag, and ^^.

cell efficiencies after subjection to 10

metallization systems also evidenced sli

moderate numbers of cycles, generally le

systems showed the largest decrease in c

cell efficiencies after 123 days of humi

again showed the largest decrease in eel

Graphs of the environmental exposur

for each fS£. the -metallization systems a

fill factors or series resistance.

17. Key Words (Selected by Aurhor(s))

•SIEnorgy — Storage i"VUH\ 10 i T(J

i . i i*y \ Grower Sourticfi rf^YYirw^i/yv-Vf e-s* f^TnTr

-Mst cr ial s —

^Vok^voHcxic, cc-Us

ation systems were subjected to temperature

rature cycling excursions were -SCrC to

re 70fC at 98% relative humidity. The seven

^ Ti/Pd/Ag , _B^ Ti/Pd/Cu , ^*X, Ni/Cu ,

Thick ^±im Ag.

ems showed slight to moderate decrease in

00 temperature cycles. Six of the seven,

ght increases in cell effj&iciencies after,

ss than 100 cycles. The copperj^based

ell efficiencies after temperature cycling.

ems showed moderate to large decreases in

dity exposure. The copper£based systems

1 efficiencies after humidity exposure.

**•«

es versus cell efficiencies are presented

s well as environmental exposures versus

18. Distributlbn Statement ' Q pC(.pVlS (dr\O.r^ "j

mcLSS &pec4roscopV 4<x1bl<?S ^doA<x\

performance. 4-esrS J

Qos4 ir~e.<dL uc^V i^"'

e.r«?v^QV s^^O-*^^.

19. Security Clossif. (of this report) 20. Security Clouif. (of this poge) 21 . No. of Poges 22. Price

Unclassified '^^^«^^Cxfi<, Unclassified 88

DIM R»/«3