Electricity from Photovoltaic Solar Cells 

Flat-Plate Solar Arrav I Project 

70 Years of Progress 

October 1985 

United States 

Department of Energy JPL 

Jet Propulsion Laboratory 

California Institute of Technology 

NASA 

National Aeronautics and 

Space Administration 
Photovoltaic Module Progress 

- 

MODULE PRICE 

5- 

10 

/ 

I I I 

1975 1985 1975 198 

Flat or nonconcentrating module prices have dropped 

as module efficiencies have increased. Prices are in 

1985 dollars for large quantities of commercial products. 

Typical module lifetimes were less than I year but 

are now estimated to be greater than 10 years. 

(Ten-year warranties are now available.) 

Technology advancement in crystalline silicon solar cells 

and modules (n on -con centrating). 

Union Carbide Corporation (UCC) funded the now 

operational silicon refinement production plant with 

1200 M T/year capacity. DOE/FSA-sponsored efforts 

were prominent in the UCC process research 

The automated machine interconnects solar cells 

and places them for module assembly. The secondgeneration machine made by Kulicke and Soffa was 

cost shared by Westinghouse Corporation 

and development. and DOE/FSA. 

Module evolution is represented by typical modules from 

the series purchased by DOEIFSA between 1975 and 1985. 

The modules were desrgned and manufactured by industry 

to FSA specif/cat/ons and were evaluated by FSA 

More technology advancements are shown on 

FRONT COVER PHOTO: 

The Block l module (1 975) being 

held in front of four Block V modules 

represents the progress of 

inside of back cover. Use of modules in photovoltaic 

power systems are shown on outside of back cover. 

a Isyear cooperative 

industry/university/DOE/FSA 

effort. 
Electricity from Photovoltaic Solar Cells 

Flat-Plate Solar Array Project 

of the 

U.S. Department of Energy's 

National Photovoltaics Program 

70 Years of Progress 

October 1985 

Elmer Christensen 

JPL NASA Jet Propulsbn Laboratory 

Cal8lornla Institute 01 Technology 

National Aeronautics and 

Space Administration 
The Role of Photovoltaics: 

An Historical Perspective 

The world’s demand for more energy continues, 

and will continue in the foreseeable future. The 

economic growth of developing nations depends upon 

increasing energy supplies. In technically advanced 

countries, long-range economic growth depends on 

the availability of inexpensive energy. Historically, in 

the United States, the main source of energy has 

changed from wood to coal to petroleum, and it is 

inevitable that change will continue as fossil fuels are 

depleted. Within a lifetime, it is expected that most 

U.S. energy will come from diverse sources, including 

renewables, instead of from a single type of fuel. 

There are many indications that the percentage of the 

total consumed energy supplied by electrical power 

will continue to increase. 

The use of photovoltaics (PV) is a promising way to 

generate electricity that could provide significant 

amounts of power in the years to come. It is a form of 

solar energy that uses solar cells to generate 

electricity cleanly and reliably, directly from sunlight 

without moving parts. PV power systems are simple, 

flexible, modular, and adaptable to many different 

applications in an almost infinite number of sizes and 

in many environments. Although photovoltaics is a 

proven technology that is cost-effective for hundreds 

of small applications, it is not yet cost-effective for 

large-scale utility use in the United States. For 

economical widespread use, the cost of generating 

PV power must continue to be decreased by reducing 

the initial system cost, by reducing land or area 

requirements, and by increasing the operational 

lifetime of its systems. 

Early needs of the space program for electrical 

power were satisfied by crystalline silicon solar cells. 

During the 1960s and 1970s, it was demonstrated 

that PV was a dependable electrical power source for 

spacecraft. In this time interval, solarcell quality and 

performance improved and the costs decreased. 

However, the costs were still much too high for 

widespread use on Earth. There was a need to 

drastically lower the manufacturing costs of solar cells 

if they were to be a practical, widely used terrestrial 

power source. 

In 1975, the U.S. Government initiated a terrestrial 

PV research and development (R&D) project with the 

objective of reducing the cost of manufacturing solar 

cells and modules. This effort, which was assigned to 

the Jet Propulsion Laboratory (JPL) in Pasadena, 

California, evolved from more than a decade and a 

half of spacecraft PV power-system experience at JPL 

and from recommendations of a conference held in 

1973 at Cherry Hill, New Jersey, on Solar 

Photovoltaic Energy. 

After about 2 years of technical progress and 

economic analyses by the Flat-Plate Solar Array 

(FSA) Project, a growing community of people 

became convinced that the Project’s goals were 

realistic and that PV power systems had the potential 

of being competitive with those based on fossil fuels. 

Additional federally supported efforts, sponsored by 

the US. Department of Energy (DOE), covering all 

aspects of PV power-system R&D, were initiated at 

the Solar Energy Research Institute (SERI), Sandia 

National Laboratories, and other organizations. These 

broadened efforts included concentrator modules, PV 

systems studies, application demonstrations, and 

research on cell materials other than silicon and 

devices such as thin-film cells. Most of the DOE funds 

have sponsored R&D in private organizations and 

universities. The result has been an effective 

Government-university-industry team that has 

cooperated to advance PV technology rapidly. 

In parallel, a small but fast-growing terrestrial PV 

industry evolved, with products that were 

economically competitive for small stand-alone PV 

power systems. A few megawatt-size utility-connected 

PV installations, made possible by Government 

sponsorship and tax incentives, have demonstrated 

the technical feasibility and excellent reliability of large 

multi-megawatt PV power-generation plants. 

Although fuel prices have leveled off, worldwide 

interest in photovoltaics continues, especially in 

countries where electricity is more expensive than in 

the United States. It is anticipated that the field of 

photovoltaics, with its tremendous potential, will 

regain its place in the sun as fossile fuel energy 

sources deplete and new energy technologies 

continue to evolve. 

ii 
Flat-Plate Solar Array 

Project Summary 

The Flat-Plate Solar Array (FSA) Project, a 

Government-sponsored photovoltaics project, was 

initiated in January 1975 (previously named the LowCost Silicon Solar Array Project) to stimulate the 

development of PV systems for widespread use. Its 

goal then was to develop PV modules with 10% 

efficiency, a 20-year lifetime, and a selling price of 

$0.50 per peak watt of generating capacity (1 975 

dollars). It was recognized that cost reduction of PV 

solarcell and module manufacturing was the key 

achievement needed if PV power systems were to be 

economically competitive for large-scale terrestrial 

use. 

The project was initiated at JPL to meet these 

goals through R&D of all phases of flat-plate module 

technology, from solarcell silicon material refinement 

through verification of module reliability and 

performance. The Project sponsored parallel 

technology efforts with periodic progress reviews and 

continuing sponsorship of only the most promising 

options. A module manufacturing cost-analysis 

capability was developed that permitted cost goal 

allocations to be made for each module technology, 

based upon potential for achievement. Economic 

analyses, done as technical progress was achieved, 

permitted assessments to be made of each technical 

option’s potential for meeting the goal and of the 

Project’s overall progress toward the national goal. 

Excellent technical progress across the entire 

project was accomplished over the years, with 

growing interest and participation by the private 

sector. More recently, effective energy conservation 

practices, a leveling of energy prices, a perception 

that energy prices are not increasing endlessly, and a 

change in Government emphasis has altered the 

picture for photovoltaics. DOE’S National 

Photovoltaics Program was redirected to longer-range 

efforts that the private sector avoids because of 

higher risk and longer payoff time. Consistent with 

these new directions, FSA has been concentrating its 

efforts on overcoming specific critical technological 

barriers that inhibit large-scale PV use. These 

activities require an in-depth, long-range, integrated 

effort that industry cannot reasonably be expected to 

undertake alone. This work is being performed by a 

team that includes universities, industry (which shares 

the cost), and Government laboratories. This team 

has worked together successfully for 10 years. 

An estimate that PV-generated power should cost 

about $0.1 5lkWh in the 1990s to be competitive in 

utility central-station generation plants is the basis for 

a DOE Five-Year Research Plan. The high cost of PV 

generating equipment, especially modules, remains 

the major limitation. Continuing solar-cell and module 

cost reductions are still the key activities. However, it 

is now known that, for a utility plant, area-related 

costs are significant enough that flat-plate module 

efficiencies must be raised to between 13 and 17% 

(more energy generated per fixed-area dollar), and 

module life should be extended to 30 years. It is 

believed by FSA reseachers that both of these 

requirements and the corresponding cost reductions 

are possible, but are achievable only with a dedicated 

effort. Present FSA activities emphasize high 

efficiency, long life, reliability, and low cost for solar 

cells, modules, and arrays. 

Major Project 

Accomplishments 

Established the basic technologies for all 

aspects of manufacturing and evaluated nonconcentrating crystalline silicon PV modules and 

arrays for terrestrial use. 

Established a new low-cost, high-purity siliconfeedstock-material refinement process. 

Made significant advances in quality and cost 

reduction of: 

Silicon sheet for solar cells 

Higher efficiency solar cells. 

Manufacturing processes 

Established PV modulelarray engineeringldesign 

and evaluation knowledge and capabilities. 

Established PV module encapsulation systems 

Devised manufacturing and life-cycle cost 

economic analysis capabilities. 

Transferred the above technologies to the 

private sector by dynamic interactive activities 

including hundreds of contracts, comprehensive 

module development and evaluation efforts, 25 

Project integration meetings, research forums, 

presentations at hundreds of technical meetings, 

and advisory efforts to industry on specific 

technical problems. 

Stimulated the establishment of a commercial 

PV industry in the United States. 

iii 
Photovoltaics 

Community 

Interactions 

Following the OPEC oil embargo of 1973, a 

conference of photovoltaics experts was held at 

Cherry Hill, New Jersey, to assess the viability of 

photovoltaics as a terrestrial electrical energy 

source. In response to a 1974 request from the 

National Science Foundation, JPL proposed a PV 

module R&D effort based upon the Cherry Hill 

conference recommendations. This was accepted in 

January 1975 and a Project, now called Flat-Plate 

Solar Array, was initiated. 

The JPL Project quickly established relationships 

with university research teams, non-profit 

organizations, and commercial firms interested in the 

development of low-cost photovoltaics. The 

fundamental strategy adopted by JPL was to involve 

industry and university teams in a highly interactive 

effort along parallel paths of technology 

development, and to select the best options. The 

effort has been highly goal oriented using a scheme 

of cost allocations to guide the progress of the 

activities. 

By 1978, the MIT-Lincoln Lab; MIT-Energy Lab; 

Brookhaven National Lab, the NASA-Lewis Research 

Center; Sandia National Lab; a newly formed SERI; 

the U.S. Army MERADCOM; and others had become 

members of the Federal program under the newly 

established U.S. Department of Energy (DOE), which 

replaced the Energy Research and Development 

Administration (ERDA). The JPLlFSA became very 

much involved through many technical interfaces 

with all of the DOE Laboratories in order to carry out 

the national mandate. 

Perhaps the most important strategy in ensuring 

technology transfer has been to directly involve the 

recipient photovoltaics community. The single, most 

prominent effort in the technology transfer strategy 

has been the conduct of 25 Project Integration 

Meetings (PIMs) over the 10-year life of the Project. 

The PlMs have been conducted to enable the rapid 

exchange of data between all FSA participants, 

assess recent progress, gain perspective on trends 

and new developments, and to guide the FSA 

Project's planning and priority adjustment. The 

communication has been open between the Federal 

program and the PV community at all levels of detail 

regarding problem-solving and choice-making. At 

each PIM, contractors presented their achievements 

and problems encountered since the previous PIM. 

These presentations to their peers and the ensuing 

discussions stimulated extensive technical 

exchanges and progress. DOE management 

participated in every PIM, lending the guidance 

needed for credible technology transfer. 

Changes in the overall political environment 

resulted in the FSA reducing its scope of activities 

starting in 1981. The DOE thrust became less 

oriented toward the direct development of the US. 

photovoltaics industry infrastructure and more toward 

the research into long-term, high-risk, high-payoff 

technical options that industry would not likely 

undertake themselves. 

Continuing under the new charter, FSA developed 

the idea of Research Forums on specific technical 

problems of special difficulty wherein a working 

group of experts both within and outside the PV 

community would convene. Of the many such forums 

held to date, every one has resulted in new ideas 

and contractual relationships to help solve the 

problem. 

The legacy of the FSA Project consists of 

significant measurable technical and economic 

achievements. An equally enduring part of the legacy 

may well turn out to be the way in which the Project 

was able to work both effectively and efficiently with 

the PV community to achieve the progress to date. 

The FSA organization has been changed 

throughout the years to meet the programmatic 

needs and has evolved to its present form as shown 

below in the Project Organization Chart. 

W T Callaphan, Manager 

D G T~s11n. Admin Mana~el 

R R McDonald. 

iv 
Contents 

Project Overview ................................................. .1 

Project Technical Thrusts. ........................................... .2 

FSA Project Technologies ........................................... .5 

Project Analyses and Integration ...................................... .7 

Silicon Material. ................................................. .12 

Silicon Sheet ................................................... .23 

High-Efficiency Solar Cells .......................................... .38 

Process Research and Development ................................... .42 

Modules and Arrays .............................................. .53 

Module and Array Evolution ......................................... .58 

Engineering Sciences and Design ..................................... .66 

Encapsulation .................................................. .70 

Reliability Physics ................................................ .77 

Module Performance and Failure Analysis ................................ .82 

Appendix A: Glossary ............................................. .88 

Appendix B: FSA Project Contractors .................................. .91 

Acknowledgment 

The progress of the past 10 years in crystalline 

silicon PV technology (nonconcentrating) has been 

accomplished only because there has been a strong 

motivation to work on a very challenging and 

worthwhile activity in which many people believe. 

Without the sustaining support of the Federal 

Government and cost sharing by private industry, 

the excellent achievements across the complete 

broad scope of activities would not have progressed 

so rapidly. The thousands of people in many diverse 

large and small organizations deserve credit for the 

excellent work performed individually and as a team. 

This report, which summarizes 10 years of FSA 

activities, was possible only with the help of many 

other members of the FSA Project. 

Prepared by the Jet Propulsion Laboratory, California Institute of Technology, 

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

Aeronautics and Space Administration. 

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

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

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

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

United States Government. Neither the United States Government nor any 

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

implied, or assumes any legal liability or responsibility for the accuracy, comp!eteness. 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 news and opinions of authors 

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

Government or any agency thereof. 

I.' This publication reports on work done under NASA Task RE-152. Amendment 

66. DOE / NASA IAA No. DE-AIOI-76ET20356. 
Project Overview 

Objectives 

To develop the flat-plate PV array technologies 

required for large-scale terrestrial use late in the 

1980s and in the 1990s: 

To advance crystalline silicon PV technologies. 

To develop the technologies required to convert 

thin-film PV research results into viable module 

and array technology. 

To stimulate transfer of knowledge of advanced PV 

materials, solar cells, modules, and arrays to the 

PV community. 

Plans 

To sponsor or cost-share efforts that industry 

reasonably would not fund because of high risks 

and/or long-time requirements: 

Perform advanced solar cell, module, and array 

technology R&D that the private sector can use 

to establish economical central station and 

rooftop applications, competitive with largesystem power-generation costs of $0.1 S/kWh in 

the 1990s. 

Emphasize efforts to reduce technical barriers 

to the development of high-performance, 

economical, and long-life solar cells, modules, 

and arrays. 

Establish the technologies for a low-cost, 

1 5 % efficient, 30-year-life, crystalline-silicon 

flat-plate module. 

In cooperation with SERI, establish the 

technology for a low-cost, 12% efficient, longlife, thin-film module. 

Develop module and array specifications, 

develop performance and lifetime criteria, and 

assess contemporary module and array 

technologies. 

Continue to fund and cost-share efforts with 

industry, universities, and other institutions. 

Continue economic analyses to evaluate 

technical options and overall progress. 

The definition of a solar cell, module and array is 

shown in Figure 1. 

DIRECT SUNLIGHT ON A SILICON SOLAR CELL 110 cm OR 4 I" DlAi 

SOLAR CELL \A SEAL 

PRODUCES ABOUT 

1 WATf OF DIRECT CURRENT ELECTRICAL ^^...r^ ,. -1 2 volt 

.I 

MODULE . MANV SOLAR CELLS IN A WEATHERPROOF UNIT 

I \. ~~*;y~;fJ;y ABOU' 

18 12 wan* ,qua.. IOOll 

ARRAY - MANY MODULES ELECTRICALLY 

AND PHYSICALLY CONNECTED TOGETHER 

Figure 1. Definitions 

1 
Project Technical Thrusts 

The three flat-plate-array technical thrusts now 

required to meet the DOE Program goal of PV power 

system generations costs of $0.1 5/kWh are: 

been set for each thrust and for specific contributing 

efforts; the contribution and potential contribution of 

individual efforts are periodically assessed, as is the 

combined contribution toward each thrust. The Project 

as a whole contributes to each thrust; the synergistic, 

integrated value of that contribution is important, and 

is likewise assessed. The goals of each thrust are the 

result of years of integrated assessments of user 

needs to meet competition and the potential of each 

technology to meet the requirements. These efforts 

are all interrelated. 

High efficiency. 

Low cost. 

Reliability. 

To achieve the overall goal, individual goals have 

Objectives: 

Lower Cost 

Higher Efficiency 

Higher Reliability 

Encapsulation I Materials & 

Processes 

ModuldArray 

Design 

Technology 

t 1- 1' 

Silicon Silicon Sheet Process Advanced Prototype 

JPL specifications 

Refinement - 

Processes 

Mutual design review 

Formation Technology Modules 

Ribbon grown to 

desired thickness 

- Sheet to solar 

cells - Designs by Industry 

+ t Fabrication by Industry 

cc into modules 

AND 

Material & Cell Cells assembled 

Characterization 

& Enhancement 1 

t ~ 

I I t I t t If 

Project Analysis and Integration 

Price allocation guidelines 

Economic analyses 

Probability of success 

Industry assessment 

~~ ~~ ~ ~ 

4 Information Flow, Both Directions 

Performance 

DurabilitylReliability 

Failure Analysis 

JPL tests . Applications 

Results to Industry 

2 
High Efficiency: Crystalline Silicon 

Factors contributing to more efficient solar cells, 

modules, and arrays: 

High-purity silicon material. 

High-quality silicon sheet as grown or 

deposited. 

Advanced solar cell designs. 

Advanced module and array designs 

Advanced cell processes. 

Advanced module processes. 

R&D is required to: 

Understand the influence on efficiency of: 

Silicon growth-related defects including: 

Impurities. 

Structural defects. 

Advanced solar cell designs. 

Advanced cell processing 

Develop advanced silicon sheet growth 

capabilities for high quality sheet that is also 

inexpensive. 

Develop advanced material and solar cell 

property measuring techniques and 

equipment. 

Develop advanced solar cell designs and 

fabrication Drocesses. 

Develop advanced module and array designs. 

High Efficiency: Thin-Film Solar Cells 

Thin-film solar cells consist of thin layers of 

semiconductor materials deposited on a 

substrate so that the cell can absorb and 

convert usable light to electricity in a small 

thickness. The potential for depositing cells and 

cell interconnections in a continuous production 

line holds promise for low-cost thin-film modules 

that require small amounts of semiconductor 

materials. 

Achievement of higher-efficiency cells and 

modules is the key to widespread use of thinfilm technology for PV power generation. 

Basic research on materials and devices leading 

to higher performance is being sponsored and/or 

performed by SERI for DOE, and by the private 

sector. 

Translation of thin-film research and the blending 

of crystalline silicon PV experience into 

commercial manufacturing technologies for thinfilm PV power units is now being accelerated by 

industry and by FSA in support of SERI. FSA is 

involved in: 

Developing thin-film cell and module process 

appropriate for large-area modules. 

Developing and verifying module and array 

technologies for reliable units. 

Low Cost: Crystalline Silicon 

Factors contributing to low-cost solar cells, 

modules, and arrays are: 

Low-cost, high-purity silicon semiconductor 

material. 

Rapid growth or deposition of silicon in sheet 

form of a quality consistent with cost-efficient 

solar cells. 

Efficient solar-cell designs that are 

inexpensive to fabricate. 

High-volume cell processes using inexpensive 

materials that have high yields of costeffective solar cells. 

Efficient module designs using inexpensive 

materials that are easily mass-produced with 

high yields of long-life, reliable modules. 

R&D is required to achieve: 

Completion of silicon refinement research. 

Cost-effective silicon sheet formation resulting 

in high-quality, inexpensive sheet. 

High-efficiency solar cell designs for costeffective processing and performance. 

Mass-production processes with high yields of 

high-efficiency, inexpensive solar cells and 

modules. 

Verification of long-life effectiveness of 

module and encapsulation materials. 

Low Cost: Thin-Film Solar Cells 

Comprehensive manufacturing technology 

efforts have not been accomplished as yet 

because material, device, and basic cell and 

module designs are still evolving. 

A thorough assessment and subsequent efforts 

are required when the research results are 

definitive. 

3 
Reliability: Crystalline Silicon 

Module and array reliability means consistent high 

Factors contributing to high reliability are: 

performance for 30 years. 

Stable, long-life materials. 

Compatible solar cell and module materials. 

Appropriate cell and module designs. 

Suitable cell and module processes. 

High quality of manufacturing workmanship. 

Capability of integration into PV power 

systems and installation. 

Types of reliability or performance degradation 

are: 

Infant mortality. 

Wear-out or aging. 

Random failures. 

Environment-induced soiling (weathervariable). 

Causes of degradation are: 

Cell cracking. 

Corrosion. 

Cell-interconnect fatigue. 

Module material aging. 

Module material fatigue. 

Insulation failure. 

Soiling or dirt accumulation on face of 

module. 

Structural failure 

Design and fabrication flaws. 

Methods of reliability verification are: 

Development of precise performancemeasurement capabilities. 

Accelerated testing of materials and modules. 

Module qualification testing. 

Field testing of modules and arrays. 

R&D is required for: 

Advanced module and array specification 

definitions. 

Material aging and stability research. 

Advanced module and array design 

technology research. 

Solarcell, module, array, and PV system 

com pati bilities. 

Advanced module design verification. 

Correlation of accelerated testing with field 

testing and with operational results. 

Reliability: Thin-Film Solar Cells 

Unknown, evaluation just beginning. 

Most of the crystalline-silicon technology, noted 

above, will apply to thin-film hardware, but 

probably with unique variations. 

4 
\ 

FSA Project Technologies 

SILICON MATERIAL 

QUARTZ (Slob IS REFINED BY REMOVING 02 AND IMPURITIES 

THE PRODUCT IS VERY HIGH PURlTY SILICON 

NEW SILICON REFINEMENT PROCESSES I ACCOMPLISHMENTS I I LARGE.SCALE SILANE PRODUCTION PLANT BY UCC IN OPERATION 

PRACTICAL CONVERSION OF SILANE TO SILICON IN FER DEMONSTRATED 1 OTHER PROCESS AND PROCESS IMPROVEMENTS ADOPTED BY INDUSTRY 

RESEARCH REQUIRED 

ESTABLISH PURITY OF SILICON FROM UCC PROCESS 

VERIFY ECONOMICS OF PROCESS 

HIGH-EFFICIENCY SOLAR CELLS 

SOLAR CELL EFFICIENCY IS A FUNCTION OF 

B SILICON SHEET QUALITY 

1 CELL DESIGN 

B CELL FABRICATION PROCESSES 1 UNIFORMITY AND REPEATABILITY OF ABOVE 

i PROBABLE MAXIMUM ACHIEVABLE 2) 25t 

U U W 

-1 '0 PRODUCTION CELLS 

W 

51 0 1975 YEAR 1985 

ACCOMPLISHMENTS 

KEY BARRIERS TO HIGHER-EFFICIENCY 

- HIGH-EFFICIENCY SOLAR CELLS MODELED 

SOLAR CELLS IDENTIFIED 

AND ANALYZED 

NEW SOLAR CELL CHARACTERIZATION 

TECHNIQUES DEVELOPED 

RESEARCH REQUIRED 

CONTINUE INVESTIGATION OF LOSS MECHANISMS 

WITHIN SOLAR CELLS 

CONTINUE DEVELOPMENT OF SOLAR CELL CHARACTERIZATION 

ANALYSIS AND MEASUREMENT TECHNIQUES 

CONTINUE RESEARCH CELL DEVELOPMENT AND TESTING 

~ 

SILICON SHEET 

SILICON IS GROWN WITH SPECIFIC ELECTRICAL AND 

CRYSTALLOGRAPHIC CHARACTERISTICS 

INGOTS ARE SLICED INTO WAFERS 

RIBBONS ARE GROWN TO DESIRED THICKNESS 

INGOT 

WAFER 

ACCOMPLISHMENTS 

NEW SEMICONTINUOUS, LARGER CZ INGOTS GROWN 

NEW SHAPED4NGOT TECHNOLOGIES EVOLVED 

FASTER, MORE EFFICIENT SAWING DEVELOPED 

GROWTH OF WIDE, FLAT-RIBBON ACHIEVED 

RESEARCH REOUIRED 

INVESTIGATE CRYSTALLIZATION RATE LIMITATIONS I AND INFLUENCE OF PRIMARY GROWTH PARAMETERS 

CONTINUE CHARACTERIZATION OF SILICON SHEET 

MATERIAL VERSUS GROWTH RATES I CONTINUE BASIC INVESTIGATION OF HlGH.QUALITY. 

I RAPID CRYSTAL GROWTH: ESPECIALLY RIBBON GROWTH 

5 

CELL AND MODULE PROCESS RESEARCH AND DEVELOPMENT 

SOLAR CELLS ARE FABRICATED FROM SILICON SHEET 

- CELLS ARE ELECTRICALLY INTERCONNECTED AND ENCAPSULATED 

IN A SUPPORTING STRUCTURE 

ACCOMPLISHMENTS ..~ 

- IMPROVED CELL SURFACE TREATMENTS, JUNCTION FORMATION, AND METALLIZATION 

HAVE RESULTED IN LESS EXPENSIVE YET MORE EFFICIENT SOLAR CELLS 

SIMPLIFIED FABRICATION SEQUENCES SUITABLE FOR AUTOMATION DEVELOPED 

kiOlOTYPE MASS-PRODUCTION EQUIPMENT DEVELOPED 

RESEARCH _. - REQUIRED 

INVESTIGATE HIGH.EFFICIENCY SOLAR CELL PROCESSING 

. INVESTIGATE ELECTRICALLY CONDUCTIVE SILICIDES, CORROSIVE 

EFFECTS ON METALLIZATION. AND THE EFFECTS ON JUNCTION FORMATION 

. CONT~NUE INVESTIGATING CRITICAL PROBLEMS OF ION IMPLANTATION, 

METALLIZATION. INTERCONNECTIONS, AND MODULE ASSEMBLY 

ORiGlNAL PAGE fS 

OF POOR QUALKY 
YODULUARRAY ENGINEERING SCIENCES AND DESIGN 

DESIGN AND TEST METHODS HAVE BEEN DEVELOPED AND USED TO IMPROVE 

THE PERFORANCE, ENVIRONMENTAL DURABILITY. AND SAFETY OF 

PHOTOVOLTAIC MODULES AND ARRAYS 

ARRAY 

STRUCTURE 

TEST 

ACCOMPLISHMENTS AND ONGOING ACrmlllES 

MODULE AND ARRAY DESIGN REQUIREMENTS GENERATED FOR FUTURE 

LARGE-SCALE APPLICATIONS 

IMPROVED DESIGN CONCEPTS, ANALYSIS, AND TEST METHODS 

AVAILABLE AND UNDER DEVELOPMENT (CIRCUIT DESIGN, SAFETY, 

STRUCTURES, RELIABILITY. ETC.) 

PERFORMANCE EVALUATION METHODS AND DESIGN AND TEST 

SPECIFICATIONS DEVELOPED THROUGH INTERACTION WITH INDUSTRY 

RESEARCH REQUIRED 

PERFORM ABOVE FUNCTIONS FOR MODULES 

MADE FROM THIN-FILM SOLAR CELLS 

RELIABILITY PHYSICS 

POWER GENERATION WITHOUT DEGRADATION REQUIRES: 

STABLE, LONG-LIFE MATERIALS 

GOOD, COMPATIBLE CELL, MODULE, ARRAY AND PV SYSTEM MATERIALS 

AND DESIGN 

GOOD FABRICATION AND INSTALLATION WORKMANSHIP 

10 

v) 

cc 

Lu 

> 

a 

'I 

d 

/ 

/ 

MODULE LIFE / 

/ / 

/ 

0 

1975 

ACCOMPLISHMENTS 

1985 

DEVELOPED ANALYSIS TECHNIOUES FOR EVALUATING PARAMETRIC 

INFLUENCES ON RELIABILITY DEVELOPED MANY TEST TECHNIOUES AND EQUIPMENT FOR 

EVALUATING MODULE RELIABILITY MODULE LIFE.TIMES HAVE INCREASED DRAMATICALLY 

RESEARCH REOUIRED 

CORRELATE FUTURE FIELD OPERATIONAL RESULTS WITH EXISTING 

DATA 

EVALUATE AND IMPROVE THIN.FILM MODULE RELIABILITY 

ENCAPSULATION 

SOLAR CELLS AND INTERCONNECTS ARE PROTECTED FROM 

THE ENVIRONMENTS 

TRANSPARENT COVER - 

INTERCONNECTIONS I 

SOLAR CELLS AND 

ELECTRICAL 

POTTANTSADHESIVES 

ACCOMPLISHMENTS 

EXPERIENCE 

BACK COVER 

INDICATES 30.YEAR 

- 

MODULE LIFE IS ACHIEVABLE 

SELECTED MATERIALS AND PROCESSES EXPECTED TO MEET GOALS 

AND AUTOMATED PROCESSING NEEDS 

MATERIALS INDUSTRY RESPONDING TO NEEDS AS DEFINED BY FSA 

RESEARCH REQUIRED 

CONTINUE INVESTIGATION OF ENCAPSULATION MATERIALS - -_ TO - 

ACHIEVE LONGTERM PHOTOTHERMAL STAB~ITY 

CONTINUE DEVELOPMENT AND USE OF ACCELERATED AND LONG-LIFE. 

DURABILITY-TESTING TECHNIQUES 

CONTINUE INVESTIGATION OF ENCAPSULANT, METALLIZATION, AND 

OTHER INTERFACIAL BONDING STABILITY AND CORROSION CRITERIA 

AND MECHANISMS 

MODULE PERFORMANCE AND FAILURE ANALYSES 

PHOTOTYPE ADVANCED MODULES HAVE BEEN DESIGNED AND MANUFACTURED 

BY INDUSTRY TO JPL SPECIFICATIONS 

MODULE ELECTRICAL PERFORMANCE AND ENVIRONMENTAL DURABILITY 

ASSESSMENT BY JPL 

PROGRESS IN MODULE PERFORMANCE 

MODULE PERFORMANCE AND DURABILITY IMPROVEMENT WITH EACH 

NEW PROCUREMENT 

LATEST MODULES (5th PROCUREMENT) INCORPORATE INNOVATIVE 

DESIGN FEATURES 

CONTINUING CHARACTERIZATION AND DIAGNOSTIC ANALYSES OF 

MODULE DEFlClENClESlFAlLURES ARE TRANSFERRED TO INDUSTRY 

PROJECT ANALYSIS AND INTEGRATION 

DEVELOPED PV ANALYSIS METHODS, ESPECIALLY FOR 

MANUFACTURING COSTS 

ESTABLISHED GOALS INTERNAL TO FSA AND MEASURED TECHNICAL 

PROGRESS BY USE OF ANALYSIS TECHNIQUES 

6 
and 

Project Analysis 

Integration 

Those responsible for initiating and organizing the 

FSA Project recognized the need for coordination 

within the Project and with private industry to 

achieve the objectives established for the Project. To 

that end, the Project Analysis and Integration (PA&) 

effort has served as a channel of communication for 

Project activities, private industry, and the US. 

Department of Energy. 

PA&I performs planning and integration activities 

to coordinate the various R&D tasks. Goals are 

established for Project R&D activities that reflect the 

objectives of the National Program. Analytical 

support is given to the Project Office, providing a 

channel for the integration of individual task work into 

the overall Project. Integration activities have also 

supported the coordination of research activities 

between the Project and other,pesearch laboratories. 

PA&I performs analytical studies to measure the 

progress toward accomplishing Project goals and to 

determine which R&D tasks are the most likely to 

result in significant performance and cost 

improvements. Guidelines for module production 

costs are established by PA&I which are consistent 

with the National Program goals. Performance and 

cost criteria are set for separate module elements, 

creating objectives for the individual R&D tasks. 

Sensitivity studies are performed to see how 

technical advances in the Project and changes in the 

economic environment affect Project objectives. 

Background 

FSA research and development has encompassed 

all phases of flat-plate module technology, from the 

basic feedstock materials for the photovoltaic 

manufacturing processes, through verification of 

module reliability and performance in the field. When 

the Project was organized in 1975, it was recognized 

that this broad spectrum of activities would require 

the Project to be somewhat unique at JPL because 

of the number of industrial organizations, Government 

laboratories, academic institutions, and other entities, 

such as utilities, with which JPL would have to 

interact. It was apparent at the outset that 

communication between JPL and the other 

participating organizations would be an important 

factor in the success of the effort. In addition, the 

Project was one of the first, and certainly the largest 

at JPL, for which technical performance was not the 

ultimate goal but only a contributing factor to the 

principal measure of success, economic 

performance. Thus, it was also apparent that 

analytical methods would be needed to translate 

technical performance into future economic 

performance. The PAM Task was formed as part of 

the original Project structure to address the problem 

of communication and to help guide the Project 

technical developments toward the National PV 

Program’s economic goals. 

Project Activities 

In 1975, analytical capabilities were needed to 

evaluate competing technologies and establish 

realistic guidelines for their development. Initially, 

cost goals were allocated to silicon material, silicon 

sheet, encapsulation materials, cell fabrication 

processes, and module assembly. As economic 

analysis capabilities were developed, thereby 

allowing Project-wide technology assessments to be 

made, these goal allocations were revised. The 

technical objectives that were established were used 

to determine the division of resources throughout the 

Project. As time passed, the completeness and 

sophistication of the analysis capabilities matured. 

The assessing of technology status and the 

tracking of progress has been one of the principal 

PA&I activities throughout the duration of the FSA 

Project. It has established a set of standards by 

which progress can be measured. The standards are 

part of an overall methodology developed by PA&I so 

that research findings could be reported in a 

common format. The methodologies are being used 

to track and assess progress within the Project so as 

to derive the maximum technical and economic 

benefits from research and development work. 

Technical reports and research activity assessments 

are an integral part of the overall Project effort. 

The FSA Project participation in the early efforts of 

DOE to establish a National PV Program Plan was 

coordinated and implemented by PA&. It authored 

several major sections of the first DOE Program 

Plan. The planning and organization of the 

Photovoltaic Technology Development and 

Applications Lead Center at JPL in 1978 was a major 

PA&I activity at that time. 

Today’s Status 

PA&I provides ongoing support to the Project in 

the form of up-to-date technology assessments. 

Manufacturing technology descriptions are revised to 

reflect new technical developments, and new 

manufacturing cost estimates are made using the 

SAMICS (Solar Array Manufacturing Industry Costing 

Standards) methodology. Examples of this analysis 

capability are the 1985 production cost estimate 

updates for Czochralski (Cz) and dendritic web silicon 

solar cell modules. 

Module cost estimates were made for state-of-theart Cz silicon solar cell technology, assuming the 

construction of a 25-MW-per-year production facility 

and current material costs. Assuming 3 shifts per 

day, full-time operation and module efficiencies of 

13.5%, production prices were estimated at $1.47 per 

7 
ECONOMIC ANALYSES 

SAMICS - THE SOLAR ARRAY 

MANUFACTURING INDUSTRY COSTING STANDARDS 

SAMlCS OUTPUTS 

sum OF DESCRIPTION OF 

SIMULATED FACTORY 

STATION RECITS (INCLUDING FINANCES) 

DIRECT WORK 

PACUOE 

MODULES 

EXAMPLE 

OPERATORSISHIFT 

INTERNAL DATA BASE 
peak watt (1 985) dollars. This represents significant 

progress over early assessments for this technology, 

Module cost estimates were made for state-of-theart, dendritic web silicon solar cell technology, 

assuming three shifts per day, full-time operation at a 

25-MW-per-year production rate and current material 

costs. Assuming module efficiencies of 13.7%, production prices were estimated at $1.48/W or more. 

or less (today’s best growth rate). 

This is based upon a web growth rate of P 0 cm2/min 

The dendritic web solar cell technology, while still 

under active development, promises to be a most 

attractive photovoltaic option. The principal 

impediment to economic feasibility of dendritic web 

has been the low crystal growth rates. If the 

technology development presently in progress is 

successful in increasing the dendritic web growth 

rates to 30 cm2/min, then the production price can 

be reduced to $0.86/Wp (1985 dollars). If modules at 

this price are placed in a PV electrical generation 

plant, electricity could be produced at $0.1 7/kWh, 

very close to the DOE Program goal of $0.1 5/kWh. 

Accom p I is h ments 

PA&I supports a systems analysis capability for 

evaluating developments within the Project and 

assessing progress. The present status of this 

capability and its accomplishments are best 

summarized in terms of the methodologies or 

analytical tools maintained for Project analysis and 

integration efforts. These analytical tools have 

provided a consistent and reliable framework for 

measuring developments throughout the Project. 

SAMICS 

One of the most important analytical tools 

developed and supported through PA&I is the Solar 

Array Manufacturing Industry Costing Standards 

(SAMICS). SAMICS has proven itself as a credible 

costing methodology for an emerging technology. 

The SAMICS methodology provides a standardized 

way for comparing cost estimates of many 

competing technical options. 

SAMICS has been used repeatedly to assess the 

cost of photovoltaic module production at output 

levels representative of commercial ventures using 

state-of-the-art technology. SAMICS has also been 

used for projections of what might reasonably be 

expected at some future date for the technology 

given successful completion of research activities. 

These snapshots of technology development in 

progress have provided FSA valuable insights into 

the likelihood of achieving desired reductions in the 

cost of photovoltaic module production. 

The SAMICS methodology is a complete procedure 

for evaluating the cost of manufacturing PV solar 

arrays. The SAMICS methodology includes: 

Computer programs designed for user 

implementation on personal computers (SAMIS 

PC, SAMPEG, IPEG-4). 

Formats for the preparation of complete 

manufacturing process descriptions. 

A detailed cost account catalog which can 

easily be modified to meet user requirements. 

A standard set of financial assumptions making 

direct technology comparisons possible. 

A report format that provides detailed value 

added estimates for all process steps. 

The option to vary any of the technical, 

financial, or overhead parameters. 

A series of competitive procurements, designated 

Block I through Block V, was initiated by FSA to 

encourage the adoption of new technology and 

stimulate reduction in the cost of photovoltaic 

module production. SAMICS has made it possible to 

assess the technical progress made. Detailed 

process descriptions reflecting the latest technology 

developments were converted into manufacturing 

cost estimates under commercial production 

conditions. These were then compared with previous 

block purchases and the cost goals of the National 

Program. 

A. -,.,.. ..’ 

SIMRAND 

Analytical tools have been developed which were 

designed specifically for support of program 

management activities. The FSA Project must 

continually make decisions regarding support levels 

for various projects. These decisions are necessarily 

based on information that is quite uncertain in 

nature. The SIMRAND (Simulation of Research and 

Development Projects) model was developed as a 

tool that allows this uncertainty to be directly 

incorporated into the information used by decisionmakers. 

The SIMRAND model assesses the probability that 

a combination of R&D activities will meet a given set 

of price goals. The descriptions developed by 

SIMRAND for each technology reflect the uncertain 

nature of the database used. Technologies can then 

be compared to see which has the highest probability 

of reaching Project goals. The decision process is 

modeled by means of alternative networks that 

represent the feasible technologies being considered. 

SIMRAND performs repeated simulations to 

a 
determine the likely distribution of outcomes for each 

technology. The network reflecting the various 

technology options is continually reevaluated during 

the simulation to find the optimum subset of research 

tasks. 

SIMRAND is a management decision methodology 

that incorporates information uncertainty. The 

SIMRAND model (Figure 2) includes: 

The capability of evaluating relatively complex 

networks of technical options representing the 

feasible choices. 

The ability to replace process parameters with 

probability distributions reflecting information 

uncertainty. 

The option to include decision-maker’s 

preferences regarding risk into the decision 

process. 

Computer program available for implementation 

on a personal computer. 

STEP 1 , STEP 2 , STEP 3 , STEP 4 , STEP 5 

SILICON CRYSTALLIZATION SAWING CELL MODULE PURIFICATION 

Figure 2. Module Production Network 

A joint JPLlSERl silicon sheet material study 

successfully used SIMRAND to incorporate 

uncertainty about potential technical progress and 

material costs into the outlook for each silicon sheet 

technology. The product was a set of distributions 

indicating the possible range of outcomes for each 

research activity. The study results were instrumental 

in redirecting the technology mix pursued by the 

National PV Program. 

The SAMICS and SIMRAND models and the 

Allocation Guidelines have been applied individually 

to specific technical problems and as an integrated 

set when longer range management decisions have 

been required. Technical options have been 

evaluated by applying the manufacturing cost 

simulations to processing sequences and comparing 

the results with the costgoal allocations. Manufacturing cost point estimates generated by SAMICS 

have been combined by using SIMRAND with the 

uncertainties associated with those estimates to form 

probability distributions of technical performance 

and/or cost. The inclusion of uncertainty has allowed 

Project management insight into the risks associated 

with the technical options being considered. 

LCP 

Assessment of a PV systems capability to 

compete with conventional energy resources requires 

simulation of its performance over its lifetime and 

recognition of regional variations in solar energy 

resources. The Lifetime Cost and Performance (LCP) 

model was developed and is now used to simulate 

the energy output of a PV power plant over its useful 

life. LCP relates the effect of hourly weather conditions, system design and component efficiencies, 

electrical design, long run effects of exposure, and 

alternative operation and maintenance strategies to 

system cost, performance, and value (Figure 3). Fixed 

array, one-axis tracking and two-axis tracking systems 

can be evaluated at different locations throughout the 

country. 

LCP simulates the performance of PV systems 

using actual solar resource measurements as 

recorded in weather data tapes. The LCP model 

includes: 

The ability to use insolation data, temperature 

measurements, and precipitation and dirt 

accumulation measurements to model location 

specific performance. 

The ability to evaluate the effects of balance of 

system component failures and electrical 

mismatches within modules. 

A computer model that can be implemented on 

a personal computer. 

LCP, which was used to assist Acurex Corp. in 

making economic assessments of alternative designs 

10 
for the first phase of the Sacramento Municipal Utility 

Districts proposed 1 00-MW PV installation. The performance of each design alternative was simulated over 

its lifetime using weather data characteristic of the 

location and the parameters of the PV array system. 

5 11111lII Ill 

JULY I 

0-1 1 I I I I I b-1 1- 

0 2 4 6 8 10 12 14 16 18 20 22 24 

HOUR OF DAY 

Figure 3. Time-of-Day Output and Customer Load 

Profile 

PVARRAY 

The PVARRAY model simulates array performance 

over time. Using PVARRAY, the power profiles of 

modules representing various state-of-the-art and 

advanced designs have been obtained and the 

effects of the current-voltage (I-V) changes resulting 

from a variety of causes have been assessed. 

PVARRAY can model the effect of specific solar cell 

failure mechanisms on system performance and correct for various failed module replacement strategies. 

PVARRAY simulates the power output of a PV 

array over its lifetime. The PVARRAY model includes: 

The ability to compare performance of different 

module designs. 

The capacity to consider alternative series 

parallel wiring schemes. 

The ability to evaluate alternative replacement 

strategies for different cell failure rates and 

diode placements. 

Computer program that can be implemented on 

a personal computer. 

The combination of PVARRAY, SAMICS, and LCP 

has been used at JPL to estimate the net present 

value of energy from a photovoltaic system over its 

useful lifetime. Manufacturing costs for the module 

production sequence have been estimated using 

SAMICS, and the life-cycle cost and performance 

have been simulated using the LCP and PVARRAY 

models. The three programs have allowed PAM to 

calculate the cost of different module designs and 

determine the time-dependent economic impact of 

design changes by simulating array performance and 

life-cycle cost (Figure 4). 

Figure 4. Cell Degradation and Lifetime Performance for Three Different Module Designs 
Silicon Material 

Semiconductorgrade silicon is the material used to 

fabricate crystalline-silicon solar cells, most solid-state 

devices, and most integrated circuits. It is very highly 

purified, with tight specifications limiting the 

concentrations of specific impurities. The 

manufacturing yields during the production of solidstate devices and the performance of the devices are 

directly related to the quality of the silicon. 

Semiconductorgrade silicon, as produced in the 

Siemens reactor for the semiconductor industry, has 

been and continues to be used by the PV industry for 

the production of the great majority of the solar cells. 

The cost of the silicon is a minor part of the cost of 

producing semiconductor devices, but it has been and 

still is a significant fraction of the cost of manufacturing 

solar cells. Integrated circuits are very small and many 

can be fabricated on one silicon wafer, but only one 

solar cell is fabricated from a wafer. The small market 

provided by solarcell users has not justified industry’s 

development of an inexpensive high-volume silicon 

production technique that would result in lowcost 

silicon material. Considerable FSA funding has been 

devoted toward advancing silicon purification 

technology and processes. 

The objective of the silicon refinement effort is to 

overcome the technical barriers to lowcost silicon 

purification in order to achieve a process capable of 

producing highquality, lowcost silicon (appropriate for 

photovoltaic use), at a price consistent with FSA 

objectives. 

The goals are to establish silicon material processes 

appropriate to: 

Purity adequate for efficient crystalline-silicon 

solar cells that meet the requirements of the FSA 

Project. 

Market price of $1 9/kg (1 985 dollars). 

High-volume production plants (c 1000 MT/ 

year). 

Background 

Silicon, the second most abundant element in the 

Earth’s crust, occurs naturally primarily as silicon 

dioxide and as silicates: sand, quartz, etc. Large 

quantities of quartzite, a crystalline form of silicon 

dioxide, are reduced in arc furnaces to metallurgicalgrade silicon (mg-Si) for use by many industries. It sells 

for $1 to $2/kg and is about 98 to 99% silicon. A 

much purer silicon is required to fabricate 

semiconductor devices and solar cells. A small portion 

(a few thousand metric tons) of mg-Si is used annually 

in processes to produce semiconductor-grade silicon. 

Most solar cells have been and still are fabricated 

from silicon with a total impurity concentration less 

than one part per million, which possesses 

semiconductor properties. In 1 975, the available 

material of this quality was produced by the Siemens 

process and was referred to as semiconductorgrade 

silicon. The price then was greater than $50/kg and 

was very sensitive to the supply-anddemand 

fluctuations. It was known that solar cells could be 

made from a less-pure silicon, HOW much less pure, 

with which impurities, and how much of each impurity 

could be tolerated, was unknown, nor was such a 

material commercially available. 

Project Activities: 

1975 TO 1981 

The status of the Siemens process for the 

production of semiconductor grade silicon was: 

Advantages 

High purity (more than adequate at that time) 

Acceptable form 

Proven process 

Potential for improvements: 

Decreased energy use 

Increased production rate 

Disadvantages 

High-energy consumption process 

Low conversion rate and yield 

Large amount of waste by-product 

High labor costs 

High product cost 

A plan was formulated within FSA to develop a 

silicon refinement process to meet PV sheet and solar 

cell requirements. The plan was to develop lowcost 

semiconductor-grade silicon processes; to develop a 

solar-grade silicon refinement process (a less-pure, but 

unspecified material that should be even less 

expensive); and to determine the effects of various 

impurities and their concentrations on silicon 

refinement processes and on solar cell performance. 

The plan consisted of three process development 

phases for both the semiconductorgrade and the solargrade process (Table 1). The intent was to establish 

the practicality of the processes by production, energy 

consumption, and economic criteria. To classify 

candidate processes and define the process 

specifications, information was required about the 

effects of specific impurities on silicon and on solar cell 

12 
Table 1. Planned Process Research Phases 

Phase Purpose 

Laboratory-Scale Demonstrate process feasibility 

Experiments 

Determine reaction kinetics and yields 

Identify reactor-material problems 

Obtain silicon deposition rates of 0.01 to 1 kglh 

Obtain product purity information 

Process Development 

Units (PDUs) 

Demonstrate practicality of reactors and critical process steps 

Obtain reactor scale-up information 

Characterize critical process components 

Establish suitability of reactor materials 

Determine process capability by chemical engineering analyses using acquired 

data and extended process design 

Perform preliminary economic analyses 

Experimental Process 

System Development 

Units (EPSDUs) 

Establish technical readiness of integrated processes 

Design and build pilot plant EPSDUs with silicon deposition rates of up to 

100 MTlyear 

Obtain operating data to characterize complete process including steady-state 

operation 

Obtain data for optimization of design and operation of a full-scale production 

plant 

Confirm product purity at steady-state conditions 

Confirm economic analyses 

performance. These purity requirements directly 

affected the process designs and, thus, the economics 

of the candidate processes. The impurity studies 

consisted of two phases: the measurements of the 

effects on the final product of impurities that are found 

in mg-Si or that might be introduced during a process 

step, and the determination of the interrelationships of 

impurities and the processing steps and their combined 

effects on solar cell performance. 

The feasibility of 11 potentially lowcost processes 

were investigated. Their technical feasibilities were 

assessed by initially studying basic chemical reactions 

and general chemical engineering designs, which led to 

an analysis of the chemistry and chemical engineering 

parameters based upon data from laboratory-scale 

experiments. After an investigation lasting 1 to 3 years, 

each of the promising specific processes was then 

evaluated for its practicality. 

Scaled-up reactors were used for chemical 

engineering studies to derive data for mass and energy 

balances, process flows, kinetics, mass transfer, 

temperature and pressure effects, and operating 

controls. This information was used in analyses and 

extrapolations that resulted in improved process 

concepts and designs. The processes being examined 

in June 1977 and a number of supporting studies are 

shown in Table 2. By this time some process 

developments had progressed rapidly. 

13 
Table 2. Silicon Material Refinement Contractors in 19 77 ~ 

Contractor Technology 

Semiconductor-Grade Production Processes 

AeroChem Research Laboratories, Princeton, 

New Jersey 

Battelle Memorial Institute, Columbus, Ohio 

Union Carbide, Sistersville, West Virginia 

Motorola, Phoenix, Arizona 

Silicon halide-alkali metal flames 

Sic I , reduction by Zn using FBR technology 

Pyrolysis of SiH, derived from redistribution of 

SiHC l3 (with a feed of metallurgical grade silicon) 

Chemical vapor transport and pyrolysis of SiF, 

generated from SiF, reaction with metallurgical grade 

silicon 

Solar-Cell-Grade Specifications - 

Northrop Research, Hawthorne, California 

C. T. Sah Associates, Urbana, Illinois 

Lifetime and diffusion length measurements 

Model and studies of effects of impurities on solar 

cell properties 

Solar cell fabrication and analysis 

Investigation of effects of impurities on solar cell 

performance 

Investigation of effects of impurities on solar cell 

performance 

Spectrolab, Inc., Sylmar, California 

Westinghouse Electric, Pittsburgh, Pennsylvania 

Monsanto Research Corp., St. Louis, Missouri 

Solar-Cell-Grade Production Processes 

AeroChem Research Laboratories, Princeton, 

New Jersey 

Dow Corning, Hemlock, Michigan 

Stanford Research Institute, Menlo Park, California 

Schumacher, Oceanside, California 

Texas Instruments, Dallas, Texas 

Westinghouse Electric, Pittsburgh, Pennsylvania 

Use of a nonequilibrium plasma jet 

Arc furnace process using purer source materials 

Na reduction of SiF4 

Reduction of SiHBr, in an FBR 

Carbothermic reduction of SiO, in a plasma 

Plasma-arc-heater reduction of SiCI, with Na 

Commercial Potential of Processes 

Lamar University, Beaumont, Texas Evaluations of relative commercial potentials of 

silicon-production processes developed under the 

Silicon Material Task 

14 
After analytical and experimental studies indicated 

that a process was feasible and potentially economical, 

and the practicality of its reactors and critical steps 

were demonstrated, a comprehensive economic 

analysis was made. Professor Carl Yaws, of Lamar 

University, and JPL personnel made these economic 

analyses which not only calculated the costs to 

produce silicon, but also made comparative 

assessments of the progress of each process. The 

most technically and economically promising 

processes were then carefully reviewed before they 

were deemed ready for design and development of a 

pilot plant. The verification of a process in a pilot plant 

was necessary to validate silicon refinement 

economics and product purity, because of limited 

capabilities in scaling-up chemical processes by 

analytical means alone. 

Purification processes involving deposition of the 

silicon from silane and dichlorosilane were 

emphasized. Because these two substances can be 

purified relatively easily and, because of their high 

reactivity, they can be more easily and completely 

decomposed or reduced to form silicon than can 

trichlorosilane, which is used in the conventional 

Siemens process. Research on other processes that 

offered promise for making a less-pure solarcellgrade 

silicon by refinement of metallurgical-grade silicon were 

continued because of the potential for even lowercost 

silicon. 

A comprehensive study of the effects of impurities 

and their concentrations on semiconductor-grade 

silicon and crystalline-solarcell performance was 

carried out by a number of organizations, with the 

major focus of this effort at Westinghouse Electric 

Corp. A functional empirical model was formulated 

based upon the relationship that impurity atoms 

primarily decrease minority carrier recombination 

lifetime, reduce the Short-circuit current of a cell, and 

essentially act independently. Experimental data 

confirmed this and provided quantitative relationships 

of the effects of specific elements on cell efficiency. 

Project Activities: 1981 to Present 

In 1981, the funds available were reduced 

considerably, which resulted in many activities being 

dropped and support being given only to the most 

promising, advanced efforts. At about the same time, 

the then newest PV system cost assessments indicated 

that higherefficiency solar cells and modules were 

necessary to offset the area costs of an array. Analysis 

indicated that the power generation cost reductions 

possible by the use of higher-efficiency solar cells were 

greater than cost reductions possible by use of solarcell-grade silicon. Cell efficiency is directly related to 

silicon purity. Therefore, solarcellgrade silicon seemed 

to be impractical economically and efforts were 

reduced. The Union Carbide Corporation silane 

process and the Hemlock Semiconductor Corp. 

dichlorosilane process developments were continued. 

In support of these efforts, a basic investigation of 

the technology of a high-performance silane fluidizedbed reactor (FBR) has been continued at JPL; research 

to characterize the aerosol phenomena involved in the 

growth of particles formed by the thermal 

decomposition of silane is being performed at the 

California Institute of Technology, and an engineering 

development of the silane-tosilicon FBR is being 

performed by Union Carbide Corporation. Continuing 

studies within FSA of the influence of impurities in 

silicon on solar cell performance are described here 

under the heading, “High Efficiency Solar Cells.” 

Today’s Status 

A silane process has been developed by the 

Union Carbide Corp. that seems to be capable of 

meeting FSA goals, including price. 

A pilot plant (1 OO-MT/year capacitv) has 

demonstrated the following (see Figure 5): 

The silane portion of the Union Carbide Corp. 

silane process has met FSA goals. 

The silane produced in the first section of the 

Union Carbide Corp. process is very high 

purity and will be a low-cost material suitable 

for amorphous-silicon solar cells and/or other 

products when and if they become largequantity products. 

The R&D results of the silane-to-silicon 

deposition FBRs of the process are good. 

Research at JPL, which established the 

fundamental technology for FBR silane-tosilicon pyrolysis, will end in 1985. The FBR 

development at Union Carbide Corp., which 

has been sponsored by DOE, will continue 

with Union Carbide Corp. funds. 

Silane process production plants (completely 

funded by Union Carbide Corp.) include: 

A 1200-MTlyear plant which started operation 

early in 1985 converting mg-Si to silane, based 

upon DOE-sponsored technology. Komatsu 

chemical-vapordeposition (CVD) reactors 

convert silane into high purity silicon at a price 

that is expected to be less than that of 

Siemens-produced silicon. It is almost at full 

capacity now (see Figure 6). 

A second 1200-MT/year plant is under 

construction with completion scheduled for 

1987. I 

1 

I 

I 

A third plant of 3000-MT/year capacity is in 

design, with completion scheduled late in 

1988. Use of a FBR is under investigation. 

The plants listed above will significantly increase 

world production of silicon. World capacity to 

produce semiconductor-grade silicon was about 

6000 MT/year at the end of 1984. 

15 
Figure 5. A 1 00-MT Silane Process Pilot Plant, Washougal, Washington 

Figure 6. A 1200-MT Silane Process Production Plant, Moses Lake, Washington 

-- 

16 
Significant Accomplishments *. 

The silane process developed at Union Carbide 

Corp. under an FSA contract seems to have the 

capability to meet the FSA goal of semiconductorgrade silicon at $1 9/kg (1 985 dollars). This process 

comprises two parts: the preparation of pure silane 

(SiH4) from metallurgical-grade silicon, and the 

deposition of high-purity silicon from the silane using a 

FBR. Operation of a 100-MT/year pilot plant has 

verified the technical and economical practicality of the 

silane portion of the process. The rate of production 

and the purity of the silane from the Union Carbide 

Corp. pilot plant are consistently high. A silane sample 

tested by Brookhaven National Laboratory was the 

purest silane it had ever tested. Good research and 

development results for lowcost pyrolysis of silane to 

silicon are coming from the collaborative fluidized-bedreactor efforts at Union Carbide Corp. and JPL. The 

Union Carbide Corp. effort is determining the steadystate operational conditions for the design of a 

production prototype reactor for silane feed 

concentrations of 20 to 25%. For more cost-effective 

pyrolysis, JPL research is being performed at silane 

concentrations up to 100 % with emphasis on 40 to 

60 % concentrations. The problems of reactor clogging, 

bed slugging, and excessive production of silicon dust 

have been solved along with raising the silane feed 

limits from 2 to 100% concentration. The Union Carbide Corp. FBR has successfully run for 66 h at 20% 

silane. Unresolved problems are suitable silicon seed 

particle material, prevention of contamination, product 

purity confirmation, and verification of production 

economics. 

The progress of 10 years of effort has resulted in 

Union Carbide Corp. building a 120O-MT/year silicon 

production plant (now in production) in which the silane 

is deposited as silicon in Komatsu chemical vapor 

deposition reactors. (These reactors were also 

operated in conjunction with the pilot plant.) The silicon 

produced is of very high purity. It is expected that the 

cost will be less than Siemens-process-produced 

silicon, but not as much less as with a FBR. 

The dichlorosilane chemical vapor deposition 

process of Hemlock Semiconductor Corp. was 

successfully carried through the demonstration of the 

practicality of the reactor and the critical process steps. 

A PDU was designed, constructed, and tested. This 

PDU was used to investigate the making of 

dichlorosilane from trichlorosilane. A pilot plant was not 

sponsored by DOE/JPL because process economics 

indicated a product price less than for the conventional 

Siemens process, but greater than that of the silane 

process. This technology is available for use in a 

production plant and Hemlock may have assimilated it 

into their ongoing production of silicon, although 

Hemlock has never divulged its commercialization 

plans to JPL. 

Numerous FSA studies also contributed to 

advancements in trichlorosilane and modified Siemensreactor technologies. A thorough examination of the 

many silicon processes has resulted in significant 

improvement in the understanding of many other 

silicon refinement technologies. The solarcellgrade 

silicon efforts were stopped because of (1) large 

budget reductions, (2) the advanced state of the Union 

Carbide Corp. silane process, (3) improved 

understanding of the need for higher-purity silicon to 

obtain high cell efficiency, and (4) the economic value 

of higherefficiency solar cells. 

A good fundamental understanding of the effects of 

impurities in silicon upon the properties of silicon 

material and upon the performance of solar cells has 

been determined by extensive studies, principally by 

the Westinghouse R&D Center, and supported by 

others. 

Silane Process 

(Union Carbide Corporation) 

Preparation of Silane from Metallurgical-Grade 

Silicon and Conversion of Silane into 

Semiconductor-Grade Silicon 

The process flow diagram is shown in Figure 7 

t- SiHpC12 1 

sic14 

IMPURITIES 

FLUIDIZED 

REACTOR 

SETTLER 

-rt 

IH2 1 TREATMENT WASTE 1 

DISTILLATION VOLATILE 

IMPURiTlES 

STRIPPER 

METAL 

CHLORIDE 

WASTE 

DISTILLATION 

SiHC13 

SiHC13 

SiHCL3 

BUTION 

REACTOR 

Uh SiHCln 

I MET Sic14 

GRADE 

Si 

J- I FLUIDIZED 

REACTOR 

REDISTRI. 

BUTION SILICON GRANULES 

REACTOR 

DISTILLATION 

lY -1 I 

SiH2CI2 

SiHCln 

Figure 7. Silane Process Flow Diagram 

17 
Past Accomplishments 

Process feasibility demonstrated in laboratoryscale experiments for silane production and for 

silane conversion to silicon by thermal 

decomposition in freespace reactors and FBRs. 

Practicality of reactors and process steps for 

continuous, steadystate silane production 

demonstrated in scaled-up process development 

units. 

Thermodynamic values and reaction kinetics 

measured in hydrochlorination and redistribution 

reactors. 

Vapor-liquid equilibria calculated for chlorosilanes. 

Detailed process design for 1 OO-MT/year EPSDU 

completed (silane portion). 

Engineering design and economic analysis for 

1 OOO-MT/year plant completed (silane portion). 

Hydrochlorination reaction characterized in 

extended studies (Solarelectronics, Inc.) consists 

of: 

Equilibrium constants and reaction kinetics 

measured over wide temperature and pressure 

ranges. 

Reaction rate equation and reaction model 

formulated. 

Suitable materials of construction for reactor 

determined. 

Conventional free-space reactor for conversion of 

silane to silicon discarded because of unsuitable 

fine powder product. 

FBR R&D plan formulated. 

More Recent Accomplishments 

Lowcost silane process technical readiness 

demonstrated by operation of pilot plant (EPSDU) 

at Washougal, Washington: 

High-purity silane produced under optimizedcontinuous steadystate conditions. 

High-purity silicon produced using Komatsu 

chemical-vapordeposition reactors. 

ORIGINAL PAGE Is 

OF #wn\ QLjAuI'y 

Engineering mode! FBR successfully operated at 

Washougal, Washington: 

Process operating parameters assessed with a 

6in.diameter FBR. 

Longduration runs with modified reactor 

extended range to continuous operation for 

66 h using 20% silane. 

Conditions for continuous, steady operation 

and for producing highpurity silicon remain to 

be defined. 

FBR research at JPL: 

Characterization for high-silaneconcentration 

operation extended to 100% silane using 

6-in.diameter reactor and cooled distributor 

plate (see Figure 8). 

Operation at various high silane concentrations 

demonstrated, e.g., 80% silane for 3 h at 

deposition rate of 3.5 kglh. 

Operating data obtained for design and 

operation of production plant. Figure 8. JPL Research Fluidized Bed Reactor 

18 
,. ” I. ... Feasibility of periodic removal of product 

demonstrated. 

Scavenging mechanism describing high bed 

deposition and very low fines formation 

demonstrated. 

Fine particle growth in silane free-space reactor 

at California Institute of Technology: 

Aerosol theory modified to describe formation 

and growth of fine particles in silane pyrolysis 

system. 

Reactor developed and experimental 

conditions investigated with the objective of 

growing silicon particles to size suitable for 

seed in an FBR. 

Purity of product from both facilities: 

Cleansing procedures established to ensure 

purity of initial seed particles. 

Quartz liners installed in FBR to prevent 

contamination from reactor wall. 

Final purity needs to be determined. 

Dichlorosilane CVD Process 

(Hemlock Semiconductor Corporation) 

Preparation of Dichlorosilane from 

Metallurgical-Grade Silicon and Deposition of 

Semiconductor-Grade Silicon from 

Dichlorosilane 

The process flow diagram is shown in Figure 9. 

Process feasibility for conversion of metallurgicalgrade silicon to dichlorosilane and for chemicalvapor deposition of dichlorosilane demonstrated. 

Practicality of redistribution and deposition 

reactors as well as integrated unit demonstrated 

in scale-up process development unit (see 

Figure 10). 

Redistribution reactor with Dowex catalyst 

characterized for kinetics and conversion yield. 

Dichlorosilane characteristics determined to 

assess hazard: 

Autoignition, explosion severity, hydrolysis, 

and explosive output measured; 

dichlorosilane found to be considerably 

more hazardous than hydrogen or 

tric hlorosilane. 

Process redesigned, including elimination of 

dichlorosilane storage, to minimize 

problems. 

Intermediate and advanced design deposition 

reactors studied: 

Objectives developed from economic goal. 

A 2 gh” cm-’ deposition rate, 40 mol % 

conversion yield, and 60 kWh/kg energy 

use. 

Results from intermediate reactor studies 

indicated reactor modifications required: 

Deposition rate, conversion yield, and 

energy usage objectives achieved 

separately. 

HCI etching step introduced to remove 

silicon wall deposits. 

Results from advanced reactor with cooled 

wall showed all objectives achievable by 

optimizing conditions. 

Semiconductor-grade purity demonstrated 

for product silicon: 

WASTE Hz TREATMENT Sic14 

SIEMENS 

REACTOR 

CONDENSER VOLATILE 

TYPE si BORON 

REMOVAL 

STRIPPER 

SiH2CLZ 

W BORON IMPURITIES 

Sic14 SiHU3 SiHC13 

SiHCl3 SiH2C12 Sic4 

SiH2C12 SiHZC12 

HCI FLUIDIZED 

BE0 - DISTILLATION HZ 

REACTOR 

- 

REDISTRIBUTION 

1 

DISTILLATION 

MET- Sic14 

GRADE Sic14 

Si SiH2C12 

Figure 9. Dichlorosilane CVD Process Flow Diagram 

19 
Figure 10. Process Development Unit 

Boron, phosphorus, carbon, and metal 

concentrations commensurate with, or less 

than, those of commercial semiconductorgrade silicon. 

Energy conversion efficiency of photovoltaic 

cells made from this material shown to be 

equivalent to that of baseline cells fabricated 

from commercial semiconductorgrade 

silicon. 

EPSDU sized to 1 OO-MT/year outlined. 

Economic analysis for a 1 OOO-MT/year plant 

indicated $34.80/kg (1 985 dollars) for product 

cost (without profit). 

Effects of Impurities in Silicon 

The objective was to relate silicon purity to the 

economics of silicon refinement processes by the 

determination of impurity effects on material and solarcell properties. The approach was to characterize 

materials and solar cells prepared from deliberately 

doped ingots. 

The studies included the following parameters: 

Deeplevel impurity concentrations in singly and 

multiply doped ingots. 

Methods and growth rates for single crystals. 

Melt-replenishment methods. 

Base dopant type and concentration (resistivity). 

Anisotropic distributions of deep-level impurities. 

Grain boundary structure (impurity interactions in 

Thermal and gettering treatments during cell 

polycrystalline cells). 

processing. 

20 
High-temperature aging of cells. 

Effects on high-efficieqcy PV, solar cells (minor 

activity). 1 

i' 

I. c. 

Tolerable concentrations of impurities were found to 

be determined by the effects of: (1) specific metallic 

impurities on the performance of cells, (2) boron and 

phosphorus on resistivity and singlecrystal yield in 

ingot growth, and (3) total impurity concentration in the 

silicon on singlecrystal growth yield. The effects of 

impurities on cell efficiency were related empirically 

through the base diffusion length and were shown to 

be speciesdependent. For example, the concentrations 

causing a 10% decrease in efficiency ranged from 

0.1 ppba for tantalum to 10 ppma for copper (see 

Figure 1 1 ). The tolerable concentrations in the polycrystalline silicon are dependent on the methods used 

for crystal growth. Crystalline structure breakdown 

occurs when the total impurity concentration exceeds a 

critical value for a particular set of conditions of ingot 

or sheet growth. For example, the critical concentration 

is in the range of 200 to 500 ppma when a 1Ocmdiameter Cz crystal is pulled at a rate of about 8 cm/h. 

(The available database for these trade-off studies has 

been incorporated into a slide rule.) The impurity limits 

can be relaxed considerably for a nearequilibrium, unidirectional solidification in which liquid-solid impurity 

segregation is the concentration controlling factor. 

Unfortunately, these studies have not been carried 

experimentally into various forms of silicon ribbon and 

solar cells made from silicon ribbons. 

In a fundamental approach to the impurity effects 

problem, a transmission-line-equivalent circuit model 

was developed by C.T. Sah to compute the exact 

steady-state characteristics of onedimensional silicon 

solar cells. The model was used for calculations of 

limiting concentrations of specific impurities for defined 

cell structures and efficiency. A model was also 

developed to study the effects of defects across the 

back-surface-field junction on the performance of highefficiency cells. A new theory to distinguish an 

acceptor-like deep level from a donor-like deep level 

was developed using measured values of the thermal 

emission and capture cross sections. This theory also 

provides information concerning the lattice distortion 

around an impurity atom before and after the capture 

or emission of an electron or hole at the impurity 

center. 

METAL IMPURITY CONCENTRATION (ppms) 

10-5 10-4 10-3 10-2 10-1 1 .o 10 Y Ill] I 1111 I Ill1 I Ill1 I Ill, I ,111 I Ill1 

- 

0 

y 0.4 - - Ni LL 

1012 1013 1014 1015 1016 1017 1018 

METAL IMPURITY CONCENTRATION (atoms/cm31 

METAL IMPURITY CONCENTRATION (ppms) 

10-5 104 103 102 10-1 1 .o 10 

Y 

v) 

co 

Ni 

AI 

>: 

p 0.6 Fe 

Y Cr 0 

Y Mn 

y 0.4 - - 

II! 

\'V 

Ti P - TYPE SILICON I 

2 0.2 

0 zz 2 0.0 

1011 1012 1013 1014 1015 1016 1017 1018 

METAL IMPURITY CONCENTRATION (atoms/cm3) 

Figure 11. Impurity Concentration Effects on Solar Cell Efficiencies, n-Type and p-Type Silicon 

21 
Conclusions 

Decrease of cell conversion efficiency is uniquely 

dependent on specific impurities and their 

concentrations. 

General empirical model correlates measured 

impurity effects on cell performance. 

Significantly less performance reduction caused 

by most deleterious elements in n-base cells. 

Reduction of electrical activity of impurities by 

precipitation in vicinity of grain boundaries in 

polycrystalline material is dependent on impurity 

diffusion coefficients. 

Removal of impurity elements by thermal and 

gettering procedures is dependent on diffusion. 

Composition of acceptable polysilicon feedstock 

depends on factors of singlecrystal growth 

technique, melt-replenishment strategy, and cellprocessing sequence. 

Long-term, ambient-temperature cell 

performance is not significantly changed further 

by presence of deeplevel impurities. 

Only about 10% variation in cell performance is 

attributable to nonuniform impurity 

concentration. 

Model and data indicate that higherefficiency 

cells are increasingly sensitive to impurities. 

Research and Development Needs 

Complete understanding of silicon particle growth 

mechanisms and particle size control in a 

fluidized bed reactor. 

Establish product purity and other characteristics 

of FBR-produced silicon. 

Complete characterization of JPL FBR for 

operation using high silane concentrations and 

producing semiconductor-grade silicon. 

Establish engineering design and operation of 

FBR for longduration steady-state production of 

semiconductor-grade silicon. 

Verify lowcost production of polysilicon in Union 

Carbide Corp. silanelFBR EPSDU. 

Define effects of silicon produced by the 

silanelFBR process on solarcell efficiency 

22 
Silicon Sheet 

In 1975, all of the silicon sheet used for 

manufacturing semiconductor devices, including 

solar cells, consisted of wafers sliced from 

Czochralski (Cz) crystals by diamond-embedded 

inside-diameter (ID) saws. This semiconductorindustries technology had evolved to a level of 

sophistication that was adequate for solar cells for 

use on spacecraft. However, the cost of producing 

these wafers was prohibitive for large-scale 

terrestrial PV uses. The cost of the silicon material 

and the processes required to produce the thinsheet constituted the majority of the manufacturing 

costs of the early terrestrial PV modules. It was 

known that solar cells could be made from silicon 

less perfect than single-crystal Cz wafers, but the 

effects on solar cell performance were not 

understood. Potentially less expensive silicon sheet 

had been experimentally grown in a laboratory; 

however, the quality of solar cells that might be 

made from these new forms of silicon was 

unknown. Therefore, an extensive investigation of 

silicon materials, its processing into thin sheets, and 

its characteristics for PV use was initiated by the 

FSA Project. 

The objective of the silicon sheet activity is to 

develop a sheet growth technology that will meet the 

cost and quality requirements for large-scale PV 

applications. Initially, the objective had been to 

define, develop, and demonstrate process 

technologies capable of producing large-area silicon 

sheet material for production of low-cost solar arrays 

at a price of $0.50 per peak watt (1975 dollars) by 

1986. 

The goals were to: 

Develop silicon sheet growth technologies to 

meet specific cost and quality criteria (including 

ribbon growth, ingot growth, and ingot slicing). 

Develop growth and slicing equipment to meet 

these criteria. 

Verify equipment performance 

The goals now are to establish crystalline silicon 

growth technology appropriate for: 

Silicon sheet of quality suitable for fabrication 

into solar cells with efficiencies UD to 20%. 

Sheet at a cost compatible with FSA goals. 

The key technical issues are to understand the 

technologies so that sheet growth can be controlled 

to meet: 

High-area throughput rates that meet quality 

and economic needs. 

A sheet quality with acceptable crystallographic 

and impurity characteristics, i.e., type, quantity, 

and uniformity of distribution. 

Material stability with little stresslstrain. 

Background 

Based upon the silicon sheet R&D performed 

during the early 1960s, considerable potential existed 

in 1975 for lower-cost silicon sheet that was 

surmised to be of adequate quality for terrestrial 

solar cells, i.e., growth of ribbons, casting of silicon, 

and advanced (less expensive) Cz and float-zone 

(FZ) crystal-growth techniques. Three new ribbon 

growth techniques, by which silicon is grown directly 

to the thickness of a solar cell, had been conceived 

and their feasibility had been demonstrated 

experimentally: the dendritic-web, Stepanov, and 

edge-defined film-fed growth (EFG) methods. 

Direct growth of silicon ribbons that did not 

require cutting to obtain the appropriate thickness for 

solar cells obviously was a way to eliminate costly 

wafering and silicon-material waste. The growing or 

casting of silicon ingots in rectangular shapes so that 

they could be cut into square or rectangular wafers, 

for more efficient solar cell packing into PV modules, 

also seemed to be economically plausible. However, 

if cast-ingot technology was to be cost-effective, the 

slicing of the large ingots into wafers had to be 

accomplished more economically than with 

contemporary single-blade ID saws. A common 

characteristic of these new silicon sheet forms was 

that they were not ideal, with imperfections and 

impurities of greater densities and more uneven 

distributions than those of Cz crystals. The electronic 

properties of these less-than-perfect forms of sheet 

were not well known in the 1970s, so many questions 

had to be answered. What would be the effect on 

solar cell performance? How would performance be 

affected by the material purity, the size and 

characteristics of the grains, and the quality of the 

crystallographic structure? What quality of silicon 

sheet could be obtained consistently from the various 

sheet-growth techniques? How would the cost to 

produce sheet using each specific technique vary 

with sheet quality? To answer these questions, 

considerable research was needed and 

manufacturing cost-analysis capabilities had to be 

developed that could be used to periodically assess 

the progress of the competing technologies. 

Project Activities: 1975 to 1981 

In 1975, under the stimulus created by the need 

for large-area low-cost silicon sheet and the 

availability of Government funds, several more 

growth techniques than mentioned above were 

devised. The emphasis was on ribbon technologies 

because they seemed to offer the greatest potential 

for inexpensive PV power. However, ingot 

23 
technologies were also funded because there was 

greater assurance that these efforts could succeed. 

An initial plan was devised that included a four-phase 

effort: research and development on sheet-growth 

methods (1 975-1 977); advanced development of 

selected growth methods (1977-1 980); prototype 

production development (1 981 -1 982); development, 

fabrication, and operation of growth production plants 

(1 983-1 986). 

Concurrently, with the early phases, a cost-tomanufacture economic analysis capability was 

developed and refined to maturity. Periodic 

assessments of the economic value of each sheet 

technology were performed, based upon the latest 

progress of sheet and other PV technologies. The 

sheet technologies showing the greatest promise for 

economic viability were selected for advanced 

development, and the less promising were dropped. 

The silicon sheet technologies funded by FSA during 

the late 1970s are shown in Tables 3 and 4. The 

lists of active contracts in June 1977 and September 

1978 show that in this period, the less promising 

ribbon techniques were dropped, and that new 

advanced Cz growth efforts (and die and container 

material studies) were initiated. Early in 1981, a 

major review of the remaining sheet techniques was 

performed. The dendritic web, EFG, and silicon-onceramic (S0C)’efforts continued to be supported. 

Also, the advanced Cz (by Kayex Corp.), heatexchange method (HEM), a Semix, Inc., casting 

process, and multiwire ingot slicing received 

continued support. 

The Cz growth technique is shown in 

Figure 12. Semiconductor silicon chunks, the 

“charge,” are melted in a quartz crucible within a 

sealed grower furnace filled with argon. A silicon 

“seed” of the appropriate crystallographic orientation 

is lowered until it is in contact with the molten silicon 

surface, and is then withdrawn slowly. The singlecrystal silicon that grows on the seed has the same 

crystallographic structure as the seed, as long as the 

furnace conditions and seed withdrawal are properly 

controlled. The dimensions of the Cz crystal or ingot 

are determined by the quantity of the silicon charge 

and the rate of seed withdrawal. The dopant and 

impurity levels in the crystal are determined by the 

trace elements in the charge, except that additional 

oxygen and carbon are introduced during the growth 

process from the quartz (Si02) crucible and the 

graphite furnace components. Control of impurities, 

structural defects, and their uniformities have been 

improved considerably by careful analysis, 

experimentation, and improved grower design and 

operational control. Goals and status are shown in 

Table 5. 

Cast-silicon ingots (Figures 13 and 14) have the 

advantage of being cast in such a form that square or 

rectangular wafers can be sliced directly from the 

ingot. Some cast ingots are large enough that they 

can be slabbed into four or nine square ingots before 

being sliced into wafers. Cast ingots are less 

exoensive than Cz ingots because their solidification 

rafes’are-faster. However, they are polycrystalline. 

The quality, size, and uniformity of grains in the ingot, 

plus the non-uniform distribution of impurities, are 

parameters that influence the electronic quality of 

specific cast ingots. The efficiencies of solar cells 

made from these materials are generally less than 

those of Cz cells, and are highly dependent upon 

the ingot quality. Goals and status are shown in 

Tables 6 and 7. 

Three ingot slicing techniques on which FSA has 

sponsored R&D are ID, multiblade, and multiwire 

(shown in Figures 15, 16 and 17). The ID and multiwire saws have cutting materials embedded in a 

softer matrix and are subject to wearout as the cutting 

particles are dulled or eroded away. The multiblade 

saw and a multiwire saw (also funded by DOE) use a 

slurry containing abrasive particles. In all cases, cooling of the cutting area with a fluid is required. The 

sawing occurs by the tearing away of silicon, causing 

surface damage to the sawed wafer. The surface 

damage is different for each sawing technique; in 

each case it must be removed, usually by etching the 

wafer in acid. Goals and status are shown in Table 8. 

The most advanced and still the most promising 

ribbon technologies are the EFG methods and the 

dendritic web methods (shown in Figures 18 and 19). 

In the EFG method, the molten silicon moves 

upward between parallel walls of a graphite die by 

capillary action. A silicon seed is lowered to make 

contact with the molten silicon in the top of the die, 

and is then pulled upward. The shape of the resulting 

ribbon grown on the seed is defined by the shape of 

the top of the die. Relatively large quantities of ribbon 

can be grown quickly, especially by pulling several 

ribbons simultaneously from the same melt. Mobile 

Solar Energy Corp. devised, with its own funds, and 

is producing nine ribbons from one melt by its 

“nonagon” concept (a tubular shape with nine flat 

sides). The quality of the EFG ribbon is its major 

limitation: it produces solar cells with lower efficiencies than are possible with Cz or FZ.wafers. The 

molten silicon reacts with the graphite dies and, as 

the silicon cools, silicon carbide precipitates are incorporated into the ribbon. The result is a ribbon that has 

a disrupted crystalline structure with uneven impurity 

distributions. Goals and status are shown in Table 9. 

Ribbon grown by the dendritic-web method is of 

higher quality than EFG ribbon and other ribbons. Its 

electronic quality can be almost as good as that of 

Cz wafers, except that it has a “twinning plane” in 

the plane of the ribbon throughout the ribbon length. 

By precise control of temperatures and temperature 

gradients, during and after seeding and growth 

initiation, parallel silicon filaments, called 

“dendrites,” can be grown in the silicon melt. Under 

carefully controlled withdrawal, the dendrites 

continue to grow and a film of molten silicon forms 

and solidifies between them. The only solid material 

that the molten silicon contacts is the quartz crucible 

which minimizes contamination and results in a 

24 
Table 3. Silicon Sheet Contractors: June 19 7 7 

Contractor Technology Area 

Ribbon Growth Processes 

Mobil-Tyco Solar Energy 

Waltham, Massachusetts 

(JPL Contract 954335) 

IBM 

Hopewell Junction, New York 

(JPL Contract 9541 44) 

RCA 

Princeton, New Jersey 

(JPL Contract 954465) 

Univ. of So. Carolina, 

Columbia, South Carolina 

(JPL Contract 954344) 

Motorola 

Phoenix, Arizona 

(JPL Contract 954376) 

Westinghouse Research 

Pittsburgh, Pennsilvania 

(JPL Contract 954654) 

Edgedefined, film-fed growth 

Capillary action shaping technique 

Inverted Stepanov growth 

Webdendritic growth 

Laser zone ribbon growth 

Dendritic web process 

Sheet Growth Processes 

Honeywell 

Bloomington, Minnesota 

(JPL Contract 954356) 

Rockwell 

Anaheim, California 

(JPL Contract 954372) 

General Electric 

Schenectady, New York 

(JPL Contract 954350) 

Univ. of Pennsylvania, 

Philadelphia, Pennsylvania 

(JPL Contract 954506) 

Dip coating of lowcost substrates 

Chemical vapor deposition on lowcost substrates 

Chemical vapor deposition on floating silicon 

substrate 

Hot-forming of silicon sheet 

Ingot Growth Process 

Crystal Systems 

Salem, Massachusetts 

(JPL Contract 954373) 

Heatexchanger ingot castinga 

~ 

Ingot Cutting 

Crystal Systems 

Salem, Massachusetts 

(JPL Contract 954373) 

Multiple wire sawinga 

Varian Breadknife sawing 

Lexington, Massachusetts 

(JPL Contract 954374) 

aSingle contract provides for both ingot casting and multiple wire sawing. 

25 
Table 4. Silicon Sheet Contractors: September 19 78 

Contractor Technology Area 

Shaped Ribbon Technology 

Mobil-Tyco Solar Energy 

Waltham, Massachusetts 

(JPL Contract 954355) 

Motorola, Inc. 

Phoenix, Arizona 

(JPL Contract 954376) 

Westinghouse Research 

Pittsburgh, Pennsylvania 

(JPL Contract 954654) 

Edgedefined film-fed growth 

Ribbon growth, laser zone regrowth 

Dendritic web process 

Supported Film Technology 

Honeywell Corp. 

Bloomington, Minnesota 

(JPL Contract 954356) 

RCA Labs 

Princeton, New Jersey 

(JPL Contract 95481 7) 

Silicon on ceramic substrates 

Epitaxial film growth on lowcost silicon substrates 

Ingot Technology 

Crystal Systems, Inc. 

Salem, Massachusetts 

(JPL Contract 954373) 

Kayex Corp. 

Rochester, New York 

(JPL Contract 954888) 

Siltec Corp. 

Menlo Park, California 

(JPL Contract 954886) 

Texas Instruments 

Dallas, Texas 

(JPL Contract 954887) 

Varian Vacuum Division 

Lexington, Massachusetts 

(JPL Contract 954374) 

Varian Vacuum Division 

Lexington, Massachusetts 

(JPL Contract 954884) 

~~ ~ 

Heat exchanger method (HEM), cast ingot, and 

multiwire fixed abrasive slicing 

Advanced Cz growth 

Advanced Cz growth 

Advanced Cz growth 

Multiblade slurry sawing 

CVD silicon nitride and carbide 

CVD silicon nitride 

Advanced Cz growth 

Die and Container Materials Studies 

Battelle Labs 

Columbus, Ohio 

(JPL Contract 954876) 

Coors Porcelain 

Golden, Colorado 

(JPL Contract 954878) 

Eagle Picher 

Miami, Oklahoma 

(JPL Contract 954877) 

RCA Labs 

Princeton, New Jersey 

(JPL Contract 954901) 

Silicon nitride for dies 

Mullite for container and substrates 

Tylan Vitreous carbon 

Torrance, California 

(JPL Contract 954896) 

26 
CZOCHRALSKI INGOT ADVANCED CZOCHRALSKI 

CRYSTAL GROWTH SYSTEM 

INGOT PULL AN0 

ROTATION 

MECHANISM 

MULTIPLE INGOT CAPABILITY 

CRYSTAL REMOVED 

HOPPER WITH SILICON INSERTED 

LOWERED TO CRUCIBLE - UNLOADED 

SECOND CRYSTAL GROWN 

CYCLE REPEATED 

CRYSTAL __ 

FURNACE ISOLATION 

VALVE 

GROWING 

CRYSTAL 

Figure 12. Advanced Czochralski Crystal Growth by Hamco Division (Kayex Corp.) 

HEAT EXCHANGER METHOD 

VIEW PORT 

e,iNSuLATION 

SILICON MELT 

SEED CRYSTAL 

HEAT EXCHANGER A$ 1 HELIUM GAS 

Figure 13. Ingot Casting by Heat Exchanger Method by Crystal Systems, Inc. 

ORIGINAL PAGE Is 

m. m WALfl 

t- -"---s%--rT7 s L - -.-ai- . . 

Figure 14. Ingot Casting by Semicrystalline Casting Process by Semix, Inc. 

27 
Table 5. Advanced Czochralski Crystal Growth by Hamco 

GOALS: 

150 kg OF INGOTS/ 

CRUCIBLE 

15-cm-DIA INGOT 

2.5 kglhr THROUGHPUT 

AUTOMATION 

15% ENCAPSULATED SOLAR 

CELL EFFICIENCY 

90% YIELD 

STATUS I RLDNEEDS 

150 kg OF INGOTS/ *INGOT QUALITY: 

CRUCIBLE STRUCTURAL PERFECTION 

15-cm-DIA INGOT 

2.2 kglhr THROUGHPUT 

GROWTH PARAMETER 

OPT1 M IZATIO N 

19861~~0~ ADD-ON PRICE. 

' (1980 $Mid 

UNDER DEVELOPMENT 

AT END OF CONTRACT 

(LATER ACHIEVED BY HAMCO) 

15% SOLAR CELL FSA PRICE 

EFFICIENCY ALLOCATION 0.112 

86% YIELD SAMICS 

PRO J ECTlON 0.1 62 

'ASSUMES 0.73 m2 OF WAFERSlkg OF INGOT 

"ENCAPSULATED CELL EFFICIENCY N.A. 

Table 6. Ingot Casting by Heat Exchanger Method by Crystal Systems, Inc. 

GOALS: 

30 cm x 30 cm x 15 cm 

SHAPED INGOT (35 kgJ 

> 90% SINGLE CRYSTAL 

2.0 kglhr GROWTH RATE 

15% ENC. SOLAR CELL 

> 95% YIELD 

EFFICIENCY 

STATUS: 

34cmx34cmx17cm 

SHAPED INGOT (35 kgJ 

> 95% SINGLE CRYSTAL 

1.9 kglhr GROWTH RATE 

15% SOLAR CELL 

EFFICIENCY 

>95% YIELD 

ASSUMES 1 m2 OF WAFERSlkg OF INGOT 

Table 7. Semicrystalline Casting Process by Semix 

GOALS: 

R&D NEEDS 

INCREASED INGOT YIELD 

(1980 SWPJ 

FSA PRICE 

SAMICS 

1986 INGOT ADD-ON PRICE' 

ALLOCATION 0.194 

PROJECTION 0.17 

~~ 

30 cm x 30 cm x 19 cm 

SHAPED INGOT (40 kg) 

SEMICRYSTALLINE 

15% SOLAR CELL 

EFFICIENCY 

98% INGOT YIELD 

2.3 kg PER HOUR 

CRYSTALLIZATION 

RATES 

STATUS: 

20x20~ 15 

(1 4 kg) 

SEMICRYSTALLINE 

15% SOLAR CELL 

EFFICIENCY 

83% INGOT YIELD 

2.3 kg PER HOUR 

R&D NEEDS 

LARGE QUANTITY OF 

LARGE AREA CELLS AT 

15% 

DEMONSTRATION OF 

HIGH THROUGHPUT 

PROCESSING 

1986 ADD-ON PRICE. 

(1 980SIWpJ 

FSA PRICE 

SEMIX PROJECTION 

ALLOCATIONS 0.256 

0 .115 

*USING 0.81 m2 OF WAFERS/kg OF INGOTS 

* ..e* 

28 
*INNER 

; 

SILICON TECHNOLOGY CORP 

I.D.* SAW 

IMBEDDED 

DIAMOND 

PARTICLES 

DIAMETER 

HOFFMAN 

MULTIBLADE SAW 

SLURRY WITH 

CUTTING 

COMPOUND 

IWAXI 

Figure 16. Multiblade Saw by P. R. Hoffman Co. 

CRYSTAL SYSTEMS, INC. 

MULTI-WIRE SAW 

MULTIPLE 

FIXED ABRASIVE 

00 GUIDE ROLLERS 

/- 

ROCKING 

INGOT 

HOLDER 

ORihitAi. PAGE 1s 

PF~WW * Figure 17. Multiwire Saw by Crystal Systems, InC. 

29 
Table 8. Advanced Wafering Technologies by Crystal Systems Inc., P. R. Hoffman, and 

Silicon Technology COrP. 

GOALS 

15 cm DIA 

20 WAFERSlcm 25 WAFERSlcm OF 

OF INGOT !NGOT 

0.5 WAFERlmin 1 WAFERlmin 

0.82 mzlkg' 1.0 mzlkg' 

10 x 10 cm square 

95% YIELD 95% YIELD 

STATUS 

(BEST SINGLE DEMONSTRATION) 

15 cm DIA 

17 WAFERSlcm 25 WAFERSlcm 

10 x 10 cm 

0.42 WAFERlmin 1.25 WAFERlmin 

0.69 m2lkg" 1.0 mYkg 

95% 95% 

'NO LONGER UNDER DEVELOPMENT 

"SQUARE METER OF WAFERSlKlLOGRAM OF INGOT 

clean, smooth ribbon. The dendrites are easily 

removed after the ribbon has cooled. The major 

disadvantage of this method is the low production 

rate of quality ribbon. Significant difficulties have 

been encountered as faster growth rates and 

sustained operation for many hours have been 

attempted. The problems of growing low-stress 

ribbons at economical growth rates continue to 

plague the web efforts. Goals and status are shown 

in Table 10. 

The silicon-on-ceramic (SOC) technique was the 

third promising non-ingot growth method. It was 

unique in that it had high growth rates; but the 

efficiency of solar cells made from it was low. 

Consequently, when the need for higher efficiency 

became widely accepted and the Project funds were 

cut, SOC efforts were dropped (see Figure 20 and 

Table 11 ). 

In 1981, the direction of the National Photovoltaics 

Program was altered to emphasize long-term, highrisk research efforts, and funding was significantly 

reduced. Concurrently, there was a growing 

awareness that PV module efficiencies had to be 

raised considerably if PV was to be cost effective for 

large uti My applications. H igher-efficiency modules 

and solar cells require higher-quality silicon sheet. 

Consequently, the production-development-oriented 

third and fourth phases were discontinued, and ingot 

growth, ingot slicing, and ribbon technologies other 

than dendritic web and EFG were no longer funded. 

The status of the sheet technologies that were 

dropped because of programmatic changes are' 

shown in Table 12, as they were at time of contract 

completion. An exception, low-angle silicon sheet 

(LASS) growth method, was funded for about 2 years. 

ADDITIONAL RAD 

REQUIRED 

DEVELOP FUNDAMENTAL UNDERSTANDING 

OF WAFERING PROCESS, FOR 

INCREASING THROUGHPUT 

INCREASED WAFER THROUGHPUT 

REDUCED EXPENDABLE COSTS 

SENSITIVITY TO EXTENDED OPERATION 

1986 WAFER ADD-ON PRICE 

(1980 UW,) 

FSA PRICE ALLOCATION 

0.081 (15 cm DIA) 

SAMICS 0.110 (15 cm DIA) 

Project Activities: 1981 to Present 

With a change in Project directions, R&D on 

specific growth technologies was then replaced by 

generic growth R&D. More recently, a concerted 

effort to increase the area growth rates ot nlgnquality dendritic web are now supported by a 

consortium of utilities, DOE and Westinghouse. 

The status of web and EFG in 1985 is shown in 

Table 13. 

As time passed, ribbon-buckling problems and 

reduced ribbon quality that were encountered when 

higher ribbon growth rates were attempted led to the 

conclusion that a more fundamental understanding of 

the impediments to high-speed silicon ribbon growth 

and quality was required. This resulted in the 

establishment of an interactive university-industryGovernment effort that continues to actively pursue 

theoretical and experimental ways to achieve highspeed, high-quality silicon crystal growth. Computer 

models have been developed and are being used to 

study the stresslstrain relationship in the silicon 

solidification zone and the nearby areas in the 

ribbon. The analytical results are then experimentally 

evaluated on ribbon-growth equipment at 

Westinghouse and Mobil Solar. In support, hightemperature silicon material properties are being 

determined and sophisticated instrumentation and 

measurement techniques continue to be developed. 

Progress and problems of the various efforts are 

discussed at frequent meetings and the results 

iterated into the appropriate activities. A low-angle 

(horizontal) silicon sheet growth effort was also 

initiated to explore the potential value of this very 

high-speed-ribbongrowth technique (Figure 21 and 

Table 14). This technique has the potential to evolve 

into a practical growth technology, but is not now 

funded by DOE because of a shortage of funds. It 

was funded in an attempt to gain generic information 

about high-speed silicon growth. 

30 
EFG* RIBBON 

DIE 

CAPILLARY 

ACTION 

MOLTEN 

SILICON 

‘EDGE-DEFINED FILM-FED GROWTH 

10-cm-WIDE RIBBON GROWER 3-to 10-cm WIDE RIBBON GROWERS 

/ 

Figure 18. Edge-Defined Film-Fed Ribbon Growth by Mobil Solar Energy Corp. 

WEB DENDRITIC 

SHEET GROWTH 

A WEB 

DENDRITE 

MOLTEN 

SILICON 

WEB RIBBON AND CELL 

,, MOVABLE DISPLACER 

SECOND GENERATION 

WEB GROWTH FURNACE 

Figure 19. Dendritic- Web Ribbon Growth by Westinghouse 

‘ FUSED-SILICA r“ TROUGH 

LIOUID-SOLID INTERFACE 

IUNDERSIDEI 

Y I CERAMIC SUBSTRATE 

YRAPHITE ROLLERS 

Figure 20. Silicon-on-Ceramic by Honeywell 

31 
Table 9. Edge-Defined Film-Fed Ribbon Growth by Mobil Solar Energy Corp. 

GOALS: ST AT U S : R&D NEEDS: I 0 SIMULTANEOUS GROWTH OF* 40 cm2/min (10 cm WIDE' 0 IMPROVED SHEET QUALITY 

I 

0 

0 

0 

0 

4 R!BBONS, 10 cm WlDE EACH, 

AT 4 cmlmin FOR 8 hr 

WITH AUTOMATIC CONTROLS 

MULTIPLE RIBBON GROWTH* 

STATION 

0.20 mm RIBBON THICKNESS 

12% ENCAPSULATED SOLAR 

CELL EFFICIENCY 

80% YIELD 

x 4.0 cmlmin) (4 hr DEMO 

OF ONE RIBBON) 

DE M 0 N STRATED ' 

(5 RIBBONS AT 5 cm WIDE) 

(3 RIBBONS AT 10 cm WIDE) 

0.15-mm RIBBON THICKNESS 

>12% SOLAR CELL* * 

EF F I C I EN CY 

INCREASED THROUGHPUT 

GROWTH SPEED 

1986 SHEET ADD-ON PRICE 

(1 980 $/WD) 

FSAPRICE 

0 SAMICS* 

ALLOCATION 0.205 

PROJECTION 0.194 

~ ~~ 

'MOBIL DEVELOPING COMPETITIVE NONAGON EFG TECHNOLOGY 

*ENCAPSULATED CELL EFFICIENCY N.A. 

Table 10. Dendritic-Web Ribbon Growth by Westinghouse 

GOALS STATUS: R&D NEEDS: (LATE 1986): 

3O-cmZlmin AREA 13-cmZ/min AREA GROWTH RATE INCREASED THROUGHPUT 

GROWTH AND STABILITY GROWTH RATE FOR 50 METER-LONG 

CRYSTAL AND CONSTANT MELT LEVEL 

FOR SHORT RIBBON LENGTHS AND 

GROWTH RATE 

1986 SHEET ADD-ON PRICE 

(1980 WWp) 

FSA PRICE 

17.5% SOLAR CELL EFFICIENCY 16.9% CELL EFFICIENCY 

PROCESS AUTOMATION AUTOMATION BEING INCORPORATED 

8-hr DEMONSTRATION RIBBON GROWTH ALLOCATION 0.292 

PROJ ECTlON 0.160 

SAMICS FOR 8-34 hr AT CONSTANT 

MELT LEVEL 

~~ 

Table 1 1. Silicon-on-Ceramic by Honeywell 

~~ ~~ 

GOALS: STATUS: ADDITIONAL R&D REQUIRED: 

<$6/m2 SUBSTRATE COST 

350-cm2/min AREA GROWTH <$6/m2 PROJECTED INCREASED THROUGHPUT 

RATE (2 SHEETS) 6O-cmVmin AREA OPTIMIZED CELL DESIGN 

0.10-mm FILM THICKNESS 

MULLITE SUBSTRATE IDENTIFIED IMPROVED SHEET QUALITY 

1986 SHEET ADD-ON PRICE 

(1980 $/Wp) 

GROWTH RATE (1 SHEET) 

<O.lO-mm FILM THICKNESS e 11 yo ENCAPSULATED SOLAR 

FSA PRICE 

SAMICS 

CELL EFFICIENCY 10.5% SOLAR CELL* 

95% YIELD EFFICIENCY ON DIP-COATED SOC ALLOCATION 0.190 

PROJECTION N.A. 

"f ~~ 

*NO LONGER UNDER DEVELOPMENT 

**ENCAPSULATED CELL EFFICIENCY N.A. 

32 
Table 12. Technology Status at Completion of Contract 

MATERIAL SOLAR CELL EFFICIENCY 

SHEET DIMENSIONS THROUGHPUT UTILI3ATION %AM1 

TECHNOLOGY (cm) (kg/hr)tor (m2/hr) m /kg ENCAPSULATED I BARE CELL 

ADD-ON 

PRICE 

$Mlp(19805) 

Adv. Wafering.' 

'HIGHER EFFICIENCY INDIVIDUAL CELLS HAVE BEEN FABRICATED 

"DATA GIVEN FOR ID WAFERING. OTHER WAFERING APPROACHES WILL ALTER THE FIGURES 

"'ADDED VALUE FOR CASTING AND WAFERING 

*"'CRYSTALLIZATION RATE kglhr 

Table 13. Technology Status of Active Contracts 

'HIGHER EFFICIENCY INDIVIDUAL CELLS HAVE BEEN FABRICATED 

"NONAGON RIBBON CONFIGURATION FUNDED BY MOBIL SOLAR HAS HIGHER VALUES. 

PULL 

DIRECTION 

HEAT FLOW MODIFIER 

CRUCIBLE 

Figure 21. Low-Angle Silicon Sheet Ribbon Growth 

by Energy Materials Corp. 

Of all of the sheet technologies, dendritic web is 

viewed today as having the greatest potential for 

high-quality sheet with the best economical value. 

However, close temperature control of the silicon 

liquid-solid interface regions in the grower are 

required. By 1981, a number of web-growth 

requirements to meet Project goals had been 

Table 14. Low-Angle Silicon Sheet Growth by 

Energy Materials Corp. 

STATUS (BEST INDIVIDUAL ACHIEVEMENT) 

RIBBON WIDTH : 15 cm 

RIBBON THICKNESS: 25 mils 

PULL SPEED: 

THROUGH PUT: 450 cm2/min 

1 RIBBON AT 85 crnlmin 

EFFICIENCY: 12.9% 

demonstrated individually. But until a few months 

ago, progress had been very slow in the efforts to 

achieve all growth requirements simultaneously and 

to grow web continuously for hours while meeting the 

requirements. To obtain a fresh look at this growth 

technique, a special JPL Web Team was formed and 

has recently made interesting advancements 

(Table 15). During the last few months, Westinghouse 

has also made good progress (Table 16). The major 

question still remains: How economically can highquality web be grown under production conditions? 

33 
Table 75. JPL Web Team 

1983 1984 

42 

5.8 

9,000 

15.9 16.2 

Objective 

The objective of the JPL Web Team is to conduct an independent R&D activity that will increase the 

likelihood that dendritic web technology will achieve the DOE goals. 

Approach 

Team effort consists of a combination of analytical and experimental work. 

Operate a modified Westinghouse web growth system. 

Measure high temperature physical properties of silicon. 

Use improved stress web model to determine a web temperature profile resulting in higher area 

growth rates of web with satisfactory quality. 

Use thermal model of web to determine a growth system thermal configuration that will yield the 

desired web temperature profile. 

Use thermal analysis of web-growth interface and the susceptor-crucible melt to determine a 

growth system thermal configuration that will improve stability of growth process. 

Design, fabricate, and test the new thermal configurations and feed results back into the models. 

Accomplishments 

High temperature properties of silicon have been measured. 

A stress model of growing web has been developed and used. 

A thermal model of growth system configuration to control web temperature has been developed. 

A thermal model of susceptor, crucible, and melt has been developed. 

Westinghouse growth system has been installed with an improved data acquisition system. 

Numerous web ribbons have been grown. 

Mid 

1985 

6.7 

27,000 

16.9 

Table 16. Dendritic- Web Growth Progress by Westinghouse (Westinghouse data) 

1980 1981 

27 

7 

4 

4.4 

3.2 

15.5 

Development 

Area Growth Rate, cm2/min 

1) Transient (Lengths Of Several Centimeters) 

2) Quasi-Steady-State (Lengths Of 30 To 100 cm) 

3) Steady-State (Meters Of Length, Hours Of Growth) 

Maximum Undeformed Width, Centimeters 

W15OUndeformed Width At 150pm Thickness 

(An Inverse Measure Of Buckling Stress) 

1982 

13 

8 

5.5 

4.9 

Maximum Area Throughput For Single Furnace in 

Maximum Demonstrated Solar Cell Efficiency, 

5-Day Week (cm2/week) 

1977 1978 

2.3 8 

2.4 3.5 

2.0 

13 14 

1979 

23 

4.0 

2.7 

15 

34 
All three ingot technologies (shown in Figures 12, 

13, and 14) are in commercial use today for 

terrestrial solar cells. The majority of the wafers for 

terrestrial solar cells are still sliced from Cz singlecrystal ingots. In 1975, the solar cells fabricated 

specifically for terrestrial use were made from the 

circular wafers sliced from Cz cylindrical crystals. 

Today, Cz crystals generally are slabbed lengthwise, 

converting cylinders into polygons that are then sliced 

into polygon wafers, thereby enabling the solar cells 

to be more efficiently “packed” into modules. Now, 

much larger Cz crystals are grown and multiple 

crystals can be grown from the same crucible by 

using melt-replenishment. Formerly, a new crucible 

was required for each crystal grown. 

Today, ID sawing remains the predominant 

commercial, and most economical, slicing 

technology. More economical sawing remains the 

major impediment to continued long-term use of ingot 

technologies for PV. Studies of the basic 

mechanisms involved in the removal of silicon, 

including the influence on cutting rates and surface 

damage of various fluids, are promising. These 

results, as yet, have not been applied in an attempt 

to develop a new saw or to improve existing saws. 

The best-quality silicon sheet today is obtained by 

FZ crystal growth; the next best is by Cz crystal 

growth. Very high-efficiency solar cells have been 

made in the laboratory from FZ wafers. However, FZ 

technology is not used commercially for PV because 

of its high cost. FZ technology development has 

never been part of the FSA Project. Excellent 

progress in reducing the cost of growing Cz crystals 

was made during the late 1970% primarily by FSAsponsored efforts. During the early 1980% continuing 

progress in improved Cz crystal quality has been 

made for and by the integrated-circuit industry to 

meet its needs for more compact and sophisticated 

devices. The achievement of even higher-quality Cz 

crystals by MCz (Cz crystals grown in a magnetic 

field), improved understanding of growth phenomena, 

and better equipment is continuing for more stringent 

future in teg rated-circu i t industry needs. These 

activities fit in well with the PV need for higherefficiency cells. However, the major obstacle to 

future use of Cz or FZ wafers for PV is the cost of 

wafering. The added cost of the wafering step is 

incidental to the cost of manufacturing integratedcircuit devices, but it looms as the single most costly 

item if Cz or FZ technology is to be used for future 

solar cells (one celllwafer) and modules. A very low 

key, but promising effort on understanding the basic 

mechanism of silicon fracturing during sawing has 

been continuing. With the uncertainties in the 

economical value of ribbon growth, it would be 

prudent to increase efforts leading to more 

economical slicing and an evaluation of the technical 

and economical potential of advanced Cz and 

advanced FZ crystal growth for higher-efficiency, 

low-cost PV cells and modules. 

35 

Progress Since 1975 

Ingot Technology 

Cz Ingots 

Ingot diameters increased from 3 to 6 in. 

Throughput rates were increased threefold. 

Ingot quality was improved 

Sheet costs were decreased to one-seventh. 

Cast Ingots 

Two concepts have proven feasible: HEM 

and Semix (semicrystalline). 

The same two techniques have advanced to 

commercial products. 

Wafering 

Developed and demonstrated increased ingot 

slicing capabilities. 

Reduced production costs 

Ribbon Technology 

Thirteen ribbon and non-ingot technologies were 

investigated. 

Two ribbon technologies continue to be 

supported: 

Dendritic web, by Westinghouse Electric 

Corp. 

EFG, by Mobil Solar Corp. (now funded only 

as generic ribbon growth). 

One ribbon technology (EFG) is now 

commercially available. One ribbon 

technology (web) is scheduled to be 

demonstrated by late 1986. 

Significant improvements in understanding 

high-speed crystalline silicon growth 

phenomena are being obtained. 

Low-angle silicon sheet, by Energy Materials 

Corp. (contract ended June 1985). 

Recent Achievements 

0 Thin, flat, essentially stress-free dendritic-web 

ribbon width has been increased to 6.7 cm 

(Westing ho u se). 
Using a new grower with dynamic controls, 

dendritic-web linear growth rates have been 

increased to 3.0 cmlmin for 150-pm-thick ribbons 

under transient growth conditions (Westinghouse). 

Dendritic-web transient area growth rate for short 

ribbon lengths was increased from 27 to 42 

cmelmin (Westinghouse: see web growth 

progress, tab!^ 16). 

Continuous web ribbon growth with melt 

replenishment was increased to 7-m length at 

7 cm2lmn using technology developed for FSA 

(Westing house). 

Integration of the theoretical studies of heat flow 

and interface stability by Massachusetts Institute 

of Technology (MIT), and plastic flow near the 

interface region by Harvard University, were integrated with residual stress data by Mobil Solar. 

Microhardness tests of silicon in various fluid 

environments has revealed that the surface 

hardness of silicon varies apparently as a function 

of the fluid's dielectric constant. Tests were 

conducted with toluene, acetone, ethanol, 

methanol, glycerol, and deionized water 

(University of Illinois at Chicago). 

A research forum and other meetings on highspeed growth and characterization of crystals for 

solar cells have resulted in a definition of the 

areas requiring research. 

A comprehensive plan for increasing ribbon 

quality and growth capability has been devised 

and is being partially implemented. 

Research and Development Needs 

Silicon sheet material R&D is needed for ribbon or 

ingot in the cost-to-produce range of $90lm2, suitable 

for production-type processing into high-performance 

17 to 20% efficiency solar cells. 

Requirements 

Diffusion length equal to or greater than 

300 pm. 

A 60- to 100-cmls bulk recombination 

velocity. 

Research Areas 

Basics of physics and chemistry of silicc 

materials at room temperature and near the 

melting point: 

Stresslstrain effects on ribbon and bulk 

silicon during growth. 

Effects of impurities on stresslstrain in silicon 

growth. 

Fracture mechanics and surface chemistry 

effects on: 

Device efficiency. 

Cell and module performance lifetime. 

Fabrication and processing rates and 

yields. 

Ribbon Technology 

A general thermal stress model of ribbon 

growth integrating creep behavior, growth 

speed, and ribbon width should be completed 

and applied to all ribbon growth processes. 

Correlation of observed buckling phenomena 

and stress model. 

Data on basic silicon material properties (i.e., 

creep behavior and stress moduli) at 

temperature range of interest (800 to 1400 "C). 

Correlation of structure (i.e., dislocations and 

grain boundaries) effects with generated stress. 

Impurity effects on stress generation 

Verification of stress model with experimental 

growth systems in industry. 

Construction of or modification of a growth 

system that integrates all information generated 

to produce optimum silicon ribbons at high 

speed. 

Ingot Technology 

Improved understanding of the effects of 

impurities on the growth of uniformly highquality single crystal ingots. 

Improved understanding of defect formation 

during single-crystal ingot growth. 

Verification of the feasibility of growth 

techniques that improve quality of ingots. 

Improved understanding of slicing phenomena 

that will lead to higher quality, more economical 

wafers. 

36 
Current Research and Plans 

Stress and strain relationships in wide, thin 

ribbons arown at hiah SDeedS: 

Modify present ribbon growth processes to use 

the data obtained on the above. 

Design, fabricate, and test low-stress ribbon 

" "I growth systems 

Develop methods of measuring thermal fields 

in arowina ribbons. Impurity effects and interface stability during ~~ 

ribbon growth: u- u 

Develop methods of measuring residual stress 

in ribbons. 

Develop methods of observing ribbon 

deformation in real time. 

Measure the hiqh-temperature mechanical 

Model heat flow and interface stability criteria 

for ribbon growth. 

Test and verify model. 

properties of silicon. ' 

Develop models for plastic deformation and 

elastic buckling in growing ribbons. a 

Verify models by relating the observed thermal 

fields to observed ribbon stress and strain. 

Use verified models to define ribbon thermal 

fields and growth dynamics that will yield lowstress growth environments. 

* 

Sheet quality effects on solar cell performance: 

Characterize ribbon material grown under 

various conditions. 

Fabricate baseline cells on ribbons grown 

under various growth-process conditions. 

Relate cell performance to ribbon growth 

process variables and specific defects. 

Optimize growth processes that compensate 

for specific ribbon defects. 
High-Eff iciency 

Solar Cells 

During the past 10 years, as FSA efforts 

continued, it has been perceived that the original 

Project goal of 1O0/o module efficiency (from 6% in 

1975) would be met as a result of the planned FSA 

efforts. It was anticipated that this would occur as 

solar cell efficiencies improved and as module 

packing factors increased, i.e., more active solar cell 

area per unit of module area. Logic indicated that 

cell efficiency should increase as low-cost cell 

design and fabrication technology matured, and as 

mass production and quality assurance were introduced. Module packing factors of 0.90 were thought 

to be achievable from the then 0.54 value. (1 .O equals 

total coverage.) These presumptions have proven to 

be accurate because, today, production modules fabricated using single-crystalline silicon cells have 

efficiencies as high as about 11 %. 

Economic analyses now indicate that module 

efficiency must be about 15% for photovoltaics to be 

economically competitive for central-station 

applications, which is now expected to be the first 

widespread use of photovoltaics in the United States. 

The achievement of 15% efficiency modules requires 

the repeatable fabrication of large-area laboratory 

solar cells with 20% or greater efficiency, 

demonstrating that high-efficiency solar cell 

principles are understood. This knowledge then must 

be converted into low-cost production cell technology 

that will yield uniform quality production cells of 17 to 

18% efficiency and, subsequently, module 

efficiencies of 15 O/O. 

The objective is to perform research that identifies 

and resolves key generic limitations to increased 

efficiency of crystalline silicon solar cells as required 

for large power generation applications. 

The goal is to establish device technology required 

for repeatable fabrication of large-area laboratory 

crystalline silicon solar cells of greater than 20% 

efficiency . 

Background 

Years ago, the need for high-power output from 

spacecraft solar cells motivated a sustained research 

for higher-efficiency cells. With the realization that 

photovoltaics had the potential to become a practical 

terrestrial power source, the research broadened. 

Consequently, the performance of silicon solar cells 

has been gradually increasing as a result of these 

systematic efforts to gain an understanding of how 

solar cells function, how to reduce cell losses by 

better cell designs, and by improved processing. A 

large number of relatively small advances, over many 

years, have added to our knowledge of solar cell 

technology and to increased cell efficiency. Early 

progress in cell designs and process techniques 

were largely empirical, which has been strengthened 

by better understanding of the underlying principles 

of operation. These activities have led to a general 

agreement regarding the factors that limit cell 

performance and to those that can be changed to 

further increase the performance of crystalline silicon 

solar cells. 

These efforts by many researchers and 

technologists over many years have gradually 

increased efficiencies from about 6% when silicon 

solar cells were devised in 1954 to the following 

efficiencies at present: 

Production cells made from Cz ingots for 

terrestrial use have efficiencies up to 14 O/O. 

Research cells made from Cz ingots have 

reached 17 to 18%. 

Research cells made from float-zone silicon 

have reached 19% efficiency. 

about 30%. 

Theoretical limit of crystalline silicon cells is 

Probably maximum achievable limit is about 

25 '/o . 

Project Activities: 1982 to Present 

The general approach being used to increase solar 

cell efficiency to greater than 20% has been 

established. The required changes in certain cell 

parameters to achieve this high efficiency are 

believed to have been identified. However, the exact 

cell parametric changes and the cell design 

modifications required so that large-area 20% 

laboratory cells can consistently be fabricated have 

not been devised. To assist in better understanding 

how a cell functions and how various parts of a cell 

influence its performance, the following figures and 

description are included. 

The main features of a crystalline silicon solar cell 

are shown in Figure 22. The cell is made from a 

semiconductor material doped (minute amounts of a 

specific element have been introduced under 

carefully controlled conditions) in a manner so as to 

make the semiconductor sheet photosensitive. The 

typical silicon cell made from p-type silicon (doped 

with boron) sheet material is modified by the diffusion 

or implantation of a phosphorus dopant to an 

extremely thin layer (now n type) on the surface that 

will become the top of the cell (emitter). The 

semiconductors must be doped correctly, so that the 

n-type material has large electron densities and the 

p-type material has large hole densities. (A hole is a 

vacancy in a silicon atom's outer electron ring after 

an electron has been freed.) After the n/p junction is 

formed, a basic requirement for PV energy 

conversion has been met by the creation of an 

38 
electrical field, which separates electrons and holes 

within the cell. Near the junction, most of the 

photogenerated holes from the n side transfer to the 

p side; similarly, most of the photogenerated electrons 

transfer to the n side. The nlp junction is permanent 

and the electrical field is stable. When light shines on 

a cell, the light that is not reflected is absorbed by 

the semiconductors. When a photon enters the cell 

and is of sufficient energy (appropriate wavelength) 

and strikes one of the four outer (valence) electrons of 

a silicon atom, that electron is dislodged. This leaves 

a silicon bond missing an electron, called a hole, and 

frees an electron to move about in the cell. Electrical current flow in a solar cell, when it does occur, is 

due to both the motion of electrons and to effective 

motion of the holes. When sunlight is absorbed in 

the cell and frees electrons and holes from their 

bonds, the electrical field separates the charges and 

establishes an electrical potential, or voltage. If the 

cell top and bottom contacts are connected to an 

external circuit, an electrical current (amperage) will 

flow as long as light continues to be absorbed by 

the cell. 

Sunlight 

1111 .. Front 

Junction 

piype ‘Back n-Type Semiconductor Contact Semiconductor 

Figure 22. Crystalline Silicon Solar Cell 

For over a decade, there have been significant 

improvements in crystalline silicon solar cell 

performance, mainly as a result of increased cell 

short-circuit current output. (Figure 23 is an I-V curve 

showing the current and voltage of a typical highefficiency solar cell fabricated at JPL.) The 

improvements nave resulted by use of shallow 

junctions, back surface fields, back surface 

reflectors, textured surfaces, and multilayer 

antireflection coatings. Shallow junctions help to 

reduce the effects of cell front surface losses. Backsurface fields reduce the effects of back-surface 

losses. Back-surface reflectors increase photon 

absorption. Textured surfaces and multilayer 

antireflection coatings reduce light reflection losses 

at the front surface. Figure 24 shows a solar cell with 

key factors that influence cell efficiency. The potential 

for further improvements in a cell’s short-circuit current are limited. However, single-junction crystalline 

silicon cell performance can be increased by increasing the cell open circuit voltage. This is the area 

of endeavor now being emphasized by FSA highefficiency silicon solar cell research. 

180 MAX POWER 

POINT 

Voc = 640.9 mvolts = 143.5mamps k-- = 539.8mvolts 

Im = 134.2mam~s 

a 

a 

0 

Pm = 72.4 mwatts 

FF = 0.787 

% EFF = 18.10 

CELL AREA 4 sa. CM 

INTENSITY 100 MWISQ. CM 

TEMP 25 C 

VOLTS 

Figure 23. Current-Voltage Curve of a High-Efficiency 

Solar Cell 

N+IPIP+ SOLAR CELL 

(THICKNESS, DEFECTS, OB. IMPURI. 

TIES, RESISTIVITY, CARRIER LIFE. 

DOPANT, DOPING METHODS, DOPING 

CONCENTRATION. DOPING PROFILE 

(GRADIENTS VS UNIFORM). LOCALIZED 

EFFECTS AT GBS. DEFECTS 

REFRACTIVE INDEX, LATTICE STRAIN, 

SURFACE PASSIVATION (SIOx. Ta205 

SIN,) 

BUSS BAR 

AR COATING 

EMITTER 

JUNCTION 

P+ BACK SURFACE FIELD 

BACK METAL CONTACT 

TION. SURFACE PREPARATION, 

SHARP VS GRADUAL (Ai. B) (EVAPORATED. SCREEN PRINTED). 

CONTACT AND GRID RESISTANCE, 

AREA COVERAGE, METALS (TbPd. 

A*. A?, Ni. Ap. Cu) EVAPORATION. 

SCREEN PRINTING SINTERING 

Figure 24. Key Factors that Influence Cell Efficiency 

The primary thrust of the FSA high-efficiency 

efforts has been and continues to be directed 

toward reducing solar cell bulk and surface losses, 

with specific focus on reducing the minority carrier 

recombination rates throughout the cell. Three 

methods being used are: increased minority carrier 

lifetime, reduced volume of silicon for bulk 

recombination, and reduced surface recombination. 

By obtaining silicon sheet with properties so that the 

39 
recombination rates in the bulk material do not 

dominate cell performance, the emphasis can then 

be on reducing recombination in the thin emitter 

region and at the cell surface. Future cells will be 

multilayer structures with each layer thickness 

designed according to the electrical properties of 

that layer and the other layers. 

Within FSA, the efforts being implemented are 

directed toward gaining a better understanding of the 

physics of carrier losses and their influence on cell 

design by the use of comprehensive modeling, by the 

development and use of new and improved 

measurement techniques, and by new and improved 

cell fabrication techniques. The surface 

recombination velocity of a solar cell (a measure of 

surface losses) is generally greater than lo5 cm/s 

An effort is under way to reduce the surface 

recombination velocity, to less than lo3 cmls byaddressing critical problems of controlling 

recombination losses at surfaces and interfaces (for 

example, the investigation of transparent conducting 

materials). To better understand and control surfaceinterface phenomena, surface passivants (such as 

SiN,, polycrystalline silicon, and ultra-thin oxide 

layers) are under development. Also, reliable 

measurement techniques to characterize passivants, 

surfaces, and interfaces are evolving. As part of 

surface loss investigations, work on transparent 

conducting materials is emphasizing study of the 

interaction of a polymer with a silicon surface and 

the effects on optical and electrical characteristics. 

Studies to understand and control bulk loss 

mechanisms by improvements in silicon sheet 

structural, electrical, and chemical properties are 

leading to improved minority carrier diffusion lengths. 

The complicated and varied nature of sheet is a 

function of its growth conditions and its processing, 

including solar cell fabrication. 

Characterization of bulk material and device 

performance evaluation are using or will use a variety 

of measurement techniques for chemical, structural, 

and electrical characterization. These include optical 

electron and ion beam analyses, X-ray characterization, light and dark current-voltage relationships, 

spreading resistance, diffusion lengths by surface 

photovoltage, scanning electron microscopy, deeplevel transient spectroscopy, secondary ion microprobe spectroscopy, and Zeeman spectroscopy. 

Measurement techniques are being developed that 

are critical to gaining a basic understanding of loss 

mechanisms, to correlating model parameters with 

measured ones, and to optimizing cell performance. 

For example, the short-circuit currentdecay (SCCD) 

method (Figure 25) has been developed at the University of Florida. It simultaneously measures the 

carrier lifetime in the bulk of the cell and the recombination velocity at the back surface interface. With 

this method, a forward bias is applied to the solar 

cell to set up a steady-state condition. Then, by 

rapidly applying a zero bias across an extremely small 

resistance, a short circuit is obtained. This causes 

the p/n junction region and the base and emitter 

regions to discharge. If the discharge times related 

to the junction and emitter regions are much smaller 

than the time related to the base region, the lifetime 

and recombination velocity can be determined. 

R1 

PULSE 

GENERATOR 

-1 I 

IT1 - TC1 2d 

5 

SOLAR 

AMPLIFIER 

R2 

Figure 25. SCCD Cell Loss Mechanism Measurement 

Method 

One of the main advantages of this technique is 

that the discharge time of the p/n junction (on the 

order of 10-11 s) is much less than the discharge 

times associated with either the emitter or the base. 

This makes it very easy to interpret the observed 

transient current. The SCCD method is simple, fast, 

and less expensive than methods used.to measure 

other key solar cell parameters. It can be routinely 

used during cell fabrication steps. It is being 

evaluated as a possible method for determining 

carrier lifetimes and recombination velocities in the 

thin emitter region. 

A highly sensitive microwave reflectance 

measuring system with computer data acquisition 

has been designed and built at JPL. The experimental 

results illustrate that the microwave technique can 

probe accurately the decay of photogenerated 

minority carriers in silicon sheet materials that 

depend on the minority carrier lifetime and surface 

recombination velocity. Under controlled surface 

conditions, one can measure bulk minority carrier 

lifetimes in silicon sheets, such as silicon wafers 

grown by Czochralski and floating-zone techniques 

and dendritic web ribbons. With known lifetime in the 

bulk material, the technique has been demonstrated 

to be an effective tool for monitoring the surface 

recombination velocity. Preliminary results reveal that 

the silicon surface polished by Syton has a small 

surface recombination velocity when it is freshly 

etched in a diluted hydrofluoric solution, but the 

recombination velocity increases drastically when the 

surface is exposed to the air. The effect becomes 

saturated after several hours. Currently, JPL 

scientists are working on an adequate analysis 

method to extract quantitative results from 

experimental data. The microwave technique is 

reliable, nondestructive, effective, and easy to use. It 

40 
can be used on as-grown and processed samples 

without any contact. 

Electron beam-induced current technique to 

measure minority carrier lifetime in n + and surface 

recombination velocity at n + front surface is being 

investigated. 

More complete analytical solar cell modeling and 

analysis capabilities development has led to the 

establishment of a program [Solar-Cell Efficiency 

Estimation Methodology and Analysis (SEEMA)] that 

uses computer models to evaluate and design 

advanced silicon cells. After an extensive search, 

FSA located two models to use in SEEMA. One of 

these, Solar-Cell Analysis Program in One Dimension 

(SCAP1 D), was designed by Purdue University. It is a 

general purpose program used to analyze and design 

silicon solar cells. The other SUPREM-II, was 

designed by Stanford University. It is used to calculate information about the emitter doping profile which 

is then used as input to the SCAPl D model. 

Researchers in FSA validated the utility of SCAP1 D 

by evaluating the metal-insulator-n/p junction (MINP) 

structure of the high-efficiency silicon cell reported 

by Martin Green of Australia. Using experimental 

data provided in the literature as input, SCAP1 D’s 

evaluation closely agreed with Green’s experimental 

results. Researchers also used SCAP1 D to perform a 

sensitivity analysis on the structure by changing 

various design parameters one at a time, while 

holding the other parameters constant. Their analyses 

indicated that a cell with the MINP structure could 

obtain 20% efficiency by increasing the bulk minority 

carrier lifetime from its present 20 pm to 250 pm. 

This would make the diffusion length much greater 

than the cell thickness of 280 mm (Figure 26). Under 

these conditions, using a back surface field would 

further improve the cell’s efficiency. 

I I I 

Lifetime ( ps) 1 

l6 t 1 

15 1111 I I I 

20 50 75 100 200 300 

Cell Thickness ( pm) 

Figure 26. Sensitivity of Efficiency to Solar Cell 

Thickness for Various Minority-Carrier Lifetime Values 

For future use, JPL is establishing improved 

versions of the two models. Purdue’s upgraded 

model, SCAP2D, performs two-dimensional analysis. 

This will enable researchers to analyze complicated 

structural designs that are difficult to reduce to 

one-dimensional equivalents. Stanford’s new 

SUPREM-Ill also has additional features for 

calculating electrical properties after each process 

step. 

Recent Accom pl ishmen t s 

New techniques to simultaneously measure bulk 

minority carrier lifetime and back surface 

recombination velocity developed, including SCCD 

and microwave techniques. 

Key barriers to obtain high-efficiency cells 

identified by use of comprehensive numerical 

analysis techniques. 

Use of new passivants for front and back cell 

surfaces showing promising results. 

High-efficiency solar cells modeled. 

Theoretical analysis of a new cell design, called 

“Floating Emitter Solar Cell Cell Transistor,” 

indicates that the cell has the potential to achieve 

greater than 20% efficiency. 

Research Needed 

Establish relationships between residue defects 

and recombination mechanisms in highest quality 

available silicon sheet. 

Investigate recombination mechanisms in low-cost 

silicon sheet in bulk, in all layers, and at all 

surfaces. 

Develop technique for measurement of front 

surface recombination velocity. 

Establish correlations between material defects, 

device fabrication steps, and device performance. 

Improve modeling capabilities. 

Develop new higher-efficiency device concepts. 

Continue characterization of silicon sheets and 

devices as their quality improves. 

Continue device modeling and experimental 

verification of high-efficiency device structures. 

Investigate cell efficiency improvements for new 

advanced cell processes. 

41 
Process Research and Development 

In 1975, the processes for fabrication of solar 

cells, for use on Earth, were either those used for 

making space solar-cells or their derivatives. These 

techniques, borrowed from diode production 

technology, were very labor intensive and material 

intensive with respect to the total product price. Acid 

etching and cleaning steps were extensively used 

along with photolithographic grid-line definition. 

The performance of individual solar cells was not 

consistent. Fabricated in batches, even the 

performances of those cells for use in space varied 

significantly. The best of these small rectangular 

“space cells” were assembled on panels, as closely 

packed as possible, to produce the maximum power 

per unit area. The lower performance cells were not 

used in spacecraft power systems because of highperformance requirements. Instead, these reject 

space cells were used for terrestrial applications. By 

that time, solar cells made specifically for terrestrial 

use had become competitive with reject space cells. 

These circular cells were made from one complete 

Cz wafer. The cell processing was similar, but 

simpler and less expensive. For example, semiconductor industry-reject wafers were used. The 

result was that terrestrial solar-cell performance was 

less than space-cell performance. The challenge in 

1975 was to reduce the cost of,terrestrial solar cells, 

but still retain good quality and performance. 

The objectives are to develop cell fabrication and 

module assembly process technologies required to 

meet cost and performance requirements for 

terrestrial PV power production. 

The goals were to: 

Identify and develop low-cost cell and module 

production processes. 

Design facilities and equipment to perform the 

processes. 

Demonstrate fabrication of low-cost PV cells 

and modules in a pilot line environment. 

Implement the transfer of mass production 

technology to industry. 

The goals now are to develop processes to: 

Fabricate high-efficiency cells from available 

silicon sheet substrates. 

Assemble cells into higher-efficiency longer-life 

modules . 

The key technical issues are to develop processes 

so that: 

Silicon sheet substrate quality is not degraded. 

A variety of high-efficiency cell designs can be 

fabricated. 

Background 

Crystalline silicon solar cells were invented at Bell 

Laboratories in the mid-1950s. These solar cells or 

solar batteries, as they were called, are still the most 

commonly used solar cells. A solar cell is fabricated 

by a series of processes, from a silicon sheet doped 

with either an n or p type impurity. The other 

type of impurity (p or n) is then added to an 

extremely thin (about 0.3 pm) layer on the surface 

that will be the top of the cell. This creates the n/p 

junction (or p/n) where the photovoltaic effect can 

occur in the solar cell. The addition of metallic 

electric current collectors, on the top and on the 

bottom of the wafer, permits electricity that is 

generated within the cell to flow from the cell. The 

top current collector consists of uniformly spaced 

fine lines that minimize the amount of sunlight that is 

blocked from entering and from being absorbed by 

the solar cell. Cell fabrication, although it seems 

simple, is sophisticated with improvements in 

process and equipment continuing even after 

30 years of progress. Completed solar cells are 

electrically interconnected and then encased to form 

a module that supports and protects the cells. 

Project Activities: 1975 to 1981 

Within FSA, the research and development 

required for solar cell fabrication and module 

assembly were conducted by a group familiar with 

the processes and equipment used to produce solar 

cells and semiconductor devices. A plan consisting 

of five phases, which was compatible with the other 

FSA activities, was devised and implemented. The 

plan consisted of: 

I. Technology Assessment: Determine availability 

and applicability of cell and module processes. 

II. Process Development: Develop applicable old 

and required new processes. 

Ill. Facility and Equipment Design: Develop 

automated equipment and facility designs. 

IV. Experimental Plant Construction: Incorporate 

previous efforts into a pilot line to define yields 

and remaining key cost factors. 

Conversion to Mass Production: Transfer 

technology to industry. 

V. 

42 
The Technology Assessment contracts awarded to 

Motorola, RCA and Texas Instruments in early 1976 

assessed the state of the art of PV process 

technologies in terms of cost, large volume 

production potential, ability to be automated, 

reliability, and process synergistics. Identification of 

those processes with the most potential for costeffective implementation and recommendations for 

further study of those processes were also 

performed. More than 50 processes covering all 

areas of cell fabrication and module assembly were 

studied. Processes evolved that clearly distinguished 

terrestrial cells from space cells. As an example, 

copper metallization was identified as being more 

cost effective for top and bottom electrical 

conductors than either solder or silver. Metallization 

methods were expanded from space-oriented 

vacuum evaporation to include electroless plating, 

electrolytic plating, thick film metal (printing), and 

reflow solder. Wafer surface treatments were 

expanded to include brushing, plasma etching, 

texture etching, and new cleaning solutions. Junction 

formation, in addition to the standard space cell 

process (gaseous diffusion), was expanded to include 

spin-on and spray-on of liquid dopants, solid dopant 

diffusion, doped oxides, ion implantation, and 

advanced ion implantation. Other good candidate cell 

processes that were added included silicon nitride 

(CVD), oxide growths, mechanical edge grinding, and 

laser scribing. Assembly methods included the use 

of: glass substrates, glass superstrates, and 

conductive adhesives; new encapsulation materials 

such as silicones, PVB, EVA; and new cell 

interconnection methods. Selected processes were 

studied regarding their “sensitivity to variables” such 

as purity or condition of input materials and supplies 

and processing parameters, primarily time and 

temperature. Efforts at JPL were concentrated on 

process verification and synergisms along with cell 

metallization analysis programs. 

The problem facing the Project rather early in its 

inception was how to compare the potential 

production costs of competing processes and 

process sequences being investigated by a number 

of different researchers. This was clearly brought out 

during the Technology Assessment phase in which 

cost comparisons between the different contractors 

on the same processes were significantly different. 

This pinpointed a need for a standard methodology 

that allowed relative comparisons of the potential 

production costs attributable to competing 

processes, and an estimate of the actual cost to 

mass produce using a specific process. 

FSA PA&I personnel are: 

Three key costing analyses methods developed by 

Interim Price Estimation Guidelines (IPEG), a 

costing system simple enough to be run on a 

hand calculator. This method uses standard 

coefficients for costing inputs such as 

equipment, direct labor, material, floor space, 

and utilities. 

43 

Solar Array Manufacturing Industry Simulation 

(SAMIS), a factory simulation which runs on a 

main frame computer. This document requires 

the filling out of detailed costing sheets called 

Format As which are then fed into a computer 

for accurate and consistent process costs. 

Solar Array Manufacturing Industry Cost 

Standards (SAMICS), a catalog of material 

costs, labor costs, and other input costs which 

assisted the contractors in inputting standard 

costs. 

These three techniques were verified and 

upgraded continually throughout the life of the 

Project and used extensively. 

At the conclusion of the assessment effort, it was 

determined that cost effective processes should: 

Consume a minimum of material and supplies. 

Use low-cost, recyclable supplies. 

Have high production yields. 

Result in high-efficiency modules. 

Before the assessment effort, automation and 

capital equipment were thought to be major cost 

factors. It was shown that when labor is a significant 

cost factor in a process, the process should be 

automated. Most semiconductor processes were, to 

a significant extent, already automated and, 

generally, capital costs for equipment could be 

distributed over large production runs resulting in low 

costs per unit. 

Requests for Process Development proposals 

were distributed to the entire PV industry and were 

broadly scoped to include new and novel processes 

and those processes identified in the Technology 

Assessment Phase as requiring further development. 

Also, unsolicited proposals from PV contractors were 

encouraged, reviewed, and acted upon. Twenty PV 

industry contractors and two universities participated 

in these studies, which reflected the thinking of the 

PV industry at that time. A partial review of some of 

the contractors and their diversified activities during 

this phase are: 

Applied Solar Energy Corp.: Low-cost copperplated contacts; ion-planted cell evaluation. 

Lockheed Missiles and Space Division: 

Sprayed-on fluorocarbon encapsulation. 

Mobil Solar Energy: Module assembly. 

Motorola: Antireflective (AR) coatings; 

palladiumlnickellsoldermetallization. 

Sensor Technology (later called Photowatt 

International): Nickellcopper metallization. 
RCA: Expitaxial cell processing. 

Solarex: Wafer thickness studies; solar breeder 

studies; high efficiency module; nickellsolder 

metallization. 

Sollos, Inc.: Nickel metallization. 

* Spectrolab: Thick-film metallization. 

Spire Corp.: Ion implantation; pulsed electron 

beam annealing. 

University of Pennsylvania: Process analysis 

and evaluation. 

Westinghouse: Cell processing using dendritic 

web. 

All contracts during this phase required process 

cost evaluations to be made using the costing 

methodology and the writing of process 

specifications. The costing documentation included 

all of the direct inputs for equipment cost and 

performance; required labor, floor space, utilities, 

maintenance, supplies and input materials for each 

process. The process specifications included detailed 

descriptions of equipment, chemicals and materials 

used, and process parameters including time, 

temperature, pressure, etc. 

what other processes were used before and after 

that process. Accordingly, an additional effort, the 

development of process sequences, was initiated. 

Discrete process steps were combined to form 

process sequences that were synergistically sound 

and cost effective, such as shown in Figure 27. 

Contracts were awarded to develop and demonstrate 

viable, cost-effective sequences. It became clear that 

there was no one single process sequence that was 

most cost effective, but rather a number of process 

sequences could be demonstrated to be viable and 

cost effective. Photowatt (then called Sensor 

Technology) demonstrated a process sequence with 

Selection of an individual process depended upon 

ENCE 1977 

LI_ 

Crll l.B 

spray-on dopant junction formation and printed metal 

mask followed by plating; Spectrolab demonstrated a 

spin-on dopant junction formation and a thick-film 

metallization system; Motorola demonstrated ion 

implantation junction forming, a CVD silicon nitride 

AR coating and plasma etching; Solarex 

demonstrated cost-effective processing of 

polycrystalline material; Westinghouse indicated that 

conventional semiconductor processes such as 

gaseous diffusion and vacuum metallization could be 

made cost effective with large volume production 

equipment. 

Collectively, the process sequences showed the 

results of the cross-fertilization of technology that 

FSA emphasized. Copper metallization was used in 

two of the sequences. RCA cleaning technology was 

used extensively, as was laser scribing and thick film 

systems. The sequences illustrated the large number 

of possible low-cost process combinations that could 

be used to make a cell. The process sequences 

demonstrated that there was a new breed of solar 

cells, terrestrial solar cells, fabricated distinctly 

different than the space cell from which it evolved. 

Most of the process R&D efforts have been 

conducted by private industry and universities 

through subcontracts with FSA which, to date, have 

included over 60 contracts. A parallel effort was the 

creation of a Process Research Laboratory at JPL in 

which the contractor process work could be verified 

and, in many cases, directly supported. Independent 

research has also been conducted in the laboratory 

by JPL personnel. 

A major tool used to transfer the evolving process 

technology to the entire PV industry and to obtain 

cross fertilization has been the distribution of process 

specifications for verification. Near the end of the 

Process Development Phase, more than 140 process 

specifications covering cell and module fabrication 

had been documented and distributed to interested 

PV industry members. A feedback request resulted in 

industry verification of more than 40 of these 

processes. The results of these efforts are shown in 

TYPICAL 12 80IWATT SEQUENCE- TYPICAL SO 70iWATT SEQUENCE. 

CELL rra 

Figure 2 7. Module Process Sequences 

44 

~ ~~_______ 
Figure 28. The JPL Process Research Laboratory was 

also very much involved in the process verification 

effort by providing a place for supplementary 

experimentation and verification. 

An important function of FSA is to promote the 

advancement of the photovoltaics industry by the transfer 

of technology 

TRANSFER 

REQUEST PROCESS 

SPECIFICATION 

FOLLOW 

UP 

TRANS F E R 

COOROINATION ECBACK 

I PIU I STATUS REPOKT 

Former Technology Transfer 

Procedures circa 1981 

#&7 - 

.VERIFY SElECTED 

-ASSESS SENSITIVITY 

*VERIFY ECONOMIC 

PROCESSES 

TO PROCESS VARIABLES 

DATA FROM 

CONTRACTOR 

Processing Technology 

Transfer Status 

1981 Under 

Available Evaluation Confirmed 

Surface Preparation 19 7 5 

Junction Formation 18 4 5 

Metallization 13 6 3 

Module Assembly 16 5 0 

Totals 66 22 13 

- - - 

Technology transfer continues but in a less 

formal manner 

RECENT TYPICAL TECHNOLOGY TRANSFER 

MELT REPLENISHMENT EQUIPMENT 

Westinghouse Electric Corp. is using equipment 

developed by Kayex Corp. and modified at Westinghouse 

to replenish silicon in a dendritic-web growth process. 

Boeing Eng'g & Construction used the JPL Process 

Research Task laboratory's equipment and processes 

to laminate CulnSe2 cells. 

THERMALLY INSULATED TOOLING FOR LAMINATORS 

Sunelco Corp., Tideland Signal Corp., Solarex Inc. and 

Boeing Eng'g & Construction have shown great interest in 

thermal insulation techniques used in the JPL Process 

Research Task's laminator (capable of producing 4 x 4-11 

modules) and are adopting them. 

Tideland has set up a production operation in Australia 

based on processes transferred from Spectrolab, Inc., 

developed under contract with JPL. 

CuInSep MODULE LAMINATION 

TERRESTRIAL MODULE PRODUCTION LINE 

UNSUPPORTED ("CREDIT-CARD") MODULE LAMINATION 

The first unsupported (no superstrate or substrate) 

module was fabricated by Spectrolab using the JPL 

Process Research Task's 4 x 4-ft laminator. 

An engineer-interactive computer program for solar-cell 

metallization design has been transferred to and is 

available from Computer Software Management and 

Information Center (COSMIC). 

Kulicke and Soffa, Inc., has built an automated machine 

for Westinghouse based on a concept developed under 

contract with JPL. 

Silver and aluminum thick-film inks are available from 

Electrink, Inc., based on formulations developed under 

JPL contracts. 

CELCAL PROGRAM AVAILABLE FROM COSMIC 

AUTOMATED ROLLING.SPOT BONDER CELL STRINGING 

THICK-FILM INK AVAILABILITY 

Figure 28. Technology Transfer to Industry 
In late 1979, a bill sponsored by Senator Tsongas 

of Massachusetts provided additional funds for nearterm reduction of solar energy production costs by 

soliciting ideas from industry. The following contracts 

were implemented: 

Organization Activities 

ARCO Solar Automated cell interconnection 

Kulicke & Soffa Automated cell interconnection 

Motorola Wax patterning. 

Thin cell processing 

Photowatt Polysilicon cell processing; 

surface texturing 

Sollos Thick-film metallization 

RCA Megasonic cleaning 

The important assembly equipment that evolved 

from the near-term effort were the automated cell 

interconnect machines by ARCO Solar and Kulicke & 

Soffa and a vacuum laminating system by ARCO 

Solar. First-generation machines are shown in 

Figure 29. 

The Facilities and Equipment Design Phase was 

never heavily stressed because most of this 

technology was already being developed for non-PV 

semiconductor devices. In addition to the special 

near-term funding, the development of other 

process equipment was funded. Important process 

equipment developments were ion implantation 

equipment for large area junction formation and a 

pulsed electron beam wafer annealing unit by Spire 

Corp., and a robotic cell interconnection and 

module assembly system by Tracor MB Associates. 

The robotic equipment was especially meaningful 

because it demonstrated that cells could be handled 

robotically in large-volume quantities without 

breakage. These machines demonstrated the 

potential for h ig h-vol ume product ion. 

1ST INTERCONNECT SOLDER 

,2ND INTERCONNECT SOLDER CELL ORIENT b FLUX APPLICAT!ON 

Figure 29. First-Generation Module Assembly 

46 
In late 1980, two Module Experimental Process 

System Development Unit (MEPSDU) contracts 

were awarded to provide the first controlled pilotline data on cell and module processing. They were 

to reflect the culmination of the technology gained 

from the earlier phases. Prior to this time, the 

contractors produced cells and modules in a 

laboratory environment and then extrapolated the 

data for input into the SAMIS factory simulation 

program. In the MEPSDU contracts, each of the two 

contractors was required to construct and operate 

a pilot line. The contractors were required to record 

the production rate, yield, and process parameters. 

Three technical demonstration runs were required, 

two of which allowed no major adjustments, 

modifications, or repairs during and between runs. 

Solarex, Inc. and Westinghouse were competitively 

selected for the MEPSDU contracts. The two 

contractors chose diametrically opposite 

approaches as shown in Figure 30. The Solarex 

process sequence would produce lower efficiency 

cells using low-cost processes and polycrystalline 

silicon substrates. The Westinghouse process 

sequence would result in higherefficiency cells 

using more expensive processes. The Solarex 

process sequence used semicrystalline silicon 

wafers, whereas the Westinghouse process 

sequence used dendritic web silicon. Ultrasonic 

bonding of cell interconnections to the solar cells 

was to be used by Westinghouse and soldered 

connections by Solarex. Vacuum lamination was the 

proposed module assembly technique for both 

processes. 

However, within 6 months the MEPSDU contracts 

were reduced in scope because of Project budget 

reductions. Within a year and a half, the MEPSDU 

efforts were cancelled entirely and the contractors 

were redirected toward research-oriented efforts in 

accordance with new Project guidelines. 

Project Activities: 1981 to Present 

The first three phases were completed, or almost 

completed, as planned with significant reductions in 

cell and module fabrication costs. Project 

redirection starting in 1981 resulted in cancellation 

of the MEPSDU and Mass Production Phases. 

Good progress had been made toward meeting 

the individual parts of the original Project goals. 

However, an end-to-end integrated sequence was 

never accomplished. Efforts were then directed 

toward basic process research and, later, toward 

higher-efficiency cell process research. With new 

emphasis on higher efficiency and longer life, 

development of more advanced and sophisticated 

technology was initiated. Also, an examination of 

the needs of the PV industry, at that time, and the 

results of the earlier efforts led to the conclusion 

that metallization and overall process yields were 

two key items in which efforts should be continued. 

APPROACH: DESIGN AND DEVELOP EXPERIMENTAL 

PROCESSING EQUIPMENT WHICH WILL HANDLE BOTH II FIRSTAND SECOND GENERATION MATERIAL. 

OBJECTIVE: VERIFY PROCESS AND EQUIPMENT 

CONCEPTS AND TRANSFER TECHNOLOGY TO INDUSTRY 

STARTING SHEET MATERIAL 

P- Each of the two 

different starting sheet SPRAY ON 

forms follow a different NaOH ETCH DOPANT DIFFUSION - HEAT ALUMINUM PRINT SINTER 

ANTI REFLECTION IA RI path in this processing 

and equipment 

experiment 

DENDRITIC WEB RIBBON GASEOUS GASEOUS 

PLASMA CLEAN DIFFUSION DIFFUSION ETCH GLASS DIP COAT 

,~ - TT ,i';OTO RESISTIPRfp~ 

P- PJUNCTION ISOLATION A-R TI-Pd-Cu 

-:- p-jg ri 1111 !I..:! !'I! 

P- PPR-METAL LASER SCRIBE COPPER PLATE REJECTION METALLIZEIVACUUMI EXPOSE-DEVELOP ETCH DIP COAT 

I1 ! 1 A FINISHED MODULES 

CELL STRINGING (SOLDER REFLOWI ASSEMBLY 

I 

Figure 30. Module Experimental Process System Development Unit 

47 
The Westinghouse MEPSDU effort was redirected 

to investigate liquid-dopant processing for junction 

formation, which ultimately replaced the gaseous 

diffusion process on the Westinghouse pilot line. The 

Solarex MEPSDU effort was redirected toward 

investigating process mechanisms affecting lifetime 

and performance of cells made from polycrystalline 

material. A second generation of automated cell 

interconnection equipment evolved from the 

Westinghouse MEPSDU effort. The initial effort was 

cost-shared with JPL and, after MEPSDU 

cancellation, was continued to completion by 

Westinghouse. Kulicke and Soffa, under contract, 

developed, designed, and fabricated the ultrasonic 

bonding machine, which is capable of 

interconnecting one cell every 3.5 seconds or a 180 

cell module every 10.5 minutes. The machine is 

shown in Figures 31 and 32. Figure 33 shows interconnected dendritic web cells. 

Figure 3 1. Second-Generation Cell Stringing Machine 

MOLECULAR BONO 

Figure 32. Rolling Spot Ultrasonic Bonding 

Figure 33. Ultrasonic Interconnect Bonding of 

Dendritic Web Solar Cells 

Metallization was determined to be the major 

remaining higher-cost factor for cell processing. 

Figure 34 shows the results of cost analysis efforts 

on metallization systems. A number of innovative 

metallization ideas were generated as a result of the 

Cell Metallization Research Forum held in 1983. The 

objectives of the Forum were to clarify and define 

the state of the art of current solar cell metallization 

systems to: report on performance experience with 

these systems, including advancements made and 

problems encountered; describe advanced 

processing techniques under development; and to 

discuss expectations for future improvements. 

Process control became a major area of concern 

as industry yields were not as high nor as predictable 

as had been expected. Industry proprietary concerns 

and lack of large-scale engineering pilot lines limited 

the availability of data. To overcome the lack of 

information and to explore processing for higherefficiency cells, process and device modeling 

capabilities were investigated. For example, SUPREM 

(developed for the semiconductor industry by 

Stanford University) provides a comprehensive 

computer process simulation program that allows the 

prediction of device structures from any fabrication 

technology sequence. An example of a device model 

is SPCOLAY, by the University of Pennsylvania, a 

one-dimensional PV cell model which has been 

particularly useful in defining the interactions 

between cell substrate material parameters and 

preferred cell processes. The soft boundary shown in 

Figure 35 defines the "Domain of Practical Significance" for those characteristics which are prevalent 

in present state-of-the-art materials. 

48 
Candidate ProcesseslSystems 

PROCESSISVSTEM cOSTlm2 DATA SOURCE 

Evaporation IEVl 

SOA ITilPdlAgl 

Advanced ITilNi .+ Cu plating') 

Screen print IPRINTI 

SOA IAg pastel 

1990 (Ag paste') 

SOA 111 pastel 

1990 [AI paste') 

1990 (MolSn'l 

Electroless plating 

SOA (Print resist. Wi.plate. Sinter. 

Advanced IPR. Wi plate. Sinter. Cu 

Wave solder) 

plating') 

Midfilm' IAgl lMlDl 

Midfilm' IMolSnl 

Ion plating' ITilNilCul IlONl 

$90 

$14 

$8 to $35 

$7 to $34 

$2.4 

$1.4 

$2.0 

$8 

$9.4 

$5 to $16 

$2 

$6 

ASEC 

Westinghouse 

2.801W. Block IV" 

JPL BPU" 

2.801W. Bloek IV 

JPL BPU 

JPL BPU. SollLos 

Solarer. Motorola 

Motorola 

Spectrolab" 

Spectrolab. Solllm 

Illinois Tool Works 

'Advancement of Photovoltaic stateof.the.art 

"Ag price from $10 lo $5010~ 

Current Results 

bOVANCfO 

0951 

PRINT RESIST 

ELECTROLESS 

NilSOLOER 0 

0 80 1A I I I I 

95 90 v 30 20 10 0 

FRONT METALLIZATION COST. $lm2 

PURPOSE OF THIS ANALYSIS: 

Compare costs and effectiveness of stateof-the-art (SOA) metallization and projected 

metallization approaches 

Estimate the potential impact of RBD in this 

area 

APPROACH: 

Use data from FSA contractors as the major 

source lor calculating cell metallization 

costs with IPEGP 

Use JPL's grid optimization model to 

establish electrical performance ratios 

based on geometrical and electrical data 

(ratio to a cell without resistive or shadow 

losses) 

Develop metallization cost vs performance 

trade-off slopes tor several PV systems; the 

cell metallization system farthest above a 

tradeofl slope has the greatest value 

Study as yet has not included process 

compatibility, reliability, or unconventional 

metallization systems 

I Figure 3 4. Metallization Cost Comparison 

BULK DIFFUSION LENGTH, prn 

Figure 35. Low- and High-Bulk Resistivity Cell 

Behavior; Effects of Bulk Resistivity, Diffusion; 

Length, and Back Surface Field 

The term "directed energy research" was coined 

to describe the increased emphasis on highefficiency processes, which covers the use of 

microwave, laser, and thermal pulse energy sources. 

All of these new energy sources are of technical 

interest because they allow many cell processes to 

be performed without heating the bulk of the 

substrate. PV cell fabrication by "cold" processing is 

used to avoid degradation of bulk substrate 

properties, particularly minority carrier lifetime. 

Results from directed energy research have been 

very gratifying. Westinghouse demonstrated that 

thermal pulse (heat lamps) can be used to 

simultaneously form n and p + junctions using liquid 

dopants. Previous processing required more 

handling, cleaning, and the use of expensive oxide 

masks. Excimer lasers can be scaled up and have 

been used by ARC0 Solar and Spire for annealing 

ion-implanted junctions. Superwave Technology 

developed a microwave-powered plasma system 

used for thin-filmed silicon nitride deposition as well 

as a passivating silicon dioxide layer. Argon lasers 

were used by Westinghouse in a successful new 

metallization method in which lasers scan wafers 

covered with metal, bonding the metal to the solar 

cell where the "laser writing" has occurred, 

eliminating the costly photolithography process 

(Figure 36). 

49 
COOAg 

I 

SCANNING MIRRORS- t- BEAM 

=&> LENS 

ii 

DEPOSITED 

METAL FILM 

\ 

SILICON 

/ SUBSTRATE 

SAMPLE BASE TEMPERATURE 75OC 

METALLIZATION PATTERNS ARE FORMED BY DIRECT WRITING 

FOCUSED LASER SPOT DECOMPOSES SPUN-ON FILM 

Figure 36. Laser Pyrolysis of Spun-On MetalloOrganic Film 

One of the new metallization research efforts 

funded was the development of metallo-organic 

decomposition (MOD) films by Purdue University. The 

MOD process uses liquid compounds containing 

metal atoms bonded to organic compounds such as 

neodecanoic acid. Figures 37 and 38 illustrate the 

wide variety of possible compounds that can be 

synthesized for research or for use in production. 

Decomposition of these compounds by the 

application of heat between 250 "C-300 "C results in 

the formation of fugitive gases such as carbon 

dioxide and deposition of a thin layer of metal. Many 

different metals have been successfully tried such as 

titanium, chromium, silver, bismuth, and palladium. 

Combinations of metals have also been successful 

with especially good results obtained from a bismuthsilver formulation. Choice of organic acid and various 

viscosity adjusters allows application of MOD inks by 

spinning, dipping, spraying, screen printing, and ink 

jet printing. Multiple layers have also been successfully deposited, and lines as narrow as 3 pm have 

been achieved. 

METAL 

ION 10. S. N. P. As) 

+ 

COMMON SOLVENT 

+ 

1 RHEOLOGY ADJUSTORS 1 

II 

PRINT, SPRAY, DIP, ETC 

+ 

II 

ICONDUCTOR/ 

R3 

R1 + R2 + R3 = C8H19 

SUBSTITUENTS 1 % ABUNDANCE 

R1 = CH3 I 

R2 = CH3 31 

R3 = C6H13 

R1 = CH3 

R2 >CH3 67 

R3 < CH6H13 

R1 >CH3 -

R2 >CH3 2 

R3 < C5H11 

Figure 38. Alternative Formations of Metallo-Organic 

Deposition Precursor Chemicals 

Today's Status 

Low-cost processes have been developed for cell 

and module fabrication steps such as: 

Surface Preparation 

Hot alkaline damage etch 

Spray, dip, roller, spin, and meniscus coating 

of antireflective films. 

Junction Formation 

Gaseous diffusion of 6-in.-diameter cells 

Liquid-dopant junctions 

Liquiddopant back surface fields. 

Aluminum-paste back surface fields 

Metallization 

Silver thick-film ink systems. 

Copper cell metallization systems. 

Pioneering of molybdenum-tin inks. 

Module Assembly 

Ultrasonic cell interconnection 

Automated cell stringing. 

EVA encapsulation processes. 

Double-chamber vacuum lamination 

Figure 3 7. Metallo-Organic Deposition Processing 

50 

The above processes seem to have the potential 

to meet the original Project goals. 
A complete processing sequence using all the 

steps, the individual yields, and the process parametric sensitivities was not completed. 

The need for higher-efficiency and longer-life 

modules, but manufactured at essentially the same 

costs has imposed the need for new, more advanced 

technology and processes. 

Accom pl is hmen t s 

The following is a list of accomplishments with 

the associated contractor(s) credited. 

Processes 

Surface Preparation 

A cost-effective etching processing for saw 

damage removal prior to junction formation 

(Sensor Technology, Lockheed). 

0 Anistropic etching process to produce large 

area textured surfaces for enhanced photon 

collection (Motorola, Sensor Technology, 

Solar Technology). 

Developed a number of antireflection 

coatings with concurrent processes, 

including multi-layer evaporation, spraying, 

dipping and spinning, and the required 

material development (RCA, Westinghouse). 

Polycrystalline silicon processing (Solarex) 

0 Microwave enhanced CVD (Superwave). 

Surface passivation of front emitter areas 

using silicon dioxide, silicon nitride, and 

silicon oxynitride (Motorola, Westinghouse, 

Spire). 

Megasonic cleaning (RCA). 

Plasma pattern etching (Motorola). 

Junction Formation 

Large area, large volume, gaseous diffusion 

processing using POCI,, and PH, and BCI, 

(Sensor Technology, Westinghouse). 

Simultaneous front and back junction 

formation using liquid dopants and infrared 

(IR) rapid heating pulse techniques 

(Westinghouse). 

Ion implantation of both front and back 

junction at a rate of 1200 waferslhour (Spire) 

Non-mass analyzed ion implantation of both 

back and front junctions (Motorola, Spire). 

Laser annealing, electron pulse beam 

annealing, and rapid thermal pulse annealing 

of ion implanted junctions as well as 

conventional thermal annealing to maintain 

lifetime of the initial silicon material (Spire, 

ARCO Solar, Lockheed). 

Spin-on, spray-on, and meniscus coating of 

liquid dopants (Spectrolab, Sensor 

Technology, Westinghouse). 

Laser scribe (Sensor Technology). 

Me tal I izat ion 

Thick film screenable cost-effective 

metallization processes using Ag, AI, AgAl, 

Cu, and MOD AgBi (Spectrolab, Bernd Ross 

Assoc., RCA, Purdue, Electrink, Sollos, Solec). 

Metallo-Organic decomposition films (MOD 

films) using low-temperature processes 

(e 300 "C) (Purdue, Electrink, Westinghouse). 

Reliable plating systems using Pd and Ni 

followed by solder build-up by immersion 

(Motorola, Solarex). 

Pyrolytic decomposition of MOD films using a 

laser as the thermal source (Westinghouse). 

Development of a generic fabrication 

process for producing MOD film (Purdue, 

Electrink). 

Diffusion barriers (Caltech) 

Module Assembly 

Ultrasonic bonding of interconnects 

(Westinghouse, Kulicke & Soffa). 

Automated soldering of interconnects (ARCO, 

Kulicke & Soffa, Foothill Engineering). 

Laminated encapsulation in a partial vacuum. 

PVB and attendant processes for 4 x 4 ft 

module (ARCO Solar, RCA). 

EVA and attendant processes (RCA). 

PV shingle assembly (General Electric). 

Silane primers, antioxidants, and UV screening agents (Springborn Laboratories) 

Electrostatic/ultrasonic edge sealing (Spire) 

(see Figure 39). 

51 
ALUMINUM FOIL 

BACK COVER 

CELL 

GLASS SUPERSTRITE aL"YIN"t4 TO 

GLASS SEAL e" E58 

ANTICIPATED BENEFITS APPLICATIONS 

0 REDUCEDH20VAPOR 0 MODULES IN SEVERE ENVIRONMENTS 

- COASTAL AND MARINE SITES 

- HOT AND HUMID REGIONS 

TRANSMISSION 

0 REDUCED 02 TRANSMISSION 

0 REDUCED AIRBORNE 0 REMOTE LOCATIONS 

POLLUTANT TRANSMISSION - REDUCED MAINTENANCE AND REPLACEMENT COSTS 0 UV AND TEMPERATURE STABLE SEAL 0 MODULES WITH SENSITIVE 

COMPONENTS 

0 EXTENDED LIFETIME FOR STANDARD MODULES 

Figure 39. Hermetic Edge Sealing 

Equipment 

Surface Preparation 

Megasonic cleaning equipment (RCA). 

Microwave enhanced CVD System 

(Supe wave). 

Junction Formation 

Ion implanter with 4-mA capability for 

phosphorous and throughput of 400 waferslh 

(Spire). 

Non-mass analyzed ion implant equipment 

(JPL, Motorola, Spire). 

Pulsed electron beam annealing system (Spire). 

Laser annealing system (Spire, ARCO Solar, 

Lock h eed). 

Meniscus coating machine for dopant 

additions to dendritic web material 

(Westinghouse). 

Metallization 

Ink jet printer-computer controlled (Purdue). 

Argon ion laser decomposition system 

(Westinghouse). 

. Dry film/mid film (Spectrolab, Applied Solar 

Energy Corp.). 

Module Assembly 

Anti-reflectance treatment of glass 

(Motorola). 

Automated cell stringing machine (ARCO 

Solar, Kulicke & Soffa). 

Ultrasonic automated cell stringing for 

dendritic web (Westinghouse, Kulicke & 

Soffa). 

A laminator capable of producing large area 

moddes (JPL, ARCO Sdar, Spire, ASEC). 

Robotic module assembly (Tracor 

MB Associates). 

Analyses 

SPCOLAY (University of Pennsylvania). PV cell 

modelling program for investigation of process 

variable sensitivities. 

Zero depth concentrator effect (JPL, Science 

Applications, J. Mark, General Electric). 

Mathematical model of the effect of light 

trapping by non-specular reflection in thick 

films. 

Grid line optimization (JPL, Texas Instruments, 

Motorola, RCA, Spectrolab). Mathematical 

models and computer optimization programs to 

allow design of minimum power loss collector 

grids. 

$2.00/watt (JPL). "Strawman" process analyses 

for low cost cell and module fabrication. 

$0.50/watt (JPL). "Strawman" process analyses 

using projected, automated processes. 

Diffusion barriers (Caltech). Development of the 

scientific basis for formation of diffusion 

barriers, especially those of amorphous films. 

Solar breeder (Solarex). An analysis of a 

photovoltaics production facility powered by 

photovoltaics . 

Wafer thickness (Motorola, Solarex). An analysis 

of wafer thickness cost tradeoffs. 

Polycrystalline cell processing '(Solarex, Sensor 

Technology). Development of techniques for 

processing polycrystalline silicon material. 

Solar Area Production System Handbook 

(Theodore Barry Associates). A compilation 

of costing variables related to large volume 

production of solar cells. 

Tandem Junction cell (Texas Instruments). An 

analysis of a novel photovoltaic device structure 

to produce electrical power. 

52 
Modules and Arrays 

In 1975, crystalline-silicon solar cells for use on 

spacecraft had already been manufactured for about 

15 years. There was significant progress made 

during these years in understanding how solar cells 

functioned and how to improve their performance. 

However, there was little information about how to 

design, fabricate, and use PV modules and arrays for 

terrestrial applications. There was an immediate 

need for module and array knowledge and 

experience. A complete technology had to be 

established including environmental and design 

specifications, engineering concepts and design 

knowledge, encapsulation materials and processing, 

module assembly techniques, quality assurance 

techniques, module performance and durability 

evaluation techniques and facilities, and failure 

analysis techniques and capabilities. An intensive 

effort to meet this need was initiated with the start of 

the FSA Project in 1975. 

The objectives of the module and array activities 

for both crystalline silicon and thin-film technologies 

are to: 

Develop design requirements and specifications 

compatible with various operational and safety 

environments for a variety of applications. 

Develop the engineering sciences technology 

required to integrate lowcost, efficient cell 

technologies into cost effective and safe 

modules and arrays that meet the operational 

requirements of future large-scale applications. 

Develop the technologies required to achieve 

and to verify reliable 30-year life flat-plate PV 

modules. 

Verify adaptability of technology to meet 

required operational conditions. 

Transfer the above technology to industry. 

The goals, as established in 1983, are to develop 

flat-plate (non-concentrating) module and array 

technology as follows: 

Modules with crystalline silicon solar cells. 

High efficiency, 15%. 

Low cost, $90/m2 (1982 dollars). 

Thirty-year reliable operation. 

Modules with thin-film solar cells. 

Efficiency, 12% 

Low cost, $70/m2 (1982 dollars). 

Thirty-year reliable operation. 

In 1975, the goals established were to develop flatplate module and array technology that industry 

could use as early as 1986 to achieve: 

Module prices less than $0.50 per peak watt 

FOB (1 975 dollars), marketing and distribution 

costs not included. 

Production rates sufficient to attain economies 

of scale. 

Operating lifetimes in excess of 20 years. 

Module efficiency greater than 10%. 

53 
Background 

Early in the 197Os, small modules were made 

using reject space cells sealed into heavy cast-glass 

modules that were durable, but expensive (Figure 40). 

However, typical mid-1970s terrestrial modules, 

designed for more economical manufacturing, had a 

lifetime ranging from 6 months tc 2 or 3 years (Figure 41). The solar cells for these modules were made 

using circular wafers directly sliced from a Cz ingot 

and processed by essentially space-cell technology. 

The electrically interconnected cells were usually 

placed on a layer of silicone rubber covering a fiberglass panel, covered with a second layer of poured 

silicone rubber and cured. The entire: process wasivery 

labor intensive and subject to product variations. The 

module designs and processing were often by trial 

and error. The products had high failure rates, caused 

by both immature designs and poor processing. 

During the past 10 years, the FSA-industry 

partnership has significantly advanced the basic 

technology for terrestrial module and array design 

and fabrication, using crystalline-silicon solar cells. 

This was accomplished by concurrent experimental 

and analytical research effor?s supported by iterative 

fabrication and testing of developmental and 

commercial modules, as described in Module and 

Array Evolution, Engineering Sciences and Design, 

Encapsulation, Reliability Physics, and Module 

Performance and Failure Analysis. The evaluation of 

modules using thin-film solar cells is just beginning, 

as is the adoption of existing knowledge and the 

development of unique new techniques for thin-film 

modules. 

Figure 40. Durable Modules Made from Space Solar 

Cells in Early 19 70s. 

54 
Figure 4 1. Mid- 19 70s Terrestrial Photovoltaic Modules 

Project Activities: 1975 to 1981 

Initially, in 1975, contemporary modules were 

purchased and evaluated; concurrently, 

developmental work was initiated on module 

specifications, designs, materials, and processing. 

These studies evaluated both the technical and 

economical feasibilities of various technical 

concepts. During the next few years, simultaneous 

assessment of the needs, capabilities, deficiencies, 

and potential solutions across the entire spectrum of 

module technologies was accomplished by an 

excellent cooperative effort by industry, universities, 

and Government. This effort resulted in a continuing 

iterative evolution of new knowledge and capabilities. 

Practical, realistic environmental protection 

requirements and approaches were developed along 

with techniques for evaluating module capabilities for 

meeting these specifications. Practical module 

design knowledge grew as encapsulation materials 

and processing methods were developed and 

evaluated, and the compatibility of the modules with 

PV power systems was established. As the 

technology evolved, it was incorporated by industry 

into durable hardware. This was accomplished by the 

Project sponsoring cell and module technology 

55 
developments, the purchase of advanced modules, 

the evaluation of these modules, and the analysis of 

module problems and failures (Figure 42). A flow of 

information from operational PV systems, as shown in 

Figure 43, helps the technology development activities shown in Figure 42. Five sequential purchases 

(“block buys” of modules designed and fabricated by 

industry to JPL specifications) have resulted in an 

effective cooperative effort bv the FSA Proiect and 

(large at the time) for the first three block buys were 

intended to increase industry production capacity as 

well as provide modules for evaluation and field testing. Block IV and V modules were purchased in 

limited quantities in the 1980s, for test and evaluation 

only. These Project efforts were instrumental in 

developing practical flat-plate module design and 

manufacturing technology and in stimulating industry 

to incomorate much of it into its commercial products. 

industry. The large quantities of modules purchased 

RELATED - 

RESEARCH 

TECHNOLOGIES 

t t 1. 

DESIGN SYNTHESIS MODULE EVALUATION L 

- PROTOTYPE MODULE 

QUALIFICATION AN0 

ACCELERATED TESTING 

ARRAY DESIGN EXPERIMENTS, 

ANALYSES, - 

REQUIREMENTS AN0 COST 

MODULE AND 

IN-FIELD EVALUATION 

EFFECTIVE SOLUTIONS PROOUCTION MODULES L A AN0 ARRAYS r 1 LONG-TERM 

OPERATIONAL 

DURABILITY MODELING CONDITIONS FA’LURE 

QUANTIFICATION OF 

DEGRADATION 

MECHANISM 

PARAMETERS 

- LIFE PREDICTION 

AN0 ACCELERATION 

TEST CAPABILITIES 

- 

30-YEAR 

TECHNOLOGY 

I 11 I 

The design requirements were periodically updated based upon new knowledge of application needs, environmental 

conditions, realism of evaluation and testing techniques and hardware performance. 

Figure 42. Module/Array Technology Development 

INFORMATION FLOW 

SYSTEM NEEDSlSENSlTlVlTlES 

ARRAY FIELD EXPERIENCE 

TECHNOLOGY CAPABILITY 

RAW WEATHER DATA 

ENGINEERING ANALYSIS 

ENVIRONMENTAL ASSESSMENT 

EXPLORATORY TESTING 

DESIGN OPTIMIZATION 

ENVIRONMENTAL STRESS LEVELS 

PERFORMANCE CRITERIA 

PERFORMANCE DATA 

TEST METHODS 

~~ 

Figure 43. Information and Flow 
Project Activities: 1981 to Present 

Since 1981, the evolution of module technology 

and the adoption by industry has continued with 

emphasis now on high-efficiency, cost-effective 

modules that meet safety and reliability requirements 

of future large-scale applications such as for utilities. 

Efforts on rooftop and residential applications are 

fewer now that it is anticipated that utility 

applications will be the first large-scale use of 

photovoltaics in the United States. 

A second parallel effort, just starting, is 

establishing the practicality of modules made using 

thin-film solar cells. This requires going through many 

of the above sequences using as much of the 

existing knowledge and capabilities as possible; 

however, many unique new techniques and new 

knowledge will have to be developed and evaluated. 

As an example, new accurate, repeatable electrical 

performance measurement techniques for thin-film 

cells and modules are being developed and verified 

so that degradation can be accurately quantified and 

performance measurements standardized. With the 

expanding availability of first-generation thin-film 

modules, performance evaluation and assessment of 

reliability characteristics are under way. 

Today’s Status 

Crystalline Silicon Modules 

Estimated life of contemporary production 

modules is more than 10 years. 

Module efficiency is as high as +1 1% in 

standard production modules. 

Encapsulation materials have been life-tested to 

20 years. 

Comprehensive terrestrial module design technology and knowledge exist, which have been 

gained from 10 years of analytical, design, 

fabrication, and test activities in active 

cooperation with industry. 

Modules of 0.9 MW sold for just under $5 per 

peak watt (1983 SMUD purchase). 

Estimated module price would be about $1.47 

per peak watt if a multi-tens of megawatts 

production plant were built with the latest 

existing technology (see PA&I section of report) 

Thin-Film Modules 

Prototype modules (other than consumer products) 

are only beginning to become available for 

evaluation: 

Reliability is unknown. 

Efficiency is about 3 to 5% 

Prices are about the same as for crystallinesilicon modules. 

Thin-film module differences requiring new or 

expanded research: 

New cell environmental durability 

(temperaturelhumiditylultraviolet) failure modes. 

Altered hot-spot heating failure mechanisms 

Short-circuit cell failure modes and effect on 

size and serieslparallel redundancy. 

New cell electrical-interconnect failure modes 

Altered glass breaking strength. 

Flexible substrate technology demands 

High-cell stresses caused by glass bending 

Nonlinear electrical response and effect on 

measurement. 

Cell-to-cell electrical variability and effect of 

electrical mismatch on circuit design. 

I 

I 

57 
Module and 

Array Evolution 

The PV terrestrial modules have evolved during the 

past 10 years so that they are now practical, reliable, 

and cost effective for many applications. There have 

been major improvements in design, fabrication, 

performance, and reliability. These improvements have 

resulted in large reductions in module prices and lifecycle PV power generation costs. An important factor 

contributing to this evolution has been the series of five 

module procurements known as "block buys." The 

module design and development were performed by 

industry to FSA specifications; the module performance 

R 

35 Watts' 

I il 

5 Watts" 21 Watts. 

sfm SOLAREX 

TEEWtOGY 

SOLAR 

WWER 

evaluations were performed by FSA. These activities 

served three functions: (1) they encouraged industry to 

incorporate the latest technology developed in the PV 

program, (2) the various modules were evaluated by 

uniform repeatable tests that enabled identification of 

design and/or fabrication deficiencies, and (3) they 

enabled a cooperative manufacturer/evaluator quick 

response to module problems that occurred. 

A module from each of the five block buys is 

shown in Figure 44. Table 17 lists the representative 

characteristics of each block of modules. The photograph, the list of characteristics, and the five trend 

charts (Figures 45 through 49) portray the progress of 

flat-plate PV modules with crystalline silicon solar 

cells during the past 10 years. 

IV 

58 Watts' 

Sl'lHt 

V 

82 Wetts' 

Figure 44. Examples of Block I Through V Modules 

GENERAL 

ELECTRIC 

58 
Table 17. Representative Characteristics of Block Modules 

9.1 

1i.e 

AREA lm21 

WEIGHT (kpl 

SUPERSTRATE OR TOP COVER 

SUBSTRATE OR BOTTOM COVER 

FRAME 

CONNECTIONS 

ENCAPSULATION SYSTEM 

ENCAPSULATION MATERIAL 

CELLS 

OUANTITV 

SIZE lmrnl 

CONFIGURATION 

MATERIAL 

JUNCTION 

PARALLEL CELL STRINGS 

INTERCONNECT REOUNOANCV 

BV-PASS OIOOES 

PACKING FACTOR 

NOCT' 

FAULT TOLERANCE 

PERFORMANCE AT 28oc CELL TEMP! 

POWER, MAX. IW 

MOOULE EFFICIENCY IWI 

ENCAPSULATED CELL EFFICIENCV lq 

10.6 

12.3 

I II I 0.1 

2 

SILICONE RUBBER 

RlGlO PAN 

NO 

TERMINALS 

CAST 

SILICONE RUBBER 

21 

OIA 76 

ROUNO 

CZ 

NIP 

NONE 

NONE 

NO 

0.54 

43 

8 

5.8 

10.6 

0.4 

5 

SILICONE RUBBER 

RlGlO PAN 

YES 

J.BOX 

CAST 

SILICONE RUBBER 

42 

OIA: 76 

ROUNO 

cz 

NIP 

NONE 

MINOR 

NO 

0.60 

44 

24 

6.7 

11.2 

111 

0.3 

5 

SILICONE RUBBER 

RlGlO PAN 

YES 

TERMINALS 

CAST 

SILICONE RUBBER 

43 

DIA 76 

ROUND 

cz 

NIP 

NONE 

MINOR 

NO 

0.65 

48 

26 

7.4 

11.5 

IV 

0.6 

9 

GLASS 

FLEXIBLE SHEET 

YES 

PIGTAILS 

LAMINATE0 

PVB 

75 

95 I95 

SHAPED 

cz NIP Pi 

3 

MUCH 

VES 

0.78 

48 

1.1 

17 

GLASS 

FLEXIBLE LAMINATE 

NO 

PLUGIN 

LAMINATED 

EVA 

117 

100 I: 100 

SHAPE0 

CZ 

NIP 

6 

MUCH 

YES 

0.89 

48 

'NOMINAL OPERATING CELL TEMPERATURE CELL TEMPERATURE IN OPEN-CIRCUITED MOOULE EXPOSEO TO BO rnwlcrn' INSOLATION IN AMBIENT OF 20~ 1 rnlr WINO VELOCITY. 

bAT 100 rnWlcrn', AM 1.5 INSOLATION. 

1 I 40k T CELL TEMP: 25°C 

T 

IV 

I 

INSOLATION: 100 mWlcm2, AM1.5 

20 

I I ESTIMATE0 

lot Ul 

1 

75 76 77 78 79 80 81 82 83 84 

YEAR 

0 I I I I I I I I 

Figure 45. Module Cost Trend 

i i 

200 I I I I I 1 I 1 

I 

1 

CELL TEMP: 25 "C 

INSOLATION: 100 mWlcm2, AM1.5 

CI 4 

" 75 76 77 78 79 80 81 82 83 84 

YEAR 

Figure 47. Module Power Trend 

V 

I I I I I I I I 1 

IV 12.5 \I 

i 

II 

I 

iCELL TEMP: 25°C 

INSOLATION: 100 mWlcm2. AM1.5 

# 

~. 15.0 - ' 

P 

I I I I I I I 1- - 

0 IV 

1: 

CELL TEMP: 25 "C 

INSOLATION: 100 mWlcm2, AM1.5 

- 

12.5- Y w

2 

2 Y 

z - 

c 

4 

2 7.5 - Q 

2~~1[~11

50 75 76 77 78 79 80 81 82 83 84 

YEAR 

Figure 48. Cell Efficiency Trend 

0 75 76 77 78 79 80 81 82 83 84 

YEAR 

Figure 49. Packing Factor Trend 

59 
The following sequence of events typifies the 

activities of a recent block buy: 

needs of the users and the desires of the 

manufacturers. 

0 FSA prepares design and test specification based 0 Development and continuing improvement in 

module evaluation capabilities and equipment 

including electrical performance, physical 

qualification tests, and correlation of both with 

field experience. 

upon prior experience. 

0 FSA conducts competitive procurement 

culminating in award of parallel contracts for 

alternate design concepts. 

0 Systematic development of module failure 

0 Contractor performs module design. 

0 FSA conducts design review. 

analysis techniques, equipment and 

supplementary analysis, and experimentation 

leading to the solution of module problems. 

Contractor fabricates 10 prototype modules. 

0 FSA performs module qualification tests (and 

failure analysis, as applicable). 

0 Contractor modifies design and/or processing 

procedures to correct problems revealed by 

qualification tests. 

0 FSA conducts design review. 

0 Contractor fabricates 10 modules. 

0 FSA performs module qualification test (and 

failure analysis, as applicable). 

0 Contractor modifies design and/or processing as 

0 FSA completes final testing. 

necessary and supplies modules for retest. 

0 FSA prepares and issues User Handbook 

describing construction details and performance 

of successful module design by each contractor. 

This series of module procurements has produced 

A systematic upgrading of module quality and 

the following results in PV module technology: 

performance by the incorporation of new 

technology into improved designs that are more 

amenable to mass production. 

Establishment and continuing upgrading of design 

and performance specifications based upon the 

0 Adoption by the international PV community of 

the FSAdeveloped specifications and design 

features of the Block V module. 

0 Variations of three of the Block V modules 

being purchased for the second phase of the 

1-MW PV central station power plant being 

The success of these activities can be attributed, at 

least in part, to the following: 

0 The competitive nature of the module 

procurements which included updated design 

and test specifications and a work statement 

indicating design parameters requiring 

improvement. 

0 Extensive advances in cell, module, and 

processing technology performed primarily by 

DOE/JPL-sponsored R&D. 

0 Participation by FSA personnel having access to 

the entire spectrum of PV technology and PV 

field operational results including items such as 

quality assurance. 

continued at the working level by the PV industry 

and FSA personnel throughout all these technical 

activities. 

The excellent cooperation established and 

Figures 50 through 54 show the five block buys 

and Tables 18 through 20 show their characteristics. 

I 60 
, h'" v 

~~ ~~~ 

ENVIRONMENTAL TESTS LIMITED TO: 

TEMPERATURE CYCLE 

HUMIDITY SOAK 

DURING PRODUCTION 

MANUFACTURERS RATINGS 

MANY DESIGN IMPROVEMENTS 

ELECTRICAL PERFORMANCE PER 

Figure 50. Block I: 19 75- 19 76, Off-the-shelf Design, 

54 kW 

FIRST LAMINATED MODULE 

CELL INTERCONNECT AND TERMINAL REDUNDANCY 

ELECTRICAL PERFORMANCE CRITERIA 

115.8 VOLTS - 60°C CELL TEMPERATURE1 

STANDARD ARRAY SIZE AND MOUNTING 

INTRODUCTION OF GROUNDING SAFETY PROVISIONS 

QA SPECIFICATION INTRODUCED 

EXPANDED ENVIRONMENTAL QUALIFICATION TESTING 

*THERMAL CYCLE 

-HUMIDITY CYCLE 

*STRUCTURAL LOADING 

Figure 5 1. Block 11: 19 76- 19 7 7, Designed to FSA 

Specification, 1 2 7 k W 

I 1 

DESIGN AND TEST SPECIFICATIONS ESSENTIALLY SAME AS BLOCK II 

IMPROVEMENTS IN DESIGN AND PRODUCTION PROCESSES RESULTING 

FROM BLOCK II EXPERIENCE 

MORE UNIFORM QA STANDARDS 

Figure 52. Block 111: 19 78- 19 79, Similar 

Specifications, 259 kW 

61 
TYPICAL DESIGN FEATURES 

- FAULT TOLERANT CELL AND CIRCUIT DESIGNS 

LAMINATED MODULE CONSTRUCTION 

LARGER POWER OUTPUT 

CELLS WITH BACK SURFACE FIELDS 

GLASS FRONT FACE 

INNOVATIVE DESIGN FEATURES 

SHAPED CELLS 

ION IMPLANTED CELLS 

SEMICRYSTALLINE CELLS 

ETHYLENE VINYL ACETATE ENCAPSULANT 

FRAMELESS MODULE 

INTEGRAL BYPASS DIODES 

, 

BATTEN - SEAM ROOFING SUBSTRATE 

Figure 53. Block IV: 1980-1981: Industry Designs Reviewed by FSA, 26 kW of Prototype Modules 

62 
* TYPICAL DESIGN FEATURES 

LARGER POWER OUTPUT 

MODULE EFFICIENCY >lo% (EXCEPT 

RIBBON CELL MODULE) 

* GLASS TOP COVER 

ETHYLENE VINYL ACETATE 

ENCAPSULANT 

COVER 

FRAMELESS MODULE 

* SHAPED CELLS (HIGHER 

PACKING FACTOR) 

* PARALLEL CELL STRINGS 

FAULT TOLERANT CELL AND CIRCUIT 

DESIGNS 

* BYPASS DIODES 

* LAMINATED COMPOSITE FILM BACK 

* LAMINATED MODULE CONSTRUCTION 

INNOVATIVE DESIGN FEATURES 

* MAJOR INCREASE IN AREA AND 

POWER OUTPUT 

MET MORE STRINGENT QUALIFICATION 

TESTS 

VIRTUAL ELIMINATION OF THE FOLLOWING CATASTROPHIC FAILURE 

MODES 

UNACCEPTABLE CELL CRACKS 

* INTERCONNECT FAILURES 

HOT-SPOT FAILURES 

- HAILDAMAGE 

* MODULE WITH CELLS MADE FROM 

SILICON RIBBON (EFG) GROWN TO THE CORRECT THICKNESS 

Figure 54. Block V: 198 1-1 985: Industry Designs Reviewed by FSA, Small Module Quantities 

for Evaluation Only 

63 
Tat 

--I-- 

MANUFACTURER I MOOEL NO. 

SENSOR TECH. I V-13.AT 

SOLAREX 

SPECTROLAB I 060513.8 

SENSOR TECH. I 20-10.1452.J 

SO1 AREX A-0221-0 

MOTOROLA P.0170.770.J 

SOLAREX A-022 1 -G 

SOLAR POWER I E.10008-F 

ARC0 SOLAR I 012110.E 

ASEC 

MOTOROLA 

PHOTOWATT 

SOLAREX 

SPIRE 

ARC! SOLAR 

G.E. 

SOLAREX 

GI.' 

SOLAREX' 

MSEC' 

SPIRE' 

60.3062.F 

47J254977Gl.C 

MSP43040.G 

ML.196 1 .O 

580.8T-L4 

58041-R.C 

058-0007.A 

004.014168.2 

47E25844962.A 

Re-1 80.1 2.0 

C.120-1 OA 

058.0008# 

? 1 c. :LJdu8e Cell and Circuit Characteristics 

CELL CIRCUIT 

SIZE BASE SERIES PARALLEL CROSS BY,PASS 

UNTY (mm) SHAPE MATL JUNCTION CELLS CELLS TIES OIOOES 

25 50 01A ROUND CZ NIP 25 - - - 

18 76 OIA NIP 18 

22 87 OIA PIN 22 

20 50 OIA NIP 20 

44 

42 

40 

44 56 OIA 

42 76 014 

40 102 OIA PIN 

120 50 OIA 40 3 

41 

- - - 

- - - - - - - - - - - - - - - - - - - - 

4 11 - 

44 56 OIA 44 - - - 

42 76 OIA ROUND Wll FLAT 42 

40 102 OIA ROUNO PIN 40 - 35 103 OIA ROUND W12 FLATS NIP 35 

19 100 OIA ROUND WI1 FLAT 

72 76 OIA ROUNO 12 6 

72 95x 95 SQUARE SEMLXTL 36 

72 95x 95 SDUARE SEMIXTL 

- - - - - 

1 

34 4 5 1 

- - 

- - - 

- - 

136 76 OIA ROUNO 

33 100 x 100 OUASISOUARE WPP+ 33 - - - 

2 35 36 

6 11 12 

3 11 2 

6 3 1 

100 x 100 OUASCSOUARE CZ NIP 36 2 34 3 

432 95 x 48 RECTANGULAR EFG NIP 36 12 2 1 

1 

91 x 91 OUASI-SOUARE CZ NIP-P+ 36 2 2 3 

108 641 64 OUASI-SOUARE cz - 72 97 x 97 IlUASI4OUARE CZ NIP 12 

1 72 

~ 72 

- 117 101 x 101 RECTANGULAR SEMbXTL NIP 13 

NOTE 'RESIDENTIAL MODULE 

Table 19. Module Performance Characteristics 

I SAMPLE PI 

AT 100 mWlcmZ, AM 1.5, 28°C CELL TEMP. 

Pmax VPma, Ipmal Voc 1st FILL MOOULE CELL 

MANUFACTURER MOOEL NO. IWI IVI IAI IVI IAI FACTOR EFF. IKI EFFCIWI 

10.2 

A-0221-0 20.5 18.0 1.14 24.3 1.43 0.59 10.7 

33.8 18.0 1.88 23.5 1.98 0.73 7.4 10.7 

lSPECTROLA8 022962.6 I 30.0 18.2 1.65 23.0 1.86 0.70 6.6 12.7 

IARCO SOLAR 10699.C I 22.8 18.2 1.25 23.3 1.38 0.71 8.4 12.2 

MOTOROLA P.0170.770-J 26.2 5.9 4.45 7.1 4.82 0.76 7.7 11.8 

111 SENSOR TECH. 20.10-1646 11.3 20.2 0.56 24.6 0.62 0.74 6.8 10.5 

SOLAREX A-0221.G 21.7 17.8 1.22 23.7 1.40 0.65 6.5 11.6 

(SOLAR POWER E.lOOO8.F I 34.8 18.3 1.90 23.6 1.97 0.75 7.7 11.2 

IARCO SOLAR OI~IO-E I 35.7 16.6 .2.15 21.0 2.42 0.70 9.6 12.6 

ASEC 60.3062-F 84.6 16.5 5.11 20.2 5.40 0.78 10.1 13.6 

GI.' 47J254977Gl.C 18.8 8.5 2.21 11.0 2.53 0.68 9.6 12.6 

MOTOROLA MSP43040.G 37.3 16.2 2.30 19.5 2.50 0.76 8.8 11.6 

PHOTOWATT ML.1961-0 38.6 5.68 6.79 6.98 7.58 0.73 7.2 11.6 

SOLAREX 580.8T.L.C 62.6 16.1 3.90 19.6 4.50 0.71 8.2 9.6 

SOLAREX' 580-8T.A.C 60.8 5.31 11.4 6.60 13.2 0.69 8.1 9.3 

SPIRE 058.0007.A 57.0 16.2 3.52 20.3 3.64 0.77 11.4 13.6 

AT 100 ..mWkm2. AM 1.5. 25'C CELL TEMP. 

lCRCO SOLAR 004014168.2 I 84.1 5.82 145 7.16 15.9 0.74 11.3 12.6 

GI.' 47E258449G2.A 81.7 17.0 4.81 20.9 5.65 0.69 10.5 11.7 

MSECa R1.180.12.D 185. 15.3 12.1 18.9 13.3 0.74 8.4 9.4 

SOLAREX C.120.10A 139. 5.84 23.8 7.47 26.7 0.70 10.3 11.7 

SPIRE' 058.0008-8 70.7 16.1 4.39 20.7 4.79 0.71 10.1 13.3 

EORMANCE 

AT 100 mWlcm'. AM 1.5. NOCTb 

pma, vb, ~p,,, vOc lSc FILL MODULE CELL NOCT~ 

IWI IAI NI IAI FACTOR EFF. IKI EFFCIKI ITI 

18.7 16.3 1.15 22.4 1.44 0.58 5.5 9.8 47 

31.3 16.6 1.89 22.0 1.98 0.72 6.9 9.7 46 

28.5 17.3 1.65 21.9 1.88 0.69 6.3 11.7 41 

20.6 16.5 1.25 22.0 1.40 0.67 7.6 11.0 50 

23.6 5.3 4.45 6.6 4.88 0.73 7.0 10.8 53 

10.2 18.6 0.5523.0 0.62 0.72 6.1 9.4 43 

19.7 16.4 1.2022.1 1.41 0.63 5.8 10.4 46 

32.2 17.2 1.87 22.0 1.98 0.74 7.1 10.3 46 

32.4 15.0 2.16 19.6 2.42 0.68 8.7 11.4 46 

77.4 15.0 5.16 19.2 5.45 0.74 9.3 12.6 47 

15.3 7.1 2.16 9.6 2.53 0.63 7.8 10.3 58 

34.3 15.1 2.27 18.4 2.52 0.74 8.1 10.6 49 

34.9 5.10 6.84 6.5 7.62 0.70 6.6 10.6 47 

57.3 14.4 3.98 18.1 4.58 0.69 7.5 8.8 49 

54.5 4.70 11.6 6.2 13.3 0.66 7.3 8.4 58 

50.8 14.2 3.58 18.6 3.67 0.74 10.1 11.9 49 

AT 100 mWkm2 AM 1.5. NOCTb 

75.0 5.20 14.4 6.5616.1 0.71 10.1 11.2 49 

65.4 13.3 4.82 17.7 5.88 0.65 8.4 9.3 65 

185. 13.2 12.5 17.8 13.7 0.87 7.5 

123. 5.18 23.7 6.7927.2 0.67 9.1 1::; 

62.7 14.5 4.32 18.8 4.84 0.69 9.0 11.8 I 47d 

:: ~ ~ 

NOTES: 'RESIOENTIAL MOOULE 

C~~~~~~~~~~~~ CELL 

d~~~~ MOUNTED 

bNOMINAL OPERATING CELL TEMPERATURE. CELL TEMPERATURE IN OPEN ClRCUlTEO MOOULE EXPOSE0 TO 60 mWm2 INSOLATION W AMBIENT OF 20°C. 1 mls WWO VELOCITY 

64 
Table 20. Module Mechanical Characteristics 

ENCAPSULANT 

RTV.615 

SYLGARO 184 

SYLGARO 184 

RN.615 

RTV.615 

SYLGARO 184 

SYLGARO 184 

PVB 

PVB 

O.C. 113.6527~ 

RTV.615 

SYLGARO 184 

SYLGARO 184 

PVB 

PVB 

G.E. SCS2402 

PVB 

PVB 

EVA 

MANUFACTURER 

ENCAPSULANT 

METHOO FRAME 

CASTING NONE 

v ALUM. 

LAMINATION ALUM. 

LAMINATION MUM. 

CASTING ST. STEEL 

NONE 

ALUM. 

NONE 

LAMINATION ALUM. 

ALUM. 

NONE 

ST. STEEL 

ALUM. 

ALUM. 

v NONE 

1 

I ISOUREX 

LENGTH 

0.57 

0.51 

0.61 

0.66 

0.582 

0.579 

1.168 

1.168 

1.168 

0.583 

0.582 

0.579 

1.168 

1.219 

1.198 

0.818 

1.198 

1.199 

1.200 

1.193 

1.200 

1.228 

1.791 

1.391 

1.134 

blE 

7;22i 

SOLAR WWR 

SENSOR TECH. 

WIDTH 

0.1 7 

0.26 

0.37 

0.12 

0.289 

0.58 

0.389 

0.388 

0.231 

0.583 

0.286 

0.579 

0.389 

0.305 

0.696 

0.669 

0.356 

0.444 

0.635 

0.628 

0.4lI 

0.633 

1.203 

0.857 

0.585 

lmlC 

0.610 

'p 

~y 

- 

" 

MOOEL NO. 

V.13.11 

785 

E.lO.229.1.5 

0605134 

20.1 0.1452.5 

A-0 22 1 .O 

E.IOOO8.C 

022962.6 

10699.C 

P-0 170.770.5 

20-1 0.1 646 

A522 1 .G 

E-1 0008f 

01211M 

80.3062f 

47J254977GkC 

MsP430406 

ML.1961-0 

58081-L.C 

580-BT.RE 

058-00074 

VOWW6W 

47E25844962.L 

Ib-180-12.0 

0584008.8 

C-120.1 OA 

SOLAREX 

SOLAR POWER 

SPECTROLAB 

ARC0 SOLAR 

MOTOROU 

SENSOR TECH. 

SOUREX 

ARC0 SOLAR 

WEC 

MOTOROLA 

PHOTOWATI 

SOUREX 

PlRE 

SOLAR mm 

G.E? 

SOUREX' 

G.E.' 

SPIRE' 

- 

MSEC' 

SOLAREX 

(R~A~ 

EL 

0.097 

0.133 

0.228 

0.1 68 

0.335 

0.454 

0.270 

0.340 

0.166 

0.335 

0.372 

0.834 

0.196 

0.426 

0.532 

0.782 

0.749 

0.504 

0.745 

0.776 

2.154 

1.331 

0.875 

o.080 

0.453 

0.454 

- 

IASS SUPERSTRATE 

1.3 RTV.615 r 1.1 SYLGARO 184 

2.6 O.C. R4.3117 

1.6 IGLASS 

13.9 

11.2 

IT0 y-- 

!9.5 

!3.8 I I 

SUBSTRATE 

OR BOTTOM COVER 

ALUMINUM 

NEMA.GI0 BOARO 

NEMA-GI0 BOARO 

ALUMINUM 

ALUMINUM 

NEMA-GI0 BOARD 

GFR POLYESTER BOAAO 

MYLAR 

TEOLAR 

STAINLESS STEEL 

ALUMINUM 

NEMA.Gl0 BOARO 

GFR POLYESTER BOAAO 

TEOlSTlTEO 

TEOLAR 

MEA0 PAN.L.BOAR0 

TEOIALlTED 

TEDiALlTEO 

TEOLAR 

TEOLAR 

MYLAR ALXOAT ~ 

TE~O' 

TEOIPETIAUTEO~" 

- 

PETIALITEO~ 

PETIMYLARITEO' 

TEOLAR 

'RESIDENTIAL MODULE %US SHINGLE YATtRlAl 

bumso MEA 

c~~~~ OIMENSION 

'PET- POLESTER FILM. WLYTHYLENE TEREPHTHALATE 

ELECTRICAL 

CONNECTIONS 

TERMINALS 

PIGTAILS 

J.BOXICABLE 

TERMINALS 

TERM IN ALS 

j40x 

J BOX 

RUG4N 

TERMINALS 

4 

t PIGTAILS 

FLATXABLE 

.!-BOX 

RUG4N 

flGTAlLS 

AGTAILS 

PLUG IN 

FLAT CABLE 

J BOX 

PLUG-IN 

PLUG-IN 

5.BOx- 

- 

PACKING 

FACTOR 

0.51 

0.61 

0.57 

0.64 

0.56 

0.69 

0.69 

0.65 

0.65 

0.56 

0.69 

0.76 

0.74 

0.76 

0.76 

0.62 

0.85 

0.87 

0.85 

0.90 

0.90 

0.89 

0.86 

0.76 

- 

0.49 

p.52 

65 
Engineering Sciences 

and Design 

At the start of the FSA Project, there was a need 

to establish a complete new PV module and array 

technology for terrestrial applications. The use of 

solar cells in space was a very different application 

than any found on Earth. Thus, there was a need to 

define potential terrestrial applications, and to 

develop the operational and environmental 

requirements and technology needed for the 

manufacture and use of PV products in these 

applications. 

The objective is to develop reliable, low-cost 

module and array engineering and design 

technologies for integrating cells and modules into 

cost-effective and safe arrays for large-scale 

applications. 

Identify and develop functional requirements 

and specifications to ensure module 

compatibilty with the various operational and 

safety environments of future PV applications. 

Develop engineering design methods and data 

and advanced safe, reliable design concepts 

focused at the needs of future large-scale PV 

applications. 

Verify the adaptability of the technology to meet 

required operational conditions. 

Background 

The use of crystalline silicon solar cells in space 

had demonstrated that PV power systems were a 

practical and reliable method of generating electrical 

power. The solar cells and the “solar panels,” as 

designed, were light-weight, resistant to radiation, 

capable of withstanding rapid heating and cooling, 

and were cost effective as used in space. The idea 

of using photovoltaics as a major power source in 

the terrestrial environment meant that the new 

functional requirements of these applications had to 

be identified and the necessary technologies 

developed. Not only were the environmental 

requirements significantly different with the addition 

of wind and moisture-related weathering, but the 

institutional requirements were vastly changed with 

added considerations of building and safety codes, 

user interaction, product liability concerns, and 

interaction with the community of installation trades 

and operation and maintenance personnel, 

Project Activities: 1975 to 1981 

Deficiencies and failures were prevalent in the first 

(pre-1975) terrestrial modules. In addition to the early 

reliability problems, the modules were found to be ill 

suited for use in the first kilowatt-scale, high-voltage 

66 

systems. Their size was too small, the mechanical 

interfaces (bolt patterns) varied from module to 

module (even within the same manufacturers model, 

they were unsafe electrically for use at high 

voltages), and they lacked means of electrically 

interconnecting them cost effectively. To define the 

needed changes, the FSA Project worked closely 

with the systems and applications researchers at 

NASA Lewis Research Center and Sandia Na?iona! 

Laboratory to understand the problems being 

uncovered and the future applications being 

identified. Next, FSA researchers contracted with 

leading industrial teams, knowledgable of the detailed 

technical characteristics of future applications, to 

identify and develop concepts and requirements for 

flat-plate PV modules specifically suited to these 

uses. Large architect and engineering (A&E) firms 

with extensive experience building large central utility 

power plants defined optimum modules and arrays 

for utility-scale applications; simultaneously, 

residential and commercial architectural firms 

developed the detailed requirements and concepts 

appropriate for these sectors, including consideration 

of applicable building and safety codes and labor 

practices. 

As the early deficiencies were gradually resolved, 

the above activities coalesced into a coherent array 

subsystem design effort (Figure 55) addressed to 

optimizing the overall module and array designs for 

the intended applications so as to reduce costs and 

increase efficiency and lifetime. The environmental 

conditions that PV modules and arrays are subjected 

to during field operation were assessed, measured, 

and defined. This covered mean operating conditions 

in addition to extremes for conditions such as UV/ 

solar weathering, temperature, temperature cycling, 

hail impact, and combinations of the above (Figure 56). Means of quantifying and reducing the 

operating temperatures of modules were developed: 

detailed wind tunnel analyses were conducted to 

define expected wind loading levels, the effectiveness 

of wind barriers, and dynamic flutter magnitudes. 

Based on the structural loading studies, new module 

structural designs were developed. These designs 

included frameless modules and low-cost groundmounted array structures (Figure 57). A new 

methodology for determining the breaking strength 

of glass was also developed. 

To improve the reliability of the arrays, complex 

circuit analyses were used to define optimum seriesparallel electrical circuit redundancy approaches at 

the array and module level, and optimum maintenance and replacement trade-offs (Figure 58); other 

design studies explored lowcost electrical interconnection techniques. 

One of the key institutional barriers uncovered in 

the electrical circuit studies was the unique electrical 

safety attributes of PV which are inconsistent with 

many standard safety practices. Unlike conventional 

electrical sources, PV modules cannot be turned off 

and they cannot generate the overcurrent needed to 
1975 TO 1984 

OBJECTIVES 

OBJECTIVE 

DEFINITION OF ARRAY DESIGN TRADE-OFFS AND GENERATION OF DESIGN GUIDELINES. RE OUIREMENTS AND CONCEPTS 

APPROACH 

DEVELOP ADVANCED DESIGN CRITERU 

DEVELOP ANALYSIS TOOLS * TESTS 

* EOUATKDNS 

* COMPUTER PROGRAMS * GENERATE REPRESENTATIVE 

DESIGNS AND DESIGN TRADE-OFF DATA 

Technology 

Thermal Design ICround) 

Thermal Design (Roof1 

PV-Thermal Modules 

Cnd-Mtd Array Structures 

Residential Arrays 

Series-Parallel Analysis 

Bypass Diode Integration 

Electrical Connectors 

Module Safety Design 

Array Safety Systems 

Flammability 

PC Interface 

V = Major Contribution 

A = Significant Contribution 

Figure 55. Engineering Sc 

Year I 

?nces Developments 

HAlL IMPACT RESISTANCE 

Figure 56. Environmental Requirements Example 

BURIEDTRUSSSTRUCTURE 

4 14 MODULE 

- 20 - 

TRUSS STRUCTURE IS BURIED IN 1 112 WIDE L 3 112 DEEP I 9 LONG TRENCHES 

REFILL AND TAMP TRENCHES USING REMOVED EARTH 

PROTOTYPE STRUCTURE VIBRATION TEST 

Figure 5 7. Advanced Array Structures 

blow fuses or circuit breakers in the event of a short 

circuit or short to ground. To develop the needed 

safety technology, JPL contracted with Underwriters 

Laboratories who developed detailed module safety 

requirements and conceptual approaches to the 

entire electrical safety system for a complete PV 

power system (Figure 59). On the basis of this work, 

requirements for UL listing of modules have been 

developed, and a new Article 690, covering required 

electrical safety features in high-voltage PV power 

systems, has been included in the 1984 revision of 

the National Electrical Code. 

67 
ORPGINAL PAGE R3 

OF POOR QUAUW 

~~ II RELIABILITYIDURABILITY II SERIESIPARALLEL NOMENCIATURE 

Figure 58. Electrical Circuit Reliability 

FRAME CIRCUIT GROUND 

STRUCTURE 

GROUND 

Figure 59. Isolation/Grounding Safety Concepts 

The most important technique for getting modulemanufacturing industry participation in the 

modulelarray design process and for transferring new 

knowledge into practice, was to make periodic 

procurements of modules. These purchases of 

advanced-design modules to successively more 

definitive and more appropriate requirements, and 

the subjection of these modules to qualification life 

tests and field tests, resiilteri in a very dyr?an?ic and 

fruitful cooperative industry/Government effort. 

There were a number of design specifications that 

evolved over the years, each building on the previous 

one together with the technology developments and 

the application and testing experience. The latest 

Block V Module Design and Test Specification was 

issued in two versions in 1981; one specification is 

for Residential Applications, and the second is for 

Intermediate Load Applications, i.e., general purpose 

uses. 

Project Activities: 1981 to Present 

Since 1981, the evolution of module and array 

requirements and technology has continued with 

emphasis on completing the technology base for 

residential applications and supporting the design 

and fielding of the first megawatt-scale utility 

applications. Detailed studies were conducted 

characterizing the operating temperatures of roofmounted modules and defining residential array 

design guidelines including array wiring practices and 

bypass-diode packaging concepts. Testing of the new 

environmentally durable encapsulants indicated 

substantial progress in the area of reliability, but a 

step backward relative to flammability and suitability 

for module applications with a high fire risk. 

Research was initiated to characterize the flame 

resistance of new module designs and to develop 

flame-resistant design approaches and encapsulant 

materials (Figure 60). 

FLAMMABILITY RESEARCH 

Figure 60. Flammability Research on 

Rooftop Module 

The need for improved integration between the 

array and the power conditioner led to extensive 

modeling of array output versus time for various sites 

in the United States. Characterizations included 

68 
statistical data on maximum array voltage, current 

and power levels and the performance efficiency of 

various maximum-power tracking algorithms. 

With the gradual completion of the engineering 

technologies for crystalline-silicon modules, 

Engineering Sciences research has gradually phased 

down in recent years and turned to the needs of the 

emerging new thin-film modules. These needs seem 

to be well served by the technology base resulting 

from the 10 years of research on crystalline-silicon 

modules and arrays. 

Significant Accomplishments 

Specific design data and recommendations that 

have evolved include: 

Module design specifications including 

environmental endurance test requirements 

Module electrical safety design requirements 

and practices (UL 1703) 

recommendations (National Electrical Code 

Article 690) 

Detailed assessments of residential and 

commercial building codes and the implications 

on the use of photovoltaics 

Safety system design concepts and 

Module product-liability guidelines 

Array wire selection and safety design 

guidelines 

Array serieslparallel electrical circuit design 

guidelines including grounding and bypass 

diode design guidelines 

Module electrical circuit design guildelines 

including interconnect redundancy and cell 

contacting techniques 

Module electrical terminal needs and designs 

Module electrical insulation system design 

guidelines and testing techniques 

Module and array thermal design guidelines for 

cooler operation, resulting in increased power 

output and longer life 

Standardized module thermal testing methods 

Fire resistant module designs and encapsulant 

materials 

Residential array design approaches 

Low-cost ground-mounted array design 

approaches including frameless modules 

Guidelines for optimum maintenance/ 

replacement of failed modules in the field 

Data on array cleaning costs and automated 

washing techniques 

Wind pressure loads on flat modules and arrays 

Structural capabilities of glass as used in 

modules, and glass sizing algorithms 

Design guidelines for optimally interfacing 

arrays with power conditioners 

69 
Encapsulation 

In 1975, the encapsulation requirements of PV cells 

for terrestrial use had not been comprehensively 

assessed, needs had not been defined, nor potential 

low-cost encapsulation materials investigated. 

Therefore, the FSA Project set out to assess the 

needs in order to identify, andlor develop iiew 

materials and new material technologies that were 

inexpensive and could protect solar cells for years. 

The assessment led to the establishment of 

encapsulation needs, goals, and detail requirements 

and a plan on how to meet the goals. 

The objectives of the encapsulation activities were 

to define, develop, demonstrate, and qualify 

encapsulation systems, and materials and processes 

that meet the FSA Project module life, cost, and 

performance goals; and to develop and validate 

encapsulation system life prediction:methodology 

based on modeling life-limiting failure modes and on 

conducting and analyzing aging tests. 

The goals are to develop encapsulation system 

Module life of 30 years (increased from 20 

years). 

Encapsulation materials, including substrate 

andlor superstrate, costs not exceeding $1 41m2 

(1 980 dollars). 

technology adequate for: 

Initial optical transmission of 90% and a loss of 

less than 5% after 20 years of use. 

Capability to withstand an electrical breakdown 

voltage of 3000 V dc. 

Structural integrity and durability to withstand 

handling and weather. 

Cost-effective processing in an automated 

factory with high yields. 

Background 

Typical terrestrial PV module fabrication prior to 

1975 was by hand assembly. The solar cells were 

interconnected by hand soldering. The strings of cells 

were encased by hand-poured silicone rubber, which 

usually resulted in non-uniform products. The 

durability of a module was unpredictable, often with 

delamination andlor cracking of the encapsulant. The 

module materials were expensive and the 

encapsulation concepts, designs, and processes 

were not amenable to mass production. Consistent 

module quality was a problem because of a lack of 

good processing knowledge, capabilities, and quality 

assurance, for example, requirements for and control 

of encapsulant curing. The product quantities were 

so small that manufacturers could not afford to do a 

thorough job of studying materials and processes. A 

comprehensive long-range encapsulation technology 

plan was needed and was formulated by the FSA 

Project. 

Project Activities: 1975 to 1981 

In the implementation of the plan to meet the 

objectives and goals, five activities were followed: (1) 

the generation of specifications and functional 

requirements for encapsulation materials; (2) the 

identification or development of low-cost materials 

satisfying the specifications and functional 

requirements; (3) engineering material usage; (4) 

recognition of life andlor weathering deficiencies in 

the low-cost materials; (5) generation of necessary 

design approaches or material modifications to 

enhance life or weathering stability; and (6) lifeprediction methodologies for encapsulation systems. 

Two basic encapsulation systems for terrestrial 

modules evolved: substrate and superstrate 

(Figure 61). This nomenclature refers to the method 

of structurally supporting the encapsulated cells and 

their electrical interconnections, i.e., sub or bottom 

side and super or top sunnyside. Today, the superstrate design is universally used because tempered 

glass is a practical top cover for use with crystalline 

silicon solar cells. Thin-film cells have the potential 

for use in flexible modules; however, a practical 

flexible top cover is unavailable. Consequently, the 

preponderance of the effort, to date, has centered 

on the technology for laminating the active components in a PV module and their function are shown 

in Figure 62. 

The key component is the pottant, which is a 

transparent, elastomeric material that encases the 

cells and their electrical circuitry. In the fabricated 

module, the pottant provides: 

Maximum optical transmission in the silicon 

solar cell operating wavelength range of 0.4 to 

1.1 pm. 

Retention of a required level of electrical 

insulation to protect against electrical 

breakdown, arcing, etc., with the associated 

dangers and hazards of electrical fires, and 

human safety. 

The mechanical properties to maintain spatial 

containment of the solar cells and 

interconnects, and to resist mechanical creep. 

The level of mechanical properties also must 

not exceed values that would impose undue 

mechanical stresses on the solar cells. 

70 
GLASS ETRUCTU RAL) 

-- SP~~~~IS~RlM 

SOLAR CELLS 

BACK COVER FiLM 

Figure 6 1. Encapsulation System Designs 

Also, the Dottant must be commercially available must also be resistant to atmospheric soiling, and be 

and be readily processible for automated fabrication. 

Because there is a significant difference between the 

thermal-expansion coefficients of polymeric materials 

and the silicon cells and metallic interconnects, 

stresses developed from the thousands of daily 

thermal cycles can result in fractured cells, broken 

interconnects, or cracks and separations in the 

pottant material. To avoid these problems, the pottant 

material must not overstress the cell and 

interconnects, and must itself be resistant to 

fracture. From the results of a theoretical analysis, 

experimental efforts, and studies of available 

materials, the pottant must be a low-modulus 

elastomeric material. 

The fop cover is in direct contact with all of the 

weathering elements: ultraviolet, humidity, water, 

oxygen, sunlight, etc., and must protect underlying 

materials from degrading effects. The outer surface 

abrasion resistant, anti-reflective, and easily 

cleanable. 

The back cover protects the cells, interconnects, 

pottant, and the substrate, if there is one, from 

weather and from electrical shorting. The cover 

should be weatherable, hard, mechanically durable 

and tough, and have a white, high-emissivity surface 

for cool module operation. 

The edge of the laminate needs to be protected so 

that delamination does not begin and destroy the 

module. A gasket is used if the module is to have a 

frame or it may be used for the installation of 

frameless modules. 

obtain good adhesion of the polymeric pottant to 

glass, solar cells, and polymeric films that are used 

for back covers. 

During module fabrication, primers are required to 

71 
MODULE ENCAPSULATION 

FUNCTIONAL DESIGN ELEMENTS 

MODULE ELEMENTS REQUIRED FUNCTlONSlCHARACATERlSTlCS 

TOP SURFACE / (AS IS. MODIFIED, 

OR COATED1 

LOW SOILING 

EASY CLEANABILITY 

ABRASION RESISTANT 

/ ANTIREFLECTIVE 

MAXIMUM LIGHT TRANSMISSION UV SCREEN 

STRUCTURAL SUPERSTRATE 1 f (IF REQUIRED) -TOPCOVER 

i c- SPACER. POROUS __ 

- POTTANT 

C- SOLAR CELLS 

POTTANT 

-SPACER. POROUS 

- DIELECTRIC 

1 

I 

- SUBSTRATE 

- BACK COVER 

PRlMERSlADHESlVES 

(AS REQUIRED1 

COMPLETED MODULE - 

AIR RELEASE 

MECHANICAL SEPARATION 

ENCASES SOLAR CELLS 

AND INTERCONNECTIONS 

HIGH LIGHT TRANSMISSION 

ELECTRICAL ISOLATION 

STRUCTURAL SUPPORT 

IIF REQUIRED) 

BACKSIDE PHYSICAL PROTECTION 

BACKSIDE WEATHERING BARRIER 

GOOD BONDING 

GOOD HEAT TRANSFER 

GOOD PROCESSABILITY 

Figure 62. Module Encapsulation Functional Design Elements 

Man: of the low-cost polymeric materials that 

satisfy module engineering and encapsulation 

processing requirements are not intrinsically weather 

stable. Therefore, emphasis has been applied to 

specific efforts to overcome life andlor weathering 

deficiencies, to enhance weather stability, and to 

develop accelerated aging techniques. Specific items 

include the development of chemically attachable 

stabilization additives (ultraviolet screening agents, 

anti-oxidants, etc.), computer-assisted kinetic 

modeling of outdoor weathering reactions, and the 

use of new and novel outdoor-heating racks and 

controlled-environment reactors as accelerated aging 

techniques. 

Transparent polymeric pottants are subject to 

three weathering actions: ultraviolet reactions, 

thermal oxidation, and hydrolysis. During the 

assessment of various pottants, each polymer was 

studied for its ability to withstand the above 

weathering actions and the cost of the product. It 

was determined that all the lowest cost polymers 

were subject to all three weathering actions, a 

number of expensive polymers were subject to no 

weathering actions, and a few intermediate cost 

polymers were resistant to weathering (at 

temperatures up to 80°C), except for ultraviolet 

weathering. Of the four intermediate cost polymers, 

two were casting liquids and two were dry films that 

could be used for fabricating laminated modules. 

The dry films are practical for PV module uses 

because of the ease and simplicity of handling and 

processing. Initially, ethylene vinyl acetate (EVA) was 

the only lowcost dry film available; therefore, 

emphasis was placed upon efforts to adopt it for PV 

use. The search for a second dry film polymer 

continued in case EVA did not fulfill the encapsulation 

requirements. Ethylene methyl acrylate (EMA) was 

discovered 2 or 3 years later, but by then it seemed 

that EVA would satisfy the needs. A lack of sufficient 

funds inhibited a thorough investigation of EMA 

although, to date, there are no known EMA 

deficiencies for use as a PV module pottant. The 

approach used was to develop an EVA that could be 

thoroughly evaluated while work progressed on an 

advanced EVA. 

Other encapsulation topics addressed related to 

life and durability included: (1) soiling, (2) electrical 

insulation, (3) primers and adhesives, (4) encapsulant 

system design analysis, and (5) material 

characterization (see Figures 63 and 64). 

72 
ENCAPSULANT SYSTEM DESIGN ANALYSIS 

RESPONSE OF MODULE COMPONENTS TO ENVIRONMENT 

WILL PROVIDE CRITERIA 

FOR MATERIAL SELECTION 

AND MODULE DESIGN 

SOLAR 

CELL 

STRESS 

WIND. 190 mph 

TEMPERATURE 40° + 90°C 

HUMIDITY Oo - 100% 

SOLAR 100 mWIcm2 

'OTTANT THICKNESS REQUIRED FOR SPECIFIC MODULE DESIGN AND ENVIRONMENT 

Figure 63. Encapsulant System Design Analysis 

MATERIAL CHARACTERIZATION 

COVER SCREEN ABSORPTION OF HARMFUL UV RADIATION: REQUIRED FOR MATERIALS SELECTION AND LIFE ASSESSMENT 

SOLAR 

ABSORPTION 

(%I 

40 

- 

m 

0 i 

UV SCREEN CUT OFF 

CURVES 

iVlNYL TlNUVlN IN MMAi 

CELL RESPONSE 

450 _I 

250 300 350 400 

WAVELENGTH. hlnml 

Figure 64. Material Characterization 

Project Activities: 1981 to Present 

The PV industry has been using .a first-generation 

EVA in commercial modules for more than 5 years. 

This material has proven to be a surprisingly stable 

polymer when correct module design and processing 

have been used. The life-limiting characteristics of 

this EVA have been found from experiments, 

qualification testing, field operation, and accelerated 

testing. With this knowledge, an advanced EVA that 

corrects minor deficiencies of the earlier EVA has 

been devised. On the basis of the results of 1 year of 

evaluating the advanced EVA, it seems to have 

corrected the previous materials deficiencies. At this 

time, there are no known reasons why this new EVA 

should not satisfy the requirements for a 30-year 

module life. However, testing needs to be done to 

verify the life of the pottant and of a complete 

encapsulation system using this pottant. 

In experimental studies of EVA aging, it seems 

that EVA does not undergo thermal oxidation at 

temperatures up to 50°C. However, it does undergo 

photo-oxidation. EVA, which yellows and degrades 

within 1000 h when subjected to normal weather 

conditions at 50°C, did not yellow or degrade in 

20,000 h when protected by an ultraviolet screening 

film. Therefore, the life of an EVA pottant in outdoor 

service at 50°C (or less) is directly related to the 

permanence of the module's protection from 

ultraviolet. 

The advanced EVA material (formulated by 

Springborn Laboratories) uses a curing agent (TBEC) 

73 
ADVANCED ADDITIVES DEVELOPMENT 

MATERIALS IMPROVEMENT USING ADDITIVES 

1 DEVELOPMENT OF CHEMICALLY BOUND UV SCREENING ADDITIVES I COPOLYMERIZE 

on 

I CHEMICALLY ATTACHED UV SCREEN IN ACRYLIC IMMAI CHAIN FOR USE IN TRANSPARENT COVER FILM na POTTAIIT UATFCIII, q 

I zFk%Z?: Ai UNlV OF MASS) 

CONCEPT DEVELOPMENTS IN 

ADVANCED STABILIZATION 

ADDITIVES PROVIDED DIRECTIONS 

FOR A MORE WEATHER STABLE, 

EASIER TO PROCESS, EVA FORMULATION 

COMPONENT 

IMPROVED ADVANCED EVA FORMULATION 

FUNCTION 

EVA - DUPONT ELVAX 150 

LUPERSOL TBEC (PENN WALT) 

UV.2098 (AM. CYANAMID) 

UV-3346 (AM. CYANAMID) 

Figure 65. Advanced Encapsulation Materials Research 

which cures faster at a lower temperature in addition 

to having a longer storage or shelf life; it also has a 

chemically attachable, nondepleting ultraviolet screen 

component and has a nondepleting polymeric weather 

stabilizer (see Figure 65). The test results to date 

have been excellent. 

The components of an encapsulation system need 

to be bonded to each other permanently. Significant 

advances have been made in this chemical bonding 

technology during the past 10 years and especially 

during the last 2 years when the number of adhesives 

recommended for use in the PV industry has been 

reduced from 23 to 3 and the adhesives themselves 

are of improved quality (see Figure 66). 

PROBLEM SOLUTION 

GENERATE ACTUAL CHEMICAL BONDS 

SUCH AS, EVA AND GLASS, BY EMPLOYING 

SILANE CHEMISTRY, WHICH RESULTS IN 

HIGH BOND STRENGTH, AND LONG.TERM 

OUTDOOR WEATHER STABILITY 

BETWEEN MATERIAL INTERFACES 

DELAMINATION OF 

WEATHERING 

0 SILANE 

CHEMICALLY BONDED 0 EVA 

INTERFACE INTERPHASE 

MOLECULAR-LEVEL MODEL 

OF A SILANE (CHEMICAL) 

BONDED INTERFACE - GLASS AND EVA 

Figure 66. Chemical Bonding Research 

BASIC RESIN 

PEROXIDE CURING AGENT 

REPLACING LUPERSOL 101 

FASTER CURE AT LOWER 

TEMPERATURES 

IKPROVED iGNG-TERM 

STORAGE STABILITY 

UV SCREENING - 

CHEMICALLY ATTACHABLE 

FOR BETTER WEATHERING 

PERMANENCE 

UV ANTI-OXIDANT, A HINDERED 

AMINE LIGHT STABILIZER 

(HALS) - 

HIGH MOLECULAR 

WEIGHT ADDITIVE FOR 

WEATHERING 

PERMANENCE 

A definition of the intrinsic dielectric strength of 

insulating materials which can be considered as a 

fundamental material property hag been proposed. 

This is being used to quantify *the aging characteristics of pottants in regard to their electrical insulation 

properties. 

Predictive aging models that connect change of 

encapsulation material properties and rates of 

changes to module performance and life have been 

developed. A correlation of accelerated outdoor aging 

to normal outdoor aging by elevating the temperature 

of encapsulants has been determined (see Figure 67). 

Equipment for accelerated testing has also been 

developed. 

Power loss of modules caused by 3 years of outdoor soiling has been reduced significantly by the use 

of fluorocarbon coatings on the module top surface. 

On glass surfaces, this amounted to a time-averaged 

loss reduction of from 2.65 to 1.55%. 

The use of flexible top cover materials is receiving 

increased interest by thin-film module manufacturers. 

However, there is no proven top layer material other 

than glass. It is the judgment of Project personnel that 

neither Tedlar nor Acrylar materials, as envisioned, 

will become an acceptable top material. Present ultraviolet additives to Tedlar are depleted too quickly and 

the mechanical properties of Acrylar are deficient for 

handling, processing, and in-field operation. Years 

ago, the study of substrate encapsulation systems 

was downgraded so that, with the available funds, 

superstrate encapsulation could be emphasized. If 

flexible module designs are to become a reality, a 

new film material must be developed for use as a top 

layer, combining properties such as long-term life, 

lowcost, weatherability, ultraviolet permanence, good 

transparency in the visible spectrum, and good 

characteristics during film processing, handling, and 

encapsulation processing. 

74 
JCREASING THE SUNS INTENSITY I SUNS), AT DSET-ARIZONA 

MATERIAL 

GLASS - LOW IRON TEMPERED ISUNAOEXI 

- LOW IRON ANNEALED 

FLUOROCARBON - ITEDLARI 

ACRYLIC - (ACRYLARI 

THIS INITIAL FSA LEARNING 

STATUS NOTES 

STRONGEST BUT DEFECT SENSITIVE 

THICKNESS DETERMINED BY HAIL OR WIND 

DuPONT UV SCREENING FILM 

3M UV SCREENING FILM 

EXPERIENCE EVOLVED TO A -. 2ND . -GENERATION - -. . -. .. OUTDOOR 

AGING CONCEPT 

=3 LED TO 

INCREASING TEST TEMPERATURE AT 1 SUN, STRINGBORN LABS, ENFIELD, CONNECTICUT 

UNITS ARE CALLED 

OUTDOOR PHOTOTHERMAL 

AGING REACTORS (OPTAR) 

UNITS OPERATE AT 

70, 90, AND 105OC 

IN A TRIAL RUN, UNITS 

SUCCESSFULLY YIELDED 

ACCELERATING AGING 

RATES OF POLYPROPYLENE 

AT 70, 90, AND 105°C, WHICH EXTRAPOLATED 

TO THE ACTUAL AGING 

TIME OF POLYPROPYLENE 

AT ROOM TEMPERATURES 

NATURAL OUTDOOR AGING 

OF POLYPROPYLENE 

200 500 2000 

AGING TIME, HOURS 

Figure 6 7. Outdoor Accelerated Aging Testing of Module Designs and Low-Cost Encapsulation Materials 

Table 2 1. Encapsulation Materials-1 985 

MODULE ELEMENT 

SUPERSTRATE 

ISTRUCTURAL) 

PLASTIC FILMS 

IUV SCREEN) 

PLASTIC FILMS 

INON UV SCREEN) 

PRIMERS 

SURFACE TREATMENTS 

PROJECTED COST IN 

PRODUCTION OUANTlTlES 

$/m* (1980 DOLLARS) I THICKNESS (mrn)* 

550 850 30 50 

5W 6W I 30 50 

100 300 0025 010 

050 070 005 008 

- - ANTI SOILING TREATMENTS 0 .' 

HIGH COST CASTING 3000 60W I5 30 LIOUID 

THERMOPLASTIC IN SAFETY GLASS LAMINATION 3 60 07 

MELTS AND CURES DURING LAMINATION 100 200 05 10 

POTTANT SILICONE RUBBER (RTV 615 SYLGARDI 

POLYVINYL BUTERAL (PVB SAFLEX) 

ETHYLENE VINYL ACETATE (EVA1 

ETHYLENE METHYL ACRYLATE ICMA) 100 200 05 10 

150 300 05 10 

0 0751m2 0 125 

.. SOFTENS AND CURES DURING LAMINATION 

ALIPHATIC POLYETHER URETHANE .@ 

FOR AIR RELEASE AND ELECTRICAL ISOLATION SPACING 

DIELECTRIC AND THERMAL MATCH 550 850 30 50 

SPACER NONWOVEN GLASS MAT [CRANEGLASI 

SUBSTRATE GLASS TEMPERED OR ANNEALtD HIGH STRENGTH 5 40 0 91 

COST AND THERMAL EXPANSION DISADVANTAGES 10 W 15 

DIELECTRIC FILM 1 Wlrn2 0 13 

DIELECTRIC FILM 1W 0 05 

BETTER THERMAL EXPANSION MATCH THAN AI 

(STRUCTURAL1 STEEL 

ALUMINUM (SHEET AND EXTRUSIONS1 

POLYESTER FILM - [MYLAR) 

FLUOROCARBON PIGMENTED ITEDLARI 

ALUMINUM FOILIPOLYMER LAMINATES 

FIBER GLASS COMPOSITE 

BACK COVER 

'. PROVIDES HERMETICITY AND DIELECTRIC 1W 200 005 013 '. 1W 200 005 013 

'' FLAME RESISTANT BACK COVER - - STEEL FOILIPOLYMER LAMINATES 

. 

" 

"'MATERIAL UNDER DEVELOPMENT AND EVALUATION FOR PV 

USED IN CURRENT PV MOWLES 

COMMERCIAL MATERIAL BEING EVALUATED FOR PV MODULES 

'0001 ~n = 0025 mm 0040 tn = 1 0 mm 

75 
Today’s Status 

Superstrate encapsulation designs, materials, 

and material processing are compatible with 

mass production requirements for module$ 

using crystalline silicon solar cells and should 

meet cost allocations. 

Encapsulation materials have been life-tested to 

20 years. 

Advanced encapsulation materials are expected 

to have a 30-year life expectancy. 

More testing and field operational results are 

needed to determine actual module life 

capabilities. 

Significant Accomplishments 

Assessed module encapsulation requirements 

and potential materials for use under terrestrial 

operating conditions. 

Developed a polymeric pottant material (EVA) 

and processing techniques suitable for mass 

production of PV modules. 

Developed back-cover concepts and materials. 

Developed primers and adhesives for glass 

superstrate modules using the above materials. 

Superstrate and substrate module design 

concepts devised and verified by passing 

module qualification tests. 

Devised photodegradation resistant additives 

for pottants and devised and carried out 

degradation tests that verified their value. 

Devised life prediction modeling and tests, and 

performed studies of module encapsulation 

lifetimes. 

Encapsulation materials life-tested to 20 years. 

Materials industry responded very well to JPLdefined material development needs and have 

established commercial products. 

Encapsulation System R&D Needs 

Thin-film module encapsulation systems. 

Flexible top cover films. 

Durable bonding technology for thin-film 

modules. 

Life verification of above and superstrate 

module encapsulation systems for crystalline 

silicon solar cell modules. 

76 
Reliability Physics 

Reliability has always been an inherent part of FSA 

activities. In fact, efforts in all parts of the Project have 

been directed toward a lowcost, long-life product. As 

terrestrial PV technology evolved, the more easy-tosolve failure mechanisms were addressed first; in 

parallel, excellent progress was made on long-term 

studies of encapsulation materials and designs for longlife modules. As each aspect of module technology 

evolved, as an entity, the need to understand all the 

degrading mechanisms and their interactions within a 

module became apparent. These efforts have been 

consolidated and are called Reliability Physics. 

The objective is to develop the reliability and 

durability knowledge required to produce long-life PV 

modules and arrays: 

Develop an understanding of module long-term 

degradation and failure mechanisms and 

techniques for reducing degradation. 

Develop a reliability prediction capability. 

Develop accelerated test capabilities. 

Verify material, device, and module reliabilities. 

Background 

In the early 198Os, when the DOE PV Program 

emphasis was shifted to electric utility applications, 

module and array lifetime goals were increased to 

30 years from 20 years. This was considered to be a 

practical goal based upon technology advancements, 

typical utility planning for 30-year operational times, 

and the economic advantages of longer operation of 

power systems. 

basic understanding be determined of all the 

phenomena involved in modulelarray degradation. A 

long-range integrated approach of analyses and 

experiments covering all important parameters that can 

shorten the life and/or reduce the power generating 

capabilities of PV arrays was established. Within FSA, 

these efforts are now grouped under Reliability Physics 

and include the durability of cells, cell metallization, cell 

interconnects (within a module), encapsulation 

materials and their use, effects on module life of 

fabrication processing; and the influence of operational 

and environmental conditions. 

The longer-life requirement necessitated that a more 

Project Activities 

During the course of the project, a steady stream of 

module failure mechanisms (Figure 68) was observed 

and identified through module testing, application 

experiments, and failure analyses. To resolve the 

reliability problems a systematic research effort was 

undertaken with parallel efforts focused on the most 

troublesome failure mechanisms. Initial emphasis, 

placed on the development of test methods useful for 

quantifying the reliability weaknesses during the 

module design phase, led to the continuing evolution 

and development of module qualification tests. These 

tests and their severity were selected to fail older 

module designs with known deficiencies and to pass 

modules with good in-the-field performance. As module 

designs and fabrication techniques improved, emphasis 

was gradually shifted toward achieving a physical 

understanding of the longer-term, less-well-understood 

failure mechanisms and devising qualification tests and 

design solutions for them. Research activities were 

initiated studying interconnect fatigue (Figure 69), 

sheet glass strength (Figure 70), module soiling (Figure 71 ), hot-spot heating (Figure 72), breakdown of 

electrical insulation materials (Figure 73) and various 

others. All of these activities have led to comprehensive amounts of directly usable data and knowledge 

as shown in Figure 74. This information has been 

quickly transferred to industry and continues to be 

incorporated into production module designs and 

fabrication techniques. 

Over the years, the reliabilityldurability research in 

FSA has evolved into a general methodology with six 

major elements: 

Identification of key degradation mechanisms. 

Establishment of mechanism-specific reliability 

goals. 

Quantification of mechanism parameter 

dependencies. 

Development of degradation prediction methods. 

Identification of costeffective solutions. 

Testing and failure analysis of trial solutions. 

Although a substantial degree of synergism exists 

among these elements, it is useful to address each 

separately. 

Identification of the failure mechanism is a critical 

step. During the early years of the Project, module 

qualification testing was effective in detecting module 

failure. With the better quality of contemporary 

modules, the best evidence of a generic problem is 

well documented field failures. This requires careful 

monitoring of field applications with statistically 

significant numbers of modules, an active problemfailure reporting system, and a comprehensive failure 

analysis capability. The voltages and currents in an 

operating system also play an important role in hotspot heating failures, shorts to ground and incircuit 

arcs, Failures that show up only after prolonged field 

exposure must be identified by a variety of accelerated 

tests, unless waiting for years is acceptable. 

77 
FAILURE 

RATE 

LIFE PREDICTION 

OBJECTIVE: DEVELOP AND VERIFY DEGRADATION RATE 

MODELS FOR EACH WEAR.OUT TYPE FAILURE 

MODULE FAILURE TYPES 

WEAR-OUT (AGING) 

- CORROSION - MATERIAL DEGRADATION - FATIGUE (CYCLING1 - WEAR (DIMENSIONAL1 - DAMAGE ACCUMULATION 

\ 

(INFANT MORTALITY) 

MANUFACTURING ' . PROCESS QUALITY . - MATERIALS ' " 

- CONTAMINATION - DIMENSIONS - ASSEMBLY - HANDLING 

OVERSTRESS 

- RANDOM 

TIME IN OPERATION 

Figure 68. Module Failure-Rate Classification 

I II 

Figure 69. Electrical Interconnect Fatigue Research 

UNIFORMLY LOADED, SIMPLY SUPPORTED. 

RECTANGULAR GLASS SHEET I 

CORNER STRESSES ARE 7 

I I11111111 I I,,,,,, I 1 I,,, 

100 lo00 lo00 1W 

LOAD INTENSITY FACTOR 

Figure 70. Glass Breakage and Sizing Research 

Figure 7 1. Module Soiling Research 

78 

T0II)INCI CA lldlt UT LO* LNOllEP CA PHOTOGRAPHS OF PARTICLES 

DEPOSITED ON MODULES 

IScannmg Electron MICIOSCOP~I 
EXPERIMENTAL APPARATUS OBSERVED MODULE 

RESPONSE vs CELL TEMPERATURE 

Figure 72. Hot-Spot Endurance Test Development 

HIGH VOLTAGE THIN FILM 

INSULATION BREAKDOWN 

RESEARCH APPARTUS 

PROBABILITY OF BREAKDOWN P 

THIN FILM 

TEST BREAKDOWN DATA 

Figure 73. Insulation Breakdown Research 

Technology 

Bond Delamination 

Interconnect Fatigue 

Galvanic Corrosion 

Electrochemical Corrosion 

Photothermal Degradation 

Structural Failure 

Hail Impact 

Glass Breakage 

Cell Fracture Mechanics 

Voltaga Breakdown 

Hot-spot Heating 

Soiling 

V = Major Contribution 

A = Significant Contribution 

Figure 74. Reliability and Durability Developments, 

1975 to 1984 

79 
Thirteen principal failure mechanisms of flat 

crystalline-silicon modules, grouped into four types of 

degradation are shown in Table 22. The key failure 

mechanisms identified to date, and target degradation 

levels for each that are consistent with a 30-year 

lifetime, are shown together with their economic 

significances and target allocation levels. The units of 

degradation in the third column provide a convenient 

means of quantifying failure levels of individual 

mechanisms according to their approximate time 

dependence. Columns 4 and 5 indicate the level of 

degradation for each mechanism that will result in a 

10% increase in the cost of delivered energy from a 

large PV system. Because the mechanisms will 

generally occur concurrently, the total cost impact is 

the sum of the 13 cost contributions. Column 6 of the 

table carries the analysis a step further and suggests a 

strawman allocation of allowable degradation among 

the 13 mechanisms to achieve a total reliability 

performance consistent with expectations of a 30-year 

lifetime. The total effect of the allowable levels is a 

20% increase in the cost of energy over that from a 

perfect, failure-free system. The distribution among the 

mechanisms reflects the judgment of FSA personnel at 

this time. It is important to update these allocations as 

field operation experience becomes available in the 

next few years. 

A specific degradation or failure mechanism is 

dependent upon a large number of field-stress 

parameters, system operating parameters, componentdesign parameters, and manufacturing-processing 

parameters. In order to understand the test data, a 

quantitative understanding of the importance of each 

principal parameter is required. This is best derived 

from understanding the chemical and physical 

processes in the degradation, plus a qualitative insight 

into the mechanism physics as can best be determined 

by specific tests and analyses. 

A specific degradation prediction can be made by 

determining the time history of an applied stress of the 

appropriate parameter. A variety of these 

environmental stress characterizations have been 

developed at JPL including hail impact probability, 

wind loading pressures, and array voltage and current 

durations. Solar radiation surface meteorological 

observations (SOLMET) weather data nave been used 

to model ultraviolet, temperature, and humidity levels 

to which modules are exposed. These models have 

been combined with other parametric variables in 

making module useful-life predictions. 

trade-offs of degradation rates, failure rates, and 

lifetimes against initial-manufacturing costs, field 

maintenance costs, and lost energy revenues. Lifecycle costing analysis integrates these variables and 

permits trade-offs to be made. In the identification of 

cost-effective solutions, it is important to minimize 

sensitivities to processing variations and design and 

material uncertainties. These analyses permit rational 

judgments and cost-effective levels of reliability to be 

selected. 

Cost effectiveness of modules and arrays requires 

The first and last step in a reliability program is 

extensive testing of the complete product. Such testing 

cuts across the entire reliability development effort 

from beginning to end. It plays a key role in the 

identification of failure mechanisms, in the illumination 

of parameter dependencies, in the selection of costeffective solutions, and in the verification of final 

designs. By the end of the reliability effort, tests with 

quantitative correlation to field applications should be 

available, based on the parameter dependencies 

determined and the prediction algorithms developed. A 

key element of complete-product testing is inclusion of 

the synergistic reactions between all of the 

components, including likely interfacing components on 

Table 22. Life-Cycle Cost Impacts and Allowable Degradation Levels 

for ' Economic 

Failure Mechanism Degradation I Typeof I Degrad. 

'k = Discount rate 

80 
the user’s side of the interface. This latter consideration 

is one of the reasons field-application data are more 

convincing than laboratory test data because they 

include interaction with the user. 

Sign if i can t Accomplishments 

Developed long-term soiling data for various 

module surface materials and synthesis of 

antisoiling surface treatments. 

Developed analysis tools, test methods, and 

design data for the control of solarcell hot-spot 

heating. 

Developed data on probability of impact of 

various-sized hailstones, and means of surviving 

hail impact. 

Developed analysis tools and design data for 

predicting the breaking load of glass sheets. 

Developed design data and test methods on the 

photothermal stability of module encapsulants. 

Developed test methods and performance data 

on the reliability of solarcell metallization 

systems. 

Characterized the electrical breakdown strength 

of polymeric dielectric films. 

Characterized the parameters involved in 

electrochemical corrosion of modules. 

Developed analysis tools and design data for 

predicting interconnect fatigue and designing 

long-life, reliable cell interconnections. 

Developed design data and qualification testing 

technique to ensure the reliability of bypass 

diodes. 

Developed design data and testing techniques on 

the fracture strength of crystalline-silicon wafers 

and cells. 

Developed a comprehensive set of module 

qualification tests, testing facilities, and failure 

analysis facilities. 

81 
Module Performance 

and Failure Analysis 

In 1975, at the start of the FSA Project, litte or no 

quamtitative da.?a were available en the electrica.! 

efficiency of the early PV modules or on the exact 

cause of the reliability problems common to modules 

in use at that time. Building on JPL’s history of 

making precise measurements and performing failure 

analyses as part of its space photovoltaic activities, 

the Project initiated the measurements, testing, and 

failure analysis of terrestrial modules in 1975. This 

was an important part of the Block I module 

procurement activities and it has continued 

throughout the duration of the Project. 

The objectives of the module performance 

measurement and failure analysis activity are to 

support the module development effort: 

By providing accurate electrical measurements 

of state-of-the-art and advanced development 

modules, including developing and providing 

international recognized electrical performance 

measurements and facilities. 

By conducting various characterization and 

environmental qualification tests including the 

development of unique testing methods and 

fac i I i ties. 

By establishing a problem-failure reporting 

procedure, conducting detailed analysis of 

module failures, and developing advanced 

failure analysis techniques and facilities. 

Background 

At the start of the JPL Project, it was recognized 

that to succeed with the development of advanced 

module technologies there must be accurate 

measurements data to continually determine the 

status of the technology, and to guarantee accurate 

communication among the large number of 

researchers involved. The need to accurately define 

costs and cost standards was the responsibility of 

the Project Analysis and Integration portion of the 

Project; the need to accurately measure efficiency 

and reliability became this activity’s responsibility. 

This evaluation function served to determine whether 

design and fabrication innovations were successful in 

their objectives, and to identify deficiencies that 

should be the object of design and fabrication 

modifications or the subject of further research. 

Project Activities 

Electrical Performance Measurement 

The electrical performance of a new module 

design is the necessary criterion for determining 

82 

whether module innovations have resulted in 

increased efficiency. Therefore, progress would have 

been impossible to assess without the development 

of precise and accurate methods of performing this 

measurement. Because the measurement of modules 

under natural sunlight is prohibitively time-consuming, 

use is made of solar simulators. Early in the Project, 

a standard irradiance (magnitude and spectral 

distribution) was defined and various solar simulators 

were put into commercial use by the many 

companies and laboratories involved in the program. 

Because none of the simulators could duplicate the 

standard spectrum, methods were developed for 

correcting the simulator measurement to the value of 

power output that would have been obtained if the 

module had been exDosed to a standard irradiance. 

In 1975, electrical performance measurement of 

solar cells and arrays had already undergone considerable development in connection with the use of 

these devices on spacecraft over the prior 15 years. 

Measurement for that application was simpler than 

measurement for the terrestrial application because 

the solar irradiance spectrum above the Earth’s 

atmosphere is more stable and more easily measured 

and described. It was also feasible to devote 

extensive resources to the measurement of a single 

spacecraft module (or array) because of its high cost 

and need as part of space mission success. 

In the terrestrial application, the irradiance 

spectrum is far more difficult to measure and define 

because it is subject to spectral absorption and 

scattering in the atmosphere and because it consists 

of both a direct and diffuse component. Because the 

commercial cost of an individual module must be 

low, the resources allotted to its measurement must 

also be low. 

The first method consisted of performing a primary 

(natural sunlight) calibration of a solar cell that was a 

sample of the cells used in the modules; it could be 

presumed to have the same spectral response. Using 

this cell as a reference cell for adjusting the 

magnitude of irradiance from any simulator, the 

resultant measurement was then independent of the 

simulator spectrum. Subsequently, a new method 

known as mismatch factor correction was developed. 

With this method, which required knowledge of the 

simulator spectrum and of the spectral response of 

both the reference cell and the module, it was 

possible to perform a computer correction of the 

measured power value even though the reference 

cell spectral response did not match that of the 

module. 

The disadvantage of the above two methods is 

that either they assume that the spectral response of 

the module is the same as that of the cell, or they 

require the difficult or impossible operation of 

measuring the spectral response of the module. A 

solution to these problems consisted of designing a 

simulatorlfilter combination that produced the 
standard spectrum. Such a system has now been 

implemented for both of the current irradiances 

defined in the US. standards documents. These are 

the airmass 1.5 direct normal irradiance (ASTM E 

891 -82) and the airmass 1.5 global spectrum 

(combining direct and diffuse components, ASTM E 

892-82). With this system, the reference cell need 

only be of the same generic material as the module’s 

cells, and very large modules can be measured 

because their spectral response need not be known. 

The simulator used in this system is known as the 

Large Area Pulsed Solar Simulator; it produces the 

spectrum shown in Figure 75 when it is not filtered. 

The filtered spectra for the direct normal case and 

the global case are shown in Figures 76 and 77, 

respectively, along with the desired standard spectra. 

These spectral matches are close enough in both 

cases so that the error due to the mismatch is not 

greater than 1%. 

180 

I I I 1 I I I I 1 

- - ---- ASTM E89142 DIRECT IRRAD - 

Figure 75. Spectral Irradiance (JPL Unfiltered 

LAPSS) 

I 

- 

- 

- 

- 

- 

- 

nd 

CL *- 

E 40- --- - 

- 

Y4- 

20- 

0 I I I I I I I I I 

Y 

- 

303 400 500 6w 700 800 900 1wO 11M 1200 1300 

Figure 76. Spectral Irradiance (AM 1.5 Direct LAPSS 

Versus ASTM AM 1.5 Direct) 

ol’ I 1 I I I I I I I‘ I I 

300 400 500 600 700 800 900 lMl0 1100 1200 1300 

WAVELENGTH (nanometers) 

Figure 77. Spectral Irradiance (AM 1.5 Global LAPSS 

Versus ASTM AM 1.5 Global) 

a3 
Qualification Testing 

In addition to determining whether a new module 

design provides a desired increase in efficiency, the 

electrical performance measurement of a module 

also serves as the principal criterion for determining 

the success of a new module in terms of its ability to 

withstand the environmental and electrical stresses 

expected in the field. An accelerated program of 

these stresses has been applied to all new modules 

developed under the FSA Project as well as to other 

commercial modules available from US. and foreign 

sources. The test program consists of a set of 

formally defined qualification tests whose description 

and sequence are illustrated and described in detail in 

Figure 78. As the figure also shows (under the 

heading "Qualification Test Specifications"), the 

specifications evolved during a series of module 

development programs over a 10-year period. The 

evolution has resulted from the information provided 

by the tests themselves, from module field test 

experience, and from module research. 

Qualification tests have been performed on 

modules of about 150 different designs. With few 

exceptions, the tests have revealed defects 

necessitating module design or manufacturing 

process changes. This is proof of the critical role of 

this type of testing in identifying module 

modifications needed, and in identifying required 

research in materials, processes, andlor design. 

URGE AREA SOUR SIMULATOR 

I 

n 

Figure 78. Module Qualification 

, .. 

a4 
Qualification tests are performed on a small 

sample of modules of any given design. The proof of 

satisfactory performance, including reliability, 

requires field testing the modules in the various 

environments in which PV power stations would be 

installed. Such a program was instituted early during 

the Project history using the sites listed in Figure 79. 

The performance observed in the tests at these sites, 

and at other array installations that were part of the 

DOE program, resulted in important modifications to 

the earlier qualification test specifications. These 

provided for better accelerated testing to reveal the 

potentiality of failures because of such phenomena 

as hail impact, thermal stress, and cell back-biasing. 

The early history of failure statistics is shown in 

Figure 80. Because of reductions in support of field 

testing, statistics are not available for the Block IV or 

Block V modules. 

Failure Analysis 

When problems occur during qualification tests, or 

perform an in-depth failure analysis to find the exact 

cause of the problem. A Problem Failure Reporting 

system was established by the FSA Project in 1975 

to provide formal reporting of all failures, regardless 

of the site of occurrence. The reports and the failed 

modules were delivered to failure analysis personnel 

who then applied a variety of sophisticated techniques (some derived from the NASA space exploration program and some developed explicitly for the 

terrestrial PV program) to isolate the specific cause of 

the failure (Figure 81). A highly detailed report was 

prepared describing each such analysis and 

presenting the results, with recommendations for 

correcting the module deficiencies. These 

Performance and Failure Analysis Reports were 

supplied to the manufacturer of the module and to 

JPL personnel responsible for module development 

research. During the various module development 

programs, such analyses were instrumental in 

correcting design and processing problems to the 

extent that new modules incorporating the 

recommended changes were then successful in 

field tests, or at array installatbns, it is necessary to passing the qualification tests. 

MODULE FIELDTESTING-16 SITES 

PASADENA. CA (JPLI 

PRIME SITE FOR FIELD 

TESTING 

L 

HOUGHTON. MI 

TYPICAL REMOTE SITE 

FIELD TEST SITES 

EXTREME WEATHER 

FORT GREELY, ALASKA (ARCTIC) 

FORT CLAYTON, CANAL ZONE (TROPIC1 

MARINE 

KEY WEST. FLORIDA 

SAN NICOLAS ISLAND, CALIFORNIA 

POINT VICENTE, CALIFORNIA 

HIGH DESERT 

ALBUQUERQUE, NEW MEXICO 

DUGWAY. UTAH 

GOLDSTONE, CALIFORNIA 

MOUNTAIN 

MINES PEAK, COLORADO TABLE MOUNTAIN, CALIFORNIA 

URBAN COASTAL 

NEW ORLEANS, LOUISIANA 

NEW LONDON. CONNECTICUT 

MIDWEST 

CRANE, INDIANA 

UPPER GREAT LAKES 

HOUGHTON, MICHIGAN 

NORTHWEST 

SEAlTLE. WASHINGTON 

URBAN SOUTHWEST 

PASADENA, CALIFORNIA 

Figure 79. Module Field Testing (1 6 Sites) 

85 
MSINAL PAGE 8 

OF POOR QUAW 

NEEDS FOR LONG-LIFE. DURABLE MODULES/ARRAYS 

MODULE AND ARRAY FIELD TESTING FIELD TESTING IN REAL-USE POWER 

SYSTEM CONFIGURATIONS REQUIRED BECAUSE MODULE QUALIFICATION TESTING HAS NOT BEEN ENTIRELY 

KNOWLEDGE OF MODULE WEAR-OUT MECHANISMS WHICH OTHER THAN CELL 

SUCCESSFUL IN PREDICTING AND/OR DUPLICATING MODULE FIELD FAILURES DETECTION OF DEFICIENCIESFAILURES 

WHICH OFTEN HAVE BEEN A SURPRISE 

PERFORMANCE VERIFICATION, UNDER OPERATIONAL CONDITIONS. OF NEW ADVANCED MODULES, WITH INNOVATIVE 

DESIGN FEATURES 

INTERCONNECT FAILURES HAVE NOT BEEN DETECTED AS Y'ET. POTENTIAL LONG-TERM OPERATION DEFICIENCIES/FAILURES ARE EVIDEN~ CRACKED CELLS 

AND COVER GLASS) 

MODULE TESTING IN ARRAYS 

REDUCTION OF SINGLE MODULE ENDURANCE FIELD TESTING IN FAVOR OF 

FAILURE. RATES FOR MODULES AT FSA FIELD TEST SITES 

211 ' 1 I ' 1 I ' ' I ' ' ' 1 

18 i 

15- 

12 ~ 

261 MODULES DEPLOVED 48 FAILURES 

/ J- BLOCK I 

326 MODULES DEPLOVED 

12 FAILURES 

I 

w I63 MODULES DEPLOVED 

i! 

n 31//-<;, 0 n 6 12 18 24 30 36 42 ~, 

TIME IN FIELD, MONTHS 

KNOWLEDGE @F MODULE/ARRAY 

PERFORMANCE AND SAFETY PROBLEMS WITH THE REMAINDER OF THE POWER SYSTEM AND WITH UTILITY INTERACTIONS 

CENTRAL STATION: ESTABLISHMENT 

OF CREDIBILITY OF MODULElARRAYS 

AS POWER GENERATORS FOR 

CENTRAL STATION USE; i.e.. 

COST EFFECTIVE WITH LONG LIFE OPERATION 

*FAILURE - OUTPOWER DECREASE 

GREATER THAN 25% 

Figure 80. Needs for Long-Life, Durable Modules/Arrays 

MODULE PROBLEMlFAlLURE ANALYSIS 

X-RAY OF OVERHEATED 

CELLS SHOWING MELTED SOLDER OIJTPUT SHOWING CRACKED CELL 

LASER SCAN OF MODULE 

PHOTOMICROGRAPH OF HAIL DAMAGE 

I..__..__.... 

OF INSULATOR 

PROBLEMSlFAlLURES AT TESTlAPPLlCATlON 

JPL AND MANUFACTURER EVALUATE 

MANUFACTURER CHANGES MODULE 

SITES REPORTED 

PIF AND DETERMINE CORRECTIVE ACTIONS 

DESIGN OR WORKMANSHIP AS NEEDED 

Figure 8 1. Module Problem/Failure Analysis 
I. . 

Significant Accomplishments 

Environmentally tested more than 150 different 

module designs to date over the 10-year period. 

Processed 1200 reports of failures, involving 

460 major failure analyses. 

Developed unique environmental testing 

facilities including hail guns, mechanical cyclicloading apparatus, hot-spot test equipment, and 

nominal operating cell temperature (NOCT) test 

racks and instrumentation. 

Established outdoor field test-rack facilities 

throughout the United States in various 

geographical climates. 

Developed module inspection techniques and 

guidelines. 

Developed a unique Large Area Pulsed Solar 

Simulator with closely matched airmass 1.5 

direct and airmass 1.5 global spectra. 

Calibrated numerous reference cells that have 

served as the primary industry standards over 

the past 10 years. 

Developed a simple, accurate method of 

secondary calibration of reference cells; have 

calibrated cells for many U.S. and foreign PV 

manufacturers. 

Developed a unique laser scanner instrument 

for the failure analysis of large-area modules. 
Appendix A: Glossary 

A 

A&E 

AM 

AR 

a-Si 

ASME 

ASTM 

BSF 

BSR 

C 

CAST 

COSMIC 

CVD 

CVT 

cz 

DCS 

DOE 

EFG 

EMA 

EPSDU 

ERDA 

EVA 

FAST 

FBR 

FSA 

FZ 

h 

HEM 

Isc 

I -v 

ID 

Ampere(s); Angstrom(s) 

Architects and engineering 

Air mass (e.g., AM1 - uni? 

Antireflective 

Amorphous silicon 

/I mi! 

American Society of Mechanical Engineers 

American Society for Testing and Materials 

Back-surface field 

Back-surface reflection 

Celsius (temperature scale) 

Capillary Action Shaping Technique 

Computer Software Management Information Center 

Chemical vapor deposition 

Chemical vapor transport 

Czochralski (classical silicon crystal growth method) 

Dic hlorosilane 

U.S. Department of Energy 

Edgedefined film-fed growth (silicon-ribbon growth method) 

Ethylene methyl acrylate 

Experimental process system development unit 

Energy Research and Development Administration 

Ethylene vinyl acetate 

Fixed Abrasive Slicing Technique 

Fluidized-bed reactor 

Flat-Plate Solar Array Project 

Float-zone (silicon crystal growth method) 

Heat transfer coefficient; hour(s) 

Heat-exchange method (silicon-crystal ingot-growth method) 

Short-circuit current 

Current-voltage 

Inside diameter 

88 
IPEG 

IR 

Solar-Cell Analysis Program in One Dimension 

89 

! 

Jsc 

JPL 

Kg 

LAPSS 

LASS 

MEPSDU 

mg-Si 

MlNP 

MOD 

MSEC 

MT 

NASA 

N EC 

NOCT 

O&M 

Pmax 

PA&I 

PDU 

PIM 

PPba 

PPm 

PU 

PV 

PVB 

R&D 

RH 

S 

SAMICS 

SAMIS 

SCAP1 D 

Improved Price Estimation Guidelines 

Infrared 

Short-circuit current density 

Jet Propulsion Laboratory 

Kilogram 

Large-area pulsed solar simulator 

Low-angle silicon sheet growth method 

Module experimental process system development unit 

Metallurgical-grade silicon 

Metal insulator, n-p 

Metallo-Organic deposition 

Mobil Solar Energy Corp. 

Metric ton(s) 

National Aeronautics and Space Administration 

National Electrical Code 

Nominal operating cell temperature 

Operation and maintenance 

Maximum power 

Project Analysis and Integration Area (of FSA) 

Process development unit 

Project Integration Meeting 

Parts per billion, atomic 

Parts per million 

Polyurethane 

Photovoltaic(s) 

Polyvinyl butyral 

Research and development 

Relative humidity 

Second(s) 

Solar Array Manufacturing Industry Costing Standards 

Standard Assembly-Line Manufacturing Industry Simulation 
SCCD Short-circuit currentdecay 

SEEMA Solar-Cell Efficiency Estimation Methodology and Analysis 

SERl Solar Energy Research Institute 

Si Silicon 

SIMRAND * 

SMUD 

* SlMulatibn of Research And Development Projects 

e * .. = 

Sacramento (California) Municipal Utility District 

SOA State of the art 

SOC Silicon-on-ceramic 

SOLMET 

STC Silicon tetrachloride 

Solar radiation surface meteorological observations 

T Temperature 

TC M Transparent conducting material 

TCS Tric hlorosilane 

UL Underwriters Laboratories, Inc. 

voc Open-circuit voltage 

Peak watt@) wP 
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95 
. 96 
More Technology Advancements 

Dendritic web silicon ribbons are grown to solarcell 

thickness. Progress is shown by experimental ribbons 

grown in 1976 and 1978 and a ribbon grown in a 

Westinghouse Electric Corporation pilot plant. 

The edgedefined film-fed growth silicon ribbons are 

grown to solarcell thickness. A DOE/FSA-sponsored 

research ribbon grown in 1976 is shown next to a 

nine-sided ribbon grown in a Mobil Solar Energy 

Corporation funded configuration. 

F INGOT t?ROWN 

USING SILICON MELT 

REPLENISHMENT 

Czochralski silicon crystals as grown are 

sawed into thin circular wafers. (Support for 

this effort was completed in 1981 .) 

Prototype modules have passed UL 790 Class A 

burning brand tests which are more severe than 

this spread of flame test. I1 

/’ ~SEAL \-- 

GLASS (STRUCTURAL1 

SPACER 

POTTANT 

SOLAR CELLSINTERCONNECTED 

SPACER 

POTTANT 

-, BACK COVER FILM 

lCOMPOSlTEl GASKET 

Typical superstrate module design is shown with the 

electrically interconnected solar cells embedded in a 

laminate that is structurally supported by glass. 

Materials and processes suitable for mass production 

have been developed using this laminated design. 

This 13.7% efficiency prototype module was made 

by Spire Corporation. If solar cells more recently 

fabricated from float-zone silicon wafers were made 

into a module, the module efficiency should be 

about 15%. 
Photovoltaic Applications 

197 'k 

buoy 

?owered 

Photovoltaic-powered corrosion protection 

of underground pipes and wells. 

Later.. . 

House in Carhsle, Massachusetts, with a 7.3-kW 

photovoltaic roof-top array. Excess photovoltaicgenerated power is sold to the utility. Power is 

automatically supplied by the utility as needed. 

A 28-kW array of solar cells for crop irrigation 

during summer, and crop drying during winter 

(a DOEIUniversity of Nebraska cooperative project) 

7985 

1.2 MW of photovoltaic peaking-power generation 

capacity for the Sacramento Municipal Utility District 

(The 8 x 16 ff panels are mounted on a north-south 

axis for tracking the sun.) 

ORlGtNAL PAGE 

mLOR PHOTOGRAPH 

JPL 400-279 10185 

(5101-279)