Handbook of the Physics of Thin-Film Solar Cells

893 Pages·2013·12.677 MB·English

by  Karl W. Böer (auth.)

Handbook of the Physics of Thin-Film Solar Cells
Karl W. Böer
Handbook
of the Physics
of Thin-Film
Solar Cells
KarlW.Böer
UniversityofDelaware
Naples,USA
ISBN978-3-642-36747-2 ISBN978-3-642-36748-9(eBook)
DOI10.1007/978-3-642-36748-9
SpringerHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2014935163
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Foreword
ItisanhonourtohavebeenaskedtopreparethisshortforewordforKarlWolfgang
Böer’s new “Handbook of the Physics of Thin-Film Solar Cells”. I first met Karl
backin1976whenIvisited“SolarOne”,thefirstsolarpoweredhousethatKarlpro-
posedbackin1972.Onthesameday,IalsovisitedtheInstituteofEnergyConver-
sion,alsofoundedbyKarlattheUniversityofDelawarein1972.Theearlyworkof
the Institute focussed on copper sulphide/cadmium sulphide (Cu S/CdS) thin-film
2
solarcells,themostpromisingthin-filmcellsofthatera.Karlalsoservedatthattime
as Chairman of Solar Energy Systems (SES) Inc. formed to commercialise these
thin-film cells in conjunction with Shell Oil. These pioneering efforts have paved
the way for the next wave of thin-film devices,those based on amorphous silicon,
cadmium telluride and copper indium gallium diselenide, each treated in separate
chaptersatsomelengthinthenewHandbook.Karlisobviouslyinastrongandwell-
informedpositiontotakeonthechallengethatpreparingsuchaHandbookpresents.
ThisisnotKarl’sfirstHandbookonthepresentscale.Hismonumentalandin-
fluential“HandbookofSemiconductorPhysics”,firstpublishedin1993andsubse-
quentlyupdated,hasclearlyearnedits placeas themostcomprehensivetreatment
of the field by a single author. The present Handbook follows in the tradition of
theearlierone,providingin-depthtreatmentofthematerialanddevicephysicsre-
quiredtofullyunderstandthetechnologyandoperationalcharacteristicsofpresent
thin-filmsolarcellsandthosethatmayemergeinthefuture.Inhistreatment,Karl
brings to bear his decades of experience as both a researcher and educator in this
field,aswellasanoftenuniqueperspectiveonmanyoftheunderlyingissues.
The Handbook is a particularly timely contribution given the rapid growth of
thephotovoltaicindustryoverthelastdecadeandthetechnology’snowestablished
positionasacost-competitive,sustainablesourceoffutureenergyonalargescale.
ThepresentHandbookwillserveasausefulreferenceforthosealreadyinthefield
wanting to improve their understanding of the underlying materials science issues
andalsoforthegrowingnumberofengineersandscientistslikelytoenterthefield
overthecomingdecade.
Bronte,Sydney MartinGreen
v
Preface
BasedonmyexperienceIhaveselectedthepartsofbasicsciencethatwerehelpful
to my own research of solar cells, specifically on CdS based thin-film cells. As
any,evensomewhatmorecomprehensivetreatises,aselectionwasmadetoinclude,
beyond basic material science and the description of space charge effects, mostly
CdS/Cu S and CdS/CdTe solar cells. These treatments are more comprehensive,
2
including most recent references. The descriptions are excellent examples for the
theoreticalanalysisofmostotherthin-filmsolarcells.Theotherthin-filmsolarcells,
theCIS-groupandamorphousSiliconarediscussedinthreeshorterchaptersatthe
endofthisbook.
Overall the book is structured to address most aspects of thin-film solar cells,
starting from the history of their development and covering all features of these
includingmostrecenttechnologies,commercialviewpoints,andglobalmaterialre-
sources.
Commercialsolarcellsarebasedonmaterialsthatarehandedtousoftenmore
thanhalfacenturyagoandgrownordepositedbyconventionalmethods,dopedto
producehomo-orhetero-junctionandthentreatedinvariouswaystoimprovetheir
performance.Allofthishasbecamearoutineandtheselectionoftheprocesseswas
doneofteninrespecttoproductioneconomics,withaneyetominimizelongterm
cell degradation. Only a few researchers were involved to search for new ways to
makethesecellsorusetotallynewmaterialsinthehopeforperformanceimprove-
ment or economic production advantages and had to look for more basic material
scienceproperties.
Thisisoneofthereasonsbehindcomposingthisbook.Theotheroneistohelp
forabetterunderstandingofsolarcelloperation.
InorganizingthisbookIwanttoprovideatoolboxforscientistsandengineers
fordesigningnewsolarcells,improvingtheconventionalonesandunderstandbetter
theiroperation.Thistoolboxneedstocoverawidefieldfromtheunderstandingof
the material science aspects of the building blocks and structure of solar cells to
theelectronicconfigurationwithitsinfluenceontheseparatepartsofthecell,and
finallyofthesolarcellitself,analyzingitandinterpretingitsperformance.
vii
viii Preface
This is a complex field with constantly new developments, and to attempt a
comprehensivedescriptionisimpossibleinareasonableandusefulsinglevolume.
Therefore a selection was made between basic facts and useful results that have
more principle value than going into detail for which a long list of cited literature
exists.Thisexcludesmoreextensivedescriptionofanalyticaltoolsthatarenotdealt
withindetailhere,andmanymorefieldsoftherapidlydevelopingscience.Butthe
bookshouldgivethefoundationfromwhichtobuildfurtherresearchinthefield.
Asanyselection,thisissubjectivetothejudgmentoftheauthorandhisspecific
experience.Itwillbethereforemoredetailedinthefieldsrelatingtocadmiumsul-
fideandotheradjacentfieldsthathaveprovenhelpfulindesigningmyresearchand
relatingtothedevelopmentofavarietyofthinfilmsolarcells.Wheneverpossible
withintheconstraintsofthisbook,importantsolarcelldevelopmentswereguidance
toselect.
Atmanyjunctures,possibleproblemsarepointedoutinconventionalmanufac-
turingprocesses,orfrommodelanalysesthatcouldleadtocostlymisjudgmentsand
canbeavoidedbyfollowingsomeofthesuggestionslistedinthisbook.
Inall,differentsegmentsofthisbookaremorehelpfultoproductionengineers
atthebench,whileothersmayguideresearchersintothedevelopmentofnewsolar
cells.Sinceitisopenendedinwhichdirectionfurtherinvestigationwilllead,Ihave
included some small sections of the book in fields that are not relevant to present
solar cells, but may stimulate excursion into adjacent fields that provide already a
wealthofevidencetostimulatecreativeadvances.Manyhelpfultablesarescattered
throughoutthetext.Theliteraturecitationsattempttobemorecomprehensive,in-
cluding historical publications of specific and related fields. An extensive subject
indexandalistingofusefulformulaeisappended.
Insummary,thisbookisacompendiumgivingacomprehensivedescriptionof
the basic physics relevant to the design and the analysis of solar cell materials. It
starts from the basics of material science, describing the material and its growth,
defect and electrical properties, the basics of its interaction with photons and the
involved statistics, proceeding to space charge effects in semiconductors and pn-
junctions.Mostattentionisgiventoanalyzehomo-andhetero-junctionsolarcells
usingvariousmodelsandapplyingthefield-of-directionanalysisfordiscussingcur-
rentvoltagecharacteristics,andhelpingtodiscovertheinvolvementofhigh-fieldef-
fectsinsolarcells.Thecomprehensivecoverageofthemaintopicsof—andrelating
to—solar cells with extensive reference to literature helps scientists and engineers
atalllevelstoreachabetterunderstandingandimprovementofsolarcellproperties
andtheirproduction.
Appreciation: The book is based on four decades of teaching material science
andsolarcells,andequallyimportanttheconstantinteractionwithmystudentsand
colleagues.Thelististoolong,butaveryfewmaybementionedwithappreciation:
UlrichKümmel,PeterVoss,GustavoDusselandHankHadleywhoworkedwithme
earlyinmycarrier.MorerecentlyIamespeciallygratefultomanycolleaguesofthe
Institute of Energy Conversion (IEC) of the University of Delaware who directed
me to recent development with extensive literature especially Bob Birkmeire, the
directorofIECandhisseniorscientists,StevenHegegedusandBrianMcCandless.
Preface ix
I appreciate the dedicated work of Anita Schwarz from the Computer Program
oftheUniversityofDelawaretofacilitatemyworkinpreparingthebook.Special
thanks are extended to Dieter Palme, a research associate over seven decades for
helpingmeinthemanyaspectsofcompletingthisbook.However,mydeepgrati-
tuderestswithmywife,Renate,whohaspatientlyforgivenmewhenattimesIwas
abitabsentmindedandhascontinuedtoencouragemethroughoutmyendeavor.
Naples,USA KarlW.Böer
Contents
PartI MaterialScienceofSolarCellsAtomicBindingForces
1 CrystalBonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 IonicBonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 CovalentBonding . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 TetrahedrallyBoundElements . . . . . . . . . . . . . . . 11
1.3 MixedBonding . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3.1 TetrahedrallyBondedBinaries . . . . . . . . . . . . . . . 11
1.4 MetallicBonding(DelocalizedBonding) . . . . . . . . . . . . . . 14
1.5 VanderWaalsBonding . . . . . . . . . . . . . . . . . . . . . . . 15
1.6 HydrogenBonding. . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.7 IntermediateValenceBonding. . . . . . . . . . . . . . . . . . . . 16
1.8 OtherBondingConsiderations . . . . . . . . . . . . . . . . . . . 16
1.9 AtomicandIonicRadii . . . . . . . . . . . . . . . . . . . . . . . 16
1.9.1 Bond-LengthRelaxationinAlloys . . . . . . . . . . . . . 19
2 CrystalStructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.1 CrystalSystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2 TheBravaisLattice . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3 CrystalClasses(ThePointGroups) . . . . . . . . . . . . . . . . . 24
2.4 TheSpaceGroups . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.5 CrystallographicClassification . . . . . . . . . . . . . . . . . . . 27
2.5.1 MorphologyofCrystals . . . . . . . . . . . . . . . . . . 27
2.6 TheReciprocalLattice. . . . . . . . . . . . . . . . . . . . . . . . 28
2.6.1 Wigner-SeitzCellsandBrillouinZones . . . . . . . . . . 28
2.7 TheSemiconductors . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.7.1 ElementSemiconductors . . . . . . . . . . . . . . . . . . 30
2.7.2 BinarySemiconductors . . . . . . . . . . . . . . . . . . . 30
2.7.3 Ternary,QuaternarySemiconductors . . . . . . . . . . . . 32
2.8 SuperlatticeStructures . . . . . . . . . . . . . . . . . . . . . . . . 35
2.8.1 SuperlatticesandBrillouinZones . . . . . . . . . . . . . 35
xi
xii Contents
2.8.2 SuperlatticeDeposition . . . . . . . . . . . . . . . . . . . 36
2.8.3 UltrathinSuperlattices . . . . . . . . . . . . . . . . . . . 36
2.8.4 IntercalatedCompounds . . . . . . . . . . . . . . . . . . 37
2.8.5 OrganicSuperlattices . . . . . . . . . . . . . . . . . . . . 37
2.9 AmorphousStructures . . . . . . . . . . . . . . . . . . . . . . . . 38
2.9.1 Glass-FormingBuildingBlocks . . . . . . . . . . . . . . 40
2.9.2 CoordinationNumberandConstraints . . . . . . . . . . . 41
2.9.3 Short-Rangevs.Intermediate-RangeOrder . . . . . . . . 41
2.9.4 NetworkStructures . . . . . . . . . . . . . . . . . . . . . 41
2.9.5 MatrixGlasses;α-Si:H . . . . . . . . . . . . . . . . . . . 42
2.10 Quasicrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3 TheGrowthofSemiconductors. . . . . . . . . . . . . . . . . . . . . . 45
3.1 Nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
3.2 GrowthHabit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.2.1 SingleCrystalGrowth . . . . . . . . . . . . . . . . . . . 46
3.2.2 CrystalGrowthTechniques . . . . . . . . . . . . . . . . . 47
3.3 Recrystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.1 SlowRecrystallization . . . . . . . . . . . . . . . . . . . 48
3.3.2 Stress-EnhancedCrystallization . . . . . . . . . . . . . . 48
3.3.3 HeatinginthePresenceofaFlux. . . . . . . . . . . . . . 49
3.3.4 LocalHeatingInducedRecrystallization . . . . . . . . . . 49
4 CrystalDefects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
4.1 DefectClassifications . . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 PointDefects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
4.3 DonorsandAcceptors . . . . . . . . . . . . . . . . . . . . . . . . 54
4.3.1 DefectNotationWithintheHostLattice . . . . . . . . . . 55
4.3.2 SubstitutionalsinAB-Compounds . . . . . . . . . . . . . 56
4.3.3 VacanciesandInterstitials . . . . . . . . . . . . . . . . . 56
4.4 LineDefects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4.1 EdgeDislocations. . . . . . . . . . . . . . . . . . . . . . 57
4.4.2 ScrewDislocations . . . . . . . . . . . . . . . . . . . . . 57
4.4.3 TheBurgersVector . . . . . . . . . . . . . . . . . . . . . 58
4.4.4 DislocationsinCompounds . . . . . . . . . . . . . . . . 58
4.4.5 PartialDislocationsinSemiconductors. . . . . . . . . . . 58
4.4.6 ElectronicDefectLevelsatDislocations . . . . . . . . . . 60
4.4.7 DislocationCounting . . . . . . . . . . . . . . . . . . . . 61
4.4.8 MotionandCreationofDislocations . . . . . . . . . . . . 62
4.4.9 DislocationVelocity . . . . . . . . . . . . . . . . . . . . 62
4.4.10 TheFrank-ReadSource. . . . . . . . . . . . . . . . . . . 63
4.4.11 ElectroplasticEffects . . . . . . . . . . . . . . . . . . . . 63
4.4.12 Disclinations . . . . . . . . . . . . . . . . . . . . . . . . 64
4.5 SurfaceDefects,PlanarFaults . . . . . . . . . . . . . . . . . . . . 64
4.5.1 StackingFaults . . . . . . . . . . . . . . . . . . . . . . . 64
4.5.2 GrainBoundaries . . . . . . . . . . . . . . . . . . . . . . 65
4.5.3 Metal/SemiconductorInterfaces . . . . . . . . . . . . . . 69