Determining Terrestrial - Jet Propulsion Laboratory Reliab... · Determining Terrestrial Solar Cell...

5101-163.

Low-Cost Solar Array Project

Determining Terrestrial Solar Cell Reliability Proceedings of Workshop Held at Clemson University Clemson, South Carolina May 1-2, 1980

Organized by E. L. Royal Jet Propulsion Laboratory J. W. Lathrop Clemson University

November 1980

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

Prepared by the Jet Propulsion Laboratory, California Institute of Technology, for the Department of Energy through an agreement with the National Aeronautics and Space Administration.

The JPL Low-Cost Solar Array Project is sponsored by the Department of Energy (DOE) and forms part of the Photovoltaic Energy Systems Program to initiate a major effort toward the development of low-cost solar arrays,

This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy, nor any of their employees, nor any of their contractors, subcontractors, or 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.

PREFACE

This document is an outgrowth of a workshop on "Determining Terrestrial Solar Cell Reliability," which was held May 1-2, 1980, at Clemson University, Clemson, South Carolina. The workshop was organized jointly by the Jet Propulsion Laboratory (JPL) and Clemson University, and was sponsored by the Low-Cost Solar Array Project (LSA) of the U.S. Department of Energy.

The purpose of the workshop was the critical review of silicon solar cell test results from a reliability testing program being carried out by Clemson University under contract to the Engineering Area of JPL/LSA. Since 1977 Clemson has conducted experimental reliability investigations on more than 1000 unencapsulated solar cells procured from seven photovoltaic industry manufacturers.

A total of 33 persons attended the workshop, representing fourteen organizations including private industry, national laboratories, and universities. This group of basic scientists, design engineers, and personnel involved in quality assurance and module/array field reliability participated actively in two days of workshop activities which included technical sessions, a tour of the test facilities, review of reliability test methods for solar cells, critique of test results, and moderated discussion sessions. The workshop provided a forum for productive discussion of various aspects of solar cell reliability by a broad spectrum of photovoltaic industry personnel. Much valuable information was exchanged, and reconunendations were made regarding the validity of reliability data obtained to date and the direction in which future work should be channeled.

Preparation of this document was a collaborative effort by the Engineering Area of the Low-Cost Solar Array Project, Jet Propulsion Laboratory, and the Department of Electrical and Computer Engineering, Clemson University. Included are reproductions of graphic presentation materials and highlights of discussions related to solar cell reliability test methods.

Inquiries regarding details of the contents or requests for additional information should be directed to Mr. E. L. Royal of JPL or Professor J. W. Lathrop of Clemson University.

iii

PARTICIPATING ORGANIZATIONS

ARCO Solar, Inc.

Battelle Columbus Laboratories

Clemson University

Jet Propulsion Laboratory

MIT Lincoln Laboratory

Mobil Solar Energy Corp.

Motorola, Inc.

National Bureau of Standards

Photon Power, Inc.

Sandia Laboratory

SERI

SES, Inc.

Spectrolab, Inc.

Solar Power Corp.

iv

I.

II.

TABLE OF CONTENTS

INTRODUCTION . . . . . . . . . . . . . . . • • • • • • • • • • • • • 1-1

Reliability Testing Approach For Evaluation of Terrestrial·Silicon Photovoltiac Cells (E.L. Royal) ••••••• 1-1

TECHNICAL SESSIONS • • • • • • • • • • • • • • • 2-1

TEST METHODS AND LABORATORY PROCEDURES

1. Principles of Acceleratred Stress Testing (J.L. Prince) •••••••••• • • • • • • • • • 2-3

2. Laboratory Methods and Procedures Used During Reliability Stress Testing of Silicon Solar Cells (J.W. Lathrop) • • • • • • • • • • • • • • • • •••• 2-15

3. Electrical Measurement Considerations for Reliability Testing of Photovoltaic Cells (C.R. Saylor) •••••••••• 2-37

TEST RESULTS, DATA MANAGEMENT AND ANALYSIS

1. Data Management and Analysis of Solar Cell Test Measurements (J.F. Christ) •••••••••••••••• 2-59

2. Studies to Determine Effects of Second Quadrant Operation on Solar Cell Reliability (R.A. Hartman) •••••• 2-69

3. Test Results from Clemson Solar Cell Stress Test Program (J.L. Prince) • • • ••••••••••••••• 2-87

III. WORKSHOP SESSIONS

HIGHLIGHTS OF DISCUSSIONS.

SUMMARY OF DISCUSSIONS

. . . . . •• 3-1

•• 3-5

LIST OF ATTENDEES •••••••••••••••••••••••••• 4-1

V

SECTION I

INTRODUCTION

Reliability Testing Approach for Evaluation of Terrestrial Silicon Photovoltaic Cells

E. L. ROYAL Jet Propulsion Laboratory

Prior to initiation of the JPL/LSA solar cell test and study program little reliability data on terrestrial photovoltaic cells existed. Indeed, no valid set of test methods with which to generate suitable high quality cell reliability attribute-type data was available. The purpose of the JPL/LSA program, which Clemson University was selected to carry out, was to develop suitable reliability test methods, and using those methods to generate data that would provide quantitative indicators or comparative measures of reliability on different types of state-of-the-art cells.

The approach selected for this program was to initiate the testing phase by accumulating data on unencapsulated cells. Some of the considerations involved included: a) selection of types and levels of stress to be applied, b) measurement precision, c) cell failure and degradation criteria, and d) sample quantities to satisfy statistical test design criteria. The cell types included in the testing program were all procured from photovoltaic manufacturers with the request that they be randomly selected.

The major interest initially was to observe to the best precision practical any change in Pmax as a function of stress and time. However, many physical changes that occur are visible but often are not quantiative.

Another objective was to develop a schedule and sequence of testing that would enable quantitative evaluation of newly developed cell types for comparative purposes. These tests should accelerate any latent life-limiting failure mechanism. The expected cell failure modes are degradation in Pmax, physical changes (cracks, discoloration, etc.) or failures (opens or shorts).

Subsequent phases of the program include testing of encapuslated cells, and study of possible reliability implications associated with cell operation in the second quadrant (reverse bias). Two new efforts being started are: a) failure analysis on cells that perform poorly in Clemson's reliability stress tests and b) comparison of laboratory test results with field failure results.

1-1

~ I

N

LOW-COST SOLAR ARRAY PROJECT

OBJECTIVES

• DEVELOP TEST METHODS/MEASUREMENT TECHNIQUES FOR USE IN PHOTOVOLTAIC CELL EVALUATION

• DETERMINE CELL FAILURE MECHANISMS AND RELIABILITY ATTRIBUTES OF STATE-OF-THE-ART PHOTOVOLTAIC CELLS

• DEVELOP A TEST PLAN/ SCHEDULE OF TESTS WHICH GENERATES MEANINGFUL RELIABILITY DATA ON PHOTOVOLTAIC CELLS

--------------------BOTTOM LINE:

• PROVIDE MEANINGFUL RELIABILITY DATA (AT CELL LEVEL) IN SUPPORT OF MODULE/ ARRAY DESIGN ACTIVITIES BEING PERFORMED BY LSA ENGINEERING AREA AND PHOTOVOLTAIC INDUSTRY AT LARGE

ELR 5/1/80

DESIGN OF

· TESTS


..

ANALYSIS OF

PRE-STRESS DATA

APPROACH

CELLS FOR TEST

PROGRAM

PRE-STRESS

REFERENCE CELLS

+- ELECTRICAL +MEASUREMENTS

INITIAL INSPECTION

OF CELLS

CELL TEST JIGS (DES I GN/

FABRICATE)

~RELIABILITY/STRESS TESTING DATA ANALYSIS STRESS TEST s

1 • STRESS TEST s2 • STRESS TEST s

3 • • Sn • • • • • • • • • • • • • • • • •

ELR 5/1/80


DESIGN OF TESTS

PROBLEMS

• CELL FAILURE MECHANISMS/FAILURE MODES LARGELY UNKNOWN

• NO ACCEPTABLE DEFINITION OF FAILURE

• TESTS MUST PRODUCE FAILURES TO BE SUCCESSFUL

CONSIDERATIONS

• UTILIZE RELIABILITY TEST METHODS DEVELOPED FOR SEMICONDUCTOR DEVICES

• DEVELOP MULTI PHASE TEST PROGRAM TO ACCOMPLISH OBJECTIVES

ELR 5/1/80

t PMAX


TYPICAL PMAX DEGRADATION-vs-TIME DATA

PROBLEM: NO ACCEPTED DEFINITION OF FAILURE

FAILURE DEFINITION POSSIBILITIES

A. % DEGRADATION FROM INITIAL PMAX VALUE

B. DEGRADES BELOW A PRESET LIMIT ON PMAX

C. OTHER

TIME (OR TEST CYCLES)

ELR 5/1/80


CELL TYPES SELECTED FOR EVALUATION

• SILICON/TERRESTRIAL/FLAT PLATE DESIGN APPLICATIONS

• PRODUCTION TYPES - WITH ONE EXCEPTION

• RANDOMLY SELECTED? (REQUESTED)

• UNENCAPSULATED; ENCAPSULATED TYPES INCLUDED IN LATEST ROUND OF TESTING .

• LEADS ATTACHED (TO FRONT SIDE CONTACTS ONLY) -LEAD ATTACHMENT METHOD USED SAME AS IN MODULE DESIGN


·cELL TEST ·JIG REQUIREMENTS {CONSIDERATIONS)

• ACCOMMODATE SEVEN (7) DIFFERENT SIZES/CONFIGURATIONS OF CELL TYPES TESTED

---, 4 IN.

-------_ _j_ ---i ROUND CELLS

3 IN.

__ _J

0-t

2 IN. _ _j_

RECTANGULAR CELLS

D SQUARE CELLS

• ACCOMMODATE DIFFERENT NUMBER OF LEADS AND ATTACHMENT ARRANGEMENTS ELR 5/1/80


TEST STRATEGY OVERVIEW {SIMPLIFIED)

CELL CELL POST-TEST PRE·STRESS MEASURE- MEASURE- /\\EASUREMENTS i\\EASUREJ\\ENTS MENTS AND MENTS AND AND M,D INSPECTION I I INSPECTION I I INSPECTION I I INSPECTION

'HSTRESS ~ HSTRESS L___i 'HSTRESS ~~ I APPLiED~ I APPLIED~ I APPLIEDJ' I

OPTION 1 I I I I I I , l~ I I

--~T •I I,. ~T •I 1,.--~T ~

I 1

I ~---------------4STRESS APPLIED]1-------1,.ji----1•~1

l !

DATA ANALYSIS

I I ', I . I r - - - - -, ['------ ___ ---- _________ -t- __ -~DATA ANALYSIS:

I I L _____ J

ELR 5/1/80

~ LOW-COST SOLAR ARRAY PROJECT

v~u .- ANALYSIS OF PRE-STRESS MEASUREMENT DATA PURPOSE

• PROVIDE REFERENCE TIME ZERO (t0) DATA - FOR COMPARISON WITH SUBSEQUENT MEASUREMENTS

• DEVELOP/ANALYZE STATISTICS FOR POPULATION OF EACH CELL TYPE

I.E. • RANDOM SAMPLE OR BIASED SAMPLE • LOT-TO-LOT VARIABILITY

• DEVELOP DATA BASE ON EACH CELL TYPE

I.E. •PARAMETER DISTRIBUTIONS • STANDARD DEVIATIONS •VISUAL INSPECTION PROF I LES •OTHER

TOOLS/SUPPORT UTILIZED

• DATA MANAGEMENT SYSTEM

• COMP LITER UTI LIZA Tl ON

• DATA STORED ON DI SK - RETRIEVAL POSSIBLE VIA DATA LINES • SAS SOFTWARE PACKAGE FOR ANALYSIS

ELR 5/1/80

..... I .....

0


ACTIVITIES SUPPORTED BY CELL TEST PROGRAM (SAMPLE)

RELIABILITY CELL TESTING FACTORS ACTIVITY

ENVIRONMENTAL ACCELERATED STRESS STRESSES TESTS:

• PHASE I • PHASE 11 •ET AL

MODULE/ ARRAY - REVERSE BIAS/HOT SPOT INDUCED HEATING TESTS STRESSES

ACTIVITY SUPPORTED

• RELIABILITY ENGINEERING

• TEST METHODOLOGY DEVELOPMENT

• STANDARDS DEVELOPMENT

MODULE/ ARRAY DES I GN

OTHER SPECIAL TESTS/STUDIES i.e. • DEGRADATION

COMPARISONS -ENCAPSULATED CELLS vs UNENCAPSULATED CELLS

• MODULE DESIGN/ENCAPSULATION TASK STUDIES

ELR 5/1/80

,_. I ..... ,_.


ELECTRICAL MEASUREMENTS -CONSIDERATIONS {PRE-STRESS AND POST-STRESS)

• CELL 1-V CURVES - HARD COPY PLOT DES I RED

• REPEATABILITY/ACCURACY SOME POTENTIAL SOURCES OF ERROR ARE:

• LIGHT IRRADIATION (FROM SIMULATOR)

• STABILITY OF SOURCE • VARIATION ACROSS TEST CELL SURFACE AREA

• REFERENCE CELL UNAVAILABILITY

• CELL TEMPERATURE CONTROL

• PROBE CONNECTIONS

• ELECTRICAL INSTRUMENTATION

• OTHER

• HIGH THROUGHPUT RATE DESIRED

• SHORT SETUP TIME/SIMPLIFIED CONTROLS

• SHORT INTERVAL TESTER

ELR 5/1/80

,_. I ,_.

N


ITEM

REFERENCE CELLS

LIGHT IRRADIATION (AS A STRESS)

CELL TEST SAMPLES

ELECTRICAL MEASUREMENTS

MECHANICAL MEASUREMENTS

CONSTRAINTS

PROBLEM

NOT AVAILABLE FOR SOME CELL TYPES

CONSIDERED NOT PRACTICAL

LIMITATIONS ON QUANTITY OBTAINED

LIMITATIONS ON AMOUNT PERFORMED

LIMITATIONS ON AMOUNT PERFORMED

CONSIDERATIONS

• DEVELOPED WORKAROUND

• SOLAR SIMULATOR USED ONLY DURING ELECTRICAL MEASUREMENTS

• SOME TYPES UNAVAILABLE IN QUANTITIES DESIRED

• TEST PLANS REVISED

• IN SERIES WITH ALL TESTS -WORKAROUNDS DEVELOPED

• IMPROVED TESTER BEING DEVELOPED

• TESTS ARE DESTRUCTIVE

• DESIGNED TESTS TO MAXIMIZE DATA RETURN

ELR ~/1/An

I-' I

I-' Lo,)

TEST PHASES

PHASE I ( STRESS TESTING AND MEASUREMENTS)

PHASE II (STRESS TESTING AND MEASUREMENTS)

MECHANICAL TESTS (CONTACT PULL STRENGTH AND METALIZATION ADHERENCE TESTS)


MULTIPHASE TEST PROGRAM

CELLS TESTED

SMALL - SIZE SAMPLE QUANTITIES

LARGE - SIZE SAMPLE QUANTITIES

MODERATE - SIZE SAMPLE QUANTITIES

PURPOSE

• GENERATE DATA TO VALi DATE APPROPRIATE (A) TESTS AND (B) STRESS LEVELS

•PROVIDE DATA TO ESTABLISH SAMPLE SIZES REQUIRED FOR PHASE II TESTING

•TEST METHODOLOGY DEVELOPMENT

• GENERATE DATA FOR RELIABILITY CHARACTERIZATION OF CELLS

• DEVELOP TECHNIQUES TO PERFORM MECHANICAL TYPE RELIABILITY TESTS

• GENERATE MECHANICAL PULL STRENGTH/ ADHERENCE DATA (AS A FUNCTION OF VARIOUS STRESSES APPLIED).

ELR 5/1/80


DEVELOPMENT OF CELL MEASUREMENT METHODOLOGY

EXAMPLES: · I MP ROVED: KELVIN PROBE ATTACHMENTS (VOLTAGE) COMBINED WITH THERMOCOUPLE PROBE

USED ON BACK SI DE OF CELL

CELL 1-V PLOT GENERATION IMPROVED:

ELH TYPE SOLAR SIMULATOR

X-Y PLOITER:FROM ORIGINAL CURVE TRACER

IMPROVED: GREATER STABILITY, MORE RELIABLE

PHOTOD IODE SENSOR TYPE NEW DEVELOPMENT: CONTROL Cl RCUI T HELPS OFFSET LACK OF A REFERENCE CELL

MICRO PROCESSOR-CONTROLLED CELL TESTER NEW DEVELOPMENT: SHOULD IMPROVE MEASUREMENT THROUGHPUT AND ACCURACY

ELR 5/1/80

SECTION II

TECHNICAL SESSIONS

TEST METHODS AND LABORATORY PROCEDURES

1.

2.

3.

Principles of Acceleratred Stress Testing (J.L. Prince) ••••••••••••••

Laboratory Methods and Procedures Used During Reliability Stress Testing of Silicon Solar Cells (J.W. Lathrop) • • • • • • • • • • • • •••

Electrical Measurement Considerations for Reliability Testing of Photovoltaic Cells (C.R. Saylor) •••••

2-1

2-3

2-15

2-37

Principles of Accelerated Stress Testing

J. L. PRINCE Clemson University

Abstract

The long history of accelerated stress testing in the integrated circuit industry has verified its value in assuring suitable product quality with respect to both design and manufacturing. It is expected that accelerated stress testing will play a similar role in helping to determine the reliability of both flat-plate and concentrator-type terrestrial solar cells. The purpose of this presentation is to lay the theoretical groundwork for the workshop by describing how time-to-failure distributions are derived and plotted, how failure mode distributions are determined, and how activation energies are calculated using long-term analagous data from integrated circuit accelerated stress testing.

2-3

N I

+:'-

RELIABILITY TESTING APPROACHES

• "USE-CONDITION" STRESSES (TEMP, HUMIDITY, ETC.)

• N .... oo FOR PRACTICAL TEST TIMES FOR MANY THINGS (e.g., IC' s, SOLAR CELLS)

• ACCELERATED STRESS TESTING

• APPLICATION OF HIGH STRESS (e.g., TEMP) TO EXCITE/ ACCELERATE USE- CONDITION FAILURE MECHANISMS

• EXTRAPOLATION OF EXPERIMENTAL RESULTS (FAILURE OR DEGRADATION RATE) BACK TO USE CONDITIONS

• MUST SAFEGUARD AGAINST INTRODUCTION OF ANOMALOUS FAILURE MECHANISMS

• MUST DETERMINE ACCELERATION FACTORS TO USE IN EXTRAPOLATION

• MUST SEPARATE INFANT MORTALITIES FROM MAIN BODY OF TEST SAMPLE

N I

l./1

NEED

• ASSUME THAT A= FAILURE RATE = 10%/20 YEARS @ 55oc

TEMP ACCELERATION, E = • 5eV

t = TEST TI ME = 5 x 1a3 HR

• AT 55°c, FOR 90% CERTAINTY OF SEEING AT LEAST 1

FAILURE, N = 13. 7 x 1a3 UNITS

• AT 75°c, N = 4. 92 x 1a3 UN ITS

• AT 115°c, N = 1. 1 x 1a3 UN ITS

• AT 165°c, N = 166 UN ITS

N I a,

ACCELERATED STRESS TESTING

• DETERMINE TIME-TO-FAILURE DISTRIBUTIONS

• CONSTANT FAILURE RATE (EXPONENTIAL DIST)

• LOGNORMAL DISTRIBUTION

• OTHER (E.G., WE I BULL, etc ••.... )

• DETERMINE FAILURE MODE DISTRIBUTIONS

• ASSOCIATE FAILURE MODES AND FAILURE MECHANISMS

N I

-..J

ACCELERATION FACTORS

• TEMPERATURE: ARRHENIUS RELATIONSHIP

REACTION RATE = R e-E/kT 0

E = ACTIVATION ENERGY: 0.3eV<E <l.85eV

• TEMP CYCLING

• HUMIDITY

• VOLTAGE

• CURRENT/CURRENT DENSITY

N I

00

CLASSICAL FAILURE RATE vs TIME BEHAVIOR: THE "BATHTUB" CURVE

LONG-LIFE DEVICES

,• INFANT I J(t) MORTALITY

I "CONSTANT" FAILURE RATE LIFE

WEAR.OUT

_______ _... ____________ ....._ ______ , I •-... , • _1".ood_;_.N"ood + 11MNIM n Infant mortality (IM): ., -~ _;_ ~

Nvooc1 + NuA

where 10ood

11M

Ngood

NIM

--------

failure rate of the 11good" devices f allure rate of tha .. Inf ant-mortality" devices number of the good da~lc11 number of the Inf ant mortality devices

TIME-DEPENDENT FAILURE MECHANISMS IN SILICON SEMICONDUCTOR DEVICES

DEVICE RELEVANT ACCELERATING ACCELERATION ASSOCIATION PROCESS FACTORS FACTORS .I!1=APPARENT ACTIVATION ENERGY)

SILICON OXIDE SURFACE CHARGE MOBILE IONS. T . BIPOLAR: £1 = 1.0 • 1.05 el AND ACCUMULATION Y, T MOS: E1 = 1.2 • 1.35 el

SILICON·SILICON · DIELECTRIC Ll E OXIDE INTERFACE BR[AKDOWN

CHARGE E. '· ass ll Ea = 1.3 eY [SLOW TRAPPING) INJECTION

METllllllTIDN ELECTRO MIGRATION '· i. I. '· j £ A = D. 5 • 1.2 e Y

GRADIENTS i TO i4 Of T AND i.

GRAIN SIZE COF.ROSION CONTAMINATION. I. Y. T STRONG H EffECT

DIEMICAL HUMIDITY (HJ Ea::0.3 · O.& eY ff OR ELECTROLYSIS) GALVANIC

'· l Y MAY HA VE THRESHOLDS

ELECTROL fflC CONTACT T .. METllS. YARlm

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C ff.GI GROWTH BOND MECHANICAL STRENGTH IHTERFAC£S fATICUE lEMPERlTURE T EXTREMES

CYCLING, BOND IN CYCLINli STRENGTH

HERW.£ TICITY SEAL llAKS PRESSURE PRESSURE D1ff £R£NT1Al, ATMOSPHERE

I .. VOLTAGE [ • ELECTRIC FIELD I • AREA T .. 1£MP£RA TUR( j • CURRENT DENSITY B • HUMIDITY

2-9

RELATIVE REACTION RATE (FAILURE RATE) FOR MECHANISM OBEYING THE ARRHENIUS

RELATIONSHIP - R---exp (-E/kT)

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d eo • ~ -)( -"' .... 10• ::; z ct a l&I 10' 2 -

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FAILURE RATES vs TIME (NORMALIZED) FOR VARIOUS LOGNORMAL DISTRIBUTIONS

LOGNORMAL PLOTS OF ACCELERATED LIFE DATA

........ u, a:

103 a---+-~----t-------t--+---+-----wiir-t----+-----1

~ s~~---+--~--~~--+-~-~ 0 ::c

w LL -_J

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------------------------------------.0 t 0.2 1 10 30 50 70 90 98 99.9

CUMULATIVE FAILURES (PERCENT)

2-12

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LOGNORMAL PLOT OF BIMODAL POPULATION

103 t----+-__._--+-----t----+-----+-----t'!,-,,.~-l--t-~-----it--+---t---1 MAIN DISTRIBUTION

~ 10 2 J----t--+--+-----.flllf~~,..__....~__.,_--+---t---+---t---t--t---t---1

::, COMBINED ~ DISTRIBUTIONS I

w ~ e' I- i O 1 ~~--+--f--t---+----t---4r--+---t----t---t---t---t-

FREAK DISTRIBUTIONS

i(j'iL..£::~-L.---U::;..._a.._.....___..____,jl--,l._l,,-L.-'----'---......_ ____ __

.5 2 •. 10 30 50 70 90 98 CUMULATIVE PERCENT FAILURES

LOGNORMAL PLOT OF IC FAILURE DATA, SHOWING ANOMALOUS BEHAVIOR

WITH TEMPERATURE .

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, ..i-~• If I Ill I I I I 91M I.IF~

Keyl '\,; . I I I II I I N Parameter

1

0

I I 11 111 ' I I I I I I i :t.CA ~ -zc;o nJ\ I ' I t I I It ' I •• I I I I t I 11! I I • I

11 111 I I I I I : II I I I ' I 'I 111 I I

I I i I 'II I I I I i ~., I I I : I 111 I I I I I 111 i I

.r. 1 2 r. 10 20 ,o •o so 50 10 ao H •~ •• ·•• ,,.s r • ..._. .. ,c.,,. %I

2-14

Laboratory Methods and Procedures Used During Reliability Stress Testing of Silicon Solar Cells

J. W. LATHROP Clemson University

Abstract

In this presentation the methods and procedures used by Clemson University to perform cell reliability stress testing are discussed. Topics included are: a) number of cells required for each of the different cell types tested, b) duration of time each group of cells is held under stress before electrical measurement checks were made, and c) criteria for selecting the stress down times. The laboratory logistics developed to keep track of cells throughout their testing history are described. The special equipment designed for each test is discussed in detail, including its cost (current dollars), capacity, and operating procudres. A secondary objective of this presentation is to describe solar cell testing problems and considerations for cell manufacturers, universities, and private industry laboratories that may be considering setting up their own cell testing facilities.

2-15

TEST PROGRAM LOGISTICS

• NUMBERED EACH CELL

• DID NOT RANDOMIZE • DID NOT CLEAN

• LOW POWER VISUAL INSPECTION

• PHOTOGRAPHS (OPTIONAL)

• ASSEMBLE INTO TEST LOTS

• QUANTITY DETERMINATION • PETR I DI SHES AND Fl LTER PAPER • LOT TRAVELERS

• STORAGE

• PREMEAS UREMENT /POST TEST • POST MEASUREMENT/PRETEST

• EQUIPMENT LOG SHEEf

2-16

BIAS-TEMPERATURE (B-T) TEST

7s'c 135 ·c 150°C.

165 °C

BIAS-TEMPERATURE- HUMIDITY (B-T-H)

PRESSURE COOKER (121 °C., 15 PSIG STEAM)

85°c/ssZ RH

TEMPERATURE-HUMIDITY (T-H) PRESSURE COOKER (121°C,J5 PSIG STEAM)

85°C/85,/oRH

POWER C'(CLE

INTERMITTENT FORWARD BIAS@ 50 °C.

THERMAL CYCLE.

THERMAL SHOCK

2-17

EYAMPLE OF A MINIMUM QUANTITY TEST

75 a C B-T 2. 5 >< 25

135 °c 13-T

150 °c 6-T

T-H PG

B-T-H as/as

TC.

TS

x~ 10

2 X

2 >(

X

J.25 X

)(

20

20

10

12,.

10

10

I0.75X 107

2..:..1s

BIAS-TEMPERATURE (B-T) TEST PROCEDURES

1. CLEAN OVEN AND JIGS. (SCRUB WITH WATER-DETERGENT MIXTURE, RINSE IN DI WATER, RINSE IN ALCOHOL.)

2. INSPECT BIAS NETWORKS, RESISTORS, AND CONNECTORS.

3. LOAD CELLS, CONTACT SPRINGS, AND GLASS SLIDES INTO JIGS. (INSPECT ELECTRICAL CONNECTIONS CAREFULLY.)

4. PLACE LOADED JIGS IN POSITION IN OVEN BEING CAREFUL TO AVOID CONTACT WI TH EACH OTHER.

5. ATTACH JIGS TO BIAS NETWORKS. (CHECK ELECTRICAL CONNECTIONS CAREFULLY TO AVOID POTENTIAL SHORTS.)

6. AFTER OVEN HAS REACHED OPERATING TEMPERATURE, ADJ UST POWER SUPPLY VOLTAGE TO ACHIEVE DESIRED CURRENT. START TEST TIMING. NOTE INFORMATION CONCERNING TEST CONDITIONS ON OVEN LOG SHEET.

7. TO STOP TEST: A) NOTE TIME AT TEMPERATURE, B) REMOVE BIAS FROM CELLS, C) TURN OFF OVEN AND ALLOW TO COOL TO ROOM TEMPERATURE.

8. TEST MAY BE INTERRUPTED FOR ADDING OR SUBTRACTING CELLS PROVI OED PROCEDURES 7) and 6) ARE FOLLOWED AND INTERRUPTION NOTED ON OVEN LOG.

2-19

8-T TEST LOGISTICS

C05T ~800

CAPACITY 18 JIGS

3>< 3 BOTTOM LAYER

3,c '3 TOP LAYER

1Sx8 = ILf 'i CELLS

EXAMPLE 3 CELL TYPES ( 3'' DIAM)

~8 CELLS EACH TYPE

6 :flG5 EACH TYPE

3G RESISTORS

72 AMPERES

TEMPERATURES NOMINAL DOWNTIMES (HOURS)

7 5 ° C GOO 1200 21./ 00 L.f800 • • · •

1'35 ° C GOO 1200 2'-100 'i800 · ·· 4

ISO °C 300 GOO 1200 2.L/00 · ·" ·

2-20

N I

N I-'

V\

f

ACCELERATED TEST BIASING

- f

11 r

-+ +

, ' I \ \ I ' ,

NORMAL OPERATION TEST CONDITION

BIAS CONDITIONS

CELL DIAMETER ITEST (A)

(IN) CALCULATED USED

2. 0.9 I

3 I • 'l 2

3.Lf 3

2-22

TEST LOT TRAVELER

LOT C' tz /c 1n · co2-11J

CUMULATIVE CYCLES

(DATE)

ELEC. MEAS.

(DATE)

TEST /Jo oc: 4-r

INSPECT

(DATE)

PHOTO

(DATE)

(; 7/nhe 1nVtaioJL-_---"-'-1A ___ V._7J_/ I-__ --~ __ @; ___ 0_,-3 __

'1 /77 /7:g, C(!..5 /o µ, /11' /_ I

/1/z/Jf J)[/r 1°ho/,1 L I

___ Si_~o_i_'Z,-_J . __ 1J/;f-l7f /JCtf ,~1,0/1r /.6:6

_..13..__.ro__._..Aas...___· __ ~ hf /79 ti2 elf , h" I 7'1 L .~ (·h

-------·- ----·· ----·- -·------- ------

2-23

VISUAL INSPECTION SHEET

FRONT

BACK

\ 9) -f,----,-\J

~::.::.:::::.:.::. --·::-·.:.~

l--1 l/ $ CELL # ----· .--

'"

1

.\;.J __ 8 1980 _ L,,L ·-·-···--·-·-·--·-·--.:_.:~ ;t J3_J980 ~ et~~all.1. ~~~

r;~t,~t~~~ _ J At_~?-~~1~:~0-~l.f.~!: e!~----~~~. 5 )~~~j~_-,_'!:~-st,o~

si,..; la.~ +o (aj

FEB 19 1980 4) /,he Me1 ... l1 "loV\e ____ . ·---

2-24

N I

N Ul

TEST

CELT. 'l' Yl' ::s

ff CELLS

START

STOP

TEST DURATION

EQUIP. NOTES

TEST IN PROCESS

\5'o 0 B--,

F-1"2. G-t 1. I f-12

_1-_s __ _ 2.S {O

,·100 et(} 1'1;gQ fM t;;.'r,o fM ct {.,. l':z. ,. --9.1., fj' ( 1 f__ ,:(, tz,

,:oa PM to~oQ eOl B~oo AM IO/'f /79 _10/11 {Z':f crt~tlz1

loOQ c;;oo 30 1-

f.S~ 3 7,b V

Ii' A

BIAS-TEMPERATURE-HUMIDITY (B-T-H) TEST PROCEDURE

1. CLEAN HUMIDITY CHAMBER MAKING SURE NO RUST IS PRESENT. (USE STAINLESS STEEL WOOL IF ABRASIVE IS NEEDED TO REMOVE RUST.)

2. CLEAN WATER SYSTEM. EMPTY THE HOLDING TANK, CLEAN, AND REFILL WITH FRESH DI WATER.

3. CLIP CELLS TO HANGERS.

4. ATTACH BIAS NETWORK.

5. CHECK WET BULB WATER LEVEL AND INITIALIZE WET AND DRY BULB CHART.

6. BRING TEMPERATURE TO 85°c AND RELATIVE HUMIDITY TO 85%. BE CAREFUL NOT TO PASS THE DEW POI NT OR CONDENSATION WILL FORM ON CELLS.

7. WHEN 85/85 CONDITIONS ARE REACHED, ADJUST POWER SUPPLY VOLTAGE TO ACHIEVE DESIRED CURRENT. START TEST TIME. NOTE INFORMATION CONCERNING TEST CONDIT IONS ON INSTRUMENT LOG SHEET.

8. TO STOP TEST, SHUT OFF MAIN BREAKER. DISCONTINUE TEST TIME. ALLOW SYSTEM TO REACH ROOM TEMPERATURE BEFORE REMOVING CELLS. ONCE STARTED, TEST SHOULD BE RUN TO COMPLETION WITHOUT INTERRUPTION.

2-26

85/85 13-T-H LOG1sr1c.s

co5T ef.eooo

CAPACITY > YO CELLS

NOMINAL

DOWNTIMES 250 500 1000 2.000

(HOURS)

2-27

PRESSURE COOKER TEST PROCEDURE

I. CLEAN CHAMBER AND JIG. (NORMALLY ALCOHOL WI PE IS SUFFICIENT.)

2. ADD DISTILLED WATER UNTIL LEVEL IS AT LEAST 2 INCHES ABOVE HEATING ELEMENT AND AT LEAST 4 INCHES BELOW CELL LOCATIONS.

3. CLIP CELLS TO JIG OR USE SLOTTED HOLDERS.

4. LOWER JIG INTO CHAMBER; CLOSE LID AND SECURE EVENLY.

5. OPEN RELEASE VALVE.

6. APPLY FULL POWER TO HEATING ELEMENT, (BYPASS VARIAC.)

7. STEAM SHOULD APPEAR IN 15 TO 20 MINUTES. ALLOW A STEADY FLOW OF STEAM TO VENT FOR 4 TO 5 MINUTES BEFORE CLOS ING RELEASE VALVE.

8. WHEN PRESSURE GAUGE REACHES APPROXIMATELY 12 PS IG, SWITCH POWER TO VARIAC SET TO GIVE 7.1 amps. START TEST TIME. ADJUST VARIAC AS NECESSARY TO MAINTAIN 15 PSIG. NOTE TEST CONDITIONS ON INSTRUMENT LOG SHEET.

9. TO STOP TEST, SHUT OFF MAIN SWITCH~ DISCONTINUE TEST TIME. ALLOW SYSTEM TO COOL TO ROOM TEMPERATURE BEFORE OPENING RELEASE VALVE. ONCE STARTED, TEST SHOULD BE RUN TO COMPLETION WITHOUT INTERRUPTION.

2-28

PRES5URE. COOKER LOGISTIC.S

COST -!f 1600

MODIFICATIONS

CAPACIT'( 20- '-10

NOMlNAL

DOWNTl·MES 50 100 200 LfOO 800

(HOURS)

2-29

THERMAL CYCLE PROCEDURE

1. CLEAN CHAMBERS WITH ALCOHOL.

2. CHECK FOR PROPER MECHANICAL CYCLING OPERATION OF MOVABLE CHAMBER.

3. SET TEMPERATURES FOR HOT AND COLD ZONES AND TURN ON POWER.

4. WHEN PROPER TEMPERATURES ARE OBTAINED, CALIBRATE STRIP CHART RECORDER.

5. LOAD CELLS IN RACK. VISUALLY CHECK EACH CELL WHILE LOADING.

6. PLACE LOADED RACK IN MOVABLE CHAMBER WHEN IT IS IN THE UPPER (HOT) POSITION.

7. CLOSE DOOR AND ALLOW TEMPERATURE TO EQUILIBRATE. SET TIMER FOR 5-MINUTE CYCLE LENGTH AND COUNTER FOR NUMBER OF CYCLES DESIRED.

8. START TEST. TEST WI LL STOP AFTER ALL CYCLES ARE COMPLETED.

9. STABILIZE CHAMBERS BY OPENING DOORS. REMOVE LOADED RACK AFTER COOLING TO ROOM TEMPERATURE.

IO. UNLOAD CELLS. VISUALLY CHECK EACH CELL WHILE UNLOADING.

2-30

TEMPERATURE CYCLE LOGISTICS

COST · 4 I0,000

CAPACITY

CYCLES PEf\

LN2. CYLINDER

>SO

15-20

TIME PER CYCLE. \0-15 Mlf\l

NOMINAL DOWNT\MES I IO 2.0 '-to

(CYCLES)

TEMPERATURE -65 TO+ ISO 0c

2-31

TEMPERATURE OF TYPE A CELL DURING THERMAL CYCLE STRESS

,..... z ~ ......-

~ r=

N I

w N

0

• I

-65 150

TEMPERATURE ( C)

N I

w w

TEMPERATURE OF TYPE A CELL DURING THERMAL CYCLE STRESS

I I I

-65 0 TEMPERATURE ( C) 145

THERMAL SHOCK PROCEDURE

1. CLEAN FLUID CONTAINERS WITH ALCOHOL.

2. ADD THERMAL FLUIDS TO CONTAINERS: HOT BATH (150°C) FLUORINERT ELECTRONIC LIQUID FC-40 COLD BATH (-65°C) FLUORINERT ELECTRONIC LIQUID FC-77

3. IMMERSE COLD BATH IN DRY ICE AND ALCOHOL MIXTURE. HEAT HOT BATH TO TEMPERATURE USING ELECTRIC BURNER. MONITOR TEMPERATURE OF BATHS US I NG THERMOCOUPLE. NOTE: PERFORM THIS OPERATION IN FUME HOOD. AVOID OPEN FLAME.

4. CUP WI RES TO EACH CELL BODY US I NG ALLI GATOR CU PS.

5. PLACE EACH CELL, ONE AT A TIME, INTO THE HOT BATH. ALLOW TO EQUILIBRATE FOR 5 Ml NUTES.

6. MOVE EACH CELL INDIVIDUALLY FROM HOT BATH TO COLD BATH. THE TRANSFER SHOULD TAKE NO MORE THAN 10 SECONDS, AND TYPICALLY TAKES ABOUT 2 SECONDS.

7. LEAVE CELLS IN COLD BATH 5 MINUTES, THEN MOVE BACK TO HOT BATH.

80 TEST SHOULD END WITH HOT BATH. CELLS ARE THEN REMOVED TO ROOM AMBIENT. THUS 1 CYCLE = H-C-H-ROOM AMBIENT, 2 CYCLES = H-C-H-C-H-ROOM AMBIENT

2-34

THERMAL SHOCK LOGISTICS

F~UID

COSI ""' f ISO/ GAL

TE.MPERATURES -G5°C TO+ ISO 0c

NOMINAL DOVJNTlMES

(C'(CLE.5)

I 10 20 Lf O

2-35

BASIC E~UIPMENT COSTS

ENVIRON MENTAL

G OVENS 'iBOO 2 HUMIDITY CHAMBERS IGOOO I PRESSURE c.ooKER I G 00

I THERMAL CYCLE. 10000

G POWER SUPPUES 3000

MEASUREMENT

CUR.VE TRACER+ ATIACH.

SHORT INTE~VAL TESTE~

TEMPERATURE. c.o~TROL

SIMULATO~

I NSPS.CTlo.J SCOPE

CAMERA

Ml SC.,

JIGS AND FIXTURES

C.E.LL STORAGE

2-36

$35 LiOO

CJ600

eooo 2.000

500

(000

500

$2.IGOO

sooo 15000

$20000 $77000

Electrical Measurement Considerations for Realiability Testing of Photovoltaic Cells

C.R. SAYLOR Clemson University

Abstract

Reliability testing of terrestrial solar cells involves repeated sequences of electrical measurement, followed by stress, followed by electrical measurement. Comparison of subsequent measurements is used to detect the irreversible changes due to the stressing. Since these changes may be small, an accurate and highly reproducible measurement system is required in order to distinguish between random errors and effects brought about by stress. Due to the sensitivity of the electrical parameters to temperature changes, one major problem in the design of a measurement system in maintaining a constant temperature for each measurement. Irregular back surfaces of many cells make it difficult to maintain an equilibrium temperature using a heat sink. Therefore a microcomputer-based system with a light shutter was designed to measure the parameters in less than one second, thus removing the problem of temperature change.

Maintaining a constant and repeatable light intensity is also necessary. This was accomplished by powering the ELH lamp simulator with regulated power supplies and allowing a one-hour warm-up period for the intensity to settle down. The microcomputer-based tester measures a solar cell in less than one second, and since a.c.-powered lights have 120 Hz ripple, d.c. power supplies were used. A reference cell of the same type as the cell under test was used to set the intensity. A photodiode built into the cell holder was used to monitor any variations in intensity between each measurement.

2-37

ELECTRICAL MEASU~EMENI CONSIDER.A-rl D"1S

• -PU~PD=-€

• To PErERMrNE THE EFFEC.,-S

OF CELL S~E. 5~ (1'!~

• C'4~E. ELEC1~1CAL -PARAMc ,-E~S

-ro ME'A~tJ~g

• M DN rae>r<: J CD.....i-rRcL MEA~LI ~EME'~

C::::.e>N D 1-r'lC=>NS

• l~URE. REP~AE>IL\-rY

11 Me:A-stltz;e ~IJFFJCIEtJ-fLY LA~GE

~LlAN-rt-rY or=' CELLCS

2-38

• I V C.LJRYE. CoN,-ArNS 1a:::)

MlJGH li--.1.l='Dl<MA~IDN

• N~ QLJA~,-,,-A-r\v'E

• G~ ~1<,A)~E.-("E~S -n-\A-. F-:>E.S-. CHA12:k:.~ \ Ze: C.ELL'.:S

PERFO~MANCE.

2-39

N I

.i::-. 0

1-V CHARACTERISTIC CURVE SHOWING ESTIMATED REPEATABILITY

I sc(±l%)

CELL VOLTAGE

PM (±0.5%)

VM (±1%)

IM (±1%)

MEASUREMENT SCHEMATIC

r------, I I I

I f I L ____ J

CB..L UNfER

IESI

BACK BIAS

- + ~ ......... -aRe«

LOW RESISTANC:E

SENSING RESISTOR

X-Y LQ\D RESIS10R

N I

.s::-N

"H" C ELL HOLDER CROSS SECTION

YACUUM

ELH LAMP SPECTRUM vs AIR MASS 1 SPECTRUM

2.0 c .. 1 1111\

500

/\- ELH LAMP SPECTNltt'\

' I

' ' \ AIR MASS I SPECTRUM

' ' ' ' ' ' \ \

'

1000 1500 2000 WAVELEN6TH (" •)

--1.500

CELL HOLDER DIAGRAM SHOWING SEPARATE VOLTAGE PROBE AND CURRENT LEADS

CELL

TH~MOCOUPLE AND \t>LTAGE

PROBE

CUR~ltNT Ll!AD

CUR~EWT LEAD

N V') I LLJ ~ a::=: l./l LLJ

c.. :E <C

TYPICAL 1-V CURVE WITH CONSTANT POWER CURVES OVERLA YED

2.0

1.8

I. 6

1.4

I. 2

I. 0

0.8

0.6

0.4

0.2 0. 0

o. 0 0.1 0.2 0.3 0.4 0.5 0.6 0. 7 VOLTS

• WA-re~ CcoLt'NC:- ('EQUILl!IRIUM)

• IHERMOCDUPLE. 'Pl2c>Bc.

• YACLJUM HOL'D Do'>/N

2-46

N I ~ -....J

HEAT FLOWS DURING CELL MEASUREMENT

QLAMP

rr------ SPRING LOADED THERMOCOUPLE

IRRADIANCE PROFILE OF ELH SOLAR SIMULATOR

Outline of 100 mm cell

46.8 48.3 46.0 48.7 47.6

46.8 49.2 51.0 52.6 53.5 53., 49.8

47.6 50.4 56.4 57.0 52.8 49.6 45.8

46.6 50.4 57.9 59.2 59.5 59.0 57.5 51.9 48.0

48.2 52.3 58.l 60.0 60.8 60.9 60.4 59.l 49.4

49.1 53.4 59.7 61.0 61.2 61.l 60.9 60.3 57.9 4.5 50.3 45.2 ~

I

48.9 53.B 60.3 61.2 61.2 60.8 60.7 60.l 58.5 5.0 50.3 ~5.3

48.2 53.0 59.2 60.3 60.3 59.8 59.6 59.l 49.8

'46.8 51.1 58.2 58.8 58.6 58.l 52.3 48.4

48.6 51.9 52.6 49.9

48.4 50.7 52.3 53.J 53.3 52.9 51.i 49.8 47.l

47.l AB.5 49.2 49.4 49.0 48.2

2-48

N I ~ \0

LONG TERM Pm VARIATION

• +1% ______ _. ______________________ ~

• • •• • • • MEAN -----~-~~-----------------~ • • -1% •• • -~ ........ --.~---- ........ - ........ ------------- ........ --,___..--

•

JUNE JULY AUGUST

a D1A=1C-LlL-r,' ,~ MAlt-l,...Ali-Jl~ C.0~$.,....A~

1 "t::IJ\ PE.AA-nJRE. 1:)U'E- -ro vcc::,T<. -rl\ERMAL

C.OW'I AG'l.

a LI 1'-1 ASL'E" ,-o HSA-r "!SIHK Et-lCAPSLllA-rE:D

CELL-S

2-50

SHOR! INTERVAL le.s-rc=~

• PURPD~E.:

• EwM1f-.lA-rE. TE.MPE~1URE. Tuo13LEM

11 1 ~~~c=: ~~~M&J'I RA-re

•A~H:

• TAKE MEAStl-REM'f=NIS < i ~f-0)>

a \.l6c:. L\~ l·tf !5t\Ll11E~

2-51

a i S'OC BO/IOA -S1W6LE B~~D CoM4PU1c~

8-b·rt:. ~ 48 ~RAMASu; :c./o l.iNE"S

a c:r. -8080 - I~ K. 'RA~~ Ao:.'E!5.5 MEMoey BoA.R]::,

14-> K x. B ( Sc.RA-rc.t-4 "PA? Ma/\o~")

• Rf:C-IZcx:>- lo4 DA1A ACQUl~rt1of--.l 'Bc)ARP I G:> - 1:>lf-F~a.t"T"IAL.. IM Pt..1-r ~t-lA,-.11-Jel.-S

. I Z - b0

ft A /D (2) I 'l- b·,-l D/ A I 45.

II oSC\ LLOScO?E.

2-52

·BLOCK DIAGRAM OF SHORT INTERVAL TESTER

I DC PO~"ER SUPPLIES

PLOTTER

VOLTAGE LEADS

DATA ACQUISITION

OSCILLOSCOPE --- BOARD

TELETYPE MICRO

COMPUTER

2-53

__......_ 4-L~'iP ELH SOLAR SIMU1.ATOR

I l I

CELL UNDER TEST

CURR.ENT LEADS

PROGRAMMABLE POWER

SUPPLY

COXTROLLER

R1' :C' - \ 2.l:X> ~-rA AcQLlLSn1ot-J

~fl.i'P

ICo K x 8 AANC>6~ A&.C~

MEMD"'Y (~1:A,-C~ 'PAO)

iSBC 50/IOA

/AP

2-54

SYS,EM FEA-rLlRES

• HArc:.P Co?Y' OF :r. y Cll~VE

• PA'PE~ TAPE OF PAJ?AME-rERS

• Au-ro MA,-,c. CA1...1131<A""f1~ C-HE:C~

• IV CUR\JE Dl4SPLA'tED CN ~OPE

• ~JU1 ru-r OF 'PA~Me-re~s

2-55

TECHNICAL SESSIONS (Contd)

TEST RESULTS, DATA MANAGEMENT AND ANALYSIS

1.

2.

3.

Data Management and Analysis of Solar Cell Test Measurements (J.F. Christ) •••••••• . . . . . . . Studies to Determine Effects of Second Quadrant Operation on Solar Cell Reliability (R.A. Hartman)

Test Results from Clemson Solar Cell Stress Test Program (J.L. Prince) • • • ••••••••

2-57

2-59

2-69

2-87

Data Management and Analysis of Solar Cell Test Measurements

J. F. CHRIST Clemson University

Abstract

Over the past two years approximately 7200 solar cell I-V characteristics were measured by the investigating team at Clemson University. Most of the analysis work was performed on selective subsets of measured data (i.e., V0 c, I 8 c, Vm, Im, and Pmax>• Temperature was also an important parameter in the overall analysis.

A need for managing this continuously growing number of data points, (now in excess of 38,000) was forseen, so a data mangement system was developed for the program which utilizes the Clemson University IBM 370 computer. A software package, called Statistical Analysis Software (SAS), is used to generate a wide range of histograms and curve plots.

For each different type of stress test, there are computer programs written using SAS, which facilitates error correction, data reduction, storage of reduced data on disk, and computer printouts of histograms, tables and plots through statistical routines. Each page of all computer printouts of data tables and histograms was labeled, and appropriate analysis keys describing each variable were added. Several examples of both manually drawn data plots and SAS printouts are shown as typical examples of the data management practices that were used.

2-59

N I

°' 0

DATA ANALYSIS AND MANAGEMENT

• ALL ELECTRICAL DATA ON DISK, ACCESSIBLE THROUGH IBM 370

• OVER 38,000 ELECTRICAL DATA POINTS ON DISK

• STATISTICAL ANALYSIS THROUGH SAS (STATISTICAL ANALYSIS SYSTEM) - - -

• DATA MANIPULATION THROUGH SAS AND IN-HOUSE SOFTWARE

-< -1--z L&.J

t-.) ct:: I ~

°' :::> - u I-:::> a.. 5 0

TYPICAL CELL OUTPUT 1-V CHARACTERISTIC WITH CONSTANT-POWER HYPERBOLAS SUPERIMPOSED

AND A FAR-FORWARD DIODE 1-V CHARACTERIST_IC

2.2

2.0 lsc + 2.0

1. 8

1. 6 IRRADIANCE = 1000 W/m2

1.4

1.2

1. 0 I SC + 1. 0

0.8 .. 0.6

0.4

0.2

0.0 1sc 0. 0 0.1 0.2 0.3 0.4 0.5 o. 6 0. 7

CELL VOLTAGE (V)

-< -I-z LL.I ~ ~ ::> u C ~ < s: ~ 0 LL.

LL.I C 0 -C

TYPE F CELL TEMPERATURE DISTRIBUTION UNDER ELECTRICAL MEASUREMENT CONDITIONS

WATER BATH TEMPERATURE 26°C

50--

45--

40-

v, 35-....J ....J LlJ u u_ 30-0

20-

15~

10-

5- -

::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: iiiii!!U!HU! ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: :::::::::::::::

::::::::::::::: ::::::::::::::: ::::::::::::::: :;;;;;;;;:;;;;; ::::::::::::::: =============== mmmmL ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: ::::::::::::::: :::::::::::::::

iiiiii!:!!!:i:! ::::::::::::::: :::::::::::::::

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:::::::::::::: iiiiiii:i!:i:!i ::::::::::::::

;;;;;;;;;;;;;;;; mmmmm ~mmmnm :::::::::::::::: ............... ::::::::::::::: mmmmm! mmmmm ·::mmmm iiiiiiiiiiiiiiii ::::::::::::::: ::::::::::::::: ::::::::::::::ti iiiiiiiiiiii!ii iiiiii!iiiiiiii mmmmn r mmmmm i.!.i.!.! .. i.!.!.!.!.!.!.!.!.!. Uii£iiiiiiiiii iiiiiiiiiiiiiii :::::::::::::::

iiiiHHi:ii:Hi ;;;;;;;:;;;;;;; iiiiiiiiiiiiiii iiiiiiiiiiiiiii Hii:i::i!:iii!: ;;;;;;;;;;;;;;; iiiiiiiiiiiiii! Jiiiiiiiiiiiiii iiii!iiiiUiHH ::::::::::::::: ;;;;;:;;~;;;;;; !i!ii!i!!Hi!ii mmmmm ::::::::::::::: ::::::::::::::: ~

i.i.i,!,i.'.:i.i.i.i.=.:i.i.i.i. ::::::::::::::: i:iii:ii:i!!i!i i=i=i··.i=i=!=i=i=!:i=i=!=i=i=·!=!= iiiiiiiiHiHU =============== !iiiiiiiiiiiiii ::::::::::::::: :::::::::::::::

::::::::::::::: ::::::::::::::: ::::::::::::::: iH!!HH!!!iH i!iiiiiiiiiiiii iiiiiiiiiiiiiii ·:·:·:·:·:·:·:·:·:·:·:·:·:··.·:

=============== unmnmm !!!!!!!!!!!!!!! !!!!!!!!!!!!!!! ,, 1===.':=:.'===.'::=.1===.'===.'===.'===.1===.1===1===!==='===i=== iiiiiiiiiiiiiii iiiiiiiiiiiii!i ;;;;;;;;;;;;;;; iHUiiiiiii!ii ~ iiii::i!:i::iii iii:iiiiiiiiiH iii!iiiii!iiH! iiiiiiiiiiii!ii

·;1i:·1:=.1== •• :=·d==.1==.===.;!==.!==.::·i.I==.:=1=.i==. ==···.:=======·.·.============::== ===========:=== mmmmm ====:::::::::== 1-....... -.... ii 0 ~-: ::::::::::::::: ::m ::::::: ::::::::::::::: i.i.i.i,;_·i.i.i.i.i.i,i.i.i.i. i.i.i.i.i.i.i.i,i.i.i, J 1:,:,:.:,:,:.:,:.:.:.:,:.:.·:,:," '------'~~:::.......,.::::::..._:::::: ~!i:iii.......,.iiiiii:i.iiL..i: .Jiiiiii-.-ifliiiiiiiiliiiii L........liiii.Hi:iiiiiiiiii.iii:Hi.i.il.-: ~iiiiil--.lii-~:!iiiL-_ .liuii.u.Uiilii.W.l.""'-

24 28 32

CELL TEMPERATURE· (°C)

2-62

Pm vs T FOR F-CELLS SHOWING AVERAGE SLOPE

p m

(mW)

240

230

220

210

200

190

180

~ F - 16

0 F - 25

0 F - 37

170 ------a.----"-----'--------ll------L----1 15 20 25 30 35 40 45

CELL TEMPERATURE (0 c)

2-63

Voe vs T FOR F-CELLS SHOWING· AVERAGE SLOPE

V oc (mV)

580

570

560

550

540

530

520

f:l. F - i6 1J F - 25

0 F - 37

510 ------------'-'---------.l'----------L------------'-------------"-----~----1. 15 20 25 30 35 40 45

CELL TF.MPERATURE (°C)

2-64

N I

°' VI

4~ER4GE CF I CECPE~~E! CF P~ A~C CL ... ULATl,E CJSTRleUTICN

PCCX AVEPAGE CF t CEC~EASE~ CF P~ AFTER X STRESS LEVELS FGT,PCX = 1 CF CELLS htT~ ~CPE T~AN Vt CECREASE AFTER X LEVELS

THE::F l(T:tC

IJ4AUfLE ~ PIE i\,.. STA~CARC CEV UT In

FCC 1 ?C -1.-:1e2cne 1.56fl!l82 FC T5Ftl ?C c.cccccccc c.cccccccc PGTlCFOl ?C c.ccccoccc o.cccccocc P(T25PC1 ?C c.cccccccc c.cccccccc FCT'5CFC l ?C c.cccccccc c.cccccccc i:cc2 ?C C.7241l'i5l 6.l805'51Ci'5 PGT5PC2 ?C ~- :!13:!1333 l8.25Hl858 FGT ICFC2 ?C ~.'2'22'22'2'll'll 18.25Hl8'5E FGT25FC2 ?C ?.'B??HH 18 .257't 18'5E! PGT!:CPC2 ?C c.cccccccc o.cccccocc HC'! ?C ,.1?'5CZ1tc;1 1c .He2c;a2c F<:T5H3 :!O ?.??'!?'!?H 1e .25H te5 e FGTlCPC3 JC ?.:!333HH lE!.25741858 PGT2SPC? ?C ?.?'!?!?!!'! 1e. 257"1B5e 1'05CFC3 ?C ?.???nB? 1S.257"1E! 1;e 1=([4 C PGT5PCtt C FGT 1CPC4 C FGT25FC4 C PG TSCP04 C

-------------- J'fPE=F LC T = 11 --------------FCC l 24 l.4C6769C5 9.03C32'H 7 PGT5PC1 ,4 1,.sccccccc H.1enc;621 PGTlCPCl 24 4.tHEHl:1 2'=.412'il452 FG125FC l 24 4. l66H667 20.41241452 FG T5CPC 1 24 c.cccccccc o.ccoccoco i:cc2 ,5 ? .ec;e 114e2 l'!.41EE:!3Cl l=GT5FC2 25 12.ccoc<::coc H.H:62"79C FGllCFC2 25 E.CCCCCCCC 27.681!74621 PGf2'!:PC2 25 ,.cccccccc 2c.cccccccc S:CT5CFC2 25 ,.cccccccc 2c.cccccccc l=CC3 25 o;. Hi9lH2 13.996qQ274 PCT~PC'! ,5 2,.cccccccc 43.~eec;e944 HTlCPC 3 25 e.cccccccc 21.E'?Pl4l:21 FC125FC3 25 4.CCCCCCCC 2c.cccccccc S:CT5CPCJ 2,; ,.cccccccc 20.cccccocc FCC4 C ,c f5S:C4 C PGTlCPC4 C P(f2":PC4 C Ftr'!CPC It C

q:42 WECNesciv. FEBRLARY 13, I

N I

(11 (11

AVE~ACE Pl'

fllPl'll .8\ERAGE Fl' AFTER X SUES~ LHELS T't PE "F LC T= lC

'vARUeLE "4 l'EAN HU.CARC CEVIAT!C,.

Pl'C !C C .192Ut49C C.Cllll5CCi Pl' l :!C c.1r;•n2cu o.c1oe1s22 Pl' 2 !C C.l'il'.H:141 c.c1e21 n 1 Fiil 3 30 c.1ei::e1c1e c.c21tr;2ee3 F,-le C

-------------- T,PE=F LCT=ll --------------

25 ,,. ,5 25

C

C.194ll'i26 C. l9C64ll 7 c.1et:e2c2c; C. tel9 l612

O.Cl232326 0.02157839 c.c2e111e4 o.c2cn6112

-------------- T,PE=f LCT=l2 --------------

25 t5 2'5 25 23

C. 2C6CC434 C.l4i4CS5le c.1e@f:cc;se c.te611eo6 C.173Cll'H

0.01393379 C.C26C;2254 C.C3't541E4 O.C3608673 c.c43c;52n

-------------- T,FE•F LCTml4 --------------

2C 20 19 1 c; 20

c.1c;ce91e1 C .1eR1U6e C.18055543 C. lECJE9549 C.t54c;CE42

C.Cl265706 C.Cl2C51tec o.01e411c;e c. oie5c;973 c.c2Huc;c;

-------------- T,FE=F LCT=l5 --------------Fll'C F foll Pl' 2 p,- ! F,-4

C. l97245P2 C.l8652lll C.lfll31Cl2 C .1 H29CH C .16';'5"'568

C.Cl 114147 o .031cc;oc; 1 o.c ]640249 C.C4Clfl5C 0 .C4077l 14

-------------- r,PE•F LCT•l1 --------------

Fl'C p,- I p,-2 FIii, Pl'4

15 1'5 ll 12 12

t. lqltf:61!49 C.19261201 c.1c;n22,;2 ColR5f:11C27 C. l 1'51:4401

o.c l2C5748 O.C2771519 C.Clf:lHH o.c11114c;2 0.04';81141

9:42 ~EChES[Av. FEeRUARV 13. 1980

!.Uttl"iPY CJ l(-n. n.t, ~H(S!, IIV!l!

-------------------------------------------------------------- llff:J --------------------------------------------------------------

DIH

LH 1 CJ 7!,C !· •1 J"4Jl JAL LH T , 75( i.-1 AflfP 60" HR t1r 1 2 7~( '-T .IIFTF R u·r HP

LH 1 3 7~C r •1 H 1f R ZI. ::-C Hr,, l 11 1 ~ 135( 1-1 JNJTIAL l , , 1 1 , ? ~c 1--1 Af 1 r II 7r.r Hit

L11 T z 1 !SC t--T ArTH' 12[~ HP L11 T ! 1!~( l·-T ,_, T f I:' Z4{'( t-R L1? T t 1 5C C f-1 JNIT JAL L1i 1 , 150( r--T ,HTFP 30[1 HRS L1Z T 2 15r.c 1-•l AFTEP t,~C, HJIS L 12 T ~ 1 '>CC f--1 AF TEP 1 n: .. II S L 1:? T 4 1 ~CC l:•T HHS: 240, 1-'IIS L14 T .: P Ht Fr.ES COOK Jti 11 IA l N l 14 1 f- TH Jr.ES COOK U TER 2{ .. ~ I

(J\ L 14 T 2 f' lti F- IIE S CC'IC·K AFTER 1~? t'I!

" L14 , 3 61"4 FHS cror u TE R ?P!- tR L14 T 4 ~TH FHS (0(11( loF HR 1,99 HR L 1 !, 1 L P. lt, t 5/f!~ H, IT J H l, !, l 1 Flt- t-5/65 AFTFR 192 i-R L 1!, T 2 ~H ~'.J/b5AFHR 516 t-R l 15 T :! 8Tt- F 5/f~ AF HR r;97 .. R L 1 !, T ,. ~ Tl- I' 5/b5 AFTER 20U, HF L 17 T C, THE Fl' AL C 'HLE l"JTJH L17 T , T t-RI' CYC , C YC -t5C/15(lC L 17 T '2 HRI' CYC 1~ CYC -65C 115:.c L17 T :! Tt<RII' CY C 2C CYC -6!>C 11 H C L17 T 4 TtRI' CYC 4~ CYC -45C ,-t:SC/15:.c LH T (; THHH St<OCK JtiJlHL L 1t: l 1 lt-EH'H SkOCIC AFTER , S11K~ L 1f' l 2 THP•H ~ 1-C"C IC AF TEP 1 ':' SI-IC! LH T 3 THE~.., PL SHOCK aFlB i.'( S .. K ~ , . ' l 1Q T C. Ut.016 ~H> f-11£ s COOK l f\ IT JAL L1Q T , U",Ei r!;[ s ((,(11( H lf R ,,

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N 74 ,,. J.!!SQ53 J.~~4H~ C.45!! J.475 ; • 216«;73 I H 27 ).54159 ). 5C ;t 5·J ,: .442 J.434 '.19CCq6 (J\ 7', 2 ': J. ~ ?OJZ .J • ..iez1;1;, c.,~1 0.411 '..176756 00

11 ~·) J.!!46S3 J. c;cr;3e1 C.44C 'l.428 : • le7743 72 ~·: .). ~44;)6 J.5CeH,7 0 .44c; l.465 '.205771 7q :2 :l.Sn45 .;. 4b73f 1 c.,?z ~.401, ;.175491) 60 ~3 0.55251 '• 'H6 l2A C. 44, 'l.460 ·.20H2A ~ 1 "14 J.~24u4 ,j.4H5ll i: o't?:: J.421 '.l76C50 ~2 1c: o. 536''H .). 5C6 Ud ~ .4'.!? IJ. 455 \. l C,f.)l:~ HJ .,~ v.5"153 J.Hne1 C.4l.! J.268 ,.lO'i33<l e4 ,7 .j.54557 .). 'il}3d'i5 C. 44c J.432 · .1c;26ZI ~5 ;,. J. 55947 ).4t3617 r. .4o;r; ).426 J. l'i44•J7 e6 J.51e249 ·J. 'tc; ~s l2 ,. <t4i: J.410 ~.l8C592 r: 1 :..; o. 55245 ·J.4C,7Hl C. 41tc; J.450 i • .ZC3012 ee "l o.5ie653 J. 5~731=? ,. 415 J.448 J.1c;au~ ~c; 4l 0. !!"106 ;. s:01c1 ~- 44.: .:. 44C •. l921e46 Q') :.1 o.54247 '•'•'il6l7 ,:.444 ).442 :.1Ci6825 CL 4,~ o.534u0 J. 4c; r.:~CJ C.4~·1 '). 45'5 . • 1c;c;745 .; l ,.~ .). 53tl49 ,. 4~(Jti 1 l. r: .4 '\t- C.407 '.117644 C,! I 7 v.5'i2J2 ).~ll2':~ ~. 4 ,r. .J.440 .1qz775 ,,4 !"t! o. '!:3951 ).'il<11'd r.,.4~ ~.45(.J ·.1H8C8

STUDIES TO DETERMINE EFFECTS OF SECOND QUADRANT OPERATION ON SOLAR CELL RELIABILITY

R. A. HARTMAN* Clemson University

Abstract

The operating point of a cell in an array can shift from the first to the second quadrant of the I-V plane when the current passing through the cell exceeds the short-circuit current. In this mode, appreciable power can be dissipated, causing an increase in cell temperature, thermal instabilities and thermal breakdown. The shift from the first to the second quadrant can be caused by shadowing, interconnect failure or differential parametric degradation.

A method to observe hot spots was developed, based on a method described by the National Bureau of Standards using temperature sensitive phosphors. A preliminary look at a limited number of cells has verified the thermal nature of the solar cell's second quadrant. Three operational ranges can be defined:

Region A. In region A the temperature of the cell is less that 1700c and uniform, but much higher than when operating normally in the first quadrant. In this region the failure modes should be the same as in the first quadrant, and the electrical characteristics of the cell should degrade in exactly the same fashion as if the cell were on accelerated stress test in the first quadrant.

Region B. In region B the temperature is greater than 1700C (melting point of solder), but less than O. In this region the temperature is also uniform (or nearly so), but the failure modes may be ones not encountered in accelerated testing, i.e., molten solder effects, blistering and delamination of the encapsulant, and cracking of cells due to thermal mismatch.

Region c. In region C the temperature reaches Oi in a localized region. Temperature is very nonuniform across the cell with a mesoplasma forming at a single spot. Unless externally limited in some fashion, catastrophic melting will occur.

A sample quantity of reverse-bias cells was observed using an infrared camera prior to final selection of the temperature-sensitive phosphor method. The development of a one dimensional finite difference model is under way. At this time, initial laboratory measured data appear to closely follow model predictions.

*Present address: Solenergy Corp., Wakefield, Mass.

2-69

N I

"' 0

APPROACH/OBJECTIVES

• DETERMINE BREAKDOWN MECHANISM

• GATHER/REPORT DATA ON CELLS IN CLEMSON PROGRAM

• IMPACT OF SECOND QUADRANT ON RELIABILITY

• DEVELOP MODEL LINKING VOLTAGE, CURRENT AND TEMPERATURE

N I

'-I .....

N CELLS IN SERIES WITH A CELL WITH AN ANOMALOUS 1-V CURVE

A

I

I \

-----, __ -- - ~

' ,. -®<B>®- __ )

' ~- .-. --- -- - - -

Nx6V

B

V

N I ......,

N

80

TYPICAL PULSED SOLAR CELL SECOND QUADRANT CHARACTERISTICS

100mW/cm2 G

60 40 20

CELL VOLTAGE ( V)

10

8

-6 <t -I-z w

4 a: a: ::::, C'..l

0

N I

....... uJ

SECOND QUADRANT SOLAR CELL CHARACTERISTICS UNDER STEADY STATE CONDITIONS

(ILLUMINATED)

50

40 30

20

·20

10

6

3

-15 ·10 ·5

CELL VOLTAGE (V)

-< -f-z LU a: a: :::::, C'...)

.....J

.....J UJ (.)

N I

...... ~

SCHEMATIC OF PHOSPHOR DECORATED MEASUREMENT METHOD

UV LIGHT ---~~--

//II/ X-Y

~.._..~ ________ ov __ M---t PLOTTER

SOLAR SPECTRUM

PROGRAM. POWER SUPPLY

FUNCTION

GENERAlOR

ILLUMINATED SECOND QUADRANT CH.ARACTERISTICS

THREE OPERATING MODE RANGES

-10 -5

CELL VOLTAGE

2-75

4

-< 1--2 LU a: C: ::::, LI

2 :j UJ LI

STEADY STATE 1-V CURVE TYPE G CELL

4

2

,...._ <t ~

I-z LaJ a:: a: ::, 0

_J _J lJJ 0

--------------__J~ I

__... ......... J. ..

-10 -5 CELL VOLTAGE (V)

2-76

0

STEADY STATE 1-V CURVE TYPE A CELL

I I

- -- -er --

L ---9 ~--i'

l ____ ---'f::::r

6

4

~ . .......I..,..,._.._~~ ~s,.:L,.---'-lir_"'"--__ J_,_ __ ,_.,..,._,,...v•-_.-,,.J._ ... _,,.,; • ._"" .,..--~ Q

-20 -15 -10 -5 0

CELL VOLTAGE (V)

2-77

1-z w 0:: a: ::::> 0

-' -' LL.I 0

N I -....

00

TEMPROBE MEASURING GRIDS

CURRENT ~ .. PROBES ~

PHOSPHOR-DECORATED CELL AT DIFFERENT SECOND-QUADRANT

OPERATING POINTS

1 2

3 4

2- 79

N I

0:, 0

PHOSPHOR DECORATION TEMPERATURE MEASURING SYSTEM

,-.. <( -.....,

I-z w a:: a: ::, (.)

....J _J lJJ (.)

LEAKAGE CURRENT vs TEMPERATURE TYPE A CELL

3

2

1-

oL--t===i===::::-:;:__.L.. __ ....__ __ 0 100 200

CELL TEMPERATURE (°C)

2-81

N I

00 N

-<C -

INTRINSIC 1-V CURVE TYPE A CELL

10 · ·.. , .. , •. ··:1-·· • .,,:-~. : ·.;t kii~t.§1,p----.. ~~·~t~~~~~,.,. ~ ~-.a::.:£. •. . 11111aa.-.. lliililll -•••••••••• •••••••••• !z ••••••••••

~ 5 111•1•1 ~ ......... . i:d ......... . u ---- ..... o••••-••• 0 50 100

CELL VOLTAGE (V)

N I

00 w

1-V CURVE AT 100°C

3

~ 2 -~ _..t-...,;.L-.;.L~: .. , .... .... _. ~ .... ... ,.,.: &."~~~t . .

~ il•••fl[ijElliilrl ~ ••••••m••• ~ ••a11111111men ~ 11111111111 •••••••••• •••••••••• •••••••••• 0 ......... .

0 10 20

CELL VOLTAGE (V)

N I

CX)

~

1-V CURVE AT 150°C

3 .-. ---.~~·L.v-6..<o:.-~ ..... - .t ........ >.....;..> •• ~.:.ll.:-1'. •,

lllillmi~i.iiiiillll < •••••••••• ;: •••••••••• as 2 •••••••••• ... ~ II!!!=······· B •••••••••• ::i ••••••••• ~ •••••••••• 1••••••••••

01-------'---_..._-o 10 20

CELL VOLTAGE (V)

1-V CURVE AT 200°C

-<(

I-z LL.J ~ 2 ~ :::> u

"' I _J CX) _J V, LL.J

u

I

0+------&.----__J._--0 10 20

CELL VOLTAGE (V)

CONCLUSION

THE LOWER THE CELL TEMPERATURE, THE LESS SEVERE THE STRESS.

THUS CELLS OPERATING IN THE SECOND QUADRANT SHOULD BE KEPT AS FAR BELOW

THE KNEE AS POSSIBLE. FURTHERMORE, SINCE THE TEMPERATURE AT THE KNEE IS ROUGHLY

Bi, INDEPENDENT OF WHETHER THE KNEE OCCURS AT LOW VOLTAGE OR HIGH VOLTAGE,

A CELL WITH A HIGH BREAKDOWN SHOULD BE ABLE TO SURVIVE SECOND QUADRANT

! OPERATION BITTER THAN A CELL WITH A LOW BREAKDOWN, ALL OTHER CHARACTERISTICS °' BEING EQUAL

THIS CONCLUSION CONFLICTS WITH SOME OF THE CURRENT THINKING,

WHICH IS BASED ON LIMITING REVERSE POWER DISSIPATION BY USE OF HIGH SHUNT

RESISTANCE, LOW KNEE DEVICES. THE FALLACY IS THAT LIMITING POWER DISSIPATION,

UNLESS IT IS ACCOMPLISHED BY A DIFFERENT MECHANISM SUCH AS A FORWARD- BIAS

DIODE, DOES NOT LIMIT TEMPERATURE. IT IS TEMPERATURE WHICH IS THE RELIABILITY

STRESS FACTOR, NOT POWER

Test Results From Clemson Solar Cell Stress Test Program

J. L. PRINCE Clemson University

Abstract

Solar cells produced by different manufacurers seacted differently to the various tests. This can be observed most easily from plots of the mean percentage decrease in Pm as a function of stress time. In B-T tests, for example, type A cells exhibited consistent degradation at all temperatures, whereas type B cells showed no degradation. Type E cells exhibited latent degradation which showed up only after an extended stress time. Not all cells in any given population degraded equally, of course, and distribution plots as a function of stress time were developed to provide valuable information concerning infant mortality (i.e., observed in F cells) and the existence of "freak" failures.

In this program the general ability of accelerated stress tests to induce cell degradation and discriminate between cell types, processing technolgies, etc., was demonstrated. This discrimination was observed on the basis of Pm degradation, visual observation, and metallization adherence degradation. Thus it is clear that taken as a whole the results of the accelerated stress tests can be used to rank-order cell types with respect to potential field reliability.

Changes due to accelerated stress could be observed visually as well as measured electrically. Visual observations, however, are qualitative and largely subjective, so their use should be restricted to understanding failure mechanisms rather than predicting reliability.

Finally, extrapolation of B-T test results to use conditions indicated the possibility of less than 10-year life in some cases.

2-87

N I

co co

TYPE

A

B

C

E

F

G

H

PHYSICAL CHARACTERISTICS OF CELLS

SIZE TH IC l<NESS AIR TECHNOLOGY (IN) (Ml LS) COATING

4, DIA 24 NO PIN

3,DIA 19 YES NIP

2, DIA 20 YES NIP

3,DIA 15 NO NIP

3.9 X 0.8 13 YES NIP EFG SI LI CON

3,DIA 12 YES NIP

2 X 2 12 YES NIP POL YS I LI CON (IMPLANTED)

METAL

SOLDER

THIN FILM Ti/Pd/Ag

SOLDER

THICK Fl LM Ag

SOLDER

SOLDER

TH IN Fl LM TilPdf Ag

LONG-TERM 75°C BIAS-TEMPERATURE TEST RESULTS

-~ -6

~ o TYPE A z A TYPE B -LU 4 Cl TYPE C en ct o TYPE E LU a:

N (.J I LU 2 CX>

\0 C

~ 2 w LI 0 a: LU Q.

z <( ·2 u.J

~

0 2000 4000 6000 soou

STRESS TIME ( hr)

LONG-TERM 135° BIAS-TEMPERATURE TEST RESULTS

30 o TYPE A

- A TYPE 8 ~ -E

a TYPE C 0.. 0 TYPE E

20 2 -w Cl')

N CZ: I \0 w 0 a:

c...l w

10 C

·I-2 w c..:, C: w 0..

2 0 <t LU

~

0 2000 4000 6000 8000

STRESS TIME ( hr)

,,..,.. ~ 0 ~

a..E

~

w CJ) <{ w n:

N u I \0 w .....

C)

._ z uJ u 0: w a..

z <t w ~

MEAN PERCENT DECREASE IN Pm FOR 165 °C BIAS-TEMPERATURE STRESS TEST

40

0 TYPE A

0 TYPE 8 20

6 TYPE C

0

500 1000

STRESS TIME (hr)

-<t -..-z U,J

a: a: => u

~ _, U,J

c..,

1.0

,.. . ;)

8960 HOURS

0 l___-1, __ --1.. __ _,_ __ _.__ __ "---~~--_...

O .2 .4 .6

CELL VO l TAG E ( V)

2-92

FAR

FORWARD

LLJ (.!)

<C 1-

100,..

75 -

z 50-LLJ u 0:::: LLJ a..

25 ..

E PRE- STRESS I3s0c

0 "-m---.•..___._._,_.._._..__._......_._._._.._._l....__._. __ ...._-.-1..__ 0.2 0.4 o. 6

NORMALIZED Pm

o. 8 1. 0

N I

\0 .i:--

L&J (.!)

< 1-z L&J u ~ L&J a..

100

75

50

25

0

~

-

-

-

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LOGNORMAL SCALE

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CUMULATIVE PERCENT ~ DEGRADATION

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BEHAVIOR OF TIME TO 10% DEGRADATION vs INVERSE TEMPERATURE

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RELATIVE STRESS TEST EFFECTIVENESS

STRESS TEST F

B-T

PRESSURE COOKER

85° C/85% R.H.

THERMAL CYCLE

THERMAL SHOCK

CELL TYPE

G

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H

RELATIVE EFFECTS OF ACCELERATED STRESS TESTS ON CONTACT INTEGRITY


CELL TYPE STRESS TEST

A B C E

8-T

PRESSURE COOKER

85°C/85°/o R.H.

POWER CYCLE

THER}'1AL CYCLE

THERMAL SHOCK 11111 lllf ill~ 2-108

RELATIVE EFFECTS OF ACCELERATED STRESS TESTS ON Pm


CELL TYPE STRESS TEST

A 8 C E

8-T

PRESSURE COOKER

85°C/85°/o R.H.

POWER CYCLE

CYCLE 11111 THEF:f\1AL SHOCK ltt!!f!!l " .. ,·,'

: : .. >\ :··:::.· .'•::

2-109

N I ..... ....

0

CONCLUSIONS

• SIGNIFICANT DEGRADATION EFFECTS OBSERVED FOR MOST CELL TYPES AND MOST STRESSES

• ELECTRICAL PARAMETER DEGRADATION, B-T AND B-T~H

• PHYS I CAL AND CONTACT ADHERENCE DEGRADATION, THERMAL CYCLE AND THERMAL SHOCK

• PHYSICAL CHANGES- METAL AND AR COATING

• INFANT MORTALITIES/FREAKS OBSERVED

• FURTHER TESTING AND ANALYSIS NEEDED FOR ACCELERATION FACTORS AND MECHANISMS

• SECOND QUADRANT EFFECTS CAN BE CORRELATED WITH CELL TEMPERATURE - NOT CELL POWER DIRECTLY

SECTION III

WORKSHOP SESSIONS - HIGHLIGHTS OF DISCUSSIONS

E. L. ROYAL Jet Propulsion Laboratory

The workshop sessions were moderated and structured, but were designed to gain maximum participation by the attendees. A number of stimulating questions were presented to the group to initiate the discussions, and members of the audience were encouraged to pose questions for discussion as well.

Dr. J. w. Lathrop of Clemson University served as moderator, and skillfully elicited the the varied viewpoints reflecting different perspectives in the photovoltaic industry. Every attendee took part in the discussions, and each participating organization contributed useful information. The workshop served as a vehicle for feedback to the photovoltaic industry of a wide range of useful technical information and data on silicon solar cell reliability.

The following pages contain some of the questions that were raised at the workshop and the highlights of the discussions that ensued.

QUESTION: What relationship does a cell test (i.e., bias-temperature) have to results seen in real world field applications of solar cells?

DISCUSSION: There is guarded optimism and hope regarding design and use of the various bias-temperature tests. These tests allow us to use Arrhenius relationships and extrapolate back to use conditions. When one attempts these extrapolations (as per with !Cs) the appropriate activation energy must be assumed. It is also necessary to work with one failure mechanism and to design the test to avoid nonlinearities. This method has been used with semiconductor devices and almost always gives pessimistic results. Arrhenius relationships and extrapolations back in time for solar cells probably would also give pessimistic results.

QUESTION: What real-time field exposure time scales can be used for the accelerated stress tests on silicon solar cells that now have been under test for two years at Clemson University?

DISCUSSION: As a result of the test program, aging time as of now appears to represent two to six years of equivalent real-time stress exposure for the bias-temperature effects.

QUESTION: How are failure and degradation results from field applications correlated with degradation observed in cell stress tests?

DISCUSSION: One problem is that many other factors such as soiling tend to mask small changes in cells that may be occurring in the field. In the field

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it is only after major degradation or a failure occurs at the module level that there are any clear indicators that cell degradation is taking place at the array level. On the other hand, cell degradation on some cell types has been observed by Clemson University investigators in their cell stress tests. A few of the attendees, however, felt that degradation of cell electrical characteristics (as a function of time) in single crystal silicon cells will not be a major problem area. The Clemson investigators are collecting data that could cause rejection of such a hypothesis.

QUESTION: Are the failure mechanisms being observed in the Clemson University cell stress tests different from some of those seen in the field?

DISCUSSION: The predominant cell failure mechanism seen to date in the field was reported to be cell cracking. This failure mechanism is not the major one seen at Clemson because cell cracking is due largely to package-dependent and electrical design factors of the module design.

The present cell failure rate being experienced in the field (mainly cell open-circuit-type failures) is only one per 10,000 per year. This low figure, however, should be viewed both in light of a) the large number of silicon solar cells now in the field and b) the comparatively small percentage of the projected 20-year lifetime that these cells have operated in the field so far. These were precautionary warnings against too much optimism based on this low (but admittedly early) cell failure rate. If indeed the lognormal statistics are followed, as with !Cs and other silicon semiconductor devices, we may only be seeing the tip of an iceberg. If that is the case, the failure rate may not peak until later, i.e., sometime near the median time. If fact, analysis of early Clemson cell testing results to date tend to support this premise; trends seen in degradation as a function of time (after 2000 hours on several cell types) are up.

QUESTION: Is there a limit on the spread in certain cell parameter distributions within a given type that one should look for?

DISCUSSION: Cells in the same production lot built by similar processes showed fairly wide spreads in the initial distribution of their electrical operating parameters. These distributions seemed to have even wider spreads after the cells were exposed to stress.

The question of mismatch in a module cell string that utilized these cells was then discussed. Differential mismatch between different cells of a given type can develop as a function of cell operating time and was observed on each cell type. It was generally agreed that differential degradation of cells of the same type is something that must be expected with present technology. There is an acceptably small amount of cell mismatch when modules are assembled at the factory. Thereafter, there will be differential degradation rates between cells which could lead to more pronounced mismatching. Modules must therefore be designed to tolerate cell mismatch or must have built-in protective features to prevent failures or excessive power losses.

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QUESTION: What is the general value of a cell reliability test program?

DISCUSSION: Cell test results alone without module-level data, are not enough to indicate conclusively that a cell type is unsatisfactory. Cell testing results do serve to help cull out the really low reliability cell types, but mostly they serve as a tool with which to make comparative judgements between different cell types that are subjected to the same stress exposures.

Module design may be a bigger factor in reliability than slight differences that may exist in results of cell reliability tests on various types of cells. Failure/degradation observed in the field often involves interactions between cells and other materials used to package the module, i.e., front surface plastic encapsulants (yellowing, delamination), substrates (outgassing when heated), etc. One big value of a cell stress testing program like the one at Clemson, is that it serves as a tool to give early warning signals on reliability and indicators of which failure mechanisms predominate in each manufacturer's cell type.

Fill factor and low shunt resistance in individual cells were cited most frequently as helpful characteristics that a module designer would particularly like to pin down in cell design.

QUESTION: What are the cost factors of degradation being observed?

DISCUSSION: One suggestion was that life-cycle cost trade-off studies be considered. One possible approach might be to use estimated accelerated factors from the tests, degradation observed in relation to test exposure time, and other particulars in a life-cycle cost analysis. One might thereby see what the performance benefits are on a life-cycle cost basis. For example, sensitivity of the system cost differences could be viewed in a manner similar to that used to optimize module series-parallel designs in module/arrays. For specific cell designs one might obtain information about the relationship of life-cycle benefits to initial cost. In that way many factors of cost which are known to affect cell reliability, such as cell metallization, production processes, etc., can be evaluated with respect to their sensitivity to reliability. The disadvantages of this suggestion were then debated. It was agreed that synergistic effects of real world environments make the results of this type of approach much more complex than those of one test exposure (i.e., bias-temperature).

QUESTION: How can a cell stress testing program be used to help bring about reliable cell design?

DISCUSSION: In addition to charting degradation rates and looking for failures, more emphasis must be placed on determining what the failure mechanisms are. This additional knowledge would facilitate making cells more reliable.

The suggestion was made that it is the duty of a cell manufacturer to uncover the failure mechanisms associated with his cell. By follow-up collaboration with the test program a manufacturer can compare test results

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on his cell type with the norm from a variety of other types tested the same way. But ultimately, the responsibility for analysis of the failure mechanism would and should lie with the cell manufacturer. Independent cell stress testing organizations, like Clemson in this case, cannot do this job as well because often they do not have the necessary experience with materials used, cell design; production processes, etc., that are important contributing factors.

A middle ground was proposed in which Clemson or a specialized cell testing laboratory could support the manufacturers in their efforts to do failure analysis. An additional input required to support failure mechanism analysis is good data from the field to compare with that generated by test exposure. This might raise a problem, since some cell manufacturers, in an effort to protect certain proprietary information on their products, prefer not to work closely with an outside organization.

One concern voiced by a representative of a cell manufacturer was that there is a possibility competitors would misuse negative test results published in Clemson test reports for a sales advantageo Although the objectives of the Clemson stress test program are clearly stated, some attendees said that is can mistakenly be considered by potential customers as being equivalent to a "Consumer's Report" type of evaluation. This is not what the Clemson cell testing program is all about. However, as a result of this discussion, Clemson agreed to a proposal for a new alternate approach to cell testing that would reduce that concern. That alternate approach involves "cost sharing" by a manufacturer. In return the manufacturer gets exclusive rights to review all of the test data and analysis results made on his particular cell prior to release for public dissemination or publication. In response to this proposal, one cell manufacturer expressed iunnediate interest, and preliminary discussions were initiated. The cost sharing details had not been worked out but initial indications were that it could be done in the interest of program objectives.

QUESTION: Are manufacturers willing to set up their own cell stress testing facilities and test cells themselves?

DISCUSSION: The suggestion was made that one or two of the larger manufacturers might consider doing this on new cell types, before they get committed to mass production. The role of cell testing by independent groups such as Clemson would continue, but emphasis would be placed on evaluation of new cell technology investigations. There is already movement in that direction, because many of the new cell types being tested by Clemson represent new technology, (i.e., new metallization systems, non-single crystalline silicon, etc.).

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SUMMARY OF DISCUSSIONS

There was general agreement on the following items:

1. A set of well designed, properly conducted reliability stress tests can serve as a useful tool .in efforts to establish reliability differences between varied cell types. The Clemson tests represent a set of "Strawman" tests for cell qualification.

2. The testing of individual cells which are forced into the reverse-bias, second quadrant mode has revealed large differences in reverse characteristics between different cell types. Test methods developed and demonstrated at Clemson will be very helpful in assessing cell hot spot temperatures. The results of reverse-bias testing at Clemson will also be important in JPL/LSA efforts to develop specification criteria that cell manufacturers and module/array designers will use to help avoid or minimize the reliability hazards of reverse-bias operation.

3. The particular photovoltaic cell type which displayed the poorest reliability record among the cells evaluated in the Clemson stress test program was also reported to have had the poorest reliability record in the field. Similarly, on the other end of the scale~ the particular cell type which attained the best reliability record in the stress test program was also reported to have performed best in the field.

Although it may not be conclusive at this point, the correlation of laboratory test results with performance in field applications provided encouragement to the cell reliability investigators. The consensus was that this work was useful and should continue with the implementation of changes suggested during the workshop.

•

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LIST OF ATTENDEES

R.R. Addiss, Jr. Solar Power Corp. 20 Cabot Rd. Woburn, MA 01801

Paul Alexander Jet Propulsion Laboratory 4800 Oak Grove Dr. MS 512-103 Pasadena, CA 91109

Tom. s. Basso Solar Energy Research Institute 1617 Cole Blvd. Golden, CO 80401

Egil Castel Photon Power, Inc. 10767 Gateway West El Paso, TX 79935

Michael W. Chappell Student, Dept. of Electical

Engineering Clemson University Clemson, SC 29631

C. P. Chen Jet Propulsion Laboratory 4800 Oak Grove Dr. Pasadena, CA 91109

John Fury Christ Dept. of Electrical and Computer


Steven Forman MIT Lincoln Laboratory P. o. Box 73 Lexington, MA 02173

Gordon B. Gaines Patel le-Columbus 505 King Ave. Columbus, OH 43201

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William Gambogi SES, Inc. Tralee Industrial Park Newark, DE 19711

C. Michael Garner Sandia Laboratories Albuquerque, NM 87185

Ron Gonsiorawski Mobil Solar Energy Corp. 16 Hickory Dr. Waltham, MA 02154

Robert Hartman Dept. of Electrical and Computer


Dexter Hawkins Dept. of Electrical and Computer

'Engineering Clemson University Clemson, SC 29631

Chuck Herrington SERI/PVPO 1617 Cole Blvd. Golden, CO 80401

Bruce Larson Motorola 5005 E. McDowell Rd. Phoenix, AZ 85203

Jay W. Lathrop Dept. of Electrical and Computer


David Loudenback SES, Inc. Tralee Industrial Park Newark, DE 19711

LIST OF ATTENDEES (Cont.)

Philip Pierce SERI 1617 Cole Blvd. Golden, CO 80401

John L. Prince Dept. of Electric! and Computer


Calvin Rogers Sandia Laboratories Kirtland Air Force Base Alburquerque, NM 87185

Ron Ross Jet Propulsion Laboratory 4800 Oak Grove Dr. Pasadena, CA 91109

Edward Royal Jet Propulsion Laboratory 4800 Oak Grove Dr. Pasadena, CA 91109

Harry Schafft National Bureau of Standards Bldg. 225, Room B310 Washington, DC 20234

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Steven Sollock Jet Propulsion Laboratory 4800 Oak Grove Dr. Pasadena, CA 91109

Shawn Solomon Arco Solar, Inc. 20554 Plummer St. Chatsworth, CA 91311

William Taylor Spectrolab, Inc. 12500 Gladstone Ave. Sylmar, CA 91342

M. Patricia Themelis MIT Lincoln Laboratory Lexinton, MA 02173

Eric Tornstrom Mobil Solar Energy Corp. 16 Hickory Dr. Waltham, MA 02154

Haskel Walker Dept. of Elect~ical and

Computer Engineering Clemson University Clemson, SC 29631

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