emerging and novel photovoltaic technologies

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WorkingpaperNo3(2007)

DigitalTimestamping

EmergingandNovelPhotovoltaicTechnolo ies


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WorkingPaperNo3(2007) EmergingandNovelPhotovoltaicTechnologies

2007TheAppliedResearchInstituteforProspectiveTechnologies1

Contents


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Introduction

Energy consumption in Europe is expected to grow by 20% between now and 2020, leading to a 14%

increaseincarbondioxide(CO2)emissions.Renewableenergy(hydroelectricity,solar,wind,bioenergyand

geothermalpower)hasan importantroletoplay inreducingemissionsofgreenhousegases(suchasCO2)and also canmake important contributions to improving the security of energy supplies in the EU by

reducingtheCommunity'sgrowingdependenceonimportedenergysources.Renewableenergysourcesare

expected tobeeconomicallycompetitivewithconventionalenergy sources in themedium to long term.

Increasingtheshareofrenewableenergyintheenergybalancealsoenhancessustainability.

EUlegislationsandcommitmentstaken1,2,3,4setsacleargoaltoattain,by2010,aminimumpenetrationof

12%(from6%)ofrenewableenergysourcesintheEUalsoaddressingtheKyotoProtocol5requirementsfor

reduction of 8% of greenhouse gas emissions. Lithuania also committed to achieve the 12% share of

renewableenergy in the totalenergybalanceuntil theyear20106and toachieve that7%of consumed

electricitywouldbegeneratedfromrenewablesources

7

.

Many initiatives and industry led efforts have already started across the EU to assess the bestmix of

research and market development supports for acceleration of photovoltaic8 (PV) development. That

predictsthatwithareasonablesetof incentivesthephotovoltaicmarket intheEUcouldgrowmorethan

30%peryearoverthenext20years,from344MWofinstalledcapacityto9600MW.

Although, a number of scientific/technical, institutional and market barriers still stand in the way of

reachingthepreferredgoalsinPVindustryandthelevelofenergymarketpenetration.

Despite of large PVmarket expansion andmany research achievementsmade over the last couple of

decades, theircostsarepresently still themainobstacle foraworldwide increasedutilisationofelectric

powerprovidedbythiscleanandrenewabletechnology.ThepriceofPVsystemsisstilltoohigh,compared

withcompetingelectricitygenerationanddistributionmethods.

Inorder tobecomecompetitivewith theconventionalenergy sources,new improved solarcellconcepts

andcosteffectiveformsofapplicationsand installationsofPVmoduleshavetobedevelopedtofacilitate

furthergrowthofthesector.Theindustryislookingtodrivemodulepricesdownfrom5.75/Wp9,to3/Wp

in2010and1.51/Wpin2030.

1Energyforthefuture:renewablessourcesofenergy(WhitePaper),CommissionoftheEuropeanCommunities,1997.2ProposalforaDirectiveoftheEuropeanParliamentandoftheCouncilonthepromotionoftheuseofenergyfromrenewablesources.23.01.2008

COM(2008)final3Energyforthefuture:renewablesourcesofenergy WhitePaperforaCommunityStrategyandActionPlan,EuropeanCommission,19974AEuropeanstrategicenergytechnologyplan(SETPLAN):'Towardsalowcarbonfuture'.Communicationfromthecommissiontothecouncil,the

EuropeanParliament,TheEuropeanEconomicandSocialCommitteeandtheCommitteeoftheregions.{SEC(2007)1508},{SEC(2007)1509},

{SEC(2007)1510},{SEC(2007)1511}.22.11.2007COM(2007)723final5KyotoProtocoltotheUNFrameworkConventiononClimateChange.1997,3rdConferenceofthePartiestotheUNFCCC.6LietuvosRespublikosSeimo2002m.spalio10d.nutarimasNr.IX1130,,Dlnacionalinsenergetikosstrategijospatvirtinimo.7LietuvosRespublikosSeimo2006m.gegus11d.nutarimasNr.443,,DlNacionalinsenergijosvartojimoefektyvumodidinimo20062010met

programospatvirtinimo.8Photovoltaiccomprisesthetechnologytoconvertsunlightdirectlyintoelectricity.Thetermphotomeanslightandvoltaic,electricity.A

photovoltaiccell,alsoknownassolarcell,isasemiconductordevicethatgenerateselectricitywhenlightfallsonit.9Wp,orWattpeak,isthecommontermforwhataPVsystemiscapableofproducingunderidealcircumstances.


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There are twomain driving forces proposed by industry10 for decreasing themanufacturing costs: by

productionvolumeandbyinnovations(Figure1).

Figure1:Manufacturingcostsdrivenbyproductionvolumeandinnovations

Thevolume isneeded to stimulatemarketpartners to lowercostand to takeadvantageofeconomyof

scale.Consistentwiththetimeneededforanymajorchangeintheenergyinfrastructure,another20to30

yearsofsustainedandaggressivegrowthwillberequiredforphotovoltaicstosubstituteasignificantshare

of theconventionalenergysources.Thisgrowthwillbeonlypossible iftheprice is reducedconsiderably

(Figure2).

103rdPVIndustryForumNewStrategiesfortheBoomingPVMarket,2006


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Figure2:Projectedcostdevelopmentuntil2040PVcompetitiveness

Though themodule price decreased throughout the last decade, the price reductionwas slower than

expected.Certainlyinthelastyearsthedemandwasveryhighandtheconstructionofmanynewfacilities,

whichwill leadtoabettereconomyofscale infuture, isnotreflectedina lowerpriceyet.Inadditionthe

introductionofnewtechnologiesisnecessarytoreallyfulfilthepromiseoflowcostsolarenergyaswellas

sound fundamental research and improvements of the process and manufacturing technologies. New

developments with respect to material use, device design and new concepts to increase the overall

efficiencyareneeded.Themodulepriceisthekeyelementinthetotalpriceofaninstalledsolarsystemas

itcostrepresentsaround5060%ofthetotalinstalledcostofaPVsystem.

Therefore,thecompetitivenessofPVsystemscouldbeachievedby:

Eitherincreasingtheefficiencyofthesolarcells(i.e.increasingtheWp/m2ofcells);

Orbydecreasingthemanufacturingcost(i.e.reducingthe/Wp).

Obviously, the increase of solar cell efficiency is an important factor for the decrease of cell costs (per

generatedWp), because it reduces the costs for feedstock, crystallization andwafering by reducing the

material consumption. Although, the increment of the efficiency of the solar cells requires new

technologicalapproachesandinputfromresearch.


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GenerationsofPVCells

Firstgenerationphotovoltaics

First generation photovoltaic modules are based oncrystal silicon solar cells and demonstrate efficiencies

from 13 to 16 % (in production), with theoretical

maximum30%.Waferbasedcrystalline(monocrystalline

or polycrystalline) silicon is the dominant technology11

(Figure 3) because of wide availability and proven

reliability. This technology is well understood as it is

founded on the knowledge and technology originally

developedfortheelectronicsindustry.Yetmanufacturing

isenergy intensive,needsnumerousprocessingsteps (=

highmanufacturing costs) and highmaterials costs (for

mSi).

Secondgenerationphotovoltaics

Inanefforttoreducemanufacturingcosts,photovoltaiccellsbasedonthinfilmtechnologycalledsecond

generation solar cellswere developed. Thinfilmmodules aremade by coating and patterning entire

sheets of substrate with micronthin layers of conducting and semiconductor materials followed by

encapsulation.This leads toaprocessthatcanbehighlyefficient inmaterialsutilisation,whilethestable

efficienciesofthesemodulesareintherangeof5to15%.Themajortechnicalobstacleshamperingmarket

penetration by this technology on largescale are as follows: (i) growing of large area thin films of

homogeneousparametersatlowcostsand(ii)poorstability.

For solar cellsofbothgenerationseffortsaremademainly in improvingperformancemostnotably in

efficiency inordertoreducemodulemanufacturingcostsperWp,whichwill leadto lowersystemcosts.Thefirstgenerationproductisnearingitsoptimumprice/efficiencyincostperdeliveredWhr.However,the

predictedmoduleefficiency increment(uptothe3050%range)after2030 it isexpectedtobearesultof

successfulimplementationofthesocalledthirdornextgenerationconcepts,allowingveryefficientuseof

availablearea.

Emerginggenerationphotovoltaics

EmerginggenerationphotovoltaicsavarietyofotherPV technologiesandconversion conceptsare the

subjectof research in Europe andworldwide. They are all aimed at amodulepriceof 0.5/Wp, resp.

systempriceof1/Wp in2030,atsuperhighefficiencies(efficienciesabove25%under1sunhavetobe

demonstratedonlablevelbefore2015)atsuperhighefficienciesoratnewapplicationpossibilities.

Thisrequiresfundamentalresearch,becausereachingthetargetsrequiresathoroughunderstandingofthe

underlyingchemistry,physics,andmaterialsproperties.Hence,thenewtechnologiesareatvariousstages

ofdevelopment: fromproofofprinciple topilotproduction.Akey factor in thedecreaseof the costsof

modules isrelatedwiththemanufacturingprocessesused.Inthiscontextthere isconsiderable interest in

11 A Vision for PV Technology for 2030 and Beyond. Preliminary Report. Photovoltaic Technology Research Advisory Council (PV-TRAC). 2004

Figure3:

Production

by

technologies

in

2003

20

40

60

80

mono c-Si multi c-Si thin film other

Marketshare(%)


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replacingsinglecrystallineorpolycrystallinesemiconductor layersbynanostructured layers,onthesame

time searching for methods and experience in different research sectors to find cheap technological

solutions.Emerginggenerationcellswillbebasedonavarietyofnewconversionconceptsandprinciples,

andcurrentlycanbeconsideredtobeatthefundamentalresearchstage(Table1).Table1:Thecharacteristicsofemerginggenerationsolarcells

Parameters

TypeofSolarCell DescriptionandmaintechnicallimitationsEfficiencyin

production(%)

Lifetime

(Years)

Bandgap

(eV)

Multi

junction

Higherefficienciescouldbeachievedbyusingstacksof

semiconductorswithdifferentbandgaps(forinstance

GaAs,InP,GaInP2/GaAs,GaAs(Sisubstrate),

GaInP2/GaAs, InGaP/InGaAs/Ge).Thesearereferredtoas"multijunction"cells(alsocalled"cascade"or

"tandem"cells).Thistechnologymakesbetteruseofthe

incominglightwherebytheconversionefficiencyis

improved.Itisthemostpromising(highefficiencies

couldbeachieved)andthemostexpensivetechnology.

Figure4:Amultijunctiondeviceisastackofindividual

singlejunctioncellsindescendingorderofbandgap(Eg).

Thetopcellcapturesthehighenergyphotonsandpasses

therestofthephotonsontobeabsorbedbylower

bandgapcells

2136 0.73.4


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Dye

sensitised

photochemic

alsolarcells

(DSC)12.

Themainmaterialforsolarcellusedislowcost

nanocrystallinetitaniumdioxidewithalargeeffective

areaandorganicdyesimmersedinanelectrolyte.The

advantageofdyecellsisthattheycanbeproducedfrom

potentiallyinexpensivematerialsandbysimple

productiontechnology.Themajorchallengeistodevelop

cellsandmodulesforpowerapplications,asthatposes

forthistypeofcellsseveretemperatureconditions.

Thoughthestabilityincreasedsignificantly,itstilldoes

notmeetthestandardsofothersolarmodules.

Figure5:Dyesensitizedorganicinorganicsolarcell13

11(0.25cm2)

and8onreal

devices

Low

Variable

(depends

onmaterial

used)

Conductive

organic

polymercells

Thesedevicesarebasedonthepropertyofsomeorganic

materialstobeconductive:conjugatedpolymers.

Amongtheconductivepolymersinvestigated,themost

promisingonesarethestructurescontainingfullerene

(C60)astheacceptormaterial.Evidentadvantagesof

thistechnologyaretheexpectedlowcostmanufacturing

andthepossibilitytomakesolarcellsbytailoringthe

requiredpropertiesbymodificationsoftheorganic

molecules.Challengesaretoincreasesmallareacell

efficienciesandstabilityunderoutdoorconditions.

>5 Low

Variable

(depends

onmaterial

used)

12PVNET,EuropeanRoadmapforPVR&D,2004.

13http://www.oechemicals.com/dictionaryMZ.html


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Figure6:Anorganicsolarcellmadeupofseveral

layers

Quantum

solarcells

Thesecellsusequantumconfinementtomodifythe

electronicstructureinordertodecouplecurrentand

voltageandthusincreasingefficiency.Theeffectis

demonstratedusingIII/Vsolarcellshavinglayerswithathicknessinthenanoscalerange.Thechallengeistofind

lowcostsystemsthatcanusethesameprinciple.

Figure7:

Electron

transport

through

astructure

of

nanoparticles(left)andmoreorderednanotubes

(center)isshown.Atright,differentwavelengthsof

lightcanbeabsorbedbydifferentsizedquantumdots

layeredinarainbowsolarcell.Imagecredit:

Kongkanand,etal.2008ACS

>5%

Variable

(depends

onmaterial

used)

Nonanomaterialsarepresentlycommerciallyusedintheapplicationfieldofphotovoltaics.14Thematerials

arestillinafundamentalresearch,proofofprincipleandtestphases.Themainchallengesforthe

applicationofnanomaterialsintheenergysectoraretheimprovementofefficiencyandreductionofcosts

aswellasreliability,safetyandlifetime.Obviously,theincreaseofsolarcellefficiencyisanimportantfactor

forthedecreaseofcellcosts(pergeneratedWp),becauseitreducesthecostsforfeedstockand

manufacturingbyreducingthematerialconsumption.Theincrementoftheefficiencyofthesolarcells

14SWOTAnalysisConcerningtheUseofNanomaterialsintheEnergySector.PreparedunderFP6SSAprojectNanoroadSME:Developmentof

AdvancedTechnologyRoadmapsinNanomaterialSciencesandIndustrialAdaptationtoSmallandMediumsizedEnterprises(ContractnoNMP4CT

2004505857)


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requiresnewtechnologicalapproachesandtheinputfromresearch(basedonprinciplesofdevicephysics,

efficienciesashighas5060%arepredicted15,16).

Newtechnologiescanbecategorisedas:17

Optionsprimarilyaimedatverylowcost(whileoptimisingefficiency):

- Advancedinorganicthinfilmtechnologies(e.g.polysiliconthinfilm,spheralCISapproaches);

- Organicsolarcells(e.g.Graetzelcell,bulkdonoracceptorheterojunctionsolarcells);

- Thermophotovoltaicsolarthermalconcentrationsystemsorcogenerationsystems.

Optionsprimarilyaimedatveryhighefficiency(whileoptimisingcost):

- Tailoring the solar spectrum with the active semiconductor layer relying on up and down

conversion layers and plasmonic effects. Surface plasmons generated upon interaction between

lightandmetallicnanoparticleshavebeenproposedasamean to increase thephotoconversion

efficiencyinsolarcellsbyshiftingenergyintheincomingspectrumtowardsthewavelengthregion

wherethecollectionefficiency ismaximalorby increasingtheabsorbancebyenhancingthe local

fieldintensity.Theapplicationofsucheffectsinphotovoltaicsisdefinitelystillinaveryearlystage.

- Novelactivelayerswithreduceddimensionality(quantumwellsquantumwiresquantumdots).

Byintroducingquantumwellsorquantumdotsconsistingofalowbandgapsemiconductorwithina

hostsemiconductorwithwiderbandgap, thecurrentmightbe increasedwhile retaining (partof)

the higheroutput voltageof thehost semiconductor. Thisapproach aims atusing the quantum

confinementeffecttoobtainamaterialwithahigherbandgap.

- The collectionofexcited carriersbefore they thermalize to thebottomof the concernedenergy

band (e.g.hot carrier cells).The reduceddimensionalityof theQDmaterial tends to reduce the

allowable phonon modes by which this thermalization process takes place and increases the

probabilityofharvestingthefullenergyoftheexcitedcarrier.For most of emerging approaches the present emphasis is on basic material development, advanced

morphological and optoelectrical characterization and modelling as to predict the behaviour and

performanceunderillumination.

Nanomaterials with potential for exploitation in active layers with reduced dimensionality are

nanocomposites, consisting either of: (i) a nonnanocrystallinematrix of onematerial filledwith either

nanoparticlesornanofibersofanothermaterial;or(ii)nanonanocompositeswiththesizeofallconstituent

materials grains in thenanometer range (e.g.quantumdots, core shellnanoparticles, carbonnanotube

polymer nanonanocomposites, metalceramic nanonanocomposites, oth.). Synthesis and assembly

strategiesofnanomaterialsaccommodateprecursors from liquid,solid,orgasphase.18

Theyemploybothchemical and physical deposition approaches and similarly rely on either chemical reactivity or physical

15M.C.Beard,K.P.Knutsen,P.Yu,J.M.Luther,Q.Song,W.K.Metzger,R.J.Ellingson,A.J.Nozik.MultipleExcitonGenerationinColloidalSilicon

Nanocrystals.NANOLETTERS.ReceivedJune22,2007.16NCPVandSolarProgramReviewMeetingProceedings,2003,USA

17AStrategicResearchAgendaforPhotovoltaicSolarEnergyTechnologyResearchanddevelopmentinsupportofrealizingtheVisionfor

PhotovoltaicTechnology.PreparedbyWorkingGroup3Science,TechnologyandApplicationsoftheEUPVTechnologyPlatform.March200718JiaGraceLu,PaichunChang,ZhiyongFan.QuasionedimensionalmetaloxidematerialsSynthesis,propertiesandapplications.Availableonline

23May2006


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compaction to integrate nanostructure building blockswithin the finalmaterial structure. Themethods

employed to produce nanostructured materials are numerous (main production processes in use are

Chemical Synthesis (solgel, thermaldecomposition areused,oth.),Coating techniques (PhysicalVapour

DepositionorChemicalVapourDeposition),andPatterning(ElectronBeamLithography;XrayLithography,

ScanningProbeMicroscopy),witheachmethodhavingadvantagesanddisadvantagesdependingon the

desired properties or application.The product quality and application characteristics of nanostructured

materialsdependstronglyon thesizedistribution,morphologyandstateofaggregation, i.e. thesizeand

number of primary particles defining the degree of aggregation. Fundamental research by process

simulation is needed to provide the understanding of the particle and nanostructure formation

mechanisms. Combinedwith detailed analysis of particle size andmorphology and their impact on the

function at hand, it is possible to customise nanostructured products, providing distinct performance

advantagesforspecificapplications.

StrategicR&DtopicsforemergingPVtechnologies

ThemainissuesforemergingPVtechnologiesare:17

Poorstabilitynotmeetingthestandardsofothersolarmodules;

Thepresent35%(exceptmultijunctions)isonlyasteppingstoneformuchhigherefficiencies;

Poorunderstandingof the separationofgeneratedchargedcarriers, transportprocessesand the

relation between structure and function. This requires better processes andmaterials based on

fundamentalchemicalandphysicalknowledgeandnewdesigns;

Thetolerancetoimpuritiesisnotknownyet(thecostrisesexponentiallywithpurityrequirements),

therefore,onecannotasureifsuchsolarcellbemadeinastandardenvironment.

Insummaryitcanbestatedthatwithintheperiod20072013,fortheemergingPVtechnologies19(Table2),

issues like efficiency improvement, stability and encapsulation and the development of firstgeneration

modulemanufacturingtechnologywillbedominant.Forthenoveltechnologies20(Table3)theemphasisin

the coming yearswill be rather on nanotechnologyrelated items (nanoparticles, growth and synthesis

methods)andthefirstdemonstrationofconceptsbasedontheuseofsuchmaterialswithinfunctionalsolar

cells.IthastobeemphasizedthatforpartoftheemergingaswellasforallthenovelPVtechnologiesthese

developmentsrequiretheoreticalandexperimentaltoolsallowingtheunderstanding,themanufactureand

thecharacterizationofthemorphologicalandoptoelectricalpropertiesonnanoscale.Forthesubsequent

timeperiods20142020andbeyond2020themostpromisingconceptsaretobeselectedandimplemented

19ThecategoryEmergingisusedforthosetechnologieswhichhavepassedtheproofofconceptphaseorcanbeconsideredaslongerterm

optionsforthetwoestablishedsolarcelltechnologiescrystallineSiandthinfilmsolarcellsforwhichclearlydefineddisruptivedevelopmentsare

stilltobemade.20ThetermNovelisusedfordevelopmentsandideaswhichcanleadtopotentiallydisruptivetechnologies,butwherethereisnotyetclarityon

practicallyachievableconversionefficienciesorcoststructure.


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withanincreasedemphasisonaspectslikecost,upscaling,manufacturingandsustainabilitywhenmoving

toverylarge>10GWp/yearproductionscenarios.

Table2:

R&D

issues

for

emerging

technologies

Basiccategory Technology Aspects 20072013 2014202020202030and

beyond

Material Deposition

technology

Device Parallel

interconnection

Performance 14%

SpheralCIS

solarcell

Cost N/A

Industrial

implementation

>12%on

industriallevel

=0.50.8/Wp

Material ImprovingpolySi

electronicquality,

depositionup

scalingPerformance 14%/monolithic

moduleprocess

Advancedinorganic

thinfilmtechnologies

Thinfilm

polycrystalline

Sisolarcells

Cost N/A

Industrial

implementation

>1214%on

industriallevel=0.50.8/Wp

Implementation

ofadvanced

conceptsofsolar

spectrum

tailoringinultra

thinsolarcellsto

reach

15years

Performance 15%

Dyesolarcells

Cost N/A

Industrial

implementation

>10%on

industriallevel=

0.50.8/Wp

Material Improvedandstablepolymers,

stabilizationof

nanomorphology

for5years

Lowcostencapsulation

materialsto

guarantee

stability>15

years

Device Printing

technology

Organic

multijunctions

Organic

multijunctions

Performance 15% >10%on

industriallevel

Organicsolarcells

Bulkheterojunction

Cost N/A =0.50.8/Wp

Implementation

ofadvanced

conceptsofsolar

spectrum

tailoringtoreach

8%

Electrical

efficiency>8%

Thermophotovoltaics

Cost N/A


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Table3:R&Dissuesfornoveltechnologies

Basiccategory Technology Aspects 20072013 20142020 20202030and

beyond

Material Deposition

technology

Nanoparticle

synthesis

Metallic

intermediate

bandbulk

materials

Morphological

and

optoelectronic

characterization

Idem

Device Firstfunctional

cellsunder1sun

orconcentration

Labtypecell

Selection

Performance N/A >30%

Novelactivelayers Quantumwells

Quantumwires

Quantumdots

Nanoparticle

inclusionin

host

semiconductor

Cost N/A N/A

Upscalingof

mostpromising

approaches

requiringlow

costapproaches

fordeposition

technology,

synthesis,cell

andmodule

technology

compatiblewith

modulecosts10%

improvementrelativeto

baseline

Updown

converters

Cost N/A N/A

Material Metallic

nanoparticle

synthesiswith

controloversize,

geometryand

functionalization

Stabilityof

boostinglayer

materials

Device First

demonstrationon

existingsolarcell

typesunder1sun

orconcentration

Labtypecell

Selectionof

mostpromising

approaches

Performance N/A >10%

improvement

relativeto

baseline

Boostingstructuresat

theperipheryofthe

devise

Exploitationof

plasmonic

effects

Cost N/A N/A

Upscalingof

mostpromising

approaches

requiringlow

costapproaches

forsynthesisof

required

materials.

Depositionor

applicationtechnologyof

peripherallayers

withmodule

costs

emerging and novel photovoltaic technologies

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