DTIC ADA575033: Soluble P3HT-Grafted Graphene for Efficient Bilayer-Heterojunction Photovoltaic Devices

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by  Defense Technical Information Center

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Soluble P3HT-Grafted Graphene for
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(cid:1)
Efficient Bilayer Heterojunction
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Photovoltaic Devices
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Dingshan Yu,† Yan Yang,‡ Michael Durstock,§,* Jong-Beom Baek,(cid:1) and Liming Dai†,*
†DepartmentofChemicalEngineering,CaseWesternReserveUniversity,Cleveland,Ohio44106,‡DepartmentofChemistryandBiochemistry,NewMexicoState
University,NewMexico88003,§MaterialsandManufacturingDirectorate,AirForceResearchLaboratory,RXBP,Wright-PattersonAirForceBase,Ohio45433,and L
(cid:1)InterdisciplinarySchoolofGreenEnergy,UlsanNationalInstituteofScienceandTechnology(UNIST),100,Banyeon,Ulsan,689-798SouthKorea
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hephotovoltaiceffectinvolvesthe
generationofelectronsandholesin ABSTRACT CHOH-terminatedregioregularpoly(3-hexylthiophene)(P3HT)waschemicallygraftedonto
2
asemiconductordeviceunderillu- carboxylicgroupsofgrapheneoxide(GO)viaesterificationreaction.TheresultantP3HT-graftedGOsheets(G-
minationandthesubsequentchargecollec-
P3HT)aresolubleincommonorganicsolvents,facilitatingthestructure/propertycharacterizationandthedevice
tionatoppositeelectrodes.1(cid:1)8Photonab-
fabricationbysolutionprocessing.ThecovalentlinkageandthestrongelectronicinteractionbetweentheP3HT
sorptionoforganicoptoelectronicmaterials
andgraphenemoietiesinG-P3HTwereconfirmedbyspectroscopicanalysesandelectrochemicalmeasurements.
oftencreatesboundelectron-holepairs
(i.e.,excitons).Chargecollection,therefore, Abilayerphotovoltaicdevicebasedonthesolution-castG-P3HT/C60heterostructuresshoweda200%increaseof
requiresdissociationoftheexcitons,which thepowerconversionefficiency((cid:2)(cid:3)0.61%)withrespecttotheP3HT/C counterpartunderAM1.5illumination
60
occursonlyattheheterojunctioninterface (100mW/cm2).
betweensemiconductingmaterialsofdif-
ferentionizationpotentialsorelectronaf-
KEYWORDS:graphene · poly(3-hexylthiophene) · end-functionalization · solution
finities.Solubleconjugatedpolymersand processing · solarcells
fullerenehavebeenwidelyusedaselectron
donors(D)andacceptors(A)toenhance venteffectorpostfabricationdiffusion,also
thechargeseparation.1(cid:1)5Thephotoin- createsproblemsinabilayerdevice.Fur-
ducedchargetransferbetweentheexcited thermore,theoverallenergyconversionef-
conductingpolymerdonorandC60accep- ficiencyofabilayerdeviceisdiminishedby
torcanoccurveryrapidlyonasubpicosec- thelimitedeffectiveinterfacialareaavail-
ondtimescalewithaquantumefficiencyof ableinthelayerstructure,andthephotoex-
closetounityforchargeseparationfrom citationsproducedfarfromtheinterfacere-
donortoacceptor.Becausetheexcitondif- combinebeforediffusingtothebilayer
fusionrangeistypicallyatleastafactorof heterojunction.However,abilayeredde-
10smallerthantheopticalabsorption vicestructurecouldbemorefavorablewith
depth,thephotoexcitationsproducedfar respecttothebulkheterojunctionforeffi-
fromtheinterfacerecombinebeforediffus- cientchargetransportsincetheseparated
ingtotheheterojunction.Inordertoover- chargecarrierscaneasilytransporttothe
comethisdeficiency,bulkheterojunction oppositeelectrodeswithaminimizedre-
photovoltaiccellsbasedoninterpenetrat- combinationpossibility.7(cid:1)9Inthisregard,
ingnetworksconsistingofapolymericelec- Wangetal.9reportedthataP3HT/PCBMbi-
trondonor[e.g.,poly(3-hexylthiophene)] layerpolymersolarcellwithaconcentra-
andaC derivativeacceptor(typically, tiongradientshowedanenhancedphoto-
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[6,6]-phenyl-C -butyricacidmethylester, currentdensityandpowerconversion
61
*Addresscorrespondenceto
PCBM)havebeendeveloped.1,6 efficiencycomparedtothoseofthebulk
[email protected](L.D.),
Thebulkheterojunctionconcepthas heterojunctionphotovoltaiccellsunderthe [email protected](M.D.).
beenwidelyusedforpolymerphotovoltaic samecondition.Ontheotherhand,theC
60 ReceivedforreviewMay20,2010
cells.Duetoitshighinterfacialcontactarea layerinbilayerdevicescouldprovideanad-
andacceptedSeptember01,2010.
betweenthedonorandacceptor,thebulk ditionalprotectiontothepolymerlayer
heterojunctionismoreeffectivecompared frompossibledegradationcausedbyoxy- PublishedonlineSeptember10,
2010.
withabilayerdevicestructure.Miscibility genandhumidity,thusimprovingthede-
10.1021/nn101671t
betweentheelectronacceptoranddonor vicestability.10Moreimportantly,thebilayer
attheinterface,eithercausedbyacosol- structureallowstheC60layertoprevent ©2010AmericanChemicalSociety
www.acsnano.org VOL. 4 ▪ NO. 10 ▪ 5633–5640 ▪ 2010 5633
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1. REPORT DATE 3. DATES COVERED
10 SEP 2010 2. REPORT TYPE 00-00-2010 to 00-00-2010
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Soluble P3HT-Grafted Graphene for Efficient Bilayer-Heterojunction
5b. GRANT NUMBER
Photovoltaic Devices
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
Ulsan National Institute of Science and Technology REPORT NUMBER
(UNIST),Interdisciplinary School of Green Energy & Inst of Advanced
Materials & Devices,100, Banyeon,Ulsan 689-798, South Korea,
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; distribution unlimited
13. SUPPLEMENTARY NOTES
14. ABSTRACT
CH2OH-terminated regioregular poly(3-hexylthiophene) (P3HT) was chemically grafted onto carboxylic
groups of graphene oxide (GO) via esterification reaction. The resultant P3HT-grafted GO sheets (GP3HT)
are soluble in common organic solvents, facilitating the structure/property characterization and the device
fabrication by solution processing. The covalent linkage and the strong electronic interaction between the
P3HT and graphene moieties in G-P3HT were confirmed by spectroscopic analyses and electrochemical
measurements. A bilayer photovoltaic device based on the solution-cast G-P3HT/C60 heterostructures
showed a 200% increase of the power conversion efficiency ( 0.61%) with respect to the P3HT/C60
counterpart under AM 1.5 illumination (100 mW/cm2).
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 8
unclassified unclassified unclassified Report (SAR)
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
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Figure1.(a)SynthesisprocedureforchemicalgraftingofCHOH-terminatedP3HTchainsontographene,whichinvolvestheSOCl
2 2
treatmentofGO(step1)andtheesterificationreactionbetweenacyl-chloridefunctionalizedGOandMeOH-terminatedP3HT(step
2).
thepolymerlayerfromdirectcontactwiththecath- tentialapplicationofgraphenesheetsinpolymer
ode,reducingtherecombinationlossandeliminating photovoltaicdeviceshasbeenprecludedbytheirpoor
anypossibleshortcircuitproblemforpolymerhybrid solubility,whichmakesthegraphenedispersionand
solarcellscontainingconductingadditives(e.g.,carbon devicefabricationverydifficult,ifnotimpossible,bythe
nanotubes,graphenesheets;seebelow).Inthisstudy, traditionalsolutionprocessingmethods(e.g.,spin-or
therefore,wefocusonthebilayerstructuretodemon- blade-coating).Fortunately,thepresenceofoxygen-
stratepotentialapplicationsofP3HT-graftedgraphene containingfunctionalgroups(e.g.,(cid:1)OHand(cid:1)COOH)
(G-P3HT)inphotovoltaicdevices. ingrapheneoxide(GO)couldnotonlyimpartsolubil-
Ascanbeseenfromabovediscussion,efficient ityforlarge-scalefilmformationthroughsolutionpro-
photovoltaiccellsshouldinvolvematerialswitharela- cesses(e.g.,spin-castingandlayer-by-layerself-
tivelyhighchargemobilityandlargeD(cid:1)Ainterfacefor assembling)27butalsofacilitatechemicalfunctionaliza-
efficientexcitiondissociationandtransport.5,11Along tionofgrapheneforvariousapplications.28Although
withcertaininorganicnanoparticles(e.g.,CdSe,12,13 someeffortshavebeenmadetowardpolymerfunction-
CdTe,5andZnO14,15nanocrystals),carbonnanotubes alizationofGOsheets,noattempthasbeenreported
(CNTs)haverecentlybeenusedinbilayerandbulkhet- forconjugatedpolymerchains.29
erojunctionpolymerphotovoltaicdevicestoprovide Inthisstudy,wehavechemicallygraftedCHOH-
2
thelargesurface/interfaceareaaswellasgoodelec- terminatedregioregularpoly(3-hexylthiophene)(P3HT)
tronicpropertiesforenhancedchargeseparationand mainlyontocarboxylicgroupsofGOsheetsviaesterifi-
transport.16(cid:1)20However,significantimprovementinthe cationreaction.TheresultantP3HT-graftedGOsheets
overalldeviceperformancehasnotbeenachievedfor (G-P3HT)aresolubleincommonorganicsolvents,facili-
CNT-basedpolymerphotovoltaiccellsduelargelyto tatingthestructure/propertycharacterizationandde-
technicaldifficultiesindispersingCNTshomogenously vicefabricationbysolutionprocessing.Inparticular,we
inthepolymermatrixwithoutanyshort-circuitprob- havefoundthatabilayerphotovoltaicdevicebased
lem,particularlyinthebulkheterojunctionphotovol- onthesolution-castG-P3HTheterostructurewithther-
taicdevices. mallyevaporatedC showedanupto200%increasein
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AsthebuildingblocksforCNTsandothercarbon thepowerconversionefficiencycomparedtoitsP3HT/
nanomaterials,thetwo-dimensional(2-D)singleatomic C counterpartunderAM1.5illumination(100mW/
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carbonsheetsofgrapheneshowremarkableelec- cm2).Theresultantpowerconversionefficiencyof
tronic,thermal,andmechanicalpropertiesattractive 0.61%indicatesthattheG-P3HT/C bilayerphotovol-
60
foravarietyofpotentialdeviceapplications.21(cid:1)25Of taicdeviceisalsomoreefficientthanitscounterpart
particularinterest,grapheneshowsthehighestroom- withelectrondonorsbasedonP3HTwithandwithout
temperaturemobilityforelectronandholetransport dopingbyCNTorinorganicnanocrystals,suchasCdTe
amongallknowncarbonnanomaterials.25Compared tetrapods.5,17(cid:1)19
withCNTs,theone-atomthicknessand2-Dcarbonnet-
workofgrapheneleadtoamuchhigherspecificsur- RESULTSANDDISCUSSION
facearea(hence,alargerinterfaceinapolymermatrix) GOwaspreparedaccordingtoamodifiedHum-
andareducedthrough-thicknessshortcircuitforthe mersmethod.30IndividualGOsheetsthusproduced
photovoltaicactivelayereveninabulkheterojunction withathicknessof(cid:2)1.3nmandsizedistributioninthe
device.22,26Therefore,thecombinationofgraphene rangeof0.2(cid:1)2(cid:3)mareshowninFigureS1oftheSup-
sheetswithconjugatedpolymersisofgreatpromise portingInformation.EndfunctionalizationofP3HTwas
forpolymer-basedphotovoltaiccells.However,thepo- carriedoutaccordingtothereportedproceduretoyield
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(cid:1)CHOHendgroups31(SchemeS1,SupportingInfor-
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mation).AsforthepreparationoftheP3HTgrafted
graphene,GOwasfirstreactedwithSOCl toobtain
2 T
acyl-chloridefunctionalizedGO.Then,acondensation
reactionbetweentheCH2OH-terminatedP3HTandthe I
acyl-chloridefunctionalizedGOwasperformedtoyield C
theG-P3HT(Figure1).ThefinalG-P3HTproductcanbe
easilyredispersedincommonorganicsolvents,such L
asTHF,chloroform,andtoluene,undersonication.
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Figure2showsX-rayphotoelectronspectroscopy
(XPS)spectraofGObeforeandaftergraftingwiththe
CHOH-terminatedP3HTchains.WhileFigure2ashows
2
onlytheCandOpeaksforGO,thecorrespondingXPS
spectrumforthepurifiedG-P3HTinFigure2breveals
thepresenceofcarbon,oxygen,andsulfur,arisingfrom
theP3HTchainsandgraphenesheets.Figure2crepro-
ducesthehigh-resolutionC1sspectrumforGO,which
exhibitsthepresenceofC(cid:1)C(284.5),C(cid:1)O(286.4),
CAO(288.0)andCOOH(289.0eV)groups.Uponpoly- Figure2.XPSspectraofGOandG-P3HT.(a,b)Surveyspectraand(c,
d)high-resolutionC1sspectra.
mergrafting(Figure2d),theintensitiesoftheCAOand
COOHpeaksdecreaseddramaticallyduetotheforma- Figure4ashowsthenormalizedabsorptionspectra
tionofesterlinkagesbetweentheCOOHgroupsofGO ofpureP3HTandG-P3HTinchloroformsolution.A
andthehydroxylgroupsoftheCHOH-terminated strongabsorptionbandat446nmattributabletothe
2
P3HT.ThepresenceoftheesterlinkagesinG-P3HTis (cid:4)(cid:1)(cid:4)*transition36wasseenforpureP3HT,whichred-
alsoevidencedbytheCAOandC(cid:1)Opeaksat288.0 shiftedtoabout458nmupongraftingontographene
and286.5eVinFigure2d,thoughthecontributionfrom inG-P3HT.However,thesimplemixtureofP3HTand
someunreactedfunctionalgroupsintheGOcannot grapheneinthesamesolventdidnotcauseanysignifi-
beruledout.Whiletheesterificationreactionbetween cantchangeintheP3HTabsorptionband(FigureS5,
theacyl-chloridefunctionalizedGOandCHOH- SupportingInformation).Theseresultsindicatethat
2
terminatedP3HTsignificantlyreducedtheCOOHcom- thereexistsastronginteractionbetweenP3HTand
ponentofGO,thesignificantdecreaseoftheC(cid:1)Ocom- grapheneinG-P3HT.Suchspecificinteractionprob-
ponentinFigure2csuggestspossibleactivationofthe ablyincreasestheelectrondelocalizationalongthe
(cid:1)OHgroupsofGObythionylchlorideandsubsequent polymerchain,thusleadingtotheobservedred-shift
graftingoftheCH2OH-terminatedP3HTontotheacti- oftheopticalabsorptionpeak.18TheUV(cid:1)visabsorp-
vated(cid:1)OHgroupofGObytheformationof tionspectraforthinfilmsofpureP3HTandG-P3HTare
O(cid:1)S(O)(cid:1)Olinkage(Figure1).32 alsoshowninFigure4a.Asexpected,theP3HTthin
TheesterlinkagebetweenP3HTandgraphenein filmshowedasignificantred-shiftofthe(cid:4)(cid:1)(cid:4)*absorp-
G-P3HTwasalsoconfirmedby1HNMRmeasurements tionupto(cid:2)512nmfromthecorrespondingsolution
(FigureS2(cid:1)S4,SupportingInformation)andfurther absorptionat446nmdue,mostprobably,tothestrong
checkedbyFouriertransforminfrared(FTIR)spectros- interchaininteractionintheregioregularP3HTthin
copy.AsshowninFigure3(solidline),thepureGO
showsthepeaksofOH(O(cid:1)Hstretchingvibrations)at
3431,CAO(carboxylicacidandcarbonylmoieties)at
1731,andC(cid:1)C(skeletalvibrationofgraphiticdomains)
at1627cm(cid:1)1,alongwiththebroadbandover
1000(cid:1)1400cm(cid:1)1arisingfromtheC(cid:1)OH(1227)and
C(cid:1)O(1075)stretchingvibrations.33FortheG-P3HT(Fig-
ure3,dashline),thebandsat1726and1242cm(cid:1)1can
beattributedtotheCAOstretchingandtheC(cid:1)O
stretchingoftheestergroups,respectively,confirming
theformationofcovalentbondingbetweengraphene
andP3HT.34Besides,thepresenceofP3HTisconfirmed
bytheappearanceofthreeabsorptionbandsat2960,
2925,and2855cm(cid:1)1associatedwithaliphaticC(cid:1)H
stretchingofP3HTandthe821cm(cid:1)1bandcorrespond-
Figure3.FTIRspectraofGObeforeandaftergraftingwith
ingtoaromaticC(cid:1)HbendingofP3HT.35 P3HTchains.
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Figure4.(a)UV(cid:1)visabsorptionspectraofpureP3HTandG-P3HT(solidanddashlines,respectively)inCHCl solutionat1
3
mg/mLandofsolidthinfilmpreparedbyspin-coatingonquartzplates.(b)PLspectraofpureP3HTandG-P3HT(solidand
dashlines,respectively)inCHCl solutionat1mg/mLand(cid:4) (cid:3)450nm.
3 ex
film.TheabsorptionbandoftheG-P3HTfilmalso Therefore,thecharge-transferinteractionindeedoc-
showedastrongred-shiftupto(cid:2)522nmfromthecor- curredbetweentheP3HTandthegrapheneinG-P3HT,
respondingsolutionabsorptionat458nm.The providinganewnonradiativedecaypathwayforphoto-
graphene-inducedenhancementinelectrondelocaliza- generatedexcitonsintheG-P3HT.Furthermore,G-P3HT
tionalongthechemicallygraftedP3HTchainswasalso alsoshowsarelativelyfastdecaytimecomparedwith
observedintheG-P3HTfilm,asevidencedbyabout10 thesimplemixtureofP3HTandgraphene(i.e.,G/P3HT)
nmred-shiftintheabsorptionbandoftheG-P3HTfilm havingaPLlifetimeof476ps(Figure5).Asaresult,
withrespecttothatoftheP3HTfilm.Theobservedred- thechemicallinkagebetweentheP3HTandgraphene
shiftsforthe(cid:4)(cid:1)(cid:4)*absorptionbandofG-P3HTinboth couldfacilitatetheexcitondissociationforthesolarcell
thesolutionandthesolidstateuponbeinggrafted application.
ontographeneindicateanenhancedelectrondelocal- Todemonstratethepotentialapplicationofthe
izationthroughchargetransferwiththechemically G-P3HTinphotovoltaiccells,wecarriedoutcyclicvolta-
conjugatedgraphenesheettoreducethebandgapen- mmetry(CV)measurementsofpureP3HTandG-P3HT
ergy;anadvantageforthephotovoltaicapplication.37,38 fordeterminingtheirbandgapenergies.TheCVofboth
AscanbeseeninFigure1,oneendoftheP3HTchain P3HTandG-P3HTrevealedwell-definedoxidationand
intheHOHC(cid:1)P3HT(cid:1)CHOHwillbegraftedontoone reductionpeaks(FigureS6,SupportingInformation).
2 2
graphenesheet,whiletheotherendofthesamepoly- Fromtheonsetofoxidation(E )andreduction(E )
ox red
merchainmayattachtothesameortoanother potentials,wecancalculatethehighestoccupiedand
graphenesheettoformacontinuousnetworkstruc- lowestunoccupiedmolecularorbitals(HOMOand
ture.Thisshouldalsofacilitatethechargetransportin LUMO,respectively)energylevelsaswellastheen-
photovoltaiccells.Figure4bshowsphotoluminescence ergygaps(E )ofthepolymers,accordingtotheprevi-
g
(PL)spectraofP3HTandG-P3HTinchloroformsolu- ouslyreportedmethod(seeexperimentalmethods).40
tionatanexcitationwavelengthof450nm.Ascanbe Theobtainedelectrochemicaldata,togetherwiththe
seen,thepureP3HTshowsastrongemissionbandover opticalabsorptiondataaresummarizedinTable1.As
550(cid:1)650nm,whichwasremarkablyreducedfor canbeseen,G-P3HTshowedalowerHOMOleveland
G-P3HTduetothePLquenchingeffectcausedby anarrowerbandgapthanthatofP3HT.Therelatively
graphene.22Thisisanadditionaladvantageforusing lowerbandgapobservedforG-P3HTprobablyresulted
theG-P3HTinphotovoltaiccells. fromtheabove-mentionedcharge-transferinteraction
PLlifetimemeasurementscanprovideanotherevi- betweenthegraphenesheetsandend-anchoredP3HT
denceforthecharge-transferinteractionbetweenP3HT chains.41
andthegrapheneinG-P3HT.IftheobservedPL
quenchingforP3HTintheG-P3HTresultsfromthe
charge-transferinteractionwithgraphene,thenthe
chemicalgraftingofP3HTchainsontothegraphene
shouldacceleratethedecayoftheP3HTemission.Toin-
vestigatecharge-transferinteraction,weperformed
time-resolvedemissionstudiesbyusinganexcitation
wavelengthof410nm.ThePLlifetimewascalculated
byfittingdatatoasingleexponentialdecayfunction.39
ThePLlifetimeprofilesforthepristineP3HTand
G-P3HTaregiveninFigure5,which,asexpected,shows
Figure5.Time-resolvedphotoluminescencespectraofthe
aPLlifetimeof507psforthepristineP3HTinCHCl
3 pristineP3HT(1mg/mL),G-P3HT(1mg/mL),andG/P3HT
andamuchshorterPLlifetimeof380psfortheG-P3HT. mixture(1mg/mL)inCHCl.
3
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TABLE1.OpticalandelectrochemicaldataofP3HTandG-P3HT R
UV(cid:1)visabsorption cyclicvoltammetry
T
CHCl3solution solidfilms Eox(V) Ered(V)
(cid:4)max(nm) (cid:4)onset(nm) Eg(eV) (cid:4)max(nm) (cid:4)onset(nm) Eg(eV) HOMO(eV) LUMO(eV) Eg(eV) I
P3HT 446 545 2.28 512 648 1.91 0.89/(cid:1)5.29 (cid:1)1.01/(cid:1)3.39 1.90 C
G-P3HT 458 557 2.23 522 666 1.85 0.99/(cid:1)5.39 (cid:1)0.84/(cid:1)3.56 1.83
L
AsschematicallyshowninFigure6a,polymer rentdensity(J )of3.5mAcm(cid:1)2,fillfactor(FF)of0.41,
sc
E
photovoltaiccellswiththestructureofITO/PEDOT: andoverallpowerconversionefficiency((cid:5))of0.61%
PSS(30nm)/G-P3HTorP3HT(60nm)/C (45nm)/Al(100 wereobtainedfortheG-P3HT/C device.Incontrast,
60 60
nm)werefabricated.Apostfabricationannealingofthe thereferencephotovoltaiccellbasedonP3HT/C
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G-P3HTactivelayerwasconductedat160°Cfor20 showedamuchlowerVoc(0.23V),Jsc(1.9mAcm(cid:1)2),
minunderN,whichshouldnotonlyimprovethemor- and(cid:5)(0.20%)butaslightlyhigherFF(0.45).Therefore,
2
phologyoftheP3HTmatrix42butmoreimportantlyre- thephotovoltaiccellbasedonG-P3HT/C60clearlyout-
movetheresidualfunctionalgroupsinthegraphene performeditscounterpartbasedonP3HT/C60.Whilethe
sheettoincreasetheconjugationlengthforenhanc- increasedJscforthephotovoltaicdevicebasedon
ingthechargetransportmobility.22Figure6bgivesthe G-P3HTcanbeattributedtotheenhancedcharge
energyleveldiagramfortheG-P3HT/C device,while transport/collectionassociatedwiththegraphene
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Figure6cshowsthecurrent(cid:1)voltage(J(cid:1)V)curvesfor sheets,theincreasedVocisnotinconsistentwiththere-
boththeG-P3HT/C andP3HT/C photovoltaiccells. ducedHOMOleveloftheP3HTuponend-anchoring
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ontothegraphenesheet,astheV forabulk-
Asexpected,noshortcircuitcurrent(J )wasobserved oc
sc
heterojunctionsolarcellnormallydependsontheen-
foreithertheP3HTorG-P3HT-baseddevicesindark.
Upon100mW/cm2(AM1.5G)illumination,however, ergydifferencebetweentheLUMOleveloftheelectron
acceptorandtheHOMOleveloftheelectrondonor.22
anopen-circuitvoltage(V )of0.43V,short-circuitcur-
oc
Consequently,significantlyimproveddeviceperfor-
mancewitharemarkableincreaseinthepowerconver-
sionefficiencyupto200%wasrepeatedlyobtained
fortheG-P3HTdeviceswithrespecttotheP3HT
counterpart.
Table2summarizestypicaldeviceperformancefor
about10bilayerphotovoltaiccellsinvestigatedinthis
study,includingthereferencedevicesfabricatedinthe
samemannerwithidenticaldeviceparametersbyusing
themixtureofgraphene(2wt%)andP3HT/C asthe
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activelayer.Inthiscase,itwasdifficulttoyieldahomo-
geneousfilmbydirectlyblendinggraphenewithP3HT
duetotheaggregationofgrapheneinthepolymerma-
trix.Thegrapheneaggregationoftenhasadeleterious
effectonthechargeseparation/transport.Hence,the
overalldeviceefficiencywasnotremarkablyimproved
(0.18%,Table2),althoughtheV exhibitedanincrease
oc
relativetothatoftheP3HT/C device(I(cid:1)Vcurve,Fig-
60
ureS7,SupportingInformation).Therefore,thedevice
basedonG-P3HT/C showedthebestoveralldevice
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performanceamongallofthedifferenttypephotovol-
taicdevicesstudiedinthepresentwork.Althoughthe
obtainedpowerconversionefficiencyof0.61%isstill
moderate,itissignificantforbilayerdevicesandalready
TABLE2.DevicePerformancesofSeveralDonors/C60
HeterojunctionDevices
VOC(V) JSC(mA/cm2) FF (cid:2)(%)
Figure6.(a)Schematicand(b)energyleveldiagramofa P3HT 0.23 1.9 0.45 0.20
ITO/PEDOT:PSS/G-P3HT/C60/Alphotovoltaicdevice.(c) G-P3HT 0.43 3.5 0.41 0.61
Current(cid:1)voltagecharacteristicsofthephotovoltaicdevices
G/P3HTmixture 0.27 1.8 0.38 0.18
usingP3HT/C orG-P3HT/C astheactivelayer.
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L considerablyhigherthanthoseofCNTorinorganic tatingthestructure/propertycharacterizationandde-
nanocrystal(CdTe)-dopedP3HT(orP3OT)/C bilayer vicefabricationbysolutionprocessing.
60
C heterojunctionsolarcells5,17(cid:1)19andevenbetterthan Detailedspectroscopicandelectrochemicalmeasure-
thatofaCNT/P3HT/C bulkheterojunctionsolarcell.20 mentsindicatedthatchemicalgraftingofP3HTonto
60
I
Furthermore,therelativelylowfillfactorfortheG-P3HT grapheneinducedastrongelectronicinteraction,lead-
T
deviceindicatesconsiderableroomforfurtherimprove- ingtoanenhancedelectrondelocalizationanda
mentinthedeviceperformance. slightlyreducedbandgapenergyforthegraphene-
R
boundP3HT,withrespecttopureP3HT.Bilayerphoto-
A CONCLUSION voltaicdevicesbasedontheG-P3HT/C60heterostruc-
Insummary,regioregularpoly(3-hexylthiophene) turesshowedahighershortcircuitcurrentandopen
(P3HT)chainshavebeencovalentlygraftedonto circuitvoltagewitha200%increaseinthepowercon-
graphenesheetsviaesterificationbetweenthecarbox- versionefficiencycomparedtoitspureP3HT/C coun-
60
ylicgroupsinGOandCHOH-terminatedP3HT.There- terpart.Therefore,theorganicsolubleG-P3HTholds
2
sultantP3HT-graftedGOsheets(G-P3HT)possessgood greatpromiseforawiderangeofpotentialapplica-
solubilityincommonorganicsolvents(e.g.,THF),facili- tionsinoptoelectronicdevices.
METHODS ElectrochemicalMeasurements.Toinvestigatetheelectrochemi-
PreparationofGONanosheets.GOsheetswerepreparedfromex- calpropertiesofP3HTbeforeandafterbeinggraftedonto
grapheneandtoestimatetheirHOMOandLUMOenergylev-
foliationofgraphiteaccordingtoamodifiedHummers
method.30Briefly,commerciallyobtainedgraphitepowder(Ald- els,cyclicvoltammetry(CV)wascarriedoutbyusingastandard
three-electrodesystem,whichconsistsofaglassycarbondiskas
rich)wasvigorouslystirredforfivedaysinamixtureofHSO
2 4 theworkingelectrode,aplatinumwireasthecounterelec-
(98%),NaNO,andKMnO.Aftercompletionofthereaction,the
3 4 trode,andasilverwireasthereferenceelectrode.Thepolymer
mixturewaswashedwith5%HSO inwater.Consequently,30%
2 4 electrodewaspreparedbydrop-castingofapolymersolution
HO wasaddedtothereactionvessel,andthemixturewas
2 2 ontotheglassycarbonelectrode.CVwasrecordedinacetoni-
stirredfor2hatroomtemperature.TheGOwasfilteredand
trilecontaining0.1Mtetrabutylammoniumhexafluorophos-
washedthreetimeswith1MHClandthreetimeswithDIwa-
phate(TBAPF,Aldrich)asthesupportingelectrolyte.Before
ter.TheGOcanbeseparatedanddriedintoabrownpowder 6
eachmeasurement,theelectrochemicalcellwaspurgedwith
form.TheAFMimageoftheobtainedGOsheetsisshowninFig-
high-purityargongasfor15min.TheHOMOandLUMOenergy
ureS1,SupportingInformation.
levelsineVaswellastheelectrochemicalenergygap(E ineV)of
PreparationofP3HT-GraftedGraphene.End-functionalizedregio- g
thesampleswerecalculatedaccordingtothefollowing
regularP3HTwithmethylenehydroxygroupswassynthesized
bythepreviouslyreportedmethod.31Thepresenceofthemeth- equations:37
ylenehydroxygroupsatendswasconfirmedbyNMRresults
(seeFiguresS2(cid:1)S4,SupportingInformation).Inatypicalexperi- EHOMO)-e(Eox+4.4) (1)
mentforthesynthesisoftheP3HT-graftedgraphene,driedGO
sample(25mg)wasrefluxedinthionylchloride(25mL)for24h, ELUMO)-e(Ered+4.4) (2)
followedbytheremovalofexcessthionylchlorideunder
vacuum.CH2OH-terminatedP3HT(100mg)in30mLTHFwas Eg)e(Eox-Ered) (3)
thenaddedthroughasyringetothethionylchloridetreatedGO
understirring,followedbytheadditionoftriethylamine(15 whereE andE aretheonsetofoxidationandreductionpo-
ox red
mL)innitrogenatmosphere.Aftersonicationfor2h,thereac- tential,respectively.
tionmixturewasvigorouslystirredfor36h,leadingtoadarksus- FabricationandCharacterizationofPhotovoltaicDevices.Indiumtin
pension.Thesolidinthesuspensionwasremovedbycentrifug- oxide(ITO,25(cid:6)/sq)coatedglassplateswereusedasthesub-
ingat5000rpmfor10min,andthesolventintheclearsolution strateforthedevicefabrication.Thesubstrateswerecleanedby
thuspreparedwaspartiallyremovedbyevaporation.Itwasfur- consecutivesonicationindetergent(MICRO-90),deionizedwa-
therpurifiedbyprecipitatinginmethanol,filtering,andsolvent- ter,isopropylalcohol,andacetoneinanultrasonicbath(VWR
washingthoroughlytoremovetheexcesstriethylamine.Thefi- model75D),followedbya10minUV(cid:1)ozonetreatment.
nalproductwasdriedinvacuumovenfor24hat60°C. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PE-
MicroscopicandSpectroscopicCharacterization.Thesurfacetopol- DOT:PSS)(BaytonP,(cid:2)30nminthickness)wasspincoatedonto
ogyofGOwasexaminedusinganatomicforcemicroscopy thecleansubstrates,whichweresubsequentlydriedat120°C
microscope(AFM,Micro40,andPacificTechnology)inthetap- for10mintoremoveresidualwater.Photovoltaicdeviceswere
pingmodeunderambientatmosphere.Thesurfacechemistry thenfabricatedinagloveboxundernitrogenatmosphereby
wasanalyzedusingaVGMicroTechESCA2000X-rayphotoelec- spincoatingaG-P3HTthinfilm(60nm)fromitschlorobenzene
tronspectrometer(XPS).Fouriertransforminfrared(FTIR)spec- solution(10mg/mL),followedbyannealingat160°Cfor20min.
trawererecordedonaPerkin-ElmerFTIRspectrometer(Spec- Thereafter,athinlayerofC (45nm)wasvacuumevaporated
60
trumONE).NMRmeasurementswereperformedinCDCl ona ontotheG-P3HTsurface,followedbyvacuumevaporationofAl
3
Bruker300MHzNMRspectrometer.Thermogravimetricanaly- (100nm)ontotheC layerthroughashadowmasktodefinean
60
ses(TGA)weremadeonaTAQ50instrumentunderN atmo- activeareaof6mm2foreachdevice.Thecurrent(cid:1)voltage(J(cid:1)V)
2
spherewithaheatingrateof20°C/min.UV(cid:1)visabsorptionspec- curvesofthephotovoltaicdeviceswererecordedonaKeithly
trawererecordedonaPerkin-ElmerLambda900UV(cid:1)vis-NIR 236source-measurementunit.Thephotocurrentwasmeasured
spectrophotometer.Photoluminescence(PL)spectroscopywas undersimulatedAM1.5Girradiation(100mW/cm2),usingaxe-
donewithaLS55spectrometer(Perkin-Elmer)atanexcitation nonlamp-basedsolarsimulator(XPS-400,SolarLightCo.).Allde-
wavelengthof450nm.Time-resolvedphotoluminescencespectro- viceswerefabricatedandtestedinoxygenandmoisture-freeen-
scopywasperformedwithatime-correlatedsinglephoton vironmentundernitrogeninsidetheglovebox.Forcomparison,
countingspectrometer.Outputofamode-lockedTi:sapphirela- bilayersolarcellsbasedonpureP3HT/C andonP3HTmixed
60
serrunningat820nmwasdoubledtogenerateexcitationpulses withgraphene(2wt%)/C werealsofabricatedandinvesti-
60
at410nm. gatedunderthesamecondition.
5638 VOL. 4 ▪ NO. 10 ▪ YU ET AL. www.acsnano.org
A
Acknowledgment.TheauthorsthankthesupportfromAFOSR Composites.Sol.EnergyMater.Sol.Cells2007,91,
(FA9550-09-1-0331)andWCUProjectthroughUNISTfromthe 1478–1482. R
MinistryofEducation,Science,andTechnologyinKorea. 17. Pradhan,B.;Batabyal,S.K.;Pala,A.J.Functionalized
CarbonNanotubesinDonor/Acceptor-typePhotovoltaic T
SupportingInformationAvailable:AFMimageofGO Devices.Appl.Phys.Lett.2006,88,093106.
nanosheets,1HNMRspectraoftheregioregularP3HTwith 18. Arranz-Andres,J.;Blau,M.J.EnhancedDevice
(cid:1)CHOor(cid:1)CH2OHendgroupsandtheP3HTgraftedgraphene, PerformanceUsingDifferentCarbonNanotubeTypesin I
absorptionspectraofthemixtureofgrapheneandP3HTinchlo- PolymerPhotovoltaicDevices.Carbon2008,46, C
roform,andtheJ(cid:1)Vcurvefromabilayersolarcellbasedon 2067–2075.
tSh1e(cid:1)mS7ix.tTuhriesoinffgorrampahteionneiasnadvaPi3laHbTle/Cf6r0eearoefsceheanrgineFviigautrheesInter- 19. ISmompraonvii,nPg.RPh.;oStoomvoalntai,iSc.RPe.;sFploanhsaeuto,fEP.;oUlym(3e-no,M. L
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