Solar Cells Operating Principles Technology and System Applications

139 Pages·1982·17.977 MB·English

by  Martin A. Green

. i-
Lib of Congrefl nC aPtuablloigciantgi oIn Dato
CL ASIF
Green,a rMi tn A.
Solar cells. ADQUIS•
ele(cPtrren icoten-iHcalsl} serie dis t saitne s oplhysical FECHA
Bpi.b liograp: hy PROCED.
21..I nPchluoidtnSedoosxel v.a orl ctealilcs. power eeneratlon.
hn,O-.o1�· -
ISBN 0·13·822s2e7r01·e3s .s2u1'8A214A-4C4 R3526
To Judy and Brie
CONTENTS
Editorial/production supervoirs dieosni gann:d interi
BARBARA BERNSTEIN
Manufacluring buyer: JoYCE LEVATINO
e 9182 by Prentice-Hall, Incl.i,r rEsng,l eNw.oJo.d 0C7632
PREFACE
xiii
All rigehstesr vred. No part of this book may
be reprodunc eadn yi form or
by nay means without. permission in writing Chapter 1S.O LAR CELLS AND SUNLIGHT 1
from the publisher.
1.1 Introduction
1.2 Outline of Solar Cell De2v elopment
Printed in the Unsi toefd ASmteartieca 1.3 Physical Source of S2u nlight
10 9 8 7 6 6 4 3 2 1 1.4 The Solar Const4a nt
1.5 Solar Intensity at the Ear5t h's Surface
1.6 Direct and Diffuse Rad6i ation
1. 1 Apparent Motion of th8e Sun
1.8 Solar lnsolation8 Data
1.9 Summary 9
PRENTICE-HALL INTERNATIONAL, INC., London
PRENTICE-HALL oF AusTRAUA PyTd.n eLyI ITED, S 1 h.1pterR E2V.I EW OF SEMICONDUCTOR PROPERTIES 13
PRENTICE-HALL OF CANADA,. oL To., Toron
PRENTICE-HAFL LI NODIA PRIVIATTEED ,L INew Delhi 2.1 Introductio1n3
PRENTICE-HALL OF JAPAN, INC., Tokyo 2.2 Crystal uSrter uacntd Orientat1i4o ns
PRENTICE-HALL OF SouTHEASLTT DA.s,iI nASg aPproEr.e
WmTEHALLS BLoIoITED, Wellin getaolna,n dN e 2.3 Forbidden Energy Gaps 17
v
vi o11 11 I Contents ii
2.4 lroh,1brli1y of Occupation of Allowed hapter p·nJ UNCTION DIODES 62
f 4.
Stt1t $ 18
2.& h·ctrons and Holes 20 4.1 Introduction 62
I 4.2 Electrostatics of Junctions 63
Dynamics of Electrons and Holes 21 p-n
2.G 4.3 Junction Capacitance 67
2.7 Energy Density of Allowed States 23
4.4 Carrier Injection
2.8 Densities of Electrons and Holes 24 68
4.5 Diffusive Flow in Quasi-Neutral
2.9 Bond Model of a Group IV
Regions 70
Semiconductor 26
4.6 Dark Characteristics 72 4.6. 1
2.10 Group Ill and V Dopants 28 2.11
Minority Canr riers i
Carrier Densities 30
Quasi-Neutral Regions, 72
2.12 Location of Fermi Level in Doped
4.6.2 Minority-Carrier Currents, 74
Semiconductors 32
4.7 Illuminated Characteristics 76
2.13 Effect of Other Types of Impurities 33
4.8 Solar Cell Output Parameters 79
2 .14 Carrier Transport 34
4.9 Effect of Finite Cell Dimensions
2.14.1 Drift, 34
2.14.2 Diffusion, 36 on 10 81
2.15 Summary 37 4.10 Summary 82
Chapter 5. EFFICIENCY LIMITS, LOSSES,
Chapter 3. GENERATION, RECOMBINATION, AND
AND MEASUREMENT 85
THE BASIC EQUATIONS OF DEVICE PHYSICS 40
5.1 Introduction 85
3.1 Introduction 40
5.2 Efficiency Limits 85
3.2 Interaction of Light with
5.2. 1 General, 85
Semiconductor 40
5.2.2 Short-Circuit Current, 86
3.3 Absorption of Light 43
5.2.3 Open-Circoulitta gVe and
3.3. Direct-Band-Gap
1 Efficiency, 86
Semiconductor,
5.2.4 Effiiecncy Limits for
43 3.3.2
Black-Boedlyl sC, 90
IndirSeecmti-cBoannddu-cGtaopr , 45
5.3 Effect of Temperature 91
3.3.3 Other Absorption Processes, 47
5.4 Efficiency Losses 92 5.4.1
3.4 Recombination Processes 50
General, 92
3.4. 1 Relaxation itbor 50E iqmuu,i l
5.4.2 Short-Circuit Current
3.4.2 Radiative Recombination, 50
Losses, 92
3.4.3 Auger Recombination, 52
5.4.3 Open-Circuit Voltage
3.4.4 Recombination through Traps, 53
Losses, 93
3.4.5 Recombination at 5S5 urfaces,
5.4.4 Factor Losses, 96
3.5 Basic Equations of Semiconductor-Device ll
5.5 Efficiency Measurement 98
Physics 56
5.6 Summary 101
3.5.1 Introduction, 56
3.5.2 Poisson's 5E6q iunoa,t
Cl111pter 6. STANDARD SILICON SOLAR CELL
3.5.3 Current Density Equations, 57
TECHNOLOGY 103
3.5.4 Continuity Equations, 57
3.5.5 Equatioent ,S 58 6. 1 Introduction 103
3.6 Summary 59 6.2 Sand to Metallurgical Grade Silicon 105
viii
Contents Co e s
nt nt ix
6.3 Metallurgical-Grade Silicon to
8.7 Optical Design 161
Semiconductor-Grade Silicon 106
8.7.1 Antireflection Coating, 161
6.4 Semiconductor-Grade Polysilicon to
8.7.2 Textured Surfaces, 164
Single-Crystal Wafers 107
8.8 Spectral Response 165
6.5 Single-Crystal Wafers to Solar Cells 108
8.9 Summary 167
6.6 Solar Cells to Solar Cell Modules 111
6.6.1 Module Construction. 111
6.6.2 Cell Operating Chnpter OTHER DEVICE STRUCTURES 170
9.
Temperature, 113
9.1 Introduction 170
6.6.3 Module Durability, 7 74
9.2 Homojunctions 170
6.6.4 Module Circuit Design. 715
6.7 Energy Accounting 117 9.3 Semiconductor Heterojunctions 172
9.4 Metal-Semiconductor
6.8 Summary 119
Heterojunctions 175
Chapter 7. IMPROVED SILICON CELL TECHNOLOGY 121 9.5 Practical Low-Resistance Contacts 177
9.6 MIS Solar Cells 178
7.1 Introduction 121 9.7 Photoelectrochemical Cells 181
7.2 Solar-Grade Silicon 121 9.7.1 Semiconductor-Liquid
7. 3 Silicon Sheet 123 Heterojunctions, 181
7.3.1 Sheet Requirements, 123 9.7.2 Electrochemical Photovoltaic
7.3.2 Ingot Technologies, 123 Cells, 181
7.3.3 Ribbon Silicon, 124 9.7.3 Photoelectrolysis Cell, 183
7.4 Cell Fabrication and 9.8 Summary 183
Interconnection 127
7 .5 Analysis of Candidate Factories 131
7.6 Summary 135 1 h.1pter 10. OTHER SEMICONDUCTOR MATERIALS 187
10.1 Introduction 187
Chapter 8. DESIGN OF SILICON SOLAR CELLS 138 10.2 Polycrystalline Silicon 187
10.3 Amorphous Silicon 190
8.1 Introduction 138
10.4 Gallium Arsenide Solar Cells 192
8.2 Major Considerations 138
10.4. 7 Properties of GaAs, 792
8.2. 1 Collection Probability of
10.4.2 GaAs Homojunctions, 193
Generated Carriers. 138
10.4.3 Ga1-xAlxAs/GaAs Heteroface
8.2.2 Junction Dept/1, 143
Cells, 194
8.2.3 Lateral Resistance of
10.4.4 AIAs/GaAs Heterojunctions,
Top Layer, 145
196
8.3 Doping of the Substrate 147
10.5 Cu2S/CdS Solar Cells 196
8.4 Back Surface Fields 149
8.5 Top-Layer Limitations 150 I 10.5.1 Cell Structure, 196
10.5.2 Operating Characteristics,
8.5.1 Dead Layers, 150
f 197
8.5.2 High-Doping Effects, 751
10.5.3 Advantages and
8.5.3 Contribution to Saturation
Disadvantages of Cu:zS/CdS
Current Density. 753
Cells, 799
Top.Contact Design 153
8.6 10.6 Summary 200
Contents Contents xi
x
Chapter 11. CONCENTRATING SYSTEMS 204 1,h111>ter 14. RESIDENTIAL AND CENTRALIZED
PHOTOVOLTAIC POWER SYSTEMS
11. 1 Introduction 204 249
11. 2 Ideal Concentrators 205 14.1 Introduction 249
11.3 Stationary and Periodically Adjusted 14.2 Residential Systems 250
Concentrators 206 14.2.1 Storage Options, 250
11 .4 Tracking Concentrators 208 14.2.2 Module Mounting, 252
11. 5 Concentrator Cell Design 209 14.2.3 Thermal Generation, 252
11 .6 U ltra-H igh-Etticiency Systems 213 14.2.4 System Configurations, 254
11.6.1 General, 213 14.2.5 Demonstration Program, 254
11.6.2 Multigap-Cell Concepts, 213 14.3 Central Power Plants 256
11.6.3 Thermophorovoltaic 14.3.1 General Considerations, 256
Conversion, 217 14.3.2 Operating Mode, 258
11.7 Summary 219 14.3.3 Satellite Solar Power
Stations, 262
14.4 Summary 263
Chapter 12. PHOTOVOLTAIC SYSTEMS: COMPONENTS
AND APPLICATIONS 222
Appendix A PHYSICAL CONST ANTS 265
12.1 Introduction 222
Appendix B SELECTED PROPERTIES OF SILICON 266
12.2 Energy Storage 223
12.2.1 Electrochemical l\1lpendix C LIST OF SYMBOLS 267
Batteries. 223
12.2.2 Large-Capacity Approaches, BIBLIOGRAPHY 269
225
12.3 Power Conditioning Equipment 226 INDEX 270
12.4 Photovoltaic Applications 227
12.5 Summary 228
Chapter 13. DESIGN OF STAND-ALONE SYSTEMS 230
13.1 Introduction 230
13.2 Solar Module Performance 230
13.3 Battery Performance 232
13.3.1 Performance Requirements,
232
13.3.2 Lead-Acid Batteries. 232
13.3.3 Nickel-Cadmium Batteries,
235
13.4 Power Control 235
13.5 System Sizing 237
13.6 Water Pumping 246
13.7 Summary 247
""
""'""'
PREFJ\CE
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xiii
xiv Preface
cunent �echnology for fabricating them, and probable technological
Ch1apte r
developments in the future. The final chapters deal with system ap­
plications, ranging from the small systems commercially available at
present to residential and central power systems that may be avail­
able in the future.
The book is intended primarily for the increasing numbers of
engineers and scientists attracted to this rapidly expanding field. As
such, it is suitable for use as a textbook for both undergraduate and
graduate courses. deliberate attempt has been made not to exclude
A
the material contained within from those readers who entering
are
the field through a different route. For example, a rather pictorial
review of the properties of semiconductors relevant to the under­
standing of solar cell operation is included. Although this may serve SOLAR CELLS
as a quick review for many readers, for other readers it may provide
a framework on which the material in subsequent chapters can be AND SUNLIGHT
supported. Irrespective of background, working through the text
and associated exercises would place the reader in a very strong posi­
tion for future activity in this area.
I would like to acknowledge the large number of people, too
numerous to mention individually, who have stimulated my interest
in solar cells over the last decade. I would particularly like to thank
Andy Blakers, Bruce Godfrey, Phill Hart, and Mike Willison for their
suggestions and indirect encouragement in this venture. Special
thanks are due to Gelly Galang for her help in preparing the manu­
script and to John Todd and Mike Willison for preparing photographs
incorporated into the text. Finally, I would like to thank Judy Green 1,1 INTRODUCTION
for her support and encouragement during the fairly intense period
l'.lular cells operate by converting sunlight directly into electricity us-
in which this book was developed.
1111{ the electronic properties of a class of material known as semicon-
Martin A. Green u<'tors. In the following chapters, this elegant energy-conversion
1
111u cess will be examined starting from the basic physical principles
solar cell operation. From this basis, the mathematical equations
ol
quantifying the energy transformation are developed. This is followed
I•\. a description of the technology used to produce present commercial
cells, based predominantly on a particular semiconductor, silicon.
•I
111ptrro vements in this technology, as well as alternative technologies
I
hold the promise of significantly lower cost, are then described.
l111d
11111 lly, Che design of solar cell systems is discussed, ranging from
I•
1111111 power supplies for remote-area use to possible future residential
1111 <'Pntral power-generating plants.
I
In this chapter, the history of solar cell development is out-
11111•<1 hridly, followed by a review of the properties of the sun and
ultulion.
It 1
1
1.2 OUTLINE OF SOLAR CELL DEVELOPMENT 1.0
e
Solar cells depend upon the photovoltaic effect for their operation. �
N
This effect was reported init.ially in 1839 by Becquerel, who observed E 0.8
�
a light-dependent voltage bet.ween electrodes immersed in an elec­
'!.
trolyte. lt was observed in an all-solid-state system in 1876 for the =
of selenium. This was followed by the development of photo­ 1;� 0.6
case
8.
cells based on both this material and cuprous oxide. Although a ..
•
silicon cell was reported in 1941, it was not until 1954 that the fore­
runner of present silicon cells was announced. Thisdevicerepresented ·..::I:!:e 0.4
a major development because it was the first photovoltaic structure ] 3000
K (lOX)
that converted light to electricity with reasonable efficiency. These ·.t:...;: : 0.2
e-
cells found application as power sources in spacecraft as early as ·e
..
1958. By the early 1960s, the design of cells for space use had stabi­
:I: 0
lized, and over the next decade, this was their major application. Ref­ 0 1.0 1.S 2.S
erence 1.1 is a good source of more detailed material up to this stage. Wavelength (µm) 3.0
The early 1970s saw an innovative period in silicon cell de­
Figure 1.1. Planckian black-body radiation distributions
velopment, with marked increases in realizable energy-conversion
for different black-body temperatures.
efficiencies. At about the same time, there was a reawakening of
interest in terrestrial use of these devices. By the end of the 1970s,
radiation emitted increase, but the wavelength of peak emission de­
the volume of cells produced for terrestrial use had completely out­
creases. example of thi.s within most of our ranges of experience
An
stripped that for space use. This increase in production volume was
that metal, when heated-, glows red and then yellow as it gets
1s
accompanied by a significant reduction in solar cell costs. The early
hotter.
1980s saw newer device technologies being evaluated at the pilot
Temperatures near the sun's center are estimated to reach a
production stage, poised to enable further reduction in costs over warm 20,000,000 K. However, this is not the temperature that de­
the coming decade. With such cost reductions, a continual expan­
termines the characteristic electromagnetic radiation emission from
sion of the range of commercial applications is ensured for this ap­
the sun. Most of the intense radiation from the sun's deep interior is
proach to utilizing the sun's energy.
absorbed by a layer of negative hydrogen ions near the sun's surface.
Region of fusion reaction, H • He
1.3 PHYSICAL SOURCE OF SUNLIGHT
�Absorption by
Radiant energy from the sun is vital for life on our planet. It deter­
H ions
mines the surface temperature of the earth as well as supplying
virtually the energy for naiural processes both on its surface and -Convective heat transfer
all
in the atmosphere.
The sun is essentially a sphere of gas heated by a nuclear fusion
reaction at its center. Hot bodies emit electromagnetic radiation with
a wavelength or spectral distribution determined by the body's tem­
perature. For a perfectly absorbing or "black" body, the spectral Photosphere
distribution of the emitted radiation is given by Planck's radiation law
(Ref. 1.2). As indicated in Fig. 1.1, this law indicates that as a body
is heated, not only does the total energy of the electromagnetic Figure 1.2. Principal features oft.he sun.
2
3
4 Solar Cells and Sunlight Chap. Sect. Solar Intensity at the Earth's Surface 5
1 1.5
These ions act as continuous absorbers over a great range of wave­ The presently accepted value of the solar constant in photo­
lengths. The accumulation of heat in this layer sets up convective voltaic work is 1. 353 kW/m2• This value has been determined by
currents that transport the excess energy through the optical barrier laking a weighted average of measurements made by equipment
(Fig. 1.2). Once through most of this layer, the energy reradiated mounted on balloons, high-altitude aircraft, and spacecraft (Ref.
is
into the relatively transparent gases above. The sharply defined level 1,3). indicated by the two uppermost curves in Fig. 1.3, the
As
where convective transport gives way to radiation is known as the 1pectral distribution of AMO radiation differs from that of an ideal
photosphere. Temperatures within the photosphere are much cooler hlack body. This is due to such effects as differing transm.issivity of
than at the sun's interior but are still a very high 6000 K. The photo­ Ih e sun's atmosphere at different wavelengths. Currently accepted
sphere radiates an essentially continuous spectrum of electromagnetic values for this distribution are tabulated in Ref. 1.3. A knowledge
radiation closely approximating that expected from a black body at of the exact distribution of the energy content in sunlight is impor­
this temperature. tant in solar cell work because these cells respond differently
to
cliCTerent wavelengths of light.
1.4 THE SOLAR CONSTANT
1.5 SOLAR INTENSITY AT THE EARTH'S
The radiant power per unit area perpendicular to the direction of the SURFACE
sun outside the earth's atmosphere but at the mean earth-sun distance
is essentially constant. This radiation intensity is referred to as the Sunlight is attenuated by at least 30% during its passage through the
solar constant or, alternatively, air mass zero (AMO) radiation, for Parth's atmosphere. Causes of such attenuation are (Ref. 1.4):
reasons that will soon become apparent.
1. Rayleigh scattering or scattering by molecules in the
2.5 atmosphere. This mechanism attenuates sunlight at all
wavelengths but is most effective at short wavelengths.
2. Scattering by aerosols and dust particles.
2.0
e 3. Absorption by the atmosphere and its constituent gases­
� oxygen, ozone, water vapor, and carbon dioxide, in
N
particular .
._E
� 1 5
e.
A typical spectral distribution of sunlight reaching the earth's
.2c i;urface is shown by the lower curve of Fig. 1.3, which also indicates
]
'E 1.0 the absorption bands associated with molecular absorption .
.� The degree of attenuation is highly variable. The most im­
.,�,
portant parameter determining the total incident power under clear
"'c 0.5 "onditions is the length of the light path through the atmosphere.
w
l'his is shortest when the sun is directly overhead. The ratio of any
111-lual path length this minimum value is known as the optical air
to
When the sun is directly overhead, the optical air mass is unity
muss.
1111cl the radiation is described as air mass one (AMl) radiation. When
Wavelength (µm)
I he :;un is an angle overhead, the air mass is given by
0 to
Figure Spectral distribution of sunlight. Shown are
1.3.
the cases of AMO and AMl.5 radiation together with the 1
radiation distribution expected from the sun if it were a Air mass= (1.1)
black body at 6000K. -­cose
6
Solar Cells and Sunlight Chap. 1
lAenMg2th. oTlf l l(H•w1w1 c·1 1•li.a,hs uiw1d·slotw wnw sa Lych aest to s bueyns t aii msv ea6rt0tei° c tahol efsf t airrouvc etmurhraees asod fi, s h ttehoige h mrta ehda.is auTtrieho netn his e
(1.2)
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1.6 DIRECT ANO DIFFUSE RADIATION
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7
8 Solar Cells and Sunlight Chap. 1
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alscoa uNaP s 1g11ifpircoapnotr toifoi ntt ob ed iffuse.
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fromd irt•sc·utn.l ighGte.n eraliltyw ,i lble richeirnt hes hortoerr
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skyi ntrucloutch1'eSur n certainwLhiecncs a lculartaidniga tlicovnd s
on inclitwsuu rfacferso md atag <'ncralrleyc ordeodn horizontal
surfacc•As .c ommon assumpLiiosnL hatd iffulsieg hitsi sotropic
(unifoirnma ldli rectioanlst)h,o ulghhc r egioonft hes kys urround­
ingt hes uni st hem osti ntenssol'u r•co <ft hirsa diation. Equinox/ Ohbosmervoenr 's
Photovoltasiycs tehmsa scodn c orl<'<'llirsautneldi gchatng en­
eralolnyl ya cceprta yssp annian lgi mit<•rdu n•f.{o cfa ngleHse.n cet,h ey
usualhlayv et ot ractkh es unt ou tilitwh ec lirt'c<o'mtp onenotfs un­
lighwti,t ht hed iffucsoem ponenwta sted'.l 'hti<sm dtso o ffstehte Summer Observer\ f acing south
advantaggaei nebdy sucht rackisnygs teomfsi ntercepmta.xiinmgu m sol slice
powerd ensibtyya lwaybse inngo rmatlo t hes un'rsa ys.
•declination of the earth 23 27'
6 =
Figure 1.4. Apparenl motion of the sun relative to a
1.7 APPARENT MOTION OF THE SUN fixed observer at latitude in the northern hemisphere.
35°
The path of the sun shown at the equinoxes and lhe
is
summer and winter solstices. The position oC the sun
Thee artshp indsa iloyn a ni maginaarxyio sr ientaitnea df ixeddi rec­ shown at solar noon on each of these days. The shadeisd
tionr elattiovt eh ep lanoef t hee arthy'<'sa rloyr biatb outth es un. circles represent the sun's position 3 h berore and after
solar noon.
Thea nglteh idsi rectmiaokne sw iiht heo rbitpalla niest hes oladre ­
clinat(i23o°n2 7' ). Perhaplse sfsa milairaert hed etaiolfts h ea ppar­
entm otioonf t hes unr elattiova e f ixeodb servoenre artrhe sulting
selectfeodri nstallatNiootno .n lyw ouldd atao n thed irecatn d
fromt her elationdsehsilp" ratbhorvd e.
diffucsoem ponenotfsl ighbte d esirabbluetd, a tao nc orresponding
Thiasp parenmto tio1ns i ndicaitneF di g1..4 fora no bserver ambientte mperatuarsew se lals w inds peeadn dd irectcioounl bde
atl atitu35d en orthO.n anyg ivedna yt,h ep lanoef t hes un'asp par­
° usedt oa dvantagAel.t hougthh eraer es tatioantvs a rioluosc ations
ento rbilti east a na ngleeq uatlo t hel atitufdreo mt heo bserver's
.U"ountdh ew orltdh adto m onitoarl tlh espea rameteprrse,s eencto n­
verticAatl .t hee quinox(eMsa rc2h1 andS eptemb2e3r) , thes un
omiefsa votrh eu seo fp hotovolstayisct eimsnr emotree gioonfst he
risedsu ee asatn ds etdsu ew ests,o t hatth ea ltituodfte h es una t
worlwdh eriet i su nliketlhyas tu cihn formatiisao vna ilable.
solanro ono n thesdea yse qual9s0° minust hel atitudAet. t he
Thea vailaibnlseo lataitoa ng ivelno catidoenp endnso to nly
summera ndw intesro lsti(cJeusn2 e1 andD ecembe2r2, respectively,
grosgse ographicfaela turseusc ha sl atituadlet,i tucdlei,m atic
•111
fort hen orthehrne misphetrhee,o pposiftoer t hes outhernt)h,e
l"lussifiacnadtp iroenv,a ilviengge tatbiuotni ,ta lsdoe pendsst rongly
altituadtse o lanro onh asi ncreaosredd e creasbeyd t hed eclination 11polno cagle ographfiecaatlu reAsl.t houguhn ablteo i ncorporate
oft hee art(2h3 °27'). r h 11 lattfeera turmeasp,s o fs olairn solatdiiosnt ribuatrieoa nv ail­
1hf ord ifferpeanrtt osf t hew orldT.h eshea veu sualbleye np re-
ii
11•1 1 by combinimnega sureidn solatdiaotnaw ithd atae stimated
l'Ci
1.8 SOLAR INSOLATION DATA f111am l argnee tworokf s tatioanrso untdh ew orlmdo nitorihnogu rs
11111 11nshinc.
Thei deasli tuatfioornt h ed esigno fp hotovolstyaisct emwso uldb e 'l'ihncf ormatmioosnt g eneraalvlayi laibslt ehe a veradgaei ly
whent herwee red etailreedc orodfst hes olairn solataitot nh es ite or on horizonstuarlf acAe w.i deluyse d source
1111 11 J!lo/Jal racliatum
a
9