DRD LINE ITEM NO. SE-7 DOE /JPL-954929-84/11

PHOTOVOLTAIC CELL RELIABILITY RESEARCH

FIFTH ANNUAL REPORT

SEPTEMBER 1984

J.W. Lathrop

Department of Electrical and Computer Engineering

Clemeon University

Clemson. SC 29631

PREPARED FOR

JET PROPULSION LABORATORY

PREPARED BY

CLEMSON UNIVERSITY
DRD Line Item No. SE-7 . DOE/JPL - 954929-84/11

ENGINEERING AREA

PHOTOVOLTAIC CELL RELIABILITY RESEARCH

INVESTIGATION OF ACCELERATED STRESS FACTORS

AND FAILURE/DEGRADATION MECHANISMS IN TERRESTRIAL SOLAR CELLS

FIFTH ANNUAL REPORT

J.W. Lathrop

Department of Electrical and Computer Engineering

Clemson University, Clemson, SC 29631

September 1984

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 work was performed for the Jet Propulsion Laboratory,

California Institute of Technology by agreement between NASA and DOE.
CLEMSON PERSONNEL

Persons contributing to the work covered in this report include:

Dr. Jay W. Lathrop

Mr. Dexter C. Hawkins

Ms. Clara White Davis

Mr. William G. Stoddard

Mr. Thomas A. Bolin

Mr. H. Jarrett Cassell, Jr.

Mr. Michael A. Cross

Mr. Thomas A. Hipp

Mr. Scott F. LaFave

Mr. Paul A. Williamson

Principal Investigator

Research Associate

Graduate Student

(cell measurement techniques)

Graduate Student

(a-Si cell testing)

Undergraduate Student

Undergraduate Student

Undergraduate Student

Undergraduate Student

Undergraduate Student

Undergraduate Student

ii
ACKNOWLEDGEMENT

——-—-..

iii
ABSTRACT

This annual report presents results of an ongoing research program into

the reliability of terrestrial solar cells. The main focus of the research is

to identify failure/degradation modes affecting solar cells and to relate

these to basic physical, chemical, and metallurgical phenomena. The program is

concerned with the reliability attributes of individual single crystalline,

polycrystalline, and amorphous thin film silicon cells. Results of subjecting

different types of crystalline cells to the Clemson accelerated test schedule

are given. Preliminary step stress results on one type of thin film amorphous

silicon (a:Si) cell indicated that extraneous degradation modes were

introduced above 140 C. The report also describes development of measurement

procedures which are applicable to the reliability testing of a:Si solar

cells. An approach to achieving the necessary repeatability, currently being

explored, of fabricating a simulated a:Si reference cell from crystalline

silicon photodiodes is described.

iv
TABLE OF CONTENTS

Section Page

CLEMSON PERSONNEL ii

ACKNOWLEDGEMENT ill

ABSTRACT iv

TABLE OF CONTENTS v

LIST OF FIGURES vi

LIST OF TABLES vii

1.0 INTRODUCTION 1

2.0 RELIABILITY RESEARCH TEST PROGRAM 5

2.1 Exploratory Reliability Testing of

Unencapsulated aiSi Cells .5

2.2 Crystalline Cell Reliability Research 15

2.3 Outdoor Real-Time Testing of Individual Cells 30

3.0 a$Si RELIABILITY INSTRUMENTATION RESEARCH 33

3.1 General Considerations 33

3.2 Simulated a:Si Standard Cells 35

3.3 Large Area Light Source 37

3.4 System Configuration and Data Acquisition Procedures 46

3.5 Second and Fourth Quadrant Measurement 52

3.6 Low Temperature Measurement Jig for Crystalline Si Cells...54

3.7 New Clemson Reliability Research Facility 59

4.0 CONCLUSIONS 61

5.0 NEW TECHNOLOGY 63

6.0 PROGRAM RESEARCH CONTRIBUTIONS 65

7.0 REFERENCES 67

APPENDIX A. Publication Abstracts 69
List of Figures

Figure Page

1. Photograph of Measurement Jig for 16-Cell a:Si Submodule 7

2. Step Stress Schedule for a:Si Cells 8

3. Histogram of Average Power Degradation of 16-Cell a:Si

Submodule Following Step Stress Schedule 11

4. IV Characteristics of atSi Cells Before and After Step

Stress Testing 13

5. Crystalline Cell Test Schedule Key to Summary Data Tables 16

6. Photograph of Outdoor Real Time Test Rack 31

7. Approach to a Simulated a:Si Standard Cell Using

Filtered Crystalline Silicon Photodiodes 36

8. Average Measured Intensity Variation of ELH Lamps 39

9. Calculated Intensities Over an Extended Area

for an 11-Lamp ELH Array 40

10. Lamp Location Diagram for 11-Lamp Array 41

11. Original Measured Intensity Variation of 11 Lamp Array 43

12. Measured Intensity Variation of 11-Lamp Array after Lamp

Tilt Adjustment 44

13. Intensity Monitoring and Adjustment System Schematic 45

14. Multivapor Lamp Spectral Characteristics 47

15. Block Diagram of the a:Si Short Interval Tester 48

16. Photograph of the a:Si Short Interval Tester 49

17. Flow Diagram of Data Taking Routine for AC Lamp Simulator 50

18. Timing Diagram of AC Light Source Data Acquisition Technique 51

19. Comparison of AC and DC Data Collection Methods 53

20. Power Supply Modification for 4th Quadrant Operation 55

21. Graphical Representation of 2nd, 3rd, and 4th Quadrant Data 56

22. Photograph of Low Temperature Measurement Jig for

Crystalline. Cells 58

23. Photograph of Clemson' s Auger Microprobe 60

vi
LIST OF TABLES

Table Page

1 Result of Step Stress Testing a:Si Cells 10

2 Unencapsulated Cell Types Classified by Primary Metallization 17

3A Unencapsulated Cell Bias-Temperature Test Results

(maximum power output, N-, 0-, and P-cells) 18

3B Unencapsulated Cell Bias-Temperature Test Results

(maximum power output, Q-, R-, and V-cells) 19

3C Unencapsulated Cell Bias-Temperature Test Results

(maximum power output, W-, and X-cells) 20

3D Unencapsulated Cell Bias-Temperature Test Results

(maximum power output, Y- and Z-cells) 21

4 Unencapsulated Cell B-T Test Results

(catastrophic mechanical change) 22

5 Unencapsulated Cell 85 C/85 %RH Test Results

(maximum power output degradation) 23

6 Unencapsulated Cell 85 C/85 %RH Test Results

(catastrophic mechanical change) 25

7 Unencapsulated Cell Pressure Cooker Test Results

(maximum power output degradation) 26

8 Unencapsulated Cell Pressure Cooker Test Results

(catastrophic mechanical change) 28

9 Summary of Unencapsulated Crystalline Cell Accelerated

Test Results 29

10 Unencapsulated Cell Real-Time Test Results

(maximum power degradation) 32

vii
1.0 INTRODUCTION

This is the Fifth Annual Report on the Investigation of Accelerated

Stress Factors and Failure/Degradation Mechanisms in Terrestrial Solar Cells,

a photovoltaic cell reliability research program which has been conducted by

Clemson University for the Department of Energy (DOE) Flat-Plate Solar Array

(FSA) Project managed by the Jet Propulsion Laboratory. The objective of the

research is the identification and characterization of fundamental physical,

chemical, and metallurgical phenomena which, acting at the cell level, could

cause photovoltaic modules to degrade with time. The approach followed is to

develop laboratory tests which will accelerate the anticipated field failure

modes, and then to subject quantities of different types of cells to these

tests. Analysis of the pre- and post-stress test data, combined with visual

observations, is then used in an effort to determine the nature of degradation

mechanisms. The program was initiated in December of 1977 and earlier reports

(1,2,3,4,5) have discussed the experimental and analytical methods employed,

the data collected, and a number of preliminary conclusions. Prior to this

reporting period, work had been concentrated entirely on crystalline silicon

cells, with testing being performed on both encapsulated and unencapsulated

configurations.

Technical activity of the reliability research program can be divided

into the stress testing of cells (Section 2.0) and the development of

instrumentation (Section 3.0). Amorphous silicon (a:Si) cell types have

recently been included in the cell test program. Very little is known

concerning the long term life of these cells and its effect on the reliability
of thin film modules, a problem now being investigated by JFL. Consequently

the Clemson program has begun a study of their reliability attributes and some

preliminary results are given in Section 2.1. The crystalline cell test

program is continuing as well and Section 2.2 gives complete test results on

10 unencapsulated cell types that had only been partially reported on in the

4th Annual Report. Preliminary results on two new silicon ribbon cell types

are also included in this section. These cell types are of particular interest

to the program because of their potential for low cost and high efficiency.

Real time outdoor testing is also underway. A test rack has been fabricated

and numbers of individual cells, both encapsulated and unencapsulated

installed. Results of this ongoing work are discussed.

Instrumentation development during the report period concentrated on a: Si

measurement methods. Because of differences in their electrical and physical

characteristics, the measurement techniques used to obtain a-Si reliability

data must be quite different from those used with crystalline cells. Of prime

concern is the absence of a stable a:Si reference cell for assuring

illumination repeatability. Section 3.2 discusses an approach to reference

cell fabrication using filtered single crystal silicon photo diodes. Because

a:Si cells have a different form factor from crystalline cells it is necessary

that the laboratory simulator be capable of uniformly illuminating a larger

area. Ideally this illumination should also correspond to the spectral

sensitivity of a:Si cells. However, cost and stability considerations favor an

extension of the modular ELH approach used successfully for many years in

crystalline reliability measurements. Development of a prototype large area

simulator for use with a:Si cells is described in Section 3.3. The final

system configuration of the a:Si measurement system and how it will be used
for data acquisition is described in Section 3.4.

Crystalline cell measurement techniques were upgraded during the

reporting period in two important ways: 1) data taking and display of the

associated IV curves was extended to include portions of the second and fourth

quadrants, and 2) a jig was designed and fabricated which allowed

characteristic curves to be measured at low temperatures. These developments

are discussed in Sections 3.5 and 3.6 respectively. Finally, the new electron

microscope facility at Clemson has become operational as discussed in Section

3.7. It is expected that the analytical capability of this instrumentation

will prove to be extremely valuable in the analysis of degraded devices,

permitting the fundamental mechanisms to be identified.
2.0 RELIABILITY RESEARCH TEST PROGRAM

2.1 Exploratory Reliability Testing of Unencapsulated a:Si Cells

In addition to requiring repeatable measurement techniques an accelerated

test program must also select stress levels which will accelerate failure

mechanisms expected in the field. If the stress level is too low the

acceleration factor will be too small and an inordinate length of time will be

required before changes are observed. On the other hand, if the stress level

is too high, extraneous degradation mechanisms will be introduced which would

never occur in the field in real time. Thus for an accelerated test program to

be effective a stress "window" must exist. One method of determining if a

window exists when there is no previous history, as is the case with a:Si

cells, is to run a step stress test. In a step stress test the component is

first measured, then subjected to a particular level of stress for a

relatively short time. The component is then measured to determine if any

changes have occurred and then subjected to a somewhat greater stress for a

similar length of time. This second level of stress should be sufficiently

high that its effect would be greatly in excess of any effects from the first

stress level. Temperature stress effects are governed by the well known

Arrenhius equation, which can be used to calculate the stress levels so there

will be little cumulative effect, provided the activation energy can be

estimated. Thus stress is increased in a step-wise fashion many times until

significant change is observed signaling the onset of an extraneous

degradation mode. In this way the test is able to give an indication of an

upper stress level for accelerating real time phenomena.

,-; •;.,-„>-. '-' ^' -'••• •»'•" - ' -
Step stress investigations were conducted in order to obtain information

concerning the permissible upper temperature stress levels for amorphous

cells. Electrical measurements were made with the 4-lamp ELH simulator and

short interval tester used for crystalline cell testing. A crystalline

standard cell was used to initialize the simulator intensity prior to each

measurement. While this method can result in absolute measurement error at any

given step, as will be discussed in Section 3, the changes being sought were

sufficiently great that it should still allow a good assessment of the

temperature at which an extraneous degradation mode occurs.

Amorphous cell samples consisting of 16 relatively small (4.5 X 1 cm)

at Si cells fabricated on a common superstrate were obtained from a commercial

manufacturer for use in the step stress investigation. This type of cell has

been designated aA, where the lower case "a" prefix indicates amorphous

silicon construction. The cells were not interconnected and could be

individually contacted from the back. Figure 1 is a photograph of the jig used

to contact the cells during measurement. A Kelvin probe arrangement of

beryllium copper contacts was used. This type of localized metallic contact

tended to scratch the thin film metallization. In addition, because the

contacts were localized the current was not well distributed across the thin

metallized areas. Future contacts will utilize conductive elastic material

which will make possible extended contacts rather than point contacts.

The sixteen a:Si cells were photographed and electrically measured. The

cells were then placed in an ambient air oven at 90 C for 20 hours and then

again measured and photographed. This sequence was repeated at 10 C intervals

through 150 C as.illustrated in Figure 2. Results of tha electrical
_.

i

Figure 1. Photograph of Measurement Jig for 16-Cell a:Si Submodule
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measurements are given in Table 1 and the histogram of Figure 3. It can be

seen that while only small changes were observed through the 130 C step, a

drastic decrease in Fm occurred as a result of the 140 C step. Individual IV

curves of the 16 cells after the 140 C step are shown in Figure 4. An

examination of color photographs taken before and after each step indicated

that a subtle color change had occurred after the 140 C step. The change was

too subtle to be reproduced in this report, but it was clear that the normally

rosy color of the cell had a decidedly greenish tinge after the 140 C step.

It is apparent from the characteristic curves of Figure 4 that a

reduction in Voc was the major reason for the observed power degradation. The

reason for this reduction is not known, but it has been determined that it is

accompanied by an increase in cell leakage of several orders of magnitude.

Additional samples and other cell constructions will need to be tested before

it can be determined whether this type of behavior is unique to these 16 cells

or to this cell type. Based on these tests, however, there would be reason to

question the validity of accelerated testing this type of cell above 130 C.

Such a restricted "window" would limit the usefulness of accelerated testing,

as discussed above, for evaluating the reliability attributes of a:Si cells.

Crystalline cells have an upper temperature limit, limited by the melting

point of solder, which is in excess of 155 C.
TABLE 1

RESULT OF STEP STRESS TESTING asSi CELLS

% decrease (average of 16 cells)

Stress Voc Isc Pm

20 hours Q 90 C -0.2 0.0 -1.4

20 hours @ 100 C -0.7 0.3 -0.7

20 hours @ 110 C -0.6 0.4 0.6

20 hours @ 120 C 0.7 0.5 1.1

20 hours @ 130 C -0.7 0.8 0.4

20 hours @ 140 C 54.5 9.7 70.9

20 hours @ 150 C 86.3 29.6 94.2

10
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Figure 4. IV Characteristics of a:Si Cells Before and After Step Stress Testing

13
"PAGE MISSING FROM AVAILABLE VERSION"
2.2 Crystalline Cell Reliability Research

Partial results were reported in the 4th Annual Report on 10

unencapsulated crystalline cell types which were undergoing accelerated

testing. Testing of these cells has now been completed and the final test

results are given in the summary data tables which follow. Table 2 classifies

the cell types according to the type of metallization each used and Figure 5

provides an overview of the summary data tables as an aid to the reader. A

brief} rather subjective, summary of results on these 10 cells is given in

Table 9.

Two new unencapsulated cell types were received during the reporting

period and were started into test. Both cell types are of ribbon construction

— one being EFG and the other dendritic ribbon. At this time insufficient

hours have been accumulated to establish any clear trends.

15
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TABLE 2.

UNENCAPSULATED CELL TYPES

CLASSIFIED BY PRIMARY METALLIZATION

CELL TYPE CONDUCTING LAYER SOLDER

N nickel plate yes

0 nickel plate yes

P nickel plate yes

Q nickel plate yes

R copper plate no

V copper plate yes

W nickel plate yes

X nickel plate yes

Y silver paste no

Z silver paste no

17
TABLE 3A

UNENCAPSULATED CELL BIAS TEMPERATURE TEST RESULTS

MAXIMUM POWER OUTPUT N-, 0-, AND P-CELLS

(Test completed)

Cell

N

N

N

0

0

0

p

p

p

Temp

C

75

135

150

75

135

150

75

135

150

Time

(hr)

600

1200

2400

4800

600

1200

2400

4800

300

600

1200

2400

600

1200

2400

4800

600

1200

2400

4800

300

600

1200

2400

600

1200

2400

4800

600

1200

2400

4800

300

600

1200

2400

Total

Cells

25

25

25

23

20

20

19

18

19

18

15

14

25

24

24

23

20

20

20

19

20

20

19

17

25

24

24

24

20

20

20

15

20

20

20

19

0-2%

23

3

4

2

19

19

14

9

15

11

5

3

19

17

15

19

-1

8

15

18

17

-1

7

20

20

7

Range

3-9%

2

8

7

9

1

1

5

8

4

7

8

6

6

9

9

5

14

9

6

1

14

9

3

8

8

5

-3

9

5

2

3

-1

5

data erra

10

of Maximum Power

10-19% 20-29%

4 10

4 9

3 9

1

2

4 2

10 3

10 2

10 6

6

10 1

12 2

16 2

6

11

10 5

1 1

1

2 1

1

LtiC

2

Degradation

30-49% 50-100%

1

2

3

2

1

12 3

18
TABLE 3B

UNENCAPSULATED CELL BIAS TEMPERATURE TEST RESULTS

MAXIMUM POWER OUTPUT Q-, R-, AND V-CELLS

(Test completed)

Cell

Q

Q

Q

R

R

R

V

V

V

Temp Time

C (hr)

75 600

1200

2400

4800

135 600

1200

2400

4800

150 300

600

1200

2400

75 600

1200

2400

4800

135 600

1200

2400

4800

150

75 600

1200

2400

4800

135 600

1200

2400

4800

150 300

600

1200

2400

Total

Cells

25

25

24

24

20

20

20

20

20

20

20

20

20

15

10

20

16

16

14

15

test

23

15

0-2%

2

13

10

5

9

10

11

4

9

Range of Maximum Power Degradation

3-9% 10-19% 20-29% 30-49% 50-100%

23

1

2

7

5

5

2

6

5

8

3

not run because of

21

3

interpretation

18

17

17

15

16

20

16

14

intei

2

17

14

9

4

18

15

7

cpretation

2

11

difficult

4

3

6

3

2

1

7

difficult

16

12

12

5

3

2

2

lack of

1

because

7

6

because

2

2

1

3

7 1

2

1 1

6

10 4

4

1

samples

of mechanical

1 2

2 1

of mechanical

23

22

23

18

18

1

20

20

2

defects

2

defects

19
TABLE 3C

UNENCAPSULATED CELL BIAS TEMPERATURE TEST RESULTS

MAXIMUM POWER OUTPUT W-, AND X-CELLS

(Test completed)

Cell Temp

C

W 75

W 135

W 150

X 75

X 135

X 150

Time

(hr)

600

1200

2400

4800

600

1200

2400

4800

300

600

1200

2400

600

1200

2400

4800

600

1200

2400

4800

300

600

1200

2400

Total Range of Maximum Power Degradation

Cells 0-2% 3-9% 10-19% 20-29% 30-49% 50-100%

25 24 1

25 6 19

25 7 18

25 3 22

20 15 5

20 17 3

20 8 12

19 8 11

20 17 3

20 17 3

20 4 15 1

20 1 15 4

25 25

25 24 1

25 22 3

25 23 2

interpretation difficult because

interpretation difficult because

2 0 4 8

20 7

interpretation difficult because

interpretation difficult because

test discontinued because of

test discontinued because of

of mechanical defects

of mechanical defects

1 5 2

4 5 4

of mechanical defects

of mechanical defects

mechanical defects

mechanical defects

20
TABLE 3D

UNENCAPSULATED CELL BIAS TEMPERATURE TEST RESULTS

MAXIMUM POWER OUTPUT Y- AND Z-CELLS

(Test completed)

Cell Temp Time Total Range of Maximum Power Degradation

C (hr) Cells 0-2% 3-9% 10-19% 20-29% 30-49% 50-100%

Y 75

Y 135

Y 150

600

1200

2400

4800

600

1200

2400

4800

300

600

1200

2400

25

25

25

25

20

20

20

20

20

20

20

20

-9

+1

-10

+1

-8

+3

-2

+4

-2

-2

+3

-1

+1

-15

-13

+1

-13

+1

-15

+2

-10

-10

-9

-1

+3

-10

-9

-7

+8

-1

+2

-2

-9

-9

-9

+7

-10

-10

-4

+7

-1

-1

+2

-1

+7

+2

Z 75 600 19 8 11

1200 15 10 5

2400 16 1 14 1

4800 15 15

Z 135 600 16 16

1200 15 11 4

2400 15 12 3

4800 15 14 1

Z 150 300 data unavailable

600 17 11 6

1200 16 7 6 3

2400 16 5 5 5

NOTEs All values are positive unless otherwise noted.

A negative value of degradation is an improvement.

21
TABLE 4

^ENCAPSULATED CELL BIAS-TEMPERATURE TEST RESULTS

CATASTROPHIC MECHANICAL CHANGE

(tests completed)

Cell

N

N

N

0

0

0

P

P

P

Q

Q

Q

R

R

R

V

V

V

U

H

W

X

X

X

Y

Y

Y

Z

Z

Z

Temp

75

135

150

75

135

150

75

135

150

75

135

150

75

135

150

75

135

150

75

135

150

75

135

150

75

135

150

75

135

150

Total #

in test

25

19

20

25

20

20

25

20

20

25

20

20

25

20

25

20

20

25

20

20

25

20

20

25

20

20

25

20

18

Defect I

Leads

M S

1

2

2

9

13

3

2

4 1

1

3

18

12 1

test not

2

4 15

1

4

5

4

10

Sate

Fr

M

2

3

2

2

4

5

1

11

7

run

1

3

3

2

2

2

1

1

2

gory — 1

acture

S

1

4

1

3

1

1

2

12

because

10

5

7

1

7

5

4

"l=moderate,

Grid

M S

20

10

5

7

5

1

11

11

14

7

3 1

of lack of

3

1

1

1

4

4

9

20

11 6

25

2

13

S=severe

Back

M S

20

10

4

2

3

3

1

10

11

1

24

7 3

samples

7

1

2

18

8

20

9

22
TABLE 5

UNENCAPSULATED CELL 85 C/85% RH TEST RESULTS

MAXIMUM POWER OUTPUT DEGRADATION

(test completed)

Cell

N

0

P

Q

R

V

Time

(hr)

250

500

1000

2000

250

500

1000

2000

250

500

1000

2000

250

500

1000

2000

250

500

1000

2000

Total

Cells

15

15

15

13

14

14

14

14

15

15

15

14

15

15

14

10

15

14

14

14

0-2%

-1

+3

-2

+2

-2

+1

-2

-2

+12

-2

+9

-1

+5

-1

+4

-3

+3

-2

+4

-1

+2

-2

+5

+1

test not

-5

+8

-5

+5

-3

+7

+11

Range

3-9%

+8

+6

+7

+7

+3

+8

+9

-7

+2

-1

+5

-2

+7

-1

+5

+11

+10

+2

of Maximum Power Degradation

10-19% 20-29% 30-49% 50-100%

+2

+3 +2

+3 +1

+2 +1

+3

+2

+1

+2

+5

+12

+9 +1

run because of lack of samples

+1

-1

+2

-1

+1

+1

+1

+1 +2

+2

23
TABLE 5 (continued)

W 250 15 +10 +5

500 15 +11 +4

1000 15 +1 +14

2000 15 +2 +13

X 250 15 +10 +5

500 14 +8 +6

1000 13 +11 +2

2000 14 +7 +7

Note: All values are positive unless otherwise noted.

A negative value of degradation is an improvement.

24
TABLE 6

UNENCAPSULATED CELL 85 C/85% RH TEST RESULTS

CATASTROPHIC MECHANICAL CHANGE

(tests completed)

Total # Defect Category — M=moderate, S=severe

Cell in test Leads Fracture Grid Back

M S M S M S M S

N

0

P

Q

R

V

W

X

Y

Z

15

15

15

15

15

15

15

15

3 1 13

1 7

3 3

1 3

test not run because of

2 2 14

1 5

cells not tested

lack of samples

5

25
TABLE 7

UNENCAPSULATED CELL PRESSURE COOKER TEST RESULTS

MAXIMUM POWER OUTPUT DEGRADATION

(tests completed)

Cell

N

0

P

Q

R

V

W

Time

(hr)

50

100

200

500

50

100

200

500

50

100

200

500

50

100

200

500

50

100

200

500

50

100

200

500

Total

Cells

8

6

8

8

10

10

10

10

10

8

(test

10

10

10

9

10

10

10

10

10

10

10

8

0-2%

6

4

1

2

4

stopped

3

1

1

test

8

6

1

4

4

1

4

Range of Maximum Power

3-9% 10-19% 20-29%

2

2

3

1

3

5

5

2

due to

6

6

3

1

1

2

3

4

1

3

5

4

operator

1

3

3

4

not run because

1

2

6

1

4

4

5

4

1

2

4

2

2

1

1

3

4

5

4

error)

1

1

of lack of

3

4

2

Degradation

30-49% 50-100%

1 2

1 2

1

1

1 1

2 1

samples

1

1

26
TABLE 7 (continued)

UNENCAPSULATED CELL PRESSURE COOKER TEST RESULTS

MAXIMUM POWER OUTPUT DEGRADATION

Cell Time Total Range of Maximum Power Degradation

(hr) Cells 0-2% 3-9% 10-19% . 20-29% 30-49% 50-100%

50 interpretation difficult because of mechanical defects

100 interpretation difficult because of mechanical defects

200 interpretation difficult because ofmechanical defects

500 test terminated because of mechanical defects

50 10 2* 8*

100 10 10*

200 10 2* 8*

500 10 10

50 8 8

100 8 8

2 0 0 8 2 5 1

500 8 8

*NOTE: Y-cell Pro values represent increases rather than decreases!

27
TABLE 8

UNENCAPSULATED CELL PRESSURE COOKER TEST RESULTS

CATASTROPHIC MECHANICAL CHANGE

Total # Defect Category — M=moderate, S=severe

Cell in test Leads Fracture Grid Back

M S M S M S M S

N 1 0 1 2 5 3

0 10 1 7 10

P operator error required premature termination of test

Q 10 2

R test not run because of lack of samples

V 1 0 1 1

W 10

X 1 0 2 7 2 8 1 0

Y 10 3

Z 10 4

28
TABLE 9

SUMMARY OF UNENCAPSULATED CRYSTALLINE CELL ACCELERATED TEST RESULTS

Cell B-T Tests PC Tests TC/TS Comments

N average average poor

0 poor average average

P good poor good

Q very poor poor poor Back contact Schottky formation

R poor — — Fracture during handling

V average average poor

W good average poor

X very poor very poor very poor Loss of grid adhesion

Y poor very poor poor Increase in Pm

Z average very poor — Fracture

29
2.3 Outdoor Real-Time Testing of Individual Cells

The outdoor test rack was installed in an effort to see if a relationship

could be established between real time testing and accelerated testing. The

real-time test program has been in operation approximately one year during

which considerable experience has been gained in outdoor test procedures. It

was found that the cells were exposed to liquid water in the form of rain and

this constituted a stress quite unlike any in the laboratory. In addition, a

number of hail storms and one freak snow caused some mechanical fracturing of

the unencapsulated cells. To avoid these effects the original rack has been

protected by the installation of a polycarbonate cover which permits free

circulation of air over the cells, but avoids the problem of liquid water in

contact with the cell surface and at the same time offers protection from

hail. Figure 6 is a photograph of the rack with its newly installed cover. The

original cells were also replaced when the cover was added.

Table 10 shows the results obtained on the initial group of

unencapsulated cells. It can be seen that several cells experienced large

amounts of degradation. In general this was due to mechanical damage induced

(we believe) as a result of water standing on the cells: one of the E-cell

leads came unattached, the N- and X-cells lost a major portion of their grids,

and the P-cell's lead, which was bonded at several points across the cell,

lost contact at many of these points. The temperature at the back of the cells

was measured at solar noon on a number of days and typically found to range

from 30 to 40 C.

30
Figure 6. Photograph of Outdoor Real Time Test Rack

31
The encapsulated cells that were subjected to outdoor testing degraded

very rapidly at first and then leveled off as can be seen from the data of

Table 10. This may be a real effect or it may be an artifact of the

measurement system. Further measurements may indicate which.

TABLE 10

UNENCAPSULATED CELL REAL-TIME TEST RESULTS

MAXIMUM POWER DEGRADATION

Cell

Type

B

C

F

E

M

N

0

P

Q

w

X

Approx Time

(hours)

7200

7300

7300

8500

7300

8900

8600

7300

7300

7300

8800

Total

Cells

1

2

1

1

1

3

2

1

2

2

1

0-2%

1

1

1

Range

3-9%

1

1

1

1

1

1

of Maximum Power Degradation

10-19% 20-29% 30-49% 50-100%

1

1 2

1

1

1

1

32
3.0 a:Si RELIABILITY INSTRUMENTATION RESEARCH

3.1 General Considerations

The ability to make repeatable electrical measurements days, weeks, and

months apart is essential to an accelerated test program. The greater the

repeatability of the measurement instrumentation the smaller the changes which

can be detected, and consequently the shorter the required acceleration time.

Under the Clemson JPL contract, a short interval measurement system was

developed for the electrical characterization of crystalline cells up to 4

inches in diameter. This instrumentation, which has been continually improved

and upgraded over the past 4 years, has proven in practice to be repeatable to

within 1%. Unfortunately, however, it cannot be used directly for a:Si cells

with this same degree of repeatability. The major difficulty involves

reproducibly setting the illumination levels over extended periods.

Reference cells have been traditionally used to adjust laboratory

simulator illumination levels to give an intensity of 1-sun. In crystalline

cell measurements the reference cell is made from a portion of a cell of the

same type as that undergoing testing and is calibrated against natural solar

radiation by a standards group at an organization such as JFL using

straightforward, but often tedious techniques. Accelerated reliability

testing, as opposed to efficiency measurement, does not require that the

illumination level be exactly 1-sun, but only that it be approximately 1-sun.

It is necessary, however, that it be repeatable. In practice, a complete cell

of the type undergoing measurement is measured at this 1-sun level (the

33
complete IV curve is taken) and set aside to be used as a standard cell

whenever it is necessary to initialize illumination levels. Unfortunately,

when working with a:Si cells it is neither possible to calibrate a reference

cell or utilize a standard cell, as is the case for crystalline cells, because

the output of a:Si cells is not necessarily stable. The output of an a:Si cell

may depend on its illumination history and other factors. Nor is it possible

to use a stable crystalline cell directly as a standard because crystalline

silicon and a:Si have different spectral sensitivities. a:Si cells are more

sensitive in the blue (uv) region of the spectrum than are crystalline silicon

cells. It is known that the ELH lamps which are used in the short interval

light source have excellent short term stability, but change their spectral

characteristics over their complete 48 hour life. Thus as ELH simulator bulbs

age and are replaced it would be possible to obtain two identical intensity

readings with a standard crystalline cell, but which would appear markedly

different if viewed by an a:Si cell.

A secondary consideration regarding illumination is the larger dimension

of many a:Si cells. Crystalline cells occupy areas normally not exceeding

4-inches x 4-inches, but a:Si submodules, which are collections of cells

monolithically interconnected on a common substrate or superstrate, may have

dimensions of up to 1-foot square with the individual cells having one

dimension as great as that of the submodule edge. Thus it is necessary to

consider an extended area light source. The Clemson accelerated test program

is primarily concerned with single a:Si cells rather than collections of

interconnected cells and consequently is not as concerned with absolute

uniformity over this extended area as with relative uniformity. In theory it

should be possible to achieve relative uniformity by making sure that any

34
variation of illumination with distance remains constant and that the cell

being measured is always geometrically positioned under the simulator in the

same way. However, in actual practice it is probably as easy to insure

absolute uniformity as relative uniformity and that is the approach that has

bee taken in designing light sources for the a:Si accelerated test program.

Much of the hardware and software used to acquire IV characteristics with

the crystalline short interval tester are directly applicable to a:Si

measurement and although a completely new system has been constructed for at Si

cells it incorporates many of the proven concepts of the previous tester.

3.2 Simulated a:Si Standard Cell

In order to achieve the necessary repeatability for reliability testing

it is absolutely essential to have a stable standard cell. Various approaches

are being followed in a number of different laboratories to simulate the

spectral characteristics of an a:Si cell using stable silicon cells. The

particular approach currently being investigated at Clemson, and which was

reported in some detail at the 24th Project Integration Meeting (PIM) in

October 1984, involves using a number of individually filtered silicon photo

diodes as illustrated in Figure 7. The output of each diode is connected to an

op-amp and the op-amp outputs are mixed together in the right proportion to

simulate an a:Si cell. The diodes, op-amps, and resistors are small and can be

mounted together in a single thermal mass. To calibrate the simulated standard

cell, the relative output of an a:Si cell, of the type to be measured, is

first measured by exposing it through the set of filters. When doing this it

is important to use a white light bias of approximately 1-sun in order to fill

35
Filter 1

Filter 2

Filter N

OUTPUT

SILICON

PHOTODIODE

Figure 7. Approach to a Simulated a:Si Standard Cell .Using

Filtered Crystalline Silicon Photodiodes

36
the traps in the amorphous film. If this is not done the measured spectral

sensitivity of the cell will be different from that which actually occurs

under terrestrial conditions. This is accomplished by using a continuous (DC)

white light and chopping (AC) the filtered light. An AC amplifier can then be

used to determine the cell's response. Following this the resistors in the

composite standard cell are adjusted so the output of each correspondingly

filtered diode is in this same proportion. To achieve adequate sensitivity the

light source used for the initial calibration should be rich in uv, such as

natural sunlight. The resultant simulated cell should have great flexibility

and allow the rapid simulation of nearly any type of construction. Such a cell

for use in accelerated testing is under development and an evaluation of its

performance will be given in the next annual report.

3.3 Large Area Light Source

Because of expense and stability considerations the approach being take

toward a large area light source is that of a multiple ELH lamp array. In such

an array it is necessary to individually adjust the intensity of each lamp in

order to produce uniform illumination over the target area. It is possible to

vary intensity by changing the distance of the lamp from the target or through

the use of neutral density filters. These approaches would require cumbersome

mechanical systems and, in the case of the filters, would not necessarily

produce a continuously variable output. Adjusting lamp voltages is the most

simple method of intensity variation, although it has the undesirable effect

of changing the spectral characteristics of the lamp by varying the filament

temperature. Under normal conditions, however, these spectral changes are

small if the voltage variation is kept within +10%, and any errors are

37
expected to be corrected by using a standard cell during initial setup.

Tungsten filament ELH lamps were chosen because, although their spectral

characteristics are not optimum, they have the advantage of being stable, low

cost, do not require ballasts, and can be operated from either the AC line or

a DC supply. In order to determine the most efficient lamp arrangement a

computer simulation was used to model the lamp characteristics. Data for this

model was obtained by recording the intensity variation of several lamps over

an eight inch square area. The lamps were free standing, not in a reflective

barrel, and the distance from the lamp to the target was adjusted from 17 to

25 inches. The data taken from four lamps was averaged to produce the

intensity variation plot shown in Figure 8. Recognizing the familiar shape of

the curve it was found that by choosing the appropriate values for the

variance and mean the intensity variation curve of a single lamp could be

mathematically represented by the normal distribution function of the form,

I/Tiro

exp

((x-X)2 + (y-Y)2

2 a

2

where a is the variance

X and Y are the means in the x and y directions.

Using this equation the intensity contribution of each lamp was obtained

and added to produce the intensity variation plot of the entire array over a

one square foot area as shown in Figure 9. The use of 11 lamps in a hexagonal

array with 6 inch spacing between the lamps and 17 inches between the light

source and target produced a fairly uniform pattern. Figure 10 is a diagram of

38
C

O<0

01o

:3wIMOou(0COccu . Ol :

 MisI: o!oco

CM

Qino»o

oCM

39
Inches]

I—I

N^X

11 '

10 -

9 •

8 -

9 -

7 •

6 -

5 -

4 -

3 -

2 -

1 -

0

• 66

• 67

• 66

• 63

• 60

• 59

• 60

•63

• 66

• 67

66

63

0

65

65

65

63

61

60

61

63

65

65

65

63

1

63

63

63

62

62

61

61

62

63

63

63

62

2

62

62

62

63

63

63

63

63

62

62

62

62

3

62

62

62

64

66

66

66

64

62

62

62

64

4

63

62

63

66

68

69

68

66

63

62

63

65

5

64

62

64

66

69

70

69

66

64

62

63 ,

66

6

63

62

63

66

68

69

68

66

63

62

63

65

7

62

62

62

64

66

66

66

64

62

62

62

64

8

62

62

62

63

63

63

63

63

63

62

62

62

9

63 65

63 65

63 65

62 63

62 61

61 60

62 61

62 63

63 65

63 65

63 65

62 63

10 11

(inches)

Figure 9. Calculated Intensities Over an Extended Area for An

11-Lamp ELH Array

40
22"

o

o

o

o

o

o

o

o

8"

8"

38"

Figure 10. Lamp Location Diagram for 11-Lamp Array

41
the 11-lamp array. Each lamp is individually adjustable with regard to angle

in order to correct for small manufacturing variations which may be present in

the lamps. Measured values of the intensity variation over a one square foot

target are given in Figure 11. Since the fabricated lamp assembly resulted in

some high intensity spots, some further physical modifications were necessary

in order to obtain a more uniform starting distribution from which the final

distribution could be veraiered through the use of small voltage excursions.

Diffusers were of little help, but by independently tilting each lamp it was

possible to achieve an initial variation which approached 10% as shown in the

pattern of Figure 12.

Once the lamps' tilt angles are adjusted, it will be necessary to be able

to individually vernier lamp voltages in such a way as to optimize

illumination uniformity. It is of course not possible to discern intensity

variations by eye so the approach that is under development is to use a 4x4

array of photodetectors. These photodetectors will in effect be multiple

filtered diodes interconnected to form simulated standard cells as described

in Section 3.2. The computer will use the output of the standard cells to

construct an array of shapes whose position varies with intensity of the proper

composition. Thus the display on the monitor will inform the operator

as to the illumination uniformity and as to which Variacs must be adjusted for

further improvement. Figure 13 is a schematic of the uniformity adjustment

system.

With the simulated standard cell there will be no difficulty in setting

the illumination level to its original value there is the possibility that a

degradation mechanism might be present which would alter only a cell's blue

42
(inches)

11 -

10

9

8

7

6

5

4 •

3 •

2

1 •

0 •

52

52

51

50

50

48

47

45

45

45

47

48

0

51

52

52

52

51

51

50

49

47

48

48

48

1

50

52

53

54

55

54

54

52

51

49

49

48

2

49

51

53

55

57

57

56

54

53

49

47

46

3

48

50

53

56

57

57

57

55

53

49

46

44

4

45

48

57

54

56

56

56

54

51

48

45

43

5

45

47

49

52

55

54

54

52

49

46

43

42

6

46

47

48

50

52

52

52

50

46

44

42

42

7

46

47

47

48

49

48

48

47

45

43

42

42

8

48

48

47

46

47

47

47

46

44

42

42

43

9

48 47

48 47

46 46

46 46

47 48

48 50

48 50

47 49

44 46

42 43

42 42

42 42

10 11

(inches)

Figure 11. Original Measured Intensity Variation of 11-Lamp Array

43
X"N

CO

<u

u

a•H

**s

11 '

10 •

9

8 •

7 •

6 -

5 -

4 -

3 -

2 -

1 •

0 •

• 60

62

• 63

• 62

• 62

62

• 62

• 62

• 62

• 62

- 62

• 60

63

64

65

63

63

62

62

62

64

65

66

62

64

66

66

65

62

62

62

62

62

66

66

62

63

66

66

64

62

61

60

60

62

64

64

61

61

64

65

64

64

62

61

61

62

62

63

60

60

62

64

64

64

63

62

62

62

62

61

59

1

60

62

63

64

64

64

64

64

63

62

61

60

I

62

63

62

62

62

63

64

64

64

63

63

52

61

62

61

61

62

63

64

63

62

64

64

64

62

62

60

60

61

63

64

63

61

64

64

63

62

61

58

60

62

63

64

62

60

62

63

62

61

60

58

60

61

62

62

60

58

58

60

59

012 3 456 7

(inches)

8 9 10 11

Figure 12. Measured Intensity Variation of 11-Lamp Array After

Lamp Tilt Adjustment

44
ELH SOIAR

SIMULATOR

o o o o

o o o o

o o o o

0)00 0

PHOTO

DIODE

ARRAY

Figure 13. Intensity Monitoring and Adjustment System Schematic

45
response and since ELH lamps are relatively weak in the blue portion of the

spectrum compared with natural sunlight, the full impact of this change might

not be observed. It is felt that such selective degradation if present would

be a second order effect. However, the possibility of enhancing the blue

portion of the spectrum through the addition of a secondary light source is

being investigated. A multi-vapor arc lamp appears to have suitable spectral

characteristics as shown in Figure 14, although some filtering will be

required to remove the peak at 600 nm.

3.4 System Configuration and Data Acquisition Procedures

A block diagram of the a:Si short interval tester is shown in Figure 15

and a photograph of the system in Figure 16. An IBM PC computer with two disk

drives and graphics printer serves as controller for the instrumentation. The

ELH lamps in the simulator may be powered by either adjustable DC supplies or

by Variacs controlling the AC line voltage. If DC were to be used program

operation would be similar to the crystalline silicon tester which uses 4 ELH

lamps, each with its own power supply. With 11 lamps in the a:Si tester,

however, the cost of power supplies (approximately $800 each) becomes

excessive. Therefore a procedure has been developed to eliminate any

illumination ripple by taking data only at the same point in the lamp's cycle.

This is accomplished by sensing the light with a separate detector and using

its output to trigger the acquisition of a data point when the illumination

reaches the 1-sun condition. Figure 17 shows the flow diagram for the data

taking routine and Figure 18 illustrates the timing diagram.

Both the cell under test and the detector are placed under the simulator,

46
tooCO0)u<aortoWl o. COB1 2Vi o0. cd00 •H

aad saa^amoueu QT
AC/DC power

digital

current source

computer

graphics printer

/ \

ELH solar

s imujator

test cell

sensing cell

A/D converter

Figure 15. Block Diagram of the a:Si Short Interval Tester

48
Figure 16. Photograph of the a:Si Short Interval Tester

49
input

sensing

eel I

vol tage

value

equal to

value

less than

revious?

input

test eel

voltag e

set

current

more

dat a

needed0

Figure 17. Flow Diagram of Data Taking Routine for AC Lamp Simulator

50
<IIoCD

OQitmH§•s- 00Os- 00i>nV) ofto(0

urtI28CDrttoS"

eH- CDOaHno(D

51-
with the detector being placed off-center since its operation does not depend

on uniformity of illumination. The computer sets the programmable power supply

to provide a particular value of current to the cell under test. The output of

the detector is sampled at an approximately 20 kHz rate until a value is

reached which corresponds to a 1-sun condition on the cell under test. When

this occurs the voltage across the cell under test is acquired and stored in

memory. The computer then advances the power supply to a new current setting

and the process is repeated until the complete IV characteristic has been

acquired. In order to check the accuracy of AC lamp operation against DC

operation data was taken using both modes as shown in Figure 19. It can be

seen that AC lamp control is able to produce quite uniform characteristics.

Because the current supply is bipolar the a:Si short interval tester when

completed will be able to obtain data in all three quadrants. It has been

found in studying crystalline cells that taking data in the far-forward and

reverse quadrants as well as the power quadrant is very helpful in determining

degradation mechanisms.

3.5 Second and Fourth Quadrant Measurement

As reported in the Fourth Annual Report, it has become apparent that some

degradation mechanisms may be more completely understood by studying the

far-forward (and reverse) regions of the cell characteristic. It is possible

to qualitatively observe these regions of the characteristic using a curve

tracer, but for quantitative data it was necessary to modify the short

interval tester. The procedure for this modification as applied to the

crystalline tester is described in this section, with a similar procedure to

be used for the a-; Si tester.

52
50

8.

jj

"i

>_x

•s

4O -

3O -

10 -

50

I

o

30 -

2O -

1O -

O-2 O.4.

Cell voltage (Volts)

O.S

OJZ O.4.

Cell voltage (Volts)

O.S

Figure 19. Comparison of AC (top) and DC (bottom) Data Collection Methods

53
Since the system A/D board was capable of converting both positive and

negative voltages, taking data in the second (reverse) quadrant simply

required a program change which caused the computer to increment the current

past Isc. Data points are recorded until the voltage limit (1.2S volts) of the

system is reached, the current limit (5 amperes) is reached, or the memory

space (1024 data points) which has been set aside for storing second quadrant

data is filled. The same number of data points are reserved in memory for each

of the three quadrants.

Fourth (far-forward) quadrant measurements require the computer to set a

negative value of current and to read the corresponding voltage through the

A/D converter. Since the current supply for the crystalline short interval

tester was unipolar (as contrasted with the a:Si supply which is bipolar) it

was necessary to use a computer controlled relay to switch polarity. A diagram

of this modification is shown in Figure 20. After all the data points are

acquired and stored in in memory, the cell characteristic is displayed on the

terminal, with all three quadrants being displayed in a single quadrant for

best graphical resolution, as shown in Figure 21.

3.6 Low Temperature Measurement Jig for Crystalline Si Cells

Cell measurements are normally made at 28 C. The jigging arrangement for

crystalline cells is such that both the front and back surfaces of the cell

are exposed to a stream of conditioned air, i.e. the cell is not in contact

with a heat sink, but quickly reaches the temperature of the air flowing over

it. However, it is possible to obtain much useful information concerning

54
CURRENT

SOURCE

COMPUTER

CELL UNDER

TEST

A/D

Figure 20. "Power Supply Modification for 4th Quadrant Operation

55
Actual Data

Video Display Trace

|

VIDEO DISPLAY

-5A

Figure 21. Graphical Representation of 2nd. 3rd .and 4th Quadrant Data

56
degradation mechanisms if it is possible to measure IV characteristics as a

function of temperature and noting the differences which result from

stressing. Elevated measurement temperatures do not present a problem as it is

possible to warm the cells by increasing the temperature of the normal 28 C

conditioning air. In a similar fashion the cell may be cooled somewhat below

28 C by chilling the conditioning air. However, when the temperature

approaches the ambient dewpoint moisture will condense on the cell surface

altering the short circuit current. At a sufficiently low temperature the

moisture will form ice crystals, further decreasing the transmission of light.

To avoid this problem the cell was mounted on a heat sink which was enclosed,

but which had a plexiglass top so the simulator could still illuminate the

cell. A photograph of the jig is shown in Figure 22. Methanol, which had been

cooled by dry ice, was pumped through the heat sink. The inside of the

enclosure was continuously purged with dry (less than 10 ppm H20) nitrogen.

When the jig was tested it was found to eliminate moisture condensation on the

cell, but at a cell temperature less than -10 C moisture began to condense on

the outside of the plexiglass. However, this could be eliminated by blowing

slightly warmed air over the outside of the jig enclosure. When making

measurements as a function of temperature at elevated temperatures a

plexiglass sheet is placed between the light source and cell even though the

enclosed jig is not used, to avoid any spectral differences.

Measurement of the characteristics of a:Si cells as a function of

temperature is expected to be more difficult because the back surface is not

necessarily a good electrical or thermal conductor as is the case in

crystalline cells. In addition, the geometry of a:Si cells has not yet been

standardized. Variable temperature measurement jigs for a:Si are presently

57
Figure 22. Photograph of Low Temperature Measurement Jig for

Crystalline Cells

58
being designed.

3.7 New Clemson Reliability Research Facility

The scanning Auger microscope mentioned in the 4th Annual Report has been

delivered and installed. Preliminary analyses have been run of selected a:Si

cells to develop proper procedures. In order to acquaint the photovoltaic

community with its capabilities a workshop is planned sometime after the first

of the year, which will include hands-on experience with the instrumentation.

A photograph of the installed Auger microprobe is shown in Figure. 23.

59
Figure 23. Photograph of Clemson's Auger Microprobe

60
4.0 CONCLUSIONS

Reliability Research Test Program (Section 2.0) ~ Preliminary step

stress tests on one submodule have shown the existence of a transition

temperature between 130 and 140 C where a sudden change is detected in the

electrical characteristics of the cells. This temperature, at which presumably

an extraneous degradation mode was introduced, is somewhat lower than the 150

C upper temperature stress used in testing unencapsulated crystalline cells,

but at the same time is somewhat higher than the 95 C upper temperature bound

for testing encapsulated cells. If this temperature proves to be a real upper

bound it will mean that a new accelerated test schedule will need to be

developed for a:Si cells.

Reliability Instrumentation Research Program (Section 3.0) — The two

factors which make possible effective accelerated testing are a repeatable

measuring system and the existence of a stress window where the stress is

sufficiently high to result in a satisfactory acceleration factor, but not so

high as to introduce extraneous degradation modes. The major reliability

instrumentation research effort during this reporting period was aimed at

achieving these two factors relative to thin film a:Si cells. The key to a

repeatable measurement system is a stable reference cell with the proper

spectral characteristics and an approach to fabricating such a cell from

single crystalline silicon is currently being investigated.

61
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5.0 NEW TECHNOLOGY

The simulated a:Si reference cell, described in Section 3.2 and

illustrated in Figure 7, and its use to obtain uniform illumination with a

multiple lamp source, as described in Section 3.3 and illustrated in Figure

13, are considered items of new technology.

63
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6.0 PROGRAM RESEARCH CONTRIBUTIONS

Since the previous Annual Report was issued the project has made a number

of documented contributions to the photovoltaic community. These are

summarized in this section with abstract material reproduced in Appendix B.

Publications and Presentations:

1. Misiakos, K., and Lathrop, J.W., "Barrier Height Determination of

Schottky Contacts Formed at the Back Contact-Semiconductor Interface of

Degraded Solar Cells," Proceedings of the IEEE Region 3 Conference

(SOUTHEASTCON), Louisville, KY, April 1984.

2. Lathrop, J.W., Misiakos, K. and Davis, C.W., "Degradation in Silicon

Solar Cells Caused by the Formation of Schottky Barrier Contacts During

Accelerated Testing," Proceedings of the 17th Photovoltaic Specialists

Conference, Orlando, FL, May 1984.

3. Lathrop, J.W. and Royal, E.L., "Assessment of Degradation in

Crystalline Silicon Solar Cells Through the use of an Accelerated Test

Program," Proceedings of International PVSEC-1, Kobe, Japan, November,

1984.

4. Lathrop, J.W. and Misiakos, K., "Degradation of Silicon Solar Cells

Due to the Formation of Schottky Barrier Contacts," Proceedings of

International PVSEC-1, Kobe, Japan, November, 1984.

Theses:

White, F.B., "Design Considerations for a Constant Flow Sulfur Dioxide

Accelerated Test System," MS Electrical Engineering Thesis, Clemson

University, December 1983.

Misiakos, K., "Schottky Barrier Formation in Terrestrial Solar Cells,"

MS Electrical Engineering Thesis, Clemson University, August 1984.

65
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7.0 REFERENCES

1. Investigation of Reliability Attributes and Accelerated Stress Factors

on Terrestrial Solar Cells, DOE/JPL-954929, 1st Annual Report, May 1979.

2. op. cit., 2nd Annual Report, April 1980.

3. op. cit., 3rd Annual Report, January 1981.

4. op. cit., 1981 Summary Report, June 1982.

5. Investigation of Accelerated Stress Factors and Failure/Degradation

Mechanisms in Terrestrial Solar Cells, DOE/JPL-954929, 4th Annual

Report, October 1983.

67
APPENDIX A

PUBLICATION ABSTRACTS

69
1st International Photovoltaic Science and Engineering Conference

Kobe, Japan November, 1984

Degradation of Silicon Solar Cells

Due to the Formation of Schottky Barrier Contacts *

Jay W. Lathrop and Konstantinos Misiakos

Electrical and Computer Engineering Department

Clemson University, Clemson, SC 29631 USA

Unencapsulated crystalline silicon solar cells, fabricated by diffusing

p-type impurities into a moderately doped n-type substrate, show severe

degradation of maximum power output when subjected to accelerated stress testing

at elevated temperatures. Examination of degraded cell IV characteristics

revealed a peculiar shape in the far forward region. Instead of the continual

downward curvature characteristic of normal cells, these degraded cells exhibited

an inflection point with an upward curvature at higher voltages. The presence of

a reverse curvature of this sort implied that stressing the cells had caused the

formation of a Schottky barrier at the ohmic back contact.

Analysis of the IV characteristics of degraded cells at different

temperatures permits modelling the Schottky barrier contact and allows

calculation of the barrier height. It was found that little or no power loss was

observed as long as the saturation current of the Schottky barrier contact was

above the short circuit current of the cell, but as the rectifying contact became

better, i.e. the saturation current became less than Isc, power loss became

apparent. This occurred at an effective barrier height of approximately 0.35 eV.

In addition there was evidence of an interfacial layer since the ideality factor

was close to 2.

It is hypothesized that the rectifying contact formation mechanism is the

saturation of dangling silicon bonds at the interface, causing a reduction in the

surface state density. A series of experiments involving different ambients and

different encapsulation conditions was performed in an attempt to define the

exact mechanism. Although not conclusive, evidence points to the dissociation of

water vapor molecules into hydrogen and oxygen at the metal surface with

subsequent diffusion of atomic hydrogen through the metal to the semiconductor

interface. This would explain the more rapid degradation observed for cells

encapsulated with organic or aluminum foil substrates compared to unencapsulated

cells. Howerver, oxygen may also play a role in causing degradation.

* This work supported in part by JPL under Contract 954929

JJ

71
1st International Photovoltaic Science and Engineering Conference

Kobe, Japan November, 1984

\

\

Assessment of Degradation in Crystalline Silicon

Solar Cells Through the Use of an Accelerated Test Program *

J.W. Lathrop

Electrical and Computer Engineering Department

Clemson University, Clemson, SC 29631 USA

E. Royal

Jet Propulsion Laboratory, MS 506-328

4800 Oak Grove Drive, Pasadena, CA 91109 USA

The reliability characteristics of solar cells intended for low-cost

terrestrial applications, either central power generation or distributed

residential usage, will be a key factor in the determination of the economic

feasibility of such systems. Typical cost models are based on a 30-year module

life, which is generally accepted to mean that the power output of an array

should not decrease by more than 10% during this period. This degree of system

reliability requires the use of very stable cells. Clemson University, under

JPL sponsorship, has had a program to assess the relative reliablity

attributes of crystalline silicon solar cell technology through laboratory

accelerated stressing since 1977. This paper discusses the methodology of

accelerated solar cell testing, the specific tests which have been developed,

and examples of the method1s usefulness as applied to state-of-the-art cells..

The program, which is continuing, has thus far tested both encapsulated

and unencapsulated individual cells. Twenty-two types of commercially

available cells have been tested in unencapsulated form and nine of these have

also been tested in encapsulated form. Seven different encapsulation systems

were evaluated which used various combinations of glass, steel, Tedlar ,and

aluminum foil with EVA and EMA potting compounds. The significant failure

modes observed are discussed and interpreted in terms of physical and chemical

effects.

* This work supported in part by JPL under Contract 954929.

72
17th Photovoltaic Specialists Conference

Orlando, FL May 1984

DEGRADATION IN SILICON SOLAR CELLS CAUSED BY THE FORMATION OF

SCHOTTKY BARRIER CONTACTS DURING ACCELERATED TESTING *

J. W. Lathrop, K. Misiakos, and C.W. Davis

Department of Electrical and Computer Engineering

Clemson University, Clemson, SC 29631

(803) 656-3375

EXTENDED ABSTRACT

Using laboratory accelerated testing procedures, Clemson University has

been conducting reliability research aimed at understanding failure/

degradation mechanisms which occur at the basic cell level. This paper

describes a new degradation mechanism recently found in one type of cell

construction. Cells subjected to accelerated stress conditions display severe

Pm degradation accompanied by the formation of a distinctly non-linear IV

characteristic indicating the formation of a rectifying Schottky barrier at

the back contact. The cell had a lightly doped substrate (no back surface

field) and relied on a high concentration of surface states, obtained by

sandblasting the substrate prior to plating the contact, to give a low barrier

height and consequent ohmic contact. This paper describes the degradation

effect and offers an explanation for its occurrence.

If the high concentration of surface states is reduced for any reason the

barrier height will rise. On lightly doped substrate material it is possible

for the barrier to become sufficiently high that Schottky barrier

rectification will occur between the back metal contact and the semiconductor

substrate. This rectification manifests itself as an unusual non-linearity in

the cell's VI characteristic, specifically as an upward concavity to the

characteristic in the 4th quadrant (V Voc). Simple computer modelling of a

degraded VI characteristics using SPICE, gave an excellent fit provided a poor

diode, i.e. one with high leakage, were in series with the back contact. .

It has been reported in the literature (1) that surface states result

from dangling silicon bonds at the surface. Presumably sandblasting, which

damages and distorts the lattice at the surface, increases the number of

surface states. It is felt that impurity atoms from the test environment, most

likely hydrogen from dissociated water vapor molecules, diffuse through the

metal contact to the semiconductor interface, tying up the unshared silicon

electrons and reducing the concentration of surface states. This increases the

barrier height and results in a rectifying Schottky barrier.

When encapsulated cells were subjected to 85 C/85%RH stress, cells having

73
an hermetic substrate such as glass or steel showed reduced Pm degradation,

whereas those with non-hermetic substrates such as Tedlar showed appreciably

greater degradation than even the unencapsulated controls. The reduced

degradation observed with hermetic substrates confirms that the effect is

moisture related. The increased effect observed with non-hermetic substrates

is reasoned to be due to the large difference between diffusion coefficients

of water vapor and hydrogen through organic materials. Water vapor is able to

easily penetrate the organic encapsulant and reach the metal surface where it

dissociates into hydrogen and oxygen molecules which, because of lower

diffusion coefficients, become trapped at the interface. A higher

concentration of hydrogen at the substrate - contact interface permits a more

rapid saturation of the silicon dangling bonds at the metal - semiconductor

interface.

Analagous reasoning can also explain the observation that foil substrates

did not offer hermetic protection. Cells encapsulated with foil substrates

degraded as fast or faster than those with non-hermetic substrates. It is

reasoned that the 1-mil thickness of aluminum encased in an organic coating

effectively blocks water molecules, but permits penetration of hydrogen atoms.

(1) Harrington, W.L. et al, "Low-energy Ion Scattering Spectrometry (Iss)

of the Si02/Si Interface," Applied Physics Letters, vol. 27, p.644

(1975)

* The research described in this paper was supported by the Jet Propulsion

Laboratory of the California Institute of Technology and was sponsored by the

Department of Energy through an interagency agreement with NASA.

74
IEEE Region 3 Conference (SOUTHEASTCON) ^ 0

Louisville, KY April 1984 t<ff5/5v

Barrier Height Determination of Schottky Contacts Formed at

the Back Contact-Semiconductor Interface of Degraded Solar Cells *

K. Misiakos and J.W. Lathrop

Department of Electrical and Computer Engineering

Clemson University, Clemson, SC 29631

(803) 656-3376

ABSTRACT

Solar cells which have degraded as a result of accelerated environmental

testing often exhibit a non-linear VI characteristic indicating the formation

of a rectifying Schottky barrier at the back contact. In order to study the

fundamental physical, chemical, and metallurgical mechanisms causing this

barrier it is necessary to be able to quantitatively determine the barrier

height. This paper describes a technique used to measure the barrier height of

a contact even though that contact is physically inseparable from the cell.

The method is based on interpreting the characteristics of degraded cells

in terms of an equivalent circuit. The equivalent circuit is shown in Figure

1, where Dl represents the cell's diffused pn junction and D2 represents the

Schottky barrier contact which has formed at the cell's back contact. It has

been found experimentally that a resistor, R2, shunting D2 must be included.

The experimentally observed VI characteristic can be interpreted in terms of

this equivalent circuit. As shown in Figure 2, 5 distinct regions of a

degraded cell's characteristic can be identified. In regions 1,2, and 3, D2 is

strongly forward biased while in regions 3,4, and 5, Dl is strongly forward

biased. In region 1, Dl is forward biased, but not conducting appreciably,

while region 2 represents the transition of Dl to strong forward conduction.

In region 5, D2 is reverse biased and region 4 represents its transition to

strong forward conduction. By generating simplified equivalent circuits for

the various regions it is possible to determine values for Rl and R2, and the

ideality factors and saturation currents of diodes Dl and D2. The barrier

height may be determined from a knowledge of the saturation current of D2 as a

function of temperature.

The paper illustrates calculation of the Schottky barrier height using

experimentally determined VI curves taken at different temperatures.

Experimental results are given showing how the barrier height of solar cell

contacts change with the amount of applied environmental stress.

* This work supported in part by JPL under Contract 954929

75