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Electrochemistry,Production,PuriﬁcationandApplications

EimutisJuzeliunas

Author

Prof.EimutisJuzeli¯unas

CenterforPhysicalSciencesand Technology

DepartmentofElectrochemical

Materials

SauletekioStr3 10257Vilnius Lithuania

CoverImage:©Georgy Shafeev/Shutterstock

Allbookspublishedby WILEY-VCH arecarefully produced.Nevertheless,authors,editors,and publisherdonotwarranttheinformation containedinthesebooks,includingthisbook, tobefreeoferrors.Readersareadvisedtokeep inmindthatstatements,data,illustrations, proceduraldetailsorotheritemsmay inadvertentlybeinaccurate.

LibraryofCongressCardNo.: appliedfor

BritishLibraryCataloguing-in-PublicationData Acataloguerecordforthisbookisavailable fromtheBritishLibrary.

Bibliographicinformationpublishedby theDeutscheNationalbibliothek TheDeutscheNationalbibliotheklists thispublicationintheDeutsche Nationalbibliografie;detailedbibliographic dataareavailableontheInternetat <http://dnb.d-nb.de>.

Allrightsreserved(includingthoseof translationintootherlanguages).Nopartof thisbookmaybereproducedinanyform–by photoprinting,microfilm,oranyother means–nortransmittedortranslatedintoa machinelanguagewithoutwrittenpermission fromthepublishers.Registerednames, trademarks,etc.usedinthisbook,evenwhen notspecificallymarkedassuch,arenottobe consideredunprotectedbylaw.

PrintISBN: 978-3-527-34897-8 ePDFISBN: 978-3-527-83190-6 ePubISBN: 978-3-527-83192-0 oBookISBN: 978-3-527-83191-3

Typesetting Straive,Chennai,India

Tothememoryofmyparents–motherZofijaandfatherEnrikas and Tomybelovedfamily–wifeKaterinaandsonsLaimisandPovilas, whoinspiredandsupported.

Contents

Preface xiii

ListofAbbreviations xv

AbouttheAuthor xix

1Introduction 1

References 3

2SiliconElectrochemistry–TowardLow-CarbonEconomy 5

2.1SiliconforEnergyStorage–ElectrochemicalBatteries 6

2.2SiliconforEnergyConversion–PhotovoltaicDevices 9

2.2.1SolartoElectricity 9

2.2.2Solar-to-ChemicalConversion 15

References 17

3BriefHistoricalOverviewofSiliconProduction. Metallurgical-GradeSilicon 21

References 26

4PhysicalandChemicalPropertiesofSilicon 27

References 30

5SiliconReﬁning:FromMetallurgical-Gradeto Electronic-Grade 33

5.1PurificationBasedonDirectSiChlorination 34

5.2TheSiemensProcess 35

5.3TheUnionCarbideProcess 37

5.4TheEthylProcess 38

5.5ElectrorefininginMoltenSalts 40

5.6ZoneRefining,AcidLeaching 43

References 45

Contents

6SiliconElectrowinningandElectrodepositionof ThinLayers 49

6.1ElectrodepositioninMoltenFluoride,Chloride,andOxide Electrolytes 49

6.2SubstrateMaterialsforSiliconElectrodeposition 55

6.3ElectrodepositionofPhotoactiveSiliconandp–nJunction 56

6.4ElectrodepositionofSiliconfromIonicLiquidsandOrganicSolvents 60

6.5PurityConcernsandSolutions 66 References 69

7PhotoelectrochemistryandNanogravimetryofSiandSi-Oxide Electrodes 77

7.1TopicalityofSiPhotoelectrochemicalResearch 77

7.2BasicParameters:Photopotential,Photocurrent,and Photocapacitance 80

7.3PhotoelectrochemicalFeaturesoftheSi-OxideElectrodes 85

7.3.1Si–SiO2 Electrode 86

7.3.2Si–HfO2 Electrode 93

7.3.3Si–Al2 O3 Electrode 105

7.4QuartzCrystalNanogravimetry 106 References 112

8Electro-DeoxidationofSolidCompoundsinMoltenSalts 121 References 127

9VoltammetryandBasicReactionsofSiliconElectrodein MoltenCaCl2 129 References 140

10Si–SiO2 ElectrodeinMoltenCaCl2 145 References 147

11FormationofSiliconOxideLayer 149 References 155

12 InSitu StudiesofSiO2 → SiConversion–SynchrotronX-ray Diffraction 159 References 162

13MoltenOxideElectrochemistryatUltra-High Temperatures 165 References 172

14SiliconSurfaceStructuring 175

14.1ElectrochemicalStructuring,PorousSilicon 175

14.2Chemical–PhysicalStructuring 178

14.2.1ChemicalEtching 178

14.2.2LaserEngineering 180

14.2.3ReactiveIonEtching 181

14.2.4PlasmaImmersionIonImplantationEtching 182

14.2.5StainEtching 182

14.2.6Metal-AssistedChemicalEtching 182

14.2.7Vapor–Liquid–SolidMethod 186

14.2.8NanostructuringBasedonPorousAluminaTemplate 186

14.3BlackSilicon 190

References 195

15ElectrochemicalSiSurfaceStructuringandFormationofBlack SiliconinHigh-TemperatureMoltenSalts 203

15.1AnodicandCathodicProcessinginMoltenCaCl2 203

15.2MicrocolumnarandAmorphousStructures 205

15.3ElectrodeoxidationofThinSiO2 Layers 207

15.4GlobularStructures 208

15.5BlackSiliconfromMoltenSalts 212

15.6ElectrochemicalSynthesisofNanowires:ImplicationsforLi-Ion Batteries 216 References 221

16SiliconCompositions–PerspectivesforSemiconductor Production 225

16.1SiliconCarbide 225

16.2Silicides 231 References 233

17SiliconPhoto-ElectrodesforWaterSplittingandTheir Protection 237

17.1Relevance,BasicPrinciples,andSemiconductorMaterialsfor Photo-Electrodes 237

17.2ProtectionofSiliconPhotoelectrodesinSolar-FuelGenerators 243

17.2.1ProtectionofSiPhotoanodes 246

17.2.2ProtectionofSiPhotoanodesforHalideReduction 247

17.2.3ProtectionofSiPhotocathodes 249

References 251

18Conclusions,Outlook,andChallenges 257

Index 263

TheodorvonGrotthussmedal.Author:PetrasRepšys.ProducedbyLietuvosmonetu ˛ kalykla(LithuanianMint)

“Thoughlightilluminatesdarkness,nothingisdarkerthanthelight.”

TheodorvonGrotthuss(1785–1822)

Preface

Siliconliesattheheartofmoderntechnology.Siliconcanbeusedinvariousfields, suchasoptoelectronics,sensors,batteries,opticalfibers,photoelectrochemical watersplitting,terahertzemitters,andnumerousotherapplications.Asanabundant,non-toxic,efficient,androbustmaterial,siliconwilldominatethesolarenergy marketatleastforthenextfewdecades.

Electrochemistrydealswiththechemicaltransformations,whichareinducedby anelectriccurrent,or viceversa –withthetransformations,whichgenerateanelectriccurrent.Theseprocessesprovideanopportunitytostoreorproduceelectricity withaminimumcarbonfootprint.Electrochemistrycan,therefore,significantly contributetolow-carboneconomy;itoffersanadvancementinsustainableenergy solutionsandenvironment-friendlytechnologies.

Intheearly2000s,V.Lehman(2002)andX.G.Zhang(2004)publishedseveral booksonsiliconelectrochemistry.Sincethen,variousbreakthroughdirectionsin siliconelectrochemistryhaveemerged.Forinstance,luminescentporoussilicon nanoparticleswereelectrochemicallyproducedandappliedasthecarriersof thedrugpayload invivo.Electrochemicalsiliconsurfacemodificationsincreased theefficiencyofphotovoltaicdevicesusedforsolarenergyharvestorforthe productionofsolarfuel.Siliconphotoelectrodeshavebeensuccessfullydeveloped forhydrogenandoxygenproductionbywatersplittingaswellasCO2 reduction. ElectrochemicallyproducedSisurfacenano-architecturesshowedanintrinsic quantumconfinementeffect.Environment-friendlyandsecuresolutionsoffered siliconelectrochemistryinhigh-temperaturemoltensalts.Electrochemicalsilicon reductionfromsilicainhigh-temperaturemoltensaltshasbeendiscovered.Electrochemicaldepositionofdopedsiliconaswellasformationofp-–njunctionhave alsobeendemonstrated.

Thisbookaimstosummarizetheexperimentalandtechnologicalworkdone inrecentdecadesonsiliconelectrochemistry,production,andpurification,highlightingsubjectsoftechnologicalsignificanceandfutureperspectives.Thebook aimstobehighlybeneficialtothecommunitiesofchemistsandmaterialscientists workinginacademiaandindustrialsectors,especiallyinthefieldofsustainable energydevelopment:photovoltaics,lightharvestingefficiency,solar-to-chemical conversion,productionofsolar-gradesiliconaswellasproductionofbatteries, photoelectrodes,orsilicon-basedsemiconductors.Thesecondarymarketofthis

xiv Preface

bookincludestheeducationandsocio-economicsectorswithfocalpointsonsuch topicalitiesasthereductioninglobalclimatechange,replacementoffossilfuelsby renewableenergy,andstrategiesoflow-carboneconomy.

Vilnius,Lithuania

June,2022

References

EimutisJuzeliunas

Lehman,V.(2002). ElectrochemistryofSilicon.Instrumentation,Science,Materialsand Applications.Wiley-VCH. Zhang,X.G.(2004). ElectrochemistryofSiliconandItsOxide.KluwerAcademic Publishers.

ListofAbbreviations

3PIthree-phaseinterface(interlines)

AFMatomicforcemicroscopy

ALacetonitrile

Al-BSFaluminumbacksidefieldtechnology

ALDatomiclayerdeposition

BCEbeforethecommon(orcurrent)era

BMIm1-butyl-3-methylimidazolium

BMPy1-butyl-3-methylpyridinium

BMPyrr N -butyl-N -methylpyrrolidinium

b-Siblacksilicon

CEcontactingelectrode

CNTcarbonnanotubes

COP21ParisClimateConference

CVcyclicvoltammetry

CVDchemicalvapordeposition

CZCzochralskiprocess

DMAE2-dimethylaminoethanethiol

DMSdimethylsulfide

DRCDemocraticRepublicoftheCongo

DREdamageremovaletching

ECTheEuropeanCommission

EDX(EDS)energy-dispersiveX-rayspectroscopy

EISelectrochemicalimpedancespectroscopy

EMIm1-ethyl-3-methylimidazolium

EMPyrrN-ethyl-N -methylpyrrolidinium

EQCMelectrochemicalquartzcrystalmicrobalance

EUtheEuropeanUnion FAPtris(pentafluoroethyl)-trifluorophosphate

FBRfluidizedbedreactor

fsfemtosecond

FTIRFouriertransforminfraredspectroscopy

FTOfluorine-dopedtinoxide

GDMSglowdischargemassspectrometry

xvi ListofAbbreviations

GDPGrossdomesticproduct

GI-XRDgrazingincidenceX-raydiffractometry

ICEinitialCoulombicefficiency

ICPinductivecoupledplasma

ILionicliquid

IPAisopropylalcohol

IRENAInternationalRenewableEnergyAgency

ISFETion-sensitivefieldeffecttransistor

ITOindiumtinoxide

LCDliquid-crystaldisplay

LIBlithium-ionbattery

M,Memetal,metallic

MACEmetal-assistedchemicaletching

MG-Simetallurgical-gradesilicon

MOEmoltenoxideelectrolysis

MSmagnetronsputtering

MTmetricton

MWTmetalwrapthrough

NHEnormalhydrogenelectrode

NMRnuclearmagneticresonance

NPsnanoparticles

nsnanosecond

NTDneutrontransmutationdoping

NWsnanowires

P-Siporoussilicon

PECphotoelectrochemicalcells,photoelectrochemistry

PEDOT:PSSpoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

PERCpassivatedemitterandrearcontact

PIIIplasmaimmersionandionimplantation

PVphotovoltaic(s)

PVDphysicalvapordeposition

QCNquartzcrystalnanobalance

R&Dresearchanddevelopment

REreferenceelectrode

RFMSradiofrequencymagnetronsputtering

RIEreactiveionetching

Si-Hhydrogen-terminatedsiliconsurface

Si-Ffluoride-terminatedsiliconsurface

Si-OHhydroxideterminatedsiliconsurface

SCRspace-chargeregion

SEMscanningelectronmicroscopy

SERSsurface-enhancedRamanspectroscopy

SoG-Sisolar-gradesilicon

SPsolubilityproduct

STCsilicontetrachloride

SWIRshort-wavelengthinfrared

TBABtetrabutylammoniumbromide

TBACtetrabutylammoniumchloride

TBMAtributyl(methyl)ammonium

TCStrichlorsilane

TEACtetraethylammoniumchloride

TEOStetraethylorthosilicate

TFOtrifluoromethylsulfonate

TFSAbis(trifluoromethylsulfonyl)amide

TFSIbis(trifluoromethylsulfonyl)imide

TG-DTAthermogravimetryanddifferentialthermalanalysis

THFtetrahydrofuran

TMAHtetramethylammoniumhydroxide

TMHA N -trimethyl-N -hexylammonium

TPACtetrapropylammoniumchloride

TRLtechnologyreadinesslevel

UPDunderpotentialdeposition

UTEultra-hightemperatureelectrochemistry

VLSvapor–liquid–soliddeposition

XRDX-raydiffraction

XPSX-rayphotoelectronspectroscopy

ListofAbbreviations xvii

AbouttheAuthor

EimutisJuzeliunas, photobyJ.Stacevi ˇ cius/ LithuanianNational Radioand Television,LRT.

ProfessorDr.EimutisJuzeliunasisaprincipalresearch associateandaheadoftheDepartmentofElectrochemical MaterialsScienceattheCentreforPhysicalSciencesand TechnologyinVilnius,Lithuania.Heworkedpreviously asarectorofKlaip ˙ edauniversity(2014–2018)andadirectoroftheInstituteofChemistryinVilnius(2001–2009). HewasaMarieCurieInternationalFellowoftheEuropeanCommissionattheUniversityofCambridge,UK (2009–2011,2013–2014)wherehecarriedoutresearchon siliconelectrochemistryinhigh-temperaturemoltensalts. HealsowasaFulbrightfellowattheVanderbiltUniversity (USA),afellowoftheAmericanChemicalSocietyatthe PennsylvaniaStateUniversity(USA),andanAlexander vonHumboldtfellowatthecompanyDECHEMAe.V. (Germany).Hismainresearchareaiselectrochemical materialsscience;currentresearchinterestsaresiliconelectrochemistryforenergy applications,environmentalandmicrobiologicaldegradationofmetals(corrosion), physicalvapordepositionofresistantalloys,andnanogravimetryofelectrochemical processes.

Introduction

Worldproductionofsilicon(Si)reached(2010–2020)abouteightmillionsofmetric tonnesinthelastdecade.Thisquantitywasproducedmainlybythecarbothermic silica(SiO2 )reduction.Theprocessrequiresalargesupplyofenergyandemits carbonoxides(COx ).Afundamentalchallengeistheelectrochemicalsiliconextractionfromsilicaorothersolidsusingelectricityinsteadofharmfulchemistries. Zerocarbonfootprintcouldbeattainedwhenusingelectronsasabsolutelyclean reductionagentsgeneratedbyrenewablesources.Electrochemicalmethodscanbe usedonawidescaleofapplications:extraction,purification,surfaceengineering, orthin-filmtechnologies.Thus,siliconelectrochemistryhasthepotentialto significantlycontributetolow-carboneconomy;thisfieldoffersanadvancement inenvironmentallyfriendlyandsecuretechnologiesofenergygenerationand storage.

Breakthroughresearchtopicshaveemergedinsiliconelectrochemistryinrecent decades.Theelectrochemicalformationofporoussilicon(P-Si)wasdiscovered in1956(Uhlir1956).Canhamreportedin1990thatavisibleroom-temperature photoluminescencefromP-SilayerformedelectrochemicallyonSiwafer(Canham 1990).ThediscoveryinspiredwidestudiesofP-Siforapplicationsinoptoelectronics,lasers,andsensors.Luminescentporoussiliconnanoparticleswereappliedas thecarriersofthedrugpayload,whoseinfraredluminescenceenabledmonitoring oftheparticles invivo (Parketal.2009).Electrochemicalnano-micro-structuring ofsiliconhasbeenwidelyinvestigated.Thesurfacemodificationsincreasedthe efficiencyofphotovoltaic(PV)devicesusedforsolarenergyharvestorforproductionofsolarfuel.Siliconphotoelectrodeshavebeensuccessfullyusedforhydrogen andoxygenproductionbywatersplittingaswellasCO2 reduction(Sunetal. 2014).ElectrochemicallyproducedSisurfacenano-architecturesshowedintrinsic quantumconfinementeffect.Electrochemicalreductionofsilicondioxidetosilicon inamoltensaltelectrolytehasbeenreported,whichformedthebasisfornew processesinsiliconsemiconductortechnologyandhigh-puritysiliconproduction (Nohiraetal.2003).Environmentallyfriendlyandsecuresolutionsofferedsilicon electrochemistryinhightemperaturemoltensalts(JuzeliunasandFray2020). Electrochemicaldepositionofdopedsiliconaswellasformationofp–njunction hasbeendemonstrated(Zouetal.2017,2019;Pengetal.2018).Theapproachhas

thepotentialofreducingcapitalcostandenergyconsumptionforfabricationof solarcellswhencomparedwiththeconventionalmanufacturingprocess.

Thisbookfeaturesrecentachievementsinsiliconelectrochemistry,particularly, inelectrochemicalsiliconextraction,purification,andprocessinginhightemperaturemoltensalts.Theintroductorypartofthebook(Chapters2–4)is devotedtogeneralaspectsofsiliconapplication.Ahistoricaloverviewofsilicon productionisprovided,anditsimportanceinalow-carboneconomyisconsidered. Chapter4addressesthephysicalandchemicalpropertiesofsilicon,whicharemost relevantforelectrochemicalmaterialsscience.Thesubsequentmaterialismore specific.Chapter5describesthemajortechnologiesusedforsiliconpurification suchasSiemens,UnionCarbide,orEthylCorporationprocesses.Thischapter alsoprovidestheprinciplesofelectrorefininginhigh-temperaturemoltensalts, highlightingtheadvantagesanddisadvantageswhencomparedwithconventional industrialprocesses.

Chapter6addresseselectrodepositionofthinlayersanddiscussesthepossibility ofreplacingmultipleprocessesofSiwaferfabricationwithone-stepelectrochemicaldeposition.Traditionalmanufacturingentailsanenergy-intensiveandenvironmentallyunfriendlyproductionofmetallurgicalgradesilicon(MG-Si),aswellas itsupgradetosolargradesilicon(SoG-Si),ingotcasting,andslicing.Electrodepositionfrommoltenfluoride,chloride,andoxideelectrolytesonvarioussubstratesis discussed.Arecentlyproposedstrategyforelectrodepositionofphotoactivesilicon andp–njunctionishighlightedindetail.Silicondepositionfromionicliquids–the room-temperaturemoltensalts–isalsodiscussedinthischapter.Significantattentionisgiventothepuritylevelofsiliconelectrodeposits,whichareessentialfor photo-electrochemicalapplications.

Chapter7disclosesphotoelectrochemical(PEC)propertiesofsilicon-oxide electrodescoatedwithultrathinfilmsofsilica(SiO2 ),hafnia(HfO2 ),andalumina (Al2 O3 ).ThepivotalconceptofPECmethodologyistoobtaininformation,which correlateswiththatofthesolid-statecellssothatthereisnopriorneedtodesigna solarcellthatcharacterizesSisurfacephoto-responsiveness.Significantattention isgiventostudiesofSi-oxideinterfacialstabilitybythequartzcrystalnanobalance (QCN)–asensitivemassdetector,whichprovidesinformationabouttheelectrode masschangewithnanogramresolution insitu andinrealtime.

Deoxidationofmetaloxidesinamoltensaltelectrolytewasdiscoveredinthe year2000(Chenetal.2000).TheprocesswasnamedtheFFCCambridgeprocess. Simplicityandrapidityoftheprocesshaveattractedglobalinterest.Over30 metalsorsemimetalswereextractedfromsolidcompoundsbythisenergy-efficient andenvironment-friendlyroute.Chapter8addressestheFFCprincipleandits applicationinsiliconreductionfromsilica.Theelectrochemicalextractionprovides agreenalternativetoconventionalcarbo-thermicsiliconproduction.Chapters9–12 providefurtherdetailsonSi–SiO2 conversioninmoltensalts.Voltammetry,basic reactions,and insitu studiesbysynchrotronX-raydiffractionarediscussed,and experimentalconditionsusedbymanyauthorsaresummarized.

Technologicalopportunitiescarryouttheoperationatultra-hightemperatures andatliquidstateofsilicafeedstock.Suchprocessesarereferredtoasmoltenoxide

References 3 electrolysis(MOE).Chapter13discussestheMOEprinciplesofsiliconextractionin aliquidstate.

Thisstudyfocusedmajorlyonelectrochemicalsurfaceengineering.Chapter 14discussesthechemical–physicalmethodsofsiliconsurfacestructuring,such aslaserengineeringandvariousetchings:chemical,photoelectrochemical,reactiveion,plasmaimmersionionimplantation,andmetal-assistedchemical.The vapor–liquid–solidmethodisalsodiscussed.

Chapter15featuresacomprehensivematerialobtainedonelectrochemical Sistructuringathigh-temperaturemoltensaltsincludingformationofblack silicon(B-Si).B-Siisanano-micro-porousmaterial,whicheffectivelyabsorbsthe lightonawiderangeofwavelengths.ElectrochemicalSistructuringinmolten saltsisattractiveduetoitsenvironmentalfriendliness,technicalsimplicity,and cost-effectiveness.

Thebookalsooutlinestheperspectivesofelectrochemicalsynthesisofsemiconductors(Chapter16),thebasicprinciplesandmaterialsforphoto-electrodes,and thepreservationofsolar-fuelgenerators(Chapter17).

Inconclusion,whilesiliconelectrochemistryoffersarangeoftechnologicalopportunities,mostofthedevelopmentsarestillontheconceptualorbench-scalelevel. Asaresult,viabletechnologicaldevelopmentsarestillpending.

References

Canham,L.T.(1990).Quantumwirearrayfabricationbyelectrochemicalandchemical dissolution. Appl.Phys.Lett. 57:1046–1048.https://doi.org/10.1063/1.103561.

Chen,G.Z.,Fray,D.J.,andFarthing,T.W.(2000).Directelectrochemicalreductionof titaniumdioxidetotitaniuminmoltencalciumchloride. Nature 407:361–364. https://doi.org/10.1038/35030069.

Juzeliunas,E.andFray,D.(2020).Siliconelectrochemistryinmoltensalts. Chem.Rev. 120:1690–1709.https://doi.org/10.1021/acs.chemrev.9b00428.

Nohira,T.,Yasuda,K.,andIto,Y.(2003).Pinpointandbulkelectrochemicalreduction ofinsulatingsilicondioxidetosilicon. Nat.Mater. 2:397–401.https://doi.org/10 .1038/nmat900.

Park,J.-H.,Gu,L.,Maltzahn,G.etal.(2009).Biodegradableluminescentporoussilicon nanoparticlesfor invivo applications. Nat.Mater. 8:331–336.https://doi.org/10.1038/ NMAT2398.

Peng,J.J.,Yin,H.Y.,Zhao,J.etal.(2018).Liquid-tin-assistedmoltensalt electrodepositionofphotoresponsiven-typesiliconfilms. Adv.Funct.Mater. 28: 1703551.https://doi.org/10.1002/adfm.201870194.

Sun,K.,Shen,S.,Liang,Y.etal.(2014).Enablingsiliconforsolar-fuelproduction. Chem.Rev. 114:8662–8719.https://doi.org/10.1021/cr300459g. Uhlir,A.(1956).Electrolyticshapingofgermaniumandsilicon. BellSystemTech.J. 35: 333.https://doi.org/10.1002/j.1538-7305.1956.tb02385.x.

4 1Introduction

Zou,X.,Ji,L.,Yang,X.etal.(2017).Electrochemicalformationofap–njunctionofthin filmsilicondepositedinmoltensalt. J.Am.Chem.Soc. 139:16060–16063.https://doi .org/10.1021/jacs.7b09090.

Zou,X.,Ji,L.,Ge,J.etal.(2019).Electrodepositionofcrystallinesiliconfilmsfrom silicondioxideforlow-costphotovoltaicapplications. Nat.Commun. 10:5772. https://doi.org/10.1038/s41467-019-13065-w.

SiliconElectrochemistry–TowardLow-CarbonEconomy

Climatechangeisonethegreatestchallengestheworldfacestoday.Therenewable energy(solar,wind,water,biomass,geothermal)isconsideredasaclimatechange imperativetoday.Excessiveexploitationoffossilenergysourceshadanegative impactonclimate,economy,andeverydaylives.Thegrowingthreatfromclimate changeisapparent,suchasnaturaldisasters,halvedmassofinlandglaciers,riseof sealevel,andextinctionofnumerousterrestrialspecies.Volatileoil,gas,andcoal pricesinrecentdecadesandconcernsabouttheirsupplyfrompoliticallyunstable countriesareamongstthedriversoftheneedforalternatives,suchasrenewables.

TheEuropeanGreenDealplanwasunveiledin2019–athree-decadeeffortto maketheclimateneutralby2050(https://ec.europa.eu/info/strategy/priorities2019-2024/european-green-deal;Tammaetal.2019;Simon2019).TheCommission’sPresidentUrsulavonderLeyencharacterizedtheinitiativeas“Europe’s manonthemoonmoment”addingthat“thegrowthmodelbasedonfossilfuels andpollutionisoutofdateandoutoftouchwithourplanet”(Simon2019).The overarchingobjectiveintheplanis“ClimateneutralEurope,”whichaimstoreach net-zerogreenhousegasemissionsby2050.Theambitionfor2030iscutting-offthe emissionby50–55%.Theplanaddressesmajoractionssuchascirculareconomy, buildingrenovation,pollution-freeenvironment,strategyofecosystemsandbiodiversity,greenandhealthieragriculture,electricvehicles,andsustainablefuels, suchasbiofuelsandhydrogen.

AnotherglobalinitiativeistheParisAgreement–aninternationaltreatyon mitigationofclimatechange(https://unfccc.int/process-and-meetings/the-parisagreement/the-paris-agreement).In2015,196partiesintheParisclimateconference(COP21)adoptedanagreementtolimitglobalwarmingbelow2 ∘ Ccompared topre-industriallevels.

Suchhighlyambitiousplansledtofundamentalchangesinthewaywegenerate andusetheenergy.Electrochemicaltechnologieshaveshowngreatpotential inadvancingtheeconomy’stransitiontowardclimateneutrality.Electrochemistryprovidessustainablesolutionsinsuchfieldsasgreen-energystorageand solar-to-chemicalorsolar-to-electricityconversion.

Materialsusedinsolardevicesplayamajorroleinthecostbreakdownoftheoverallutilizationprocessofsolarenergy.Creationandsynthesisofneweffectivematerialsforsolar-energyapplications,therefore,isveryhighontheagendaofmaterials Silicon:Electrochemistry,Production,PurificationandApplications,FirstEdition.EimutisJuzeliunas. ©2023WILEY-VCHGmbH.Published2023byWILEY-VCHGmbH.

2SiliconElectrochemistry–TowardLow-CarbonEconomy

scientistsandengineers.Vastmajorityofthesolarcellsareproducedfromsilicon wherethetotalwafercostdominatesintheoverallcellcostbalance.Itisassumed thatsilicon,beinganontoxic,efficient,androbustmaterial,willplayakeyrolein thesolarenergymarketforthenextfewdecades(Schmalenseeetal.2015;Green 2016;Polmanetal.2016).Mitigationofclimatechange,asaglobaltask,couldbe achievedbyusingtechnologiesbasedontheEarth-abundantmaterials.TheabundanceofsiliconintheEarth’scrust(27%)makesitpossibletoexpandtheapplication ofthismaterialtotheterawattscaleby2050(Schmalenseeetal.2015).

2.1SiliconforEnergyStorage–Electrochemical Batteries

Electrochemistrydealswithrelationshipbetweenelectricityandchemicalchange. Typicalexampleofanelectrochemicaldeviceusedineverydaylifeisabattery–the device,whichgenerateselectricitybychemicalreactions.Batteriesarewidely usedinportableelectronics,medicaldevices,ore-mobilityincludingelectriccars. Aviceversaprocessiswhenelectricitygenerateschemicalreactions,forinstance, electroplatingofmetals.Theprocessiswidelyappliedfortheproductionofcoatings inordertoprotectmetalsandalloysfromcorrosionortoimprovetheiraesthetic appearance,aswellastodecoratethem.

Batteriesrepresentakeytechnologyforlow-carboneconomytoreduceCO2 emissionsfromtransport,power,andindustrialsectors.Batteriesareessential devicesusedtostorestationaryenergyfromsustainablesourcessuchassolaror wind.Toreachthesustainabilitytargets,batteriesmustexhibitultra-highenergy andpowerperformanceclosetotheoreticallimits.Otherrequirementsinclude outstandinglifetime,reliability,safety,andrecyclability.Importantrequirementis alsoscalabilitytoelectricitygridlevel,aswellascost-effectivenessandsustainable batteryproduction.ThecosttargetsetbytheEuropeanCommission(EC)forthe next-generationbatteriesofstationaryenergystorageisbelow0.05€/kWh/cycleby 2030.Thegrowthinglobalbatterydemandisanticipatedtomultipliedbyafactor 14× from2018to2030.Thegreatestpartofthisdemandgoestotheelectricmobility sector(Edströmetal.2020).

Lithium-ion(Li-ion)basedtechnologydominatesthecurrentbatterymarket. Lithium-ionbatteries(LIBs)arestate-of-the-arttechnologyforportableelectronics andelectricvehicles.TheNobelPrizeinchemistryhasbeenawardedtoJ.B. Goodenough,M.S.Whittingham,andA.Yoshitoin2019forthedevelopmentof LIBs.Thesebatteries,however,haveseveralshortcomings,especiallyforstationary energystorageapplications.

ApartfromLi,cobalt(Co)isakeyelectrodematerialinLi-ionbatteries.Forcathodeproduction,variousco-containingmaterialshaverecentlybeeninvestigated: LiCoO2 ,LiNix Coy Al1 x y O2 ,LiNix Mny Co1 x y O2 ,etc.(Huangetal.2021;Chuetal. 2020;Liuetal.2019a,b).Presently,about60%ofminedcobaltisusedtoproduce theLIBselectrodes.Congo(TheDemocraticRepublicoftheCongo,theDRC) alongwithZambia,Madagascar,ZimbabweandtheRepublicofSouthAfricaare

2.1SiliconforEnergyStorage–ElectrochemicalBatteries 7 miningabout70%ofworldcobalt.TheDRCisamainglobalproducer,whichmines about60%oftheworld’scobaltfeedstock.TheAmnestyInternationalorganization reportedin2016ontheviolationofhumanrightsinthiscountry–childlaborin health-endangeringmines(https://www.amnesty.org/en/documents/afr62/3183/ 2016/en).

TheECissuesperiodicallythecommunicationswithalistofcriticalraw materials,whichspecifiesthematerialsthataremostimportantfortheEC economically,alongsidehavinghighsupplyrisk.Asof2020,thelistalsoincludes thematerials,fromwhichkeycomponentsofLIBsareproduced:lithium,cobalt, andgraphite(Communication2020).Thelistindicates100%relianceofthe EuropeanUnion(EU)importonlithium,86%oncobalt,and98%onnatural graphite.ThecommunicationstatesthattheEUdemandforlithiumforelectric vehiclebatteriesandenergystoragewillincreaseby18timesin2030andalmost upto60timesin2050.Thecorrespondingfiguresforcobaltare5and15times, respectively.ThelimitedavailabilityoflithiummakesitdoubtfulwhetherLIBs manufacturingcanscaleuptosignificantlylargerproductionvolumes.

TheWorldBankprojectedthatthescenarioofglobalwarmingbelow2∘ (COP21) willincreasethedemandinmetalsforbatteryapplicationsatthelevelof1000%by 2050(WorldBank2017).Thelistofrelevantmaterialsforbatterymanufacturing includesmetalssuchasAl,Co,Fe,Pb,Li,Mn,andNi.

AnimportantconstraintofthestationaryenergystorageusingLIBsisthe continuousconsumptionofLi-ionelectrolyte,whichlimitsbothcycleandcalendar life.Anotherrestrictionliesinthelimitationsofoperatingtemperaturewindow. Theoperationrequirescomplexthermalmanagement,whichisimpracticalfor stationarystorageapplications,particularlyin“hot”countries.Thesebatteriesalso containhardlyrecyclablematerials,suchaslithium.LIBsarestillfacingsafety issues.Also,LIBsarerelativelyexpensive.

Thus,thereisgreatdemandandagreatchallengetocreateeffectivepost-Li electrochemicalenergystoragesystems.Analternativesuggestssodiumwithan electrochemicalchargetransferreactionNa ↔ Na+ + e ,whichcouldbeperformed inasolidcompound,forinstance,inachloride.Oceansprovideunlimitedsodium source,whichisforfree.

Metal-airbatteriesexploreairoxygenasamajorreactant:

Thisapproachenablesbatteryweightreduction,atthesametime,increasing thecapacityforenergystorage.Air–metaltechnologiesareattractivebecausethey aregreen,safe,andcost-efficientintermsoffeedstock.Thetechnologiespropose usageofanunlimitedsourceofoxygenfromtheatmosphere,whichisforfree. Themetal–airbatteriescanutilizevariousmetalelectrodes,e.g.Li,Zn,Al,Fe, Mg(Tongetal.2021).Ofthesemetals,Li-airbatteryhasthehighesttheoretical energydensityandpracticaloperationvoltage(13000Whkg 1 and2.4–2.7Vvs. standardhydrogenelectrode(SHE),respectively).Lithiumisfollowedbyaluminum (8073Whkg 1 and1.2–1.6V)andmagnesium(6815Whkg 1 and1.2–1.4V).Iron hastheleastperformance(764Whkg 1 and0.8V),however,thecostofironmetal

O2 + 2H2 O + 4e ↔ 4OH (2.1)

2SiliconElectrochemistry–TowardLow-CarbonEconomy

anodeissubstantiallylesswhencomparedtoothermetals.Tongetal.summarized thatthecostofironisabout200timeslesswhencomparedwiththecostoflithium andabout15timeslesswhencomparedwithmagnesiumoraluminum(Tongetal. 2021).TheauthorsalsoestimatedthatMg,Al,andZnelectrodesarenearly10times cheaperthantheLimetalelectrodes.Li–airbatteriesarelimitedduetodendrite formation,poor-cyclingefficiency,anddifficultiesinfindingasuitablehighlystable electrolyte.Fe–airbatteryisveryattractiveintermsofexcellentresource-efficiency. Notealsothattheelectrodescanbepreparedcombiningthemintheformofalloys, forinstance,Mg–Al,Mg–Al–Zn,Mg–Li,Mg–Zn,etc.

SiliconisanattractivematerialforLIBsanodeproduction.TheLi–Sibinary systemischaracterizedbyexceptionallyhighLiinsertioncapacity.OneSiatomcan accommodate4.4LiatomsformingthealloyLi22 Si5 withtheoreticalspecificinsertioncapacityof4200mAhg 1 .TheanalogousvalueforLi15 Si4 is3576mAhg 1 (Ashurietal.2016;Huangetal.2021).Suchcapacityvaluesarethehighestamong allanodematerialsusedinLIBs.Siliconoutperformsintermsofcapacitytheconventionallyusedgraphite(372mAhg 1 forLiC6 )(Liangetal.2014).Itisassumed conceptuallythatelectrodecapacitycanbesubstantiallyincreasedwhenmoving fromclassicalintercalationreactiontoalloyingreaction(Maetal.2014).Lithium canreactwithSi-formingLi22 Si5 alloy,whilegraphiteaccommodatesmuchless lithiumformingLiC6 .However,achallengeisdisintegrationoftheelectrodedueto nearby300–400%volumechangeduringthelithiation–delithiationprocess,thatis, theexpansion–contractioncycles(Ashurietal.2016;Lietal.2017;Zuoetal.2017). Thisdetrimentalprocessreducestheintrinsicelectricalconductivityandcauses theinterfacialinstability,whichleadstocapacityfading.

Therestorationofelectricpropertiesafterthedamage,socalledself-healing,isof paramountimportanceinenergy-storagedevice.ThiswasstatedintheBATTERY 2030+ RoadmapoftheEC:“Establishinganewresearchcommunitythatincludes awiderangeofR&Ddisciplinestodevelopself-healingfunctionalitiesforbatteries” (Edströmetal.2020).Scientistsareinsearchofeffectivemeansofsurfaceengineeringthatbuffersthevolumechanges,forinstance,bygraphenefilms,silicon-carbon nanocomposites,Sinanowires,nanotubes,andothernanoparticles,solidcore–shell structures,porousSidesigns,controllablepores,orpatternedsiliconfilmson foreignmetalsubstrates,etc.Theseeffortshavebeensummarizedinreviewarticles (Liangetal.2014;Maetal.2014;Ashurietal.2016;Lietal.2017;Zuoetal.2017; Huangetal.2021).

Oxidizedsilicon(SiOx )hasalsobeenproposedasanalternativetopureSi.Such electrodesshowalowervolumechangewhencomparedtopuresilicon(Liuetal. 2019a).However,siliconmonoxidepossessessomeproblematicfeatures,suchas lowintrinsicelectricalconductivity,non-negligiblevolumechange,andlowinitial Coulombicefficiency(ICE)(Liuetal.2019a).Thus,thereisneedtoovercomethese drawbacksinordertousethematerialforLIBspractically.TheimprovementofICE couldbeachievedbypre-litiationmethods,forinstance,usingstabilizedlithium metalpowder(Huangetal.2021).Variousstrategieshavebeenproposedtoreduce theSiOvolumechange:(i)SiOdisproportionationintoafractionofSinanocrystals byhigh-energymilling(Hwaetal.2013)orheatingat900 ∘ C(Huangetal.2021);

2.2SiliconforEnergyConversion–PhotovoltaicDevices 9 (ii)synthesisofporousSiOx viachemicaletching(Yuetal.2014);(iii)compositesof disproportionatedSi–SiOx withcarbon(Si–SiOx –C)(Yamadaetal.2011;Choietal. 2012;Kimetal.2013);(iv)core–shellporoussiliconwithnitrogen-dopedcarbon layer(p-Si/NC)(Xingetal.2018);(v)siliconmonoxidecoatedwithN-dopedcarbon (SiO-NC)producedusingN-containingionicliquid(Leeetal.2013)andanalogous systemusingpitchandmelanineascarbonandnitrogensources(Huangetal.2021).

Thematerialsbasedonporoussilica(SiO2 )havealsobeenstudiedasanalternative tographiteanodesinLIBs.Poroussilicahasshownimprovedcyclingstability;its dischargepotentialwassimilartothatofpureSi.LowCoulombicefficiencyofSiO2 wasmodifiedbypreparingacarbon-coatedC/SiO2 composition(Bugaetal.2021). Thethincarbonlayerofthecompositiondiminishedinterfacialimpedanceofsilica, wherebythecapacityincreasesto714mAhg 1 .

Despiteintensiveresearch,mostoftheproposedmethodsofthevolumechange bufferingremainonabenchscale.Vitalindustrialapplicationsarestillpending.

2.2SiliconforEnergyConversion–Photovoltaic Devices

2.2.1SolartoElectricity

Solarenergycouldeffectivelyactasasubstitutetofossilfuels.Thesolarconstant, whichmeasuresthequantityofsolarenergytoasquaremeteroftheEarth surface,isestimatedtobeashighas1367Wm 2 ,accordingtotheWorldEnergy Council.Theconstanttranslatesintototalamountof3400000EJinayear (EJ–Exajoule,1EJ = 1018 J)(Breeze2019).Thesolarradiation,whichreachesthe earth,coversmorethan7000timestheworld’senergyneeds,thatis,theannual globalprimaryenergyconsumption.Breezeestimatesthatif0.1%ofthesolar energywasconvertedintoelectricitywith10%efficiency,itwouldprovidearound 10000GWenergy,whichexceedstheworldenergyneeds(Breeze2019).Solar energyprovidestheopportunityfordecentralizedsupplyofenergyactuallyat anyplacearoundtheglobe.Itcanbeaccumulated(asaheat)and/orconverted toelectricitywithnopollutingemissionstotheenvironment.Otheradvantages includeno-noiseoperationandunproblematicdecommissioning.

Photovoltaic(PV)electricitygenerationisarapidlyexpandingindustry,which aimstoacceleratethedevelopmentofclean,sustainable,andefficientenergy technologieswiththescenariostoreachsolarenergydominationintheelectricity market.Infact,creationofefficientandlow-costdevicesofsolarenergyharvesting playsadeterminingroleinpolitical-sociallandscapeoftwenty-firstcentury.There isapublicawarenessinthePVtechnologyasaproviderofclean,sustainable,and secureenergyfromthemostabundantsource,whichisforfree.Itisnotsurprising, therefore,thatworldPVproductionhasrecentlyexperiencedclosetoexponential growth.

Itisnowcloseto70yearssincetheinventionofthefirstreasonablyefficient siliconcellsin1954(Green2005).Thefirstsilicon-basedmoduleforoutdoorapplicationswasproducedatBellLaboratoriesin1955.Themodulewasanassembly

2SiliconElectrochemistry–TowardLow-CarbonEconomy

of48sub-modulesandattained2%efficiency.Thereaderinterestedinhistorical developmentofsilicon-basedPVdevicesisreferredtoM.Green’spaper,which reviewsimprovementinenergyconversionaswellaspricesofcommercialmodules overa50-yearperiod(Green2005).

TheInternationalRenewableEnergyAgency(IRENA)comprisingofabout 170membercountriesdeclaresitsmission“supportingcountriesintheirtransition toasustainableenergyfuture”(www.irena.org).IRENAisanintergovernmental organization–adriverofactions,whichadvancethetransformationoftheglobal energysysteminthepursuitoflow-carboneconomy.Thisorganizationpromotes adoptionofallformsofrenewableenergy.IRENAissuedaglobalenergytransformationpaper,whichaimstohighlightthefutureofsolarPV,includingthekey aspects,suchasthedeployment,investment,technology,gridintegration,and socio-economicaspects(IRENA2019).IRENA’sanalystsforeseethatsolarPVby 2050wouldrepresentthesecond-largestpowersourcebehindwindpowercovering aquarterofthetotalelectricityneeds.ThetotalgrowthofsolarPVcapacityis expectedtorisefrom480GWproducedin2018to8519GWprojectedby2050.Asia (mostlyChina)hasadominatingpositionintermsofinstalledcapacityandwill continuetodominateinthePVmarketreachingover50%shareintheworld’s productionby2050.ThepapershowsevolutionofPVindustrystartingfrommass productionofsolarcellsin1963to480GWglobalcapacitybytheendof2018. ImportantmilestonesintheprovidedtimelineofPVdevelopmentincludethe beginningofmassproductionofsolarcells(1963),thefirstsolarbuilding(1973), thefirstsolarplaneflightaroundtheworld(2016),andtheattainingoftheglobal installedsolarcapacityof480GW(2018).Effortshavebeenmadetobeatthecost ofphoto-electricitydown,belowthatofthepowergeneratedfromfossilornuclear fuels.Costreductionofphoto-electricityispermanentlyhighonagendaofPV engineersandmaterialsscientists.TheIRENA’spapershowsathree-timesreductionintheauctionpriceduringaneight-yearperiod,thatis,from241USD/MWh in2010to85USD/MWhin2018(IRENA2019).

AnalystsestimatedthePVcapacitygrowthuntil2050(Figure2.1)Itisexpected thatinstallationswillreach2840GWgloballyby2030.Thefigurewillriseupto 8519GWby2050assumingutility-scale(60–80%)anddistributedrooftop(40–20%). Thismeans18timeshigherproductionin2050whencomparedtotheamountproducedin2018.

Vastmajorityofthesolarcellsareproducedfromsilicon.Variousstructural formsofsiliconareusedinsolarcellproduction:mono-Si,poly-Si,andamorphous. Crystallinesiliconhasabout95%shareofthePVsolarproduction(IRENA2019; Fraunhofer2019).Thankstotechnologicalmaturityandafallinthepriceofraw material,siliconwillkeepitsdominatingpositioninthePVmarket.Increased outputsoftheSi-basedpanelsalsostrengthenpositionsofsiliconoverother material.Forinstance,anaverageefficiencyofmulti-crystallinepanelsincreased from ∼13%in2006to ∼17%recently,andthispositivetendencyisexpectedto continue(IRENA2019;Fraunhofer2019).AstudyoftheMassachusettsInstitute ofTechnology(MIT)EnergyInitiativehasshownthatsilicon,asanabundant, non-toxic,efficientandrobustmaterial,willmaintaintoppositioninthesolar

Figure2.1 PVcapacitygrowthexpectedby2050,asreportedbyIRENAanalysts. Source:IRENA2019/IRENA/Publicdomain.

energymarketatleastforthenextfewdecades(Schmalenseeetal.2015).Lowering ofthecostofSi-basedPVdevicesremainsverycrucial.

ProductionofPVmoduleisamultistepprocesscomprisingsuchprocedures asproductionofmetallurgical-gradesilicon(MG-Si),upgradingtosolar-grade silicon(SoG-Si),ingotcasting,andslicing.Traditionalmanufacturingincludes highenergy-intensiveproductionandpurificationofsilicon,suchascarbothermic siliconreductionfromsilicaandpurificationtosolargradebytheSiemensprocess. Thecostalsoincreasesduetosignificantmateriallosswhensawingthesilicon ingotintowafers.SubsequentproductionofthePVcellfromSiwaferalsoincludes numeroussteps.

First-generationtechnologies(conventionalsolarcellarchitecture)explorecrystallinesiliconPVpanels,whoseevolutionhascoveredawholescopeoftechnical maturity(TechnologyReadinessLevels,TRLs):basicresearchandtechnologydevelopment(R&D),demonstration,pilotlines,systemlaunchandoperations,enterinto marketandpenetration,andmarketmaturity.Therearemanyotherkindsofsolar paneltechnologies.Onemodificationbecomingmorecommonisthepassivated emitterandrearcontact(PERC)cells.EfficiencyofthePERCcellsis6–12%higher whencomparedwithconventionalcells.Themodificationhasanextralayerwithin thebackside,whichallowsthereflectionofsomeraysbackintothecell.TheadvantagesofthePERCcellincludesthereductioninrecombination,enhancementof lightabsorption,andhigherinternalreflectivity.Fromaneconomicalpointofview, itisimportantthatthePERCcells,beingamodificationoftheconventionalcells,

Figure2.2 MajorstepsofsolarcellproductionfromsiliconwaferaccordingtoAlback surfaceﬁeld(Al-BSF)technology.Source:Adoptedfromthetechnologyroutescheme appliedinthe“SoliTekR&D”(Vilnius,Lithuania).CourtesyofDr.J.Denafas.

donotrequiregreatinvestmentinacquiringadditionalequipment.Thesecellshave recentlybecomethenewindustrystandard.

Figure2.2demonstratesthemajorstagesofthealuminumbacksidefield(Al-BSF) technology.Theprocessstartsfromthoroughwaferqualitycheck:inspectionof as-cutwafermaterial,geometry,surfacedefects,andmicro-cracks.Baresilicon reflectsmorethan30%oftheincidentlight(Singhetal.2010;Szlufciketal.2005), thus,antireflectiontexturesareformedinacidic(typicallyinhydrofluoricacid,HF) solutions.Furtherstepsincludep–njunctionformation(e.g.byphosphorousdiffusion),chemicaledgeisolation,removalofphosphoroussilicatelayer,passivationof silicon(e.g.bySiNx layer),andpreparationofmetalliccontacts.TheBSFformation providesagoodcontactbetweenSiandAlwithlimitedpenetrationoftheAlSi alloythatisformedduringthefiringprocess,anditenablesasignificantreduction inthethicknessofAldepositedintherearcontact.Thetechnologyofpassivated emitterandrearcell(PERC)includessomeadditionalsteps:(i)edgeisolationand polishingoftherearside,(ii)backsidepassivationbytheAlOx layer,and(iii)its selectiveremoval(e.g.bylasercontactopening).

Aninnovativeapproachismetalwrapthrough(MWT)back-contactsolarcell technology.Inthistechnology,thepositiveandnegativeelectrodesarearranged ontherearsideofthecell.Thebackcontactsenableavoidingofapplicationof busbarsonthefrontside.Duetothis,theshadedareaisreduced,andtheconversionefficiencycanbeincreasedbyafewpercent.TheMWTprocessingisclose toconventionalfabricationsequences;thus,itdoesnotrequiresubstantialadditionalinvestment.PhotographinFigure2.3demonstratestheexamplesofthesolar cellsproducedin“SoliTekR&D”(thelimitedliabilitycompany(LLC)inVilnius, Lithuania)usingMWTandAl-BSFtechnologies.

Itisobviousthateachofthetechnologicalstepsdiscussedabovecontributestothe cellcostand,inturn,tothecostofphoto-electricity.Thin-filmtechnologiesprovide

Figure2.3 SolarcellsproducedaccordingtoAl-BSFtechnology(MWTcellontheleftside andtheothertwoareAl-BSFsamples).Thephotographalsoshowstheproductionunitin the“SoliTekR&D”(Vilnius,Lithuania).Source:CourtesyofDr.J.Denafas.

thepotentialforreductionofthephoto-electricityproductioncost.Depositingthin siliconlayersinsteadofthickindividualcells,whichhavetobeconstructed,framed, andwiredtogether,couldreducetheconsumptionofmaterialandenergy.

Thinfilmsprovideopportunitytoproducelargecompletemoduleswithimproved appearanceformanyvisualapplications(e.g.architecturalglass,liquidcrystaldisplays[LCD]).Mercaldoetal.presentedanalysisofarchitecturalissuesofthin-film PVs,inparticular,thoserelatedtoapplicationsoftransparentandconductiveoxides andfilmsofsiliconsolarcells(Mercaldoetal.2009).Modernarchitecturaltheories lookatbuildingsaslivingorganismsthatshouldbeable–duringtheirlife–to generatetheenergyneededtobeinoperation.Thin-filmtechnologyissuitable

2SiliconElectrochemistry–TowardLow-CarbonEconomy

tosatisfysuchadvancedarchitecturaltheory.Itisachallenginggoaltoformthin siliconfilmswitheffectivelightabsorbanceontechnicallyimportantmetallic substratessuchassteel,aluminumalloys,andcopper.Thismakesitpossibleto combinestructuralmaterialswithsiliconforfacingpanelsonbuildings,which couldbeusedtoharnesssolarenergy.Puttingthesilicononthinmetallicfoilselectrochemicallywouldexpandtheapplicationfieldstoawiderangeofsubjectswith flexiblegeometriesaswellasonthemetal-coatedglass.Electrodepositionofthin Silayers,includingtheelectrochemicalformationofap–njunction,whichcould replacemultipleprocessesofSiwafertreatment,willbediscussedinChapter6.

Themaindisadvantageofsiliconisitspoorintrinsicabilitytoabsorbthe light–morethan30%ofincidentlightisreflectedifsiliconsurfaceisnotspecificallytexturedorcoatedbyantireflectioncoatings.Also,high-processingcomplexity ofsiliconsignificantlycontributestothecostofPVdevices.Light-harvestingefficiencyoftheSi-basedphotoelectrochemical(PEC)cellscanbeachievedbycreating siliconnano-microarchitectures,whichenhancelightabsorbance.Tothisend, varioussurfaceengineeringtechniqueshavebeenapplied,suchasfemtosecond laserengineering,annealinginvacuum,coatingbyatomiclayerdeposition(ALD) aswellasvariousetchings:chemical,reactiveion,inductivelycoupledplasma,and cryogenic.Thesemethods,however,arecostlyandtechnicallysophisticated.As aresult,thecost-competitivesolutionsforindustrial-scalesolarenergyapplicationsarelimited.Furthermore,etchingsusuallyinvolvetoxicchemicals,suchas hydrofluoricacid,andratherexpensivecatalysts,suchasgold.

Electrochemicalmethodsofsiliconsurfaceengineeringareveryattractivedue toenvironmentalfriendliness,technicalsimplicity,andcost-effectiveness.Various methodsofelectrochemicalSisurfacetexturinginaqueousandmoltensaltelectrolyteswillbesurveyedinChapters14and15.

Non–siliconbasedthin-filmtechnologieshavebeenintensivelyinvestigatedas alternativestosiliconPVs.Particularly,therehavebeengreatexpectationsfrom perovskite-basedcells.ThistypereferstoabroadclassofABX3 structures,which originatefromthemineralcalciumtitaniumoxide(CaTiO3 ).ThecomponentA canbeeitherorganicions,suchasmethylamonium(MA,CH3 NH3 + )orformamidinium(FA,CH(NH2 )2 + ),orinorganicions,suchascesium(Cs+ )orrubidium (Rb+ ).ThecomponentBreferstoasmallerdivalentcation,inmostcaseslead (Pb2+ )orsometimestin(Sn2+ ).ThecomponentXreferstohalogenanions,typically iodide(I ),bromide(Br ),orchloride(Cl ).SuchABX3 structuresshowahigh lightabsorptionundervisiblelightaswellasanextremelylongchargediffusion length.ThesefeaturespresupposethehighPVperformanceoftheperovskite-based cells.MorethanadozenoffirmsfromtheUSA,SouthKorea,China,UK,Poland, JapanSwitzerland,andNetherlandsstrivedtocommercializetheperovskitesolar cells(Extance2019).Thechallengetoovercome,perovskite-cellshavelimited lifetimeduetosensitivitytoairandmoisture.Anotherissueliesinthestructural transformationsandperformancefadingwhenthecrystalswarmupandcools down.Tobecompetitive,theoperationaldurabilityofperovskitecellshouldbe comparablewiththedurabilityofthesilicon-basedcellswitha25-yearwarranty. Thisis“nowlookingincreasinglyunlikely”accordingtoMartynGreenfromthe

2.2SiliconforEnergyConversion–PhotovoltaicDevices 15 UniversityofNewSouthWalesinSydney,Australia(Extance2019).Another limitationliesinscalingupmatters:efficiencyofthecellsdoesnotreplicatewhen theirsizeincreased.Increasedefficiencyuptotheoreticallimitof45%canbe achievedintandemcells,thatis,asystemofperovskiteandsiliconlayers.Such strategyisunderactiveinvestigation(Extance2019).

TherapidexpansionofPVresultedinanincreasingamountofend-of-lifesolar panels,whichposesahazardforfuturePVdevelopment.Itisachallenginggoal tofindsolutionsforPVmodulesrecycling.Aninnovativeopportunityhasrecently proposedagroupconsistingof25researchersfromresearchestablishmentsin Singapore,China,andJapan(Caoetal.2022).Theiridealiesintheconversionof PVwasteintofeedstockforthermoelectrics(Figure2.2).Suchconversionisinline withtheprinciplesofcirculareconomy:eliminationofwasteandpollutionaswell ascirculationofproductsandmaterials.

Figure2.4outlinesthemajorconversionstepsofSi-basedsolarcellsintoSisuitable forthermoelectricapplication.PolycrystallineSisolarcellswereusedasafeedstock. AluminumandsilverwereremovedfromthecellbyleachinginHCl,HNO3 ,and H3 PO4 .Thedriedcellswerepulverizedintofinepowderbyaballmillingusing theballsmadeoftungstencarbide.Thiswasdoneinanargonatmospheretoavoid siliconoxidationbyair.Furthermore,dopants(PandGe)wereaddedandthepowderswerehomogenizedbyballmilling.Finally,thepowderswereconsolidatedby sparkplasmasinteringat1150 ∘ C,andthespecimenswerecutintodesiredshapes. Thestrategyprovidedlow-costandenvironmentallyfriendlyrecyclingofsiliconfor circulareconomyapplications.Theeconomicissuesoftheproposedprocessrelate tominimizingthecostassociatedwiththeapplicationofGeandPdopantsaswell astheballmillingandsparkplasmasintering.Costminimizationisexpectedwhen thetechnologyisusedonalarge-scaleapplication(economyofscale).

2.2.2Solar-to-ChemicalConversion

Collectionofthesolarenergyinchemicalformhelpstoconserveenergyandmakes itsustainable.Electrochemicalsolarenergycapturinginchemicalbonds(e.g. hydrogenproductionfromwater)mimicstheprocessofphotosynthesisinplants. Productionofhydrogenincombinationwithsolarenergyoccurswithnear-zero greenhousegasemissions.Whencapturedinchemicalform,theenergycanbe convertedtopowerinfuelcellswithspecificallyhighefficiency(upto85%)or incombustionengines.Itisalsopossibletocombinehydrogengenerationwith afuelcelltogenerateelectricityandreplacethewaterthathasbeenconsumed. Electrochemicalhydrogengenerationmeanstheusageofelectronsasabsolutely cleanagents,albeitatslightlylowerefficiencybutwithoutpollutantemissions.The electrochemicalprocessofferssuchadvantagesashighpurity,flexibility,supply on-siteandon-demandaswellasfaststart-upandshutdownopportunities.Of particularinterestissolar-to-fuelprocess,whichoccursdirectly(wireless)onthe photo-electrodes.

Solar-to-chemicalconversionoffersimportantopportunityforsolarenergyaccumulationandtransmission.Inthisrespect,themostattractiveprocessishydrogen