CreatingPreliminarySpecificationsforPrintedPhotovoltaicsBy:AndreaHoAdvising:MalcolmG.Keif,Ph.D.GraphicCommunicationDepartmentCollegeofLiberalArtsCaliforniaPolytechnicStateUniversity,SanLuisObispoMarch2010©2010AndreaHo

CreatingPreliminarySpecificationsforPrintedPhotovoltaics

AndreaHo

GraphicCommunicationDepartment,March2010

Advisor:MalcolmG.KeifPh.D.

Thepurposeofthisstudyistoexposethetraditionalprintingindustrytoan

emergingfieldofprintedelectronics.Withthepopularityofelectronicdocuments

andtheinternet,thebulkoftheprintingindustrywilleventuallymoveawayfrom

thetraditionalinkandpaperprintingandmovetowardsmoretechnical,functional

applicationsthatwillmeettheupcomingneedsofsociety.Printedphotovoltaics,or

solarpanelsisanexampleofprintedelectronicsthatcanbemanufacturedby

traditionalprintingprocesses.

Thisstudyinvestigatediftheflexographyprintingprocesswasaviablewaytomass

manufactureprintedphotovoltaics.Ifso,whatspecificationsandtoleranceand

specificationswillbeexpectedfromthepressandtheprintedoutputtocreatea

successfulphotovoltaic.Factorssuchasinkfilmthickness,inkuniformity,and

registrationwereexplored.

TheresultsofthisstudyshowthatFlexographyisapossiblewaytomass

manufacturephotovoltaics.However,pressmodificationsandadditionalmaterial

developmentsareneededinordertosuccessfulcreateafunctionalphotovoltaic.

TableofContents

IntroductionandPurposeofStudy ....................................................................................................1

LiteratureReview.......................................................................................................................................3

ResearchMethods......................................................................................................................................8

Results ..........................................................................................................................................................11

Conclusions ................................................................................................................................................1σ

Bibliography ..............................................................................................................................................1ψ

FiguresandTables

Figure1 ...........................................................................................................................................................6

Figure2 ...........................................................................................................................................................7

Table1..........................................................................................................................................................14

ChapterOne:IntroductionandPurposeofStudy

CreatingPreliminarySpecificationsforPrintedPhotovoltaics

Thesignificanceofsolarpowerenergyhasbeengainingmediaattentioninrecent

years.Thesunisaplentifulsourceofenergythatisavailablejustaboutanywherein

theworld,andmostimportantly,itcostsnothingtogenerate.Thenecessityfor

efficientandaffordableenergyisevidentthroughthegrowingprevalenceofthe

sustainabilitymovement,theever‐presentclimatechange,andourdependenceon

limitedfossilfuels.Theseirreplaceableresourcesandourimpactontheearth

increasetheneedforanenvironmentallyfriendlysystemtoharvestenergy.One

solutionforthisproblemissolarpanels.Currently,solarpanelsareanexcellentway

tocollectenergy,however,theyaretoocostlytomanufacture.Onesolutionisto

printsolarpanelswithtraditionalprintingpressesalreadyusedtomanufacture

newspapersandbooks.Byprintingsolarpanels,highproductioncostswouldbe

savedandsolarpanelswillbeaccessibletoeveryone.

Conventionalsolarpanels,alsoknownasphotovoltaics,aremanufacturedby

harvestingandcuttingsiliconwafers,whichcanbeanextremelyslowand

expensiveprocess.Printing,ahigh‐speedprocess,isalogicalsolutiontothemass

manufactureneedsofthephotovoltaicindustry.However,inordertosuccessfully

manufacturephotovoltaicsbyaprintedprocess,oneneedstodevelopspecifications

orspecificguidelinesforprinterstofollow.Certainperformancerequirementswill

1

beneededfromthepress,substrates,andink.Thesespecificationswillultimately

pushtheprintingindustryintonewsectorsandpromotemassmanufacture

photovoltaicgrowth.

Thisresearchprojectwillaskthequestion,isflexographyaviablewaytomass

manufactureprintedphotovoltaics.Ifso,whatspecificationsandtoleranceand

specificationswillbeexpectedfromthepressandtheprintedoutputtocreatea

successfulphotovoltaic.Thegoalandoutcomeofthisstudywastofindpreliminary

specificationsandtolerancesforprintedphotovoltaicsbytheflexographyprocess.

Factorssuchasinkfilmthickness,inkuniformity,andregistrationwereexplored.

Thepurposeofthisstudyistoexposethetraditionalprintingindustrytoan

emergingfield.Withthepopularityofelectronicdocumentsandtheinternet,the

bulkoftheprintingindustrywilleventuallymoveawayfromthetraditionalinkand

paperprintingandmovetowardsmoretechnicalapplicationsthatwillmeetthe

upcomingneedsofsociety.Additionally,photovoltaicsareimportantgloballyas

environmentpreservationisbecomingmoreandmoreofanecessity.

2

ChapterTwo:LiteratureReview

BackgroundInformationofPrintedElectronicsMarketandPhotovoltaics

HistoryofPhotovoltaics

ScientistsatBellLaboratoriesaccidentallydiscoveredsolarcelltechnologyin1953.

GeraldPearsonandCalvinFullerdiscoveredawaytocontrolthedoping,or

introductionofimpurities,necessarytotransformsiliconfromapoortosuperior

conductorofelectricity.Someoftheexperimentsthescientiststestedfor

conductingelectricityincludedexposingthesilicontothesunwhileconnectingitto

adevicethatmeasuredelectricalflow.“Thesiliconthatwasdopedhadanelectric

outputalmostfivetimesgreaterthanthebestmaterialproducedbefore.”(Sun&

Sariciftci,2005)However,thecommercialsuccessofsolartechnologyfailedto

expandatthetimeduetothecostofsolarcells.

Thefirstpracticalapplicationofthesiliconsolarcellwasin1955withtheAmerican

government’sintentiontolaunchaspacesatellite,theVanguard.TheVanguardwas

thefirstworkingsatelliteequippedwithsolarcells.(Sun&Sariciftci,2005)The

precedingsatelliteswerepoweredbybatteriesandlostallstoredpowerwithinina

week’stime.Themillionsofdollarsinvestedintoasatellitethatonlyfunctionedfor

aweekonbatterieswasnotcosteffective.TheVanguardwasinstalledwithsilicon

solarcellsforpowerbecausetherewasaneedforareliable,longlastingenergy

sourceforthesatellite.(SunandSariciftci)

3

Whilethepracticalapplicationofsolarcellsboostedthemoraleofadvocatesforthe

technology,theexpensiveproductioncoststillkeptitfromcommercialsuccess.

Researchconcentrationwentintomaterialsandmakinglowergrade,lessexpensive

silicon.Thefirstmajorbuyersofsolarcellswereoilcompanies.Theoilrigsinthe

GulfofMexicowerefittedwithwarninglightsandhornsforsafetythatreliedon

batteriestopowerthesystem.Thesebatteriesneededmaintenanceandhadtobe

replacedapproximatelyeveryninemonths.Duetotheoilrigstructureand

placementofthebattery,alargeexpensewasrequiredforaroutineprocedure.

Solarmoduleswouldhelpsavetremendouscostbyeliminatingtheneedforthe

expensiveequipment.By1980,photovoltaicswerethestandardpowersourcefor

warninglightsandhornsfortheoilrigsontheGulfofMexico.(Sun&Sariciftci,

2005)

Fasttrackingtothepresent,thephotovoltaicsindustryhasexperienceda

tremendousgrowthrateof20percentannuallyinthelastdecade.(KahnandDas)

Thesolartechnologyhasbeengainingtremendousattentionforits“green”

purposes.Withtheincreasedconcernoverclimatechangeandgreenhousesgasses

thateffectourenvironment,photovoltaicsarereceivinggreatinterest.Solar

electricitygenerationhasnonoise,harmfulemissions,orpollutinggasses.Thereare

nomovingpartsanditrequiresverylowmaintenancetokeepthesystemsrunning.

(Krebs)“Itisestimatedthattheglobalsolarenergymarketwillreach$34billionin

2010and$100billionin2050.”(Kahn&Das,2007)

4

HowSolarPanelsWork

Solarpanelsworkbyconvertingsunlightintoelectricity,orphotonsintoelectrons.

Thisispossiblebythestructureandmaterialsinphotovoltaiccells,“photo”

meaninglight,and“voltaic”meaningelectricity.Thephotovoltaiccellsarepacked

intomodules,whicharesimplygroupsofcellselectricallyconnectedandpackaged

intooneframeorasolarpanel.(Aldus,2000)

Howthephotovoltaiccellmakesthisconversionpossibleisintheunique

semiconductormaterial,ortheactivelayerinthephotovoltaicstructure.Whenthe

photonfromthesunhitsthesolarpanel,eitheroneofthreethingscanhappen:

1. Thephotonpassesstraightthroughthesolarpanel.Thisisduetothelowenergy

photonnotbeingabletoexciteanyelectronsandserveanyfunctionforthesolar

cells.

2. Thephotonsreflectoffthesurfaceofthesolarpanelandareunabletobeused.

3. Thephotonisabsorbedbythesolarcell.Thisiswhatneedstohappentobeable

tocreateacurrent.Whenthephotonisactuallyabsorbed,theenergyfromthe

sunisthentransferredtothesemiconductormaterial.Thephotonexcitesthe

electronfromoneenergystagetoanother,allowingittofreelymovearoundin

thesemiconductormaterial.Thesefreelymovingelectronsarecalledfree

carriers.Theelectronsthenflowinacertaindirection,generatingthecurrent.

5

Thecurrentandvoltagefromtheelectronsmakethepowercalledelectricity.

(Aldus,2000)

Tocreateaphotovoltaictherearemultiplelayersofmaterialsthatservedifferent

functionstomakeaworkingsystem(seefigure1).Conventionalphotovoltaicsare

manufacturedbyhandusingveryexpensive,highlysensitivematerialsandthat

needspecializedtoolstoproducethem.(NanoMarkets)Newdevelopmentsinthe

markethaveevaluatedprintingasapossiblemeansofproductioninthefuture.

Variouslayerswouldbepatternedontoatransparentsubstrateandwiththe

correctspecificationsandtolerances,createworkingphotovoltaics.

(figure1)NanoMarkets2009

6

FlexographyProcess

Flexographyisaprintingprocesswhereinkistransferredfromtheinktraybyan

aniloxroll,toaraisedimageareacalledablanket,andeventuallytoasubstrate.The

aniloxrollisapatternedroll,whichfunctionstotransferinkfromtheinktraytothe

blanket.Theaniloxrollthentransferstheinktotheprintingplateandthentothe

substrate.Flexographyisonepossibleprintingmethodtoproducephotovoltaics.

In order to determine if flexography is a viable way to produce photovoltaics,

specifications and tolerances need to be determined first and evaluated against

existing flexography presses to determine if it is possible to mass-produce

photovoltaics by this method. For this particular study, focus was on creating an

organic photovoltaic with conventional materials. (see figure 2)

P3HT:PCBM

PET or equivalent

PEDOT:PSSITO or ZnO or silver grid

-

Silver or aluminum+

(figure2)Asampleofthedevicearchitecturethatwasusedforthisstudy.

IllustrationbyMalcolmKeif.

7

Chapter Three: Research MethodsDerivingSpecificationsofPrintedPhotovoltaics

Thekeytomakingphotovoltaicswidelyusediswithlessexpensiveproduction,

enablingthedistributiontoawideraudience.Interestisbeinggainedinprintingas

ameansofmanufacturingbecauseoftheroll‐to‐rollcapabilitiesofproduction.

However,inordertoprintphotovoltaicssuccessfully,specificationssuchasthose

developedbyASTMandISOneedtobeestablished.Togainspecificationsfor

printingphotovoltaics,eliteandspecializedinterviewingwasusedtogather

research.Thepremisebehindofthisresearchapproachtothisprojectwasthatthe

subjectofphotovoltaicsissoinnovativethatagreatdealofknowledgewasnot

publishedyet,butratherbeingdiscoveredcurrently.Thespecificationsforprinted

photovolaticsarethestarttothehighvolumedistributionofrenewableenergy.

AccordingtoHarveyLevensoninSomeIdeasAboutDoingResearchinGraphic

Communication,“Eliteandspecializedinterviewingiswhereeachandevery

intervieweeistreatedasaspecialandimportantindividual.Inthisparticulartype

ofinterview,theinterviewerwillaskprecise,open‐endedquestionsthatareopento

refinementastheresearchandinterviewcontinues.”Hecontinuesontoexplain,

“whatissoughtinEliteandSpecializedInterviewingiscomprehensibility,

plausibility,andconsistency,notduplicationofresponses.”Furthermore,the

8

intervieweebecomespartoftheresearch“team”ratherthanasubject.(Levenson)

Discussionswithintervieweesweretolearnhowaphotovoltaicsystemworksand

askthemtohelpfilloutatableofspecifications.Thespecificationsandtolerances

aretherangeofparticularvariablesthatarerequiredinordertohaveaworking

photovoltaiccellsuchaskinkfilmthickness,annealingtime,andoxygentrapping

specifications.Thespecificationswerethencompiledintoaspreadsheet.

Variousexpertsintheindustrythathaveexperienceandknowledgein

manufacturingphotovoltaicswereinterviewed.ThefirstintervieweewasSarah

Mednick,aformerCaliforniaPolytechnicStateUniversitygraduateandcurrent

UniversityofCalifornia,SantaBarbaragraduatestudentintheHeegerResearch

Group.Herfocusofstudyisonphotovoltaics.Thepurposeforinterviewingherwas

togainknowledgeonhowphotovoltaicsfundamentallyworkandhaveherfillouta

tableofspecificationsthatwascreated.

ThesecondcontactwasDr.DavidSime,directoroftechnologytransferatSoligie,a

privatelyheldphotovoltaiccompany.Theircompanyincorporatesflexographic

processtoprintphotovoltaics.Hehelpedwiththespecificationssheetand

developedtolerances.

9

ThethirdcontactwasMaryBoone,amarketdevelopmentdirectoratPlextronics,a

privatelyheldphotovoltaiccompany.Sheevaluatedthespreadsheetandgave

commentsonthevarioussections.

ThefinalinterviewconductedwaswithEitanZeira,thevicepresidentofprinted

photovoltaicsatKonarka,aprivatelyheldphotovoltaiccompany.Helookedatthe

spreadsheetasawholeandgavefeedbackonspecificquestionsandoverallprinting

problemsforprintableelectronics.

Thefollowinginformationwaspursuedduringtheeliteandspecializedinterview:

1. Whatistherelationofbandgaptotheefficiencyofthesolarcell?

2. Whatisthedifferencebetweenpolymerphotovoltaicsandorganic

photovoltaics?

3. Whatdefinesthegenerationofaphotovoltaiccell?

4. Whatfactorsaffecttheefficiencyofaphotovoltaiccell?

5. Towhatextentdoesinkfilmthickness,adhesionspecification,andoxygen

trappingspecificationshaveanaffectontheefficiencyofaphotovoltaic?Ifso,

howcriticalisittokeepthematerialsandprocessingunderthetolerances?

6. Whatisthemostimportanttolerancetokeepinaphotovoltaiccell?

7. Whatroledoesflexographyhaveintheprintedelectronicsindustry?

10

ChapterFour:Results

ResultsofEliteandSpecializedInterviewing

Afterinterviewingexpertsinthisindustry,specificationsandtolerancesnecessary

forasuccessful,functioningphotovoltaicwerederived.Duetotheproprietary

natureoftheindustry,therewerevaryingopinionsonwhatspecificationswere

crucialandonthenumbersthemselves.Fourinterviewswereadministeredbyelite

andspecializedinterviewingprocess.

ThefirstinterviewwaswithSarahMednick,agraduatestudentatUniversityof

California,SantaBarbaraandfellowoftheHeegerResearchGroup.Sarahprovideda

sitetourofherlaboratoryaswellasadetailedexplanationofhowphotovoltaics

wereprocessedattheuniversity.Sarah’sexpertisewasinlabtestingphotovoltaics

sosomeofthespecificationsshegavewerealittleunreasonableforprinted

photovoltaics.Theinterviewwithherwasconsideredasastartingpointto

understandhowphotovoltaicsactuallyfunctionandhowtheyaretraditionally

fabricated.Shepointedouttheimportanceofproperlydryingorannealing

materialstomakethecircuitfunction.

ThesecondinterviewwaswithDavidSime,directoroftechnologytransferat

Soligie,aprivatelyheldphotovoltaiccompany.Theircompanyincorporatesthe

flexographicprocesstoprintphotovoltics,sohisknowledgewasvaluableforthis

11

particularpaper.Anin‐personinterviewwasconductedatthePrintedElectronics

USAconferenceinSanJose,CaliforniaonDecember3rd.Dr.Simewasabletolookat

thespreadsheetwiththedifferentcategoriesthatweredeemedimportantbySarah

Mednick.Hepointedoutthatthesubstratethicknesswouldbeconsiderablythinner

foraprintedphotovoltaicthanonespin‐coatedontoglass,typicaltothelabwork

SarahdidatUCSB.Hewasalsoabletogivetransparencyandsurfaceuniformity

specificationswhilepointingouttheimportanceofproperlydrying,orannealinga

material.Dr.Simefurtherexpandedthespreadsheetfromwhatwasderivedwith

Sarah.ThemajorityofthespreadsheetwasbuiltupontheinformationDr.Sime

gave.

ThethirdinterviewwaswithMaryBoone,amarketdevelopmentdirectorat

Plextronics.ShewasabletolookoverthespecificationsthatDr.Simegavebyemail

andwasabletogiveinputofsurfaceuniformityandoxygentrapping.Mary’s

feedbackwassomewhatlimited.

ThefinalphoneinterviewconductedwaswithEitanZeira,thevicepresidentof

printedphotovoltaicsatKonarka,aprivatelyheldphotovoltaiccompany.Hewas

abletoanswerspecificquestionspertainingtothespreadsheetandoverall

questionswithprintingtrends.Manyoftheconclusionsbasedonprintingpress

capabilitiessuchaspinholefreelayersweredevelopedwhileinterviewingEitan.

12

ChapterFive:Conclusions

ConclusionsofDerivedSpecifications

Thecontentattainedduringtheinterviewswasprocessedandthencompliedintoa

spreadsheet.(seetable1)Thespreadsheetcontainsthedeterminedcriticalfactors

necessaryforaflexiblephotovoltaicprintedonaflexographicpress.Inklayer

thickness,surfaceuniformity,contactangle,registration,annealing,conductivity,

oxygentrapping,opticaltransparency,andadhesionwereconcludedtobethemost

importantspecificationstoaheadto.Thesespecificationsweredeterminedfora

conventional,organicpolymerphotovoltaic.

Layerthicknesswasdeterminedtobeoneofthemostcriticalspecificationsfor

efficiencyofthephotovoltaic.Thematerialswouldfunctionatoptimalconductance

withcorrectthicknesses.Ideally,thesemiconductorlayershouldbearound150

nanometers.

Surfaceuniformitywasdeemedofupmostimportanceduetothepotentialpin

holingthatcouldbecausedbytheprintingpress.Anypinholespresentwouldcause

ashortinthecircuit,causingittobecomeinactive.Thefilmsubstrateshouldbeat

3‐5Rswhereasthesemiconductorlayerwillbesolventdependant.Whilethe

uniformityofalllayersisnotnecessarilycritical,theymustbepinholefree.

13

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Table 1 : Preliminary Speci�ications for Printed Photovoltaics

14

Contactangleisdeterminedbyhowresistanttheinkistobeadinguponthe

substrate.Thisinturnalsocreatespinholes.Havingthecorrectsurfaceenergyhelps

theinkstowetoutproperly.Contactanglesshouldbe30degreesorless.The

surfaceenergyofthefluidhastobehigherwitheachlayerthatisadded.A

minimumof5dynedifferencesshouldbebetweenthedifferentlayers.

Registrationwasfoundtobenotasimportantwithphotovoltaicscomparedto

transistorsoranyotherprintedelectronicdevice.Theelectrodedoesnotshortwith

bleedingbeyondtheactivelayer.

Dryingandannealingisaconsiderablechallengeforprinting.Themajorityofthe

materialsrequireextremelyhightemperaturestoannealtheink.Additionally,to

properlyanneal,theinksneedacertainperiodoftime,whichmayprovedifficultfor

aprintingpressrunningattopspeed.Itwasdiscoveredthatsomeprintersmodify

theirpressestohaveenhanceddryersorofflineprocessesthatwillannealthe

material.Additionalproblemsincludedthelimitonsubstratesthatcouldbeused.If

theinkneeds140°Ctodry,manypaperandpolymersubstratewouldlosestability.

Conductivityisoneofthemostimportantspecificationsbecauseitdetermines

efficiencyoftheelectrode.Whenthematerialisdepositedontothesubstrate,it

15

shouldbeinamannerwheretheinkwillfunctionatoptimalconditions.The

mobilityoftheP3HT:PCBMshouldbe10^‐3cm2/Vs.

Oxygentrappingisneededwithcertainmaterialstoproperlyadheretothe

substrateanddrycorrectly.ThematerialP3HT:PCBMisbestappliedinanitrogen

environment.Itmaybenecessarytouseanitrogenairknifeandchambertoapply

theink.

Boththecathodeandsubstratehavetobehighlytransparenttoallowsunintothe

circuit.Thesubstrateshouldbeabove95%transparentandthecathodeas

transparentaspossible,around80%.

Inkadhesiondetermineshowwelltheinkstickstothesubstrate.Thiswill

determinetheintegrityandstabilityofthephotovoltaicovertime.Adhesioncanbe

determinedbytheASTMcrosshatchtapetest.

Initially,registrationandsurfaceuniformitywasthoughtofasthemostimportant

specificationsaprintingpresshadtomeet.However,afteralltheinterviews,itwas

apparentthatdryingandannealingaswellassurfaceuniformitywouldbethe

greatestchallengeforatraditionalflexographypress.

16

Thehardestchallengeforprintwouldbeabletocreateathin,pinholefreelayer.

Ideally,therewouldbesomesortofsystemtoevaluatethelayersasitisbeingin

production.Theresolutionlevelsoftraditionalpressesneedtobeincreasedto

avoidpinholing.Microflexographyormicrogravurecanberesearchedforthese

purposes.

Takingthespecificationsintoaccount,isitfeasibleforflexographytobeameansof

massproductionforprintedphotovoltaics?Itispossible,howeverfrom

interviewingexpertsintheindustry,itiscommontohavehybridtechnologiesto

takethestrengthsfromeachindividualprintingprocess.Additionally,press

modificationssuchasextendeddryersandoxygentrappingchambersareneeded.

Theflexographyindustryshouldfocusonbuildingstrengthstomanufacture

functionalprintingproducts.Researchinnewmethodstoincreaseinkfilm

thicknesscontrolandimprovingdryingcapabilitiesisneeded.

Printedphotovoltaicswilleventuallychangetheentireenergymarketandhelpwith

theaffordabilityofsolarpower.Flexographycanbeahugeleaderinthisupcoming

change.Thefirststepistodiscoverandunderstandtheimportantvariablesina

photovoltaicandbuildequipmentthatcanmanufactureaqualityproductatalow

cost.Thisinturn,willrevolutionizetheindustry.

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Aldus,Scott.“HowSolarCellsWork.”1April2000.HowStuffWorks.1July2009<http://science.howstuffworks.com/solar‐cell.htm>.

Kahn,BruceandRaghDas.PrintedanThinFilmPhotovoltaicsandBatteries.Cambridge:IDTechEx,2007.

Krebs,FrederikC.PolymerPhotovoltiacsAPracticalApproach.Bellingham:SocietyofPhoto‐OpticalInstrumentationEngineers,2008.

Levenson,Harvey.SomeIdeasAboutDoingResearchinGraphicCommunication.TheGoodNeighborPress&Services,2009

Sun,Sam‐ShajingandNiyaziSedarSariciftci.OrganicPhotovoltaics:Mechanisms,Materials,andDevices.BocaRaton:CRCPress,2005.

NanoMarkets.NanoMarkets:PrintableElectronics‐PrintableElectronicsMarketOutlook.11February2008.22October2009<http://www.nanomarkets.new/product/prod_detail.cfm?prod=3&id=250>.

Figure1.Diagram.GlenAllen,VA:NanoMarkets,2009.Internet.

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