ABSTRACT The objective of this study was to investigate the technology used by Spectrolab Inc. to manufacture photovoltaic modules that have provided twenty years of reliable service at Natural Bridges National Monument in southeastern Utah. A field survey, system performance tests, and a series of module and materials tests have confirmed the durability of the modules in the array. The combination of manufacturing processes, materials, and quality controls used by Spectrolab resulted in modules that have maintained a performance level close to the original specifications for twenty years. Specific contributors to the durability of the modules included polyinyl-butyral (PVB) encapsulant, expanded metal interconnects, silicon oxide anti-reflective coating, and excellent solder/substrate solderability

B R Hansen

D L King

F M Hosking

J A Kratochvil

M A Quintana

M B Pandit

N G Dhere

R W Johnson

CiteSeerX

t
.
DIAGNOSTIC ANALYSIS OF SILICON PHOTOVOLTAIC
MODULES AFTER 20-YEAR FIELD EXPOSURE
M. A. Quintana, D. L. King, F. M. Hosking, J. A. Kratochvil, R. W. Johnson, B. R. Hansen
Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185
N. G. Dhere and M. B. Pandit, Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922-5703
ABSTRACT
The objective of this study was to investigate the
technology used by Spectrolab Inc. to manufacture
photovoltaic modules that have provided twenty years of
reliable service at Natural Bridges National Monument in
southeastern Utah. A field survey, system performance
tests, and a series of module and materials tests have
confirmed the durability of the modules in the array. The
combination of manufacturing processes, materials, and
quality controls used by Spectrolab resulted in modules
that have maintained a performance level close to the
original specifications for twenty years. Specific
contributors to the durability of the modules included
polyinyl-butyral (PVB) encapsulant, expanded metal
interconnects, silicon oxide anti-reflective coating, and
excellent solder/substrate solderability.
INTRODUCTION
Over twenty years ago the Department of Energy
and the Department of Interior agreed to install the
world’s largest, at the time, photovoltaic system at
Natural Bridges National Monument (NBNM), managed
by the National Park Service [1]. The system was to
provide all electrical power to the park. Individual PV
arrays manufactured by Spectrolab Inc., ARCO Solar,
and Motorola were installed at NBNM in 1980 and are
shown in Figure 1. Technology vaned but all modules
were monocrystalline silicon and all three module-types
were representative of JPL Block [1[ or earlier
.s@/vo2000- z337L
Pracfantrari at ‘JRm IFFF Phntnunkie Snnrialictc Cnnfnrnnrn. Anrhnmnn Al llWl fWM-fK419711_ln
-. .r. ~ . . .,_
<., .-’ J!P>-:.; ~,,;- :, !.,>.,:r,
—. ,.
..$.,.;: .Jfi,,.., ,+ :. .. !,./>,,,,,.Z-,,.+,J<.,. ,+ -,,., .:f:?:~~;.?. :%.,,. .V: .?. +;2T< -,.\ .,... ....p_ ,,: ....,, .- —-- .,. ~ ,: ., ,-, T- ---— - - -. –—
Fig. 1. Three arrays at NBNM include the Spectrolab
array in the background.
classification. Recently, arrays manufactured by Motorola
and ARCO were disabled or dismantled due to increased
maintenance requirements and/or significantly decreased
power production. While additional arrays with newer
technology have been recently added to the NBNM
system, the original Spectrolab amay continues to provide
useful power to the park.
The Spectrolab array consisted of ten strings of 70
Block II modules nominally rated at 28.5W at 1000 W/cm*
and 41° NOCT [2]. NBNM system performance tests
sponsored by Sandia in 1985, 1992, and 1997 repeatedly
showed that the array strings, while degrading in total
output power, continued to maintain the shape of their I-V
curves well and maintained high fill factors of about 70Y0.
A single field-aged module and two unexposed modules
were studied to determine the technological reasons for
this success.
Fig. 2. Glass breakage and delamination on Spectrolab
module
FIELD OBSERVATIONS
A field survey of the NBNM array was performed in
1999 and a Spectrolab module was obtained for
diagnostic work. Later, two unexposed modules were
obtained from NBNM for comparative electrical
performance testing. The laminates were constructed with
a single untempered-glass sheet, PVB encapsulant, 120
(5-cm diameter) c-si cells, another layer of PVB, and a
Mylar@ backsheet. The backsheet of the modules were
painted white to enhance the amount of light reflected
back onto the cells [3].
Notable features in the array included a significant
number of modules with cracked glass, Figure 2,
DISCLAIMER
This report was-prepared as an account of work sponsored
by an agency of the United States Government. Neither
the United States Government nor any agency thereof, nor
any of their employees, make any warranty, express or
implied, or assumes any legal liability or responsibility for
the accuracy, completeness, or usefulness of any
information, apparatus, product, or process disclosed, or
represents that its use would not infringe privately owned
rights. Reference herein to any specific commercial
product, process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily constitute
or imply its endorsement, recommendation, or favoring by
the United States Government or any agency thereof. The
views and opinions of authors expressed herein do not
necessarily state or reflect those of the United States
Government or any agency thereof.
DISCLAIMER
Portions of this document may be illegible
in electronic image products. Images are
produced from the best avaiiable original
document.
.
presumably due to hail damage or thermal cycling. A
study performed shortly after the installation identified
twenty-five cracked modules. It was hypothesized that
the cracks occurred due to flaws along the edges of the
glass in conjunction with excessive tensile stress caused
by diurnal temperature cycling [4]. NBNM, located in
southeastern Utah, has a climate that is hot and dry in
the summer and cold and dry in the winter. Our field
survey noted that despite the cracks in the glass surface
of several modules and, in some cases missing glass
shards, there was minimal delamination or corrosion
caused by air and/or water intrusion. The encapsulant in
a majority of the modules had yellowed to vatying
degrees. Some modules had nonuniform yellowing while
a small fraction had no yellowing at all. Yellowing was
initially attributed to moisture penetration at the glass
fracture lines [4]. However, the field-aged module (20
years) used in this study was nonuniformly yellowed and
the glass had not been fractured.
TEST RESULTS
The three Spectrolab modules were subjected to
electrical performance tests, dark I-V tests and infrared
thermal scans. Table 1 shows a summay of module
electrical performance parameters. Electrical
performance of the field-aged module was consistent
with JPL Block II module specifications adjusted for
temperature and solar spectrum. In a more direct
comparison, performance of the field-aged module was
about 10’%0lower than the two unexposed modules. A
closer analysis shows that l= on the aged module is --7’?10
lower than that of the unexposed modules. This was
attributed to an optical loss, likely from the yellowing of
the encapsulant. The remaining loss in power, -2.5%,
resulted from decreased fill factor. The Vw of the field-
aged module remained unchanged suggesting that the
fundamental performance of the silicon cells has
remained stable. A 1997 system assessment showed
that five of the ten module strings at NBNM were still
performing at -80% of their initial field rating.
Performance of the entire array had degraded -50% from
the initial rating. Degradation was attributed to specific
problems such as complete failure of individual modules
in strings. The field data for functional module strings
Table 1. Spectrolab module electrical perfonman~
original JPL rating and Sandia data for 20-year module
and two unexposed modules.
I I Isc (A) I Voc (V) lPmp (WI FF
JPUBlock II
Rating, NOCT 1.88 21.9 28.5 0.69
JPL Data at
ASTM-SRC 1.96 23.9 33.3 0.71
Aged module,
20 years 1.91 23.6 31.8 0.70
along with our assessment of individual modules
suggests: 1) about half of string-level power loss can be
attributed to module degradation, 2) the remainder of the
string-level power loss was likely due to increased
resistance in module wiring and terminations.
INFRARED IMAGING AND DARK I-V
An infrared (IR) scan of the field-aged module showed
uniform heating of the three parallel cell-strings while a
steady current of 4.4 amps was circulated through the
forward-biased module. Cell temperatures ranged from
about 27.O”C to a maximum of 29.20C. There was an
absence of localized hot spots that could indicate high
resistance solder-joints. iR scans of the unexposed
modules showed comparable temperatures and thermal
uniformity. Dark I-V tests of the field-aged module yielded
a cell series resistance (1%.)of 53 mfl This is higher than
contemporary products, but consistent with modules made
with small cells. The RS of the two unexposed modules
was 36 mfl The apparent 47% increase in Rs of the field-
aged module over twenty years corresponds to a 2.5%
drop in fill factor, consistent with I-V data in Table 1.
Fig. 3. Front-side solder joints and interconnects.
SOLDER-JOINT METALLURGY
Metallurgy of six solder-joint samples was examined
by cross-sectioning and microanalysis. Interconnect
material, Figure 3, was similar to a material commonly
known as “expanded metal”. Expanded metal is a sheet
or coil of metal that is slit and drawn into rows of diamond
shaped openings. Spectrolab soldered these
interconnects onto small pads on the cells rather than
across the entire length of the cell. Multiple openings in
the interconnect material provided a short escape path for
L-EW24
Fig. 4. Cross-section of solder-joint shows copper
filament, solder, metallization, and silicon from top to
bottom.
IJraetnntt=r-i at 7Rh IFFF Dhntnwdt=ir .Cnnri4iete Cnnfnrnnms Anehnmna Al nWl .WlfLAO1991(lfl
,..=,.7,v --,w.-.~ -,-, ~-”-,,-- .2.T= ,.4 . .L,, ..,..>. .. . ?., -. :. \ -,. >$...!-. ., .,. ,.,.,,..,.,, ., ..,- -— --’ “’—-
.. ,
gases generated during the soldering process, thereby
minimizing voids. Voids have been identified as initiators
frequently linked to solder-joint failure. Outgassing
efficiency of the expanded metal interconnects may be
an advantage over the solid ribbons currently used by the
PV industry. Finally, soldering filaments rather than long
lengths of flat ribbon also created a large number of
transitional fillets that minimized stresses resulting from
differential expansion. Sustained electrical performance
over twenty years and metallurgical characteristics
demonstrated the value of this interconnecting technique.
Grinding the joints to expose cross-sections was
difficult because the width of interconnect filaments was
small, 250-500~ (.010-.020 in.). The cross-section of a
solder-joint across the width of a filament in Figure 4
showed smooth transitional fillets. Joints studied had a
small number of voids but no cracks. Cell metallization
was thinner, 7-8 ~, and uniform in thickness, indicating
that the metallization was deposited through a very
controlled process. Figure 4 shows a grainy, almost
ctystal-like structure in the metallization, suggesting that
the metallization was screen-printed. The thin
metallization and a well-controlled soldering process may
have contributed to a small interrnetallic layer. Excessive
intermetallic reactions may contribute to solder dewetting
and brittle joints. Solder-joints were uniformly thicker, 12-
75 pm, than those observed in today’s modules. Joints
exhibited good wetting, suggesting clean surfaces, use of
an effective flux, an effective soldering process, and good
solderability of the substrate by the solder alloy.
CELL TO ENCAPSULANT INTERFACE
Characteristics of the cell to encapsulant interfaces
were studied using adhesional strength tests, scanning
electron microscopy (SEM), X-ray photoelectron
spectroscopy (XPS), and Auger electron spectroscopy
(AES) at the Florida Solar Energy Center (FSEC).
Samples were cored using a 1.9-cm O. D. coring drill.
Aluminum nuts were glued to the samples using an ultra-
high vacuum-compatible (UHV) conductive epoxy glue.
The glue and relatively large sample diameter were
required for XPS analysis. Forty samples, located from
the penphey to the center, were extracted from twenty
Prac+antad 4 %@ IFFF Dhnfnwdtaim .Srm&diete (lnnfarnnra. Anrhnmnn Al IXWI IWMJKI19’)lM
,-. . ~.,>, ., ~.. ~ ,. j , .<;,/ ; :::,~,.~.,;y+,,y: .. ,..t$,f~.J., . .,...>.f .. ..; .< : ‘-,. ; .:y:@.,~’&+: -——, ,$ ., , . . . .. . .. .“, . .. . +’ : : :+; ,r’,-w. ,+., , ,:.;+: ,..
—— -------
Fig. 5. SEM shows pyramidal texture on cell.
cells. Samples were extracted by W.Sting, and a torque
sensor was used to measure the force necessary to cause
failure at the front cell/encapsulant interface. The average
adhesional shear strength was 4.5 ~1 .OMPa. Baseline
data on unexposed modules was unavailable but the
measured values were about 80% of the adhesional
strength of new modules made with EVA.
Twelve silicon cell samples were analyzed by Auger
electron spectroscopy (AES), Table 2. Typically a spot
between two gridlines was chosen for analysis. Figure 5
is a SEM micrograph that shows the surFace texture of a
cell. An AES survey was cam”ed out on the cell surface
over a range of kinetic energies from 50 to 2230 eV. An
AES line-scan over a 102-pm length between gridlines,
showed significant concentrations of potassium, sodium
and tin distributed uniformly over the cell surface.
Presence of tin and silver between gridlines was attributed
to diffusion from the metallization over twenty years of
service.
A depth profile, taken by sputtering the cell surface at
a rate of 100 ~ per minute, showed the presence of-1650
~ thick silicon-oxide layer at the surface, Figure 6. In
addition, this analysis provided a better understanding of
the impurities at the cell surface. Concentrations of
sodium and tin decreased rapidly to negligible levels within
a depth of 50 A. The source of sodium is believed to be
the glass superstrata. Concentrations of carbon and
potassium fell slowly over a depth of 250 A. The
potassium peak reappeared and became more intense at
the interface between the silicon-oxide and the elemental
silicon. Presence of potassium was residue likely from the
cell texturing process. As expected, phosphorous (n-type
dopant) concentration near the silicon-oxide/cell interface
was about 0.5-1 atomic 9!o. Phosphorous was only
present at negligible levels at the cell surface indicating it
had not diffused through the silicon-oxide layer.
Table 2 AES survey of a Spectrolab cell lists impurities
found on the surface.
Ca;on 267 11.0
Sodium 985 3.4
Potassium 249 2.1
Tin 421,431 1.1
Silver 350 0.4
Twelve PVB encapsulant samples were analyzed by
XPS, Figure 7. The small amount of silicon on the
encapsulant surface may have originated from a primer, if
one was used to improve encapsulant adhesion, or from
the silicon-oxide on the silicon cell. Silver and lead, both
with atomic concentrations ranging from 0.1-0.5 %, were
detected only on samples near the solder-joints.
Presence of silver in an encapsulant sample was
attributed to migration from the thick-film silver grid lines.
. I
Similarly, there were traces of lead, presumably from the
solder-joint, on the encapsulant surface.
The Spectrolab
CONCLUSIONS
array at NBNM recently completed
400
300
100
-Na -P;
0 500 1000 1500 2000 2500
Depth (A)
Fig. 6. AES depth profile of a c-Si sample
Low to moderate concentrations of impurities such
as sodium, tin, potassium, silver, and lead were detected
at the cell/encapsulant interface. The effect of potassium
on adhesional characteristics is being investigated.
Presence of 3-4’?40sodium and 1-2% tin may have
reduced adhesional strength; however, encapsulant
adhesion after twenty years was still nearly equivalent to
unexposed EVA encapsulated modules currently
manufactured. The factors that may have assisted in
maintaining an adequate level of adhesional strength are:
inert silicon-oxide AR coating, a thick and semi-
impermeable painted Mylar@ backing, and PVB
encapsulant. In contrast, significant loss of EVA
adhesional strength in field-deployed PV modules has
been attributed to significant concentrations of
phosphorous, sodium, titanium, and other impurities at
the cell/encapsulant interface [5, 6, 7’J.
➤✎✍✍✍✍
Prncfnnfnrl d %7h IFFF Dhntrwdt=ie .Snna.isliete Cnnfma”ma- Anehm-arm Al flWl .WlrUlW291fN1
. . . .. .W, ~...: .- . >...,..?+., .++-.,.”:,,,,:.>.:.7..,::,”, ‘=~ / .,;.. .-.. J;,, ?2%.%2$+%2> :. +:.>..-,+: m.aY’T5?R--TT -- ‘1;.’: .. ., --- --’- ‘-”--
. .
Eleme* Comxrhtion
.
Cls 70.9 al.%
6
0 Is 25,0 al% $
S12P3 3713L%
Ag 3(I5 0,3 al%
Pb 417 0.1 at%
H
~
%
1100 880 660 440 220 0
Binding Energy (eV)
Fig. 7. XPS spectra of a PVB encapsulant sample
twenty years of service with a high degree of module
durability. National Park Sewice personnel are pleased
with the array’s performance and low maintenance.
Results of our field survey and laboratory tests indicated
that the array should continue to provide useful power to
the park. Typical module degradation mechanisms such
as delamination, corrosion, and solder-joint failure have
not contributed to module degradation over twenty years.
Using the unexposed modules as a baseline, we believe
that the individual module output at NBNM has been
degrading at a rate of about 0.5% per year. A high level of
durability was achieved by using PVB encapsulant, a
silicon-oxide AR coating, expanded metal interconnects,
cell metallization with good solderabilty, and a compatible
solder alloy. It is also important to recognize that even
though module degradation rates were low, other factors
resulted in a 2.5% per year array-level degradation.
Clearly, degradation rates this high are still unacceptably
high for future systems.
ACKNOWLEDGEMENTS
The authors are grateful to the staff at NBNM, especially
Joe Joliet and Bob Lovato for their work and cooperation,
and to Ken Ronney, Spectrolab Inc. for his historical
perspective. Sandia National Laboratones is a multi-
program laboratory operated by Sandia Corporation, a
Lockheed Martin subsidiary, for the U.S. Department of
Energy under contact #DE-AC04-94AL85000.
REFERENCES
[1] F. J. Solman, “The 100 kWp Photovoltaic Power
System at Natural Bridges National Monument”, 15ti
IECEC Conference, Seattle, WA, 16-22 August 1980
[2] M. 1. Smokier, “User Handbook for Block II Silicon
Solar Cell Modules”, Jet Propulsion Laboratory, Oct. 1977
[3] Personal communication; Ken Ronney, Spectrolab Inc.
[4] S. E. Forman, M. P. Themelis, ‘Reliability of Terrestrial
Photovoltaic Modules at Various DOE/MiT Lincoln
Laboratory Test Sites”, 15ti IEEE PVSC, May 1981
[5] N. G. Dhere and N. R. Raravikar, “Adhesional
Strength and Surface Analysis of a PV Module Deployed
in Harsh Coastal Climate”, Solar Energy Materials and
Solar Cells, Proc. 1I* PVSEC, September 1999.
[6] N. G. Dhere, M. E. Wollam, and K. S. Gadre,
“Correlation Between Surface Carbon Concentration And
Adhesive Strength At The Si Cell/Eva interface In A PV
Module”, Proc. 26th IEEE PVSC, Anaheim, CA, (1997),
pp. 1217-1220.
~ N. G. Dhere, K. S. Gadre, and A. M. Deshpande,
“Durability Of Photovoltaic Modules”, 14th European
PVSEC, Barcelona, Spain, (1997), pp. 256-259.


Diagnostic analysis of silicon photovoltaic modules after 20-year field exposure,”

Quintana, M. A.

King, D. L.

Hosking, F. M.

Kratochvil, J. A.

Johnson, R. W.

Hansen, B. R.

Dhere, N. G.

Pandit, M. B.

UNT Digital Library

t.DIAGNOSTIC ANALYSIS OF SILICON PHOTOVOLTAICMODULES AFTER 20-YEAR FIELD EXPOSUREM. A. Quintana, D. L. King, F. M. Hosking, J. A. Kratochvil, R. W. Johnson, B. R. HansenSandia National Laboratories, PO Box 5800, Albuquerque, NM 87185N. G. Dhere and M. B. Pandit, Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922-5703ABSTRACTThe objective of this study was to investigate thetechnology used by Spectrolab Inc. to manufacturephotovoltaic modules that have provided twenty years ofreliable service at Natural Bridges National Monument insoutheastern Utah. A field survey, system performancetests, and a series of module and materials tests haveconfirmed the durability of the modules in the array. Thecombination of manufacturing processes, materials, andquality controls used by Spectrolab resulted in modulesthat have maintained a performance level close to theoriginal specifications for twenty years. Specificcontributors to the durability of the modules includedpolyinyl-butyral (PVB) encapsulant, expanded metalinterconnects, silicon oxide anti-reflective coating, andexcellent solder/substrate solderability.INTRODUCTIONOver twenty years ago the Department of Energyand the Department of Interior agreed to install theworld’s largest, at the time, photovoltaic system atNatural Bridges National Monument (NBNM), managedby the National Park Service [1]. The system was toprovide all electrical power to the park. Individual PVarrays manufactured by Spectrolab Inc., ARCO Solar,and Motorola were installed at NBNM in 1980 and areshown in Figure 1. Technology vaned but all moduleswere monocrystalline silicon and all three module-typeswere representative of JPL Block [1[ or earlier.s@/vo2000- z337LPracfantrari at ‘JRm IFFF Phntnunkie Snnrialictc Cnnfnrnnrn. Anrhnmnn Al llWl fWM-fK419711_ln-. .r. ~ . . .,_<., .-’ J!P>-:.; ~,,;- :, !.,>.,:r,—. ,...$.,.;: .Jfi,,.., ,+ :. .. !,./>,,,,,.Z-,,.+,J<.,. ,+ -,,., .:f:?:~~;.?. :%.,,. .V: .?. +;2T< -,.\ .,... ....p_ ,,: ....,, .- —-- .,. ~ ,: ., ,-, T- ---— - - -. –—Fig. 1. Three arrays at NBNM include the Spectrolabarray in the background.classification. Recently, arrays manufactured by Motorolaand ARCO were disabled or dismantled due to increasedmaintenance requirements and/or significantly decreasedpower production. While additional arrays with newertechnology have been recently added to the NBNMsystem, the original Spectrolab amay continues to provideuseful power to the park.The Spectrolab array consisted of ten strings of 70Block II modules nominally rated at 28.5W at 1000 W/cm*and 41° NOCT [2]. NBNM system performance testssponsored by Sandia in 1985, 1992, and 1997 repeatedlyshowed that the array strings, while degrading in totaloutput power, continued to maintain the shape of their I-Vcurves well and maintained high fill factors of about 70Y0.A single field-aged module and two unexposed moduleswere studied to determine the technological reasons forthis success.Fig. 2. Glass breakage and delamination on SpectrolabmoduleFIELD OBSERVATIONSA field survey of the NBNM array was performed in1999 and a Spectrolab module was obtained fordiagnostic work. Later, two unexposed modules wereobtained from NBNM for comparative electricalperformance testing. The laminates were constructed witha single untempered-glass sheet, PVB encapsulant, 120(5-cm diameter) c-si cells, another layer of PVB, and aMylar@ backsheet. The backsheet of the modules werepainted white to enhance the amount of light reflectedback onto the cells [3].Notable features in the array included a significantnumber of modules with cracked glass, Figure 2,DISCLAIMERThis report was-prepared as an account of work sponsoredby an agency of the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warranty, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this document may be illegiblein electronic image products. Images areproduced from the best avaiiable originaldocument..presumably due to hail damage or thermal cycling. Astudy performed shortly after the installation identifiedtwenty-five cracked modules. It was hypothesized thatthe cracks occurred due to flaws along the edges of theglass in conjunction with excessive tensile stress causedby diurnal temperature cycling [4]. NBNM, located insoutheastern Utah, has a climate that is hot and dry inthe summer and cold and dry in the winter. Our fieldsurvey noted that despite the cracks in the glass surfaceof several modules and, in some cases missing glassshards, there was minimal delamination or corrosioncaused by air and/or water intrusion. The encapsulant ina majority of the modules had yellowed to vatyingdegrees. Some modules had nonuniform yellowing whilea small fraction had no yellowing at all. Yellowing wasinitially attributed to moisture penetration at the glassfracture lines [4]. However, the field-aged module (20years) used in this study was nonuniformly yellowed andthe glass had not been fractured.TEST RESULTSThe three Spectrolab modules were subjected toelectrical performance tests, dark I-V tests and infraredthermal scans. Table 1 shows a summay of moduleelectrical performance parameters. Electricalperformance of the field-aged module was consistentwith JPL Block II module specifications adjusted fortemperature and solar spectrum. In a more directcomparison, performance of the field-aged module wasabout 10’%0lower than the two unexposed modules. Acloser analysis shows that l= on the aged module is --7’?10lower than that of the unexposed modules. This wasattributed to an optical loss, likely from the yellowing ofthe encapsulant. The remaining loss in power, -2.5%,resulted from decreased fill factor. The Vw of the field-aged module remained unchanged suggesting that thefundamental performance of the silicon cells hasremained stable. A 1997 system assessment showedthat five of the ten module strings at NBNM were stillperforming at -80% of their initial field rating.Performance of the entire array had degraded -50% fromthe initial rating. Degradation was attributed to specificproblems such as complete failure of individual modulesin strings. The field data for functional module stringsTable 1. Spectrolab module electrical perfonman~original JPL rating and Sandia data for 20-year moduleand two unexposed modules.I I Isc (A) I Voc (V) lPmp (WI FFJPUBlock IIRating, NOCT 1.88 21.9 28.5 0.69JPL Data atASTM-SRC 1.96 23.9 33.3 0.71Aged module,20 years 1.91 23.6 31.8 0.70along with our assessment of individual modulessuggests: 1) about half of string-level power loss can beattributed to module degradation, 2) the remainder of thestring-level power loss was likely due to increasedresistance in module wiring and terminations.INFRARED IMAGING AND DARK I-VAn infrared (IR) scan of the field-aged module showeduniform heating of the three parallel cell-strings while asteady current of 4.4 amps was circulated through theforward-biased module. Cell temperatures ranged fromabout 27.O”C to a maximum of 29.20C. There was anabsence of localized hot spots that could indicate highresistance solder-joints. iR scans of the unexposedmodules showed comparable temperatures and thermaluniformity. Dark I-V tests of the field-aged module yieldeda cell series resistance (1%.)of 53 mfl This is higher thancontemporary products, but consistent with modules madewith small cells. The RS of the two unexposed moduleswas 36 mfl The apparent 47% increase in Rs of the field-aged module over twenty years corresponds to a 2.5%drop in fill factor, consistent with I-V data in Table 1.Fig. 3. Front-side solder joints and interconnects.SOLDER-JOINT METALLURGYMetallurgy of six solder-joint samples was examinedby cross-sectioning and microanalysis. Interconnectmaterial, Figure 3, was similar to a material commonlyknown as “expanded metal”. Expanded metal is a sheetor coil of metal that is slit and drawn into rows of diamondshaped openings. Spectrolab soldered theseinterconnects onto small pads on the cells rather thanacross the entire length of the cell. Multiple openings inthe interconnect material provided a short escape path forL-EW24Fig. 4. Cross-section of solder-joint shows copperfilament, solder, metallization, and silicon from top tobottom.IJraetnntt=r-i at 7Rh IFFF Dhntnwdt=ir .Cnnri4iete Cnnfnrnnms Anehnmna Al nWl .WlfLAO1991(lfl,..=,.7,v --,w.-.~ -,-, ~-”-,,-- .2.T= ,.4 . .L,, ..,..>. .. . ?., -. :. \ -,. >$...!-. ., .,. ,.,.,,..,.,, ., ..,- -— --’ “’—-.. ,gases generated during the soldering process, therebyminimizing voids. Voids have been identified as initiatorsfrequently linked to solder-joint failure. Outgassingefficiency of the expanded metal interconnects may bean advantage over the solid ribbons currently used by thePV industry. Finally, soldering filaments rather than longlengths of flat ribbon also created a large number oftransitional fillets that minimized stresses resulting fromdifferential expansion. Sustained electrical performanceover twenty years and metallurgical characteristicsdemonstrated the value of this interconnecting technique.Grinding the joints to expose cross-sections wasdifficult because the width of interconnect filaments wassmall, 250-500~ (.010-.020 in.). The cross-section of asolder-joint across the width of a filament in Figure 4showed smooth transitional fillets. Joints studied had asmall number of voids but no cracks. Cell metallizationwas thinner, 7-8 ~, and uniform in thickness, indicatingthat the metallization was deposited through a verycontrolled process. Figure 4 shows a grainy, almostctystal-like structure in the metallization, suggesting thatthe metallization was screen-printed. The thinmetallization and a well-controlled soldering process mayhave contributed to a small interrnetallic layer. Excessiveintermetallic reactions may contribute to solder dewettingand brittle joints. Solder-joints were uniformly thicker, 12-75 pm, than those observed in today’s modules. Jointsexhibited good wetting, suggesting clean surfaces, use ofan effective flux, an effective soldering process, and goodsolderability of the substrate by the solder alloy.CELL TO ENCAPSULANT INTERFACECharacteristics of the cell to encapsulant interfaceswere studied using adhesional strength tests, scanningelectron microscopy (SEM), X-ray photoelectronspectroscopy (XPS), and Auger electron spectroscopy(AES) at the Florida Solar Energy Center (FSEC).Samples were cored using a 1.9-cm O. D. coring drill.Aluminum nuts were glued to the samples using an ultra-high vacuum-compatible (UHV) conductive epoxy glue.The glue and relatively large sample diameter wererequired for XPS analysis. Forty samples, located fromthe penphey to the center, were extracted from twentyPrac+antad 4 %@ IFFF Dhnfnwdtaim .Srm&diete (lnnfarnnra. Anrhnmnn Al IXWI IWMJKI19’)lM,-. . ~.,>, ., ~.. ~ ,. j , .<;,/ ; :::,~,.~.,;y+,,y: .. ,..t$,f~.J., . .,...>.f .. ..; .< : ‘-,. ; .:y:@.,~’&+: -——, ,$ ., , . . . .. . .. .“, . .. . +’ : : :+; ,r’,-w. ,+., , ,:.;+: ,..—— -------Fig. 5. SEM shows pyramidal texture on cell.cells. Samples were extracted by W.Sting, and a torquesensor was used to measure the force necessary to causefailure at the front cell/encapsulant interface. The averageadhesional shear strength was 4.5 ~1 .OMPa. Baselinedata on unexposed modules was unavailable but themeasured values were about 80% of the adhesionalstrength of new modules made with EVA.Twelve silicon cell samples were analyzed by Augerelectron spectroscopy (AES), Table 2. Typically a spotbetween two gridlines was chosen for analysis. Figure 5is a SEM micrograph that shows the surFace texture of acell. An AES survey was cam”ed out on the cell surfaceover a range of kinetic energies from 50 to 2230 eV. AnAES line-scan over a 102-pm length between gridlines,showed significant concentrations of potassium, sodiumand tin distributed uniformly over the cell surface.Presence of tin and silver between gridlines was attributedto diffusion from the metallization over twenty years ofservice.A depth profile, taken by sputtering the cell surface ata rate of 100 ~ per minute, showed the presence of-1650~ thick silicon-oxide layer at the surface, Figure 6. Inaddition, this analysis provided a better understanding ofthe impurities at the cell surface. Concentrations ofsodium and tin decreased rapidly to negligible levels withina depth of 50 A. The source of sodium is believed to bethe glass superstrata. Concentrations of carbon andpotassium fell slowly over a depth of 250 A. Thepotassium peak reappeared and became more intense atthe interface between the silicon-oxide and the elementalsilicon. Presence of potassium was residue likely from thecell texturing process. As expected, phosphorous (n-typedopant) concentration near the silicon-oxide/cell interfacewas about 0.5-1 atomic 9!o. Phosphorous was onlypresent at negligible levels at the cell surface indicating ithad not diffused through the silicon-oxide layer.Table 2 AES survey of a Spectrolab cell lists impuritiesfound on the surface.Ca;on 267 11.0Sodium 985 3.4Potassium 249 2.1Tin 421,431 1.1Silver 350 0.4Twelve PVB encapsulant samples were analyzed byXPS, Figure 7. The small amount of silicon on theencapsulant surface may have originated from a primer, ifone was used to improve encapsulant adhesion, or fromthe silicon-oxide on the silicon cell. Silver and lead, bothwith atomic concentrations ranging from 0.1-0.5 %, weredetected only on samples near the solder-joints.Presence of silver in an encapsulant sample wasattributed to migration from the thick-film silver grid lines.. ISimilarly, there were traces of lead, presumably from thesolder-joint, on the encapsulant surface.The SpectrolabCONCLUSIONSarray at NBNM recently completed400300100-Na -P;0 500 1000 1500 2000 2500Depth (A)Fig. 6. AES depth profile of a c-Si sampleLow to moderate concentrations of impurities suchas sodium, tin, potassium, silver, and lead were detectedat the cell/encapsulant interface. The effect of potassiumon adhesional characteristics is being investigated.Presence of 3-4’?40sodium and 1-2% tin may havereduced adhesional strength; however, encapsulantadhesion after twenty years was still nearly equivalent tounexposed EVA encapsulated modules currentlymanufactured. The factors that may have assisted inmaintaining an adequate level of adhesional strength are:inert silicon-oxide AR coating, a thick and semi-impermeable painted Mylar@ backing, and PVBencapsulant. In contrast, significant loss of EVAadhesional strength in field-deployed PV modules hasbeen attributed to significant concentrations ofphosphorous, sodium, titanium, and other impurities atthe cell/encapsulant interface [5, 6, 7’J.➤✎✍✍✍✍Prncfnnfnrl d %7h IFFF Dhntrwdt=ie .Snna.isliete Cnnfma”ma- Anehm-arm Al flWl .WlrUlW291fN1. . . .. .W, ~...: .- . >...,..?+., .++-.,.”:,,,,:.>.:.7..,::,”, ‘=~ / .,;.. .-.. J;,, ?2%.%2$+%2> :. +:.>..-,+: m.aY’T5?R--TT -- ‘1;.’: .. ., --- --’- ‘-”--. .Eleme* Comxrhtion.Cls 70.9 al.%60 Is 25,0 al% $S12P3 3713L%Ag 3(I5 0,3 al%Pb 417 0.1 at%H~%1100 880 660 440 220 0Binding Energy (eV)Fig. 7. XPS spectra of a PVB encapsulant sampletwenty years of service with a high degree of moduledurability. National Park Sewice personnel are pleasedwith the array’s performance and low maintenance.Results of our field survey and laboratory tests indicatedthat the array should continue to provide useful power tothe park. Typical module degradation mechanisms suchas delamination, corrosion, and solder-joint failure havenot contributed to module degradation over twenty years.Using the unexposed modules as a baseline, we believethat the individual module output at NBNM has beendegrading at a rate of about 0.5% per year. A high level ofdurability was achieved by using PVB encapsulant, asilicon-oxide AR coating, expanded metal interconnects,cell metallization with good solderabilty, and a compatiblesolder alloy. It is also important to recognize that eventhough module degradation rates were low, other factorsresulted in a 2.5% per year array-level degradation.Clearly, degradation rates this high are still unacceptablyhigh for future systems.ACKNOWLEDGEMENTSThe authors are grateful to the staff at NBNM, especiallyJoe Joliet and Bob Lovato for their work and cooperation,and to Ken Ronney, Spectrolab Inc. for his historicalperspective. Sandia National Laboratones is a multi-program laboratory operated by Sandia Corporation, aLockheed Martin subsidiary, for the U.S. Department ofEnergy under contact #DE-AC04-94AL85000.REFERENCES[1] F. J. Solman, “The 100 kWp Photovoltaic PowerSystem at Natural Bridges National Monument”, 15tiIECEC Conference, Seattle, WA, 16-22 August 1980[2] M. 1. Smokier, “User Handbook for Block II SiliconSolar Cell Modules”, Jet Propulsion Laboratory, Oct. 1977[3] Personal communication; Ken Ronney, Spectrolab Inc.[4] S. E. Forman, M. P. Themelis, ‘Reliability of TerrestrialPhotovoltaic Modules at Various DOE/MiT LincolnLaboratory Test Sites”, 15ti IEEE PVSC, May 1981[5] N. G. Dhere and N. R. Raravikar, “AdhesionalStrength and Surface Analysis of a PV Module Deployedin Harsh Coastal Climate”, Solar Energy Materials andSolar Cells, Proc. 1I* PVSEC, September 1999.[6] N. G. Dhere, M. E. Wollam, and K. S. Gadre,“Correlation Between Surface Carbon Concentration AndAdhesive Strength At The Si Cell/Eva interface In A PVModule”, Proc. 26th IEEE PVSC, Anaheim, CA, (1997),pp. 1217-1220.~ N. G. Dhere, K. S. Gadre, and A. M. Deshpande,“Durability Of Photovoltaic Modules”, 14th EuropeanPVSEC, Barcelona, Spain, (1997), pp. 256-259.

Diagnostic Analysis of Silicon Photovoltaic Modules after 20--Year Field Exposure

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=1AC4FBE2955DEFC18DB008121A447E43?doi=10.1.1.1069.2745&rep=rep1&type=pdf

Diagnostic analysis of silicon photovoltaic modules after 20-year field exposure,”

Abstract

Similar works

Full text

Available Versions

CiteSeerX

UNT Digital Library