MaterialsChemistryandPhysics132 (2012) 131–137
ContentslistsavailableatSciVerseScienceDirect
Materials
Chemistry
and
Physics
jo u r n al h om ep a ge : w w w . e l s e v i e r . c o m / l o c a t e / m a t c h e m p h y s
Effects
of
annealing
on
the
polymer
solar
cells
based
on
CdSe–PVK
electron
acceptor
Tzong-Liu
Wang
a,∗,
Chien-Hsin
Yang
a,
Yeong-Tarng
Shieh
a,
An-Chi
Yeh
b,
Chin-Hsiang
Chen
c,
Tsung-Han
Ho
daDepartmentofChemicalandMaterialsEngineering,NationalUniversityofKaohsiung,Kaohsiung811,Taiwan,ROC bDepartmentofChemicalandMaterialsEngineering,ChengShiuUniversity,Kaohsiung833,Taiwan,ROC cDepartmentofElectronics,ChengShiuUniversity,Kaohsiung833,Taiwan,ROC
dDepartmentofChemicalandMaterialsEngineering,NationalKaohsiungUniversityofAppliedSciences,Kaohsiung807,Taiwan,ROC
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received4May2011
Receivedinrevisedform22October2011 Accepted11November2011
Keywords: Compositematerials
Photoluminescencespectroscopy Atomicforcemicroscopy Annealing
a
b
s
t
r
a
c
t
CdSe–poly(N-vinylcarbazole)(CdSe–PVK)nanocompositewassynthesizedandutilizedastheelectron acceptorintheactivelayerofpolymersolarcells.Thephotovoltaicpropertiesofthepolymersolarcells basedonpoly(3-hexylthiophene)(P3HT):CdSe–PVKastheactivelayerwereinvestigatedindetail.The effectsofannealingtemperature(100–200◦C)andtime(5–60min)onthedeviceperformancewere studied.Atannealingtemperatureof150◦Cfor30min,thedevicedemonstratedanoptimalefficiency
of0.235%underAM1.5(100mWcm−2)solarsimulatedlightirradiation.Theimprovedefficiencyunder theoptimalconditionswasconfirmedbythehighestlightharvestinUV–visspectraduetotheincreased crystallinityofP3HTafterthermalannealing.Photoluminescenceofthesedevicesalsoexhibitedthatthe quencheffectincreaseswiththeincreasingofannealingtemperature,indicatingthatthecharge sepa-rationbetweenelectron-donating(P3HT)andelectron-accepting(CdSe–PVK)moleculeswasincreased afterheattreatment.Atomicforcemicroscopy(AFM)imagesshowedthatthephasesegregationand 3DinterpenetratingnetworksofP3HT:CdSe–PVKwereresponsiblefortheenhancementofthedevice efficiency.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Over the past decades, polymer solar cells (PSCs) based on conjugated polymers have attracted considerable attention because of theirpotential use for future cheapand renewable energyproduction[1–3].Efficientpolymer-basedsolarcells uti-lizedonor–electronacceptor(D–A)bulkheterojunction(BHJ)films asactivelayers[1,2].Thedonoristypicallyakindofconjugated polymer,whiletheacceptorisgenerallyatypeoforganicor inor-ganicmolecule.TheD–ABHJstructureenablesavailabilityofthe acceptormoleculesincloseproximitytotheelectrondonor poly-mers,andtherebyfacilitateschargetransferfromexcitedpolymer chainstotheelectron acceptormolecules.Recently, many bulk heterojunction solar cells based onblends of conjugated poly-mersandinorganicnanocrystalsthatofferhighelectronmobility orimprovedspectralcoveragehavebeeninvestigated[4–9]. Fur-thermore, it has been stated that semiconductor nanocrystals (colloidally synthesized quantum dots, QDs) have the poten-tialtoincrease theefficiencyof conversionof solarphotonsto
∗ Correspondingauthor.Tel.:+88675919278;fax:+88675919277. E-mailaddress:tlwang@nuk.edu.tw(T.-L.Wang).
electricityuptoabout66%,andcanovercometheefficiencylimit causedbycarrierthermalizationintheconventionalsolarcells[10]. Inparticular,sphericalsemiconductornanoparticles,suchasCdSe quantumdots,havebeenthesubjectofextensivestudiesoverthe pastdecadebecauseoftheiruniqueopticalandelectronic prop-erties [11–13].Since CdSe hashighelectron mobility, we have employedthisspecificpropertyforimprovingdeviceefficiencyof hybridCdSe/conjugatedpolymerphotovoltaic(PV)systeminthis work.
AlthoughCdSe/conjugatedpolymersolarcellshavebeen stud-ied in several groups [5,14–18], the efficiency of photovoltaic devices is majorly limited by the low compatibility between inorganicCdSenanoparticlesandtheconjugatedpolymers. Con-sequently, a good dispersion of the CdSe nanocrystals in the polymerdonorphaseisrequiredfortheCdSe/conjugatedpolymer nanocompositeinsolarcellstocreatealargerinterfacialsurface area,enhancingthechargetransferbetweenpolymermatrixand nanocrystals.
In our previous report, we have described the synthesis of CdSe–PVK nanocomposite via ATRP approach [19]. In compari-sonwithCdSeorPVK,thisnewnanocompositepossessesunique opticalproperties.Furtherefforthasbeenmadetousethis com-positeastheelectronicacceptorintheactivelayerofPSCs[20].
0254-0584/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.11.008
132 T.-L.Wangetal./MaterialsChemistryandPhysics132 (2012) 131–137
More recently,organicsolar cells madefrom blendsof poly(3-hexylthiophene)(P3HT)and[6,6]-phenyl-C61-butyricacidmethyl
ester(PCBM)exhibitedthatboththeexternalquantumefficiency (EQE) and power conversion efficiency (PCE) were remarkably improved by thermal annealing and/or electrical aging on the devices[21,22].Thermalannealingconditionwasthekeyvariable forimprovingshortcircuitcurrent,fillfactor, andthereforethe efficiencyofthedevice.Herein,acomprehensivestudyof ther-malannealingconditions ispresentedonthepowerconversion efficienciesofPSCsbasedontheactivelayerofP3HT:CdSe–PVK. Inaddition,therelationshipbetweendeviceperformanceandthe morphologyofactivelayersisalsoinvestigatedbyUV–vis, photo-luminescence,andAFM.
2. Experimental
Thepolymerphotovoltaiccellinthisstudyconsistsofalayer ofP3HT/CdSe–PVKblendthinfilmsandwichedbetween transpar-entanodeindiumtinoxide(ITO)andmetalcathode.Thedevice structureisITO/PEDOT:PSS/P3HT:CdSe–PVK/Al.P3HT(FEMTech., Mn=16,900)actsas thep-typedonorpolymer andCdSe–PVK (Mn=950 for PVK) as the n-type acceptor in theactive layer. Beforedevicefabrication,theglasssubstratescoatedwithindium tin oxide (ITO) were first cleaned by ultrasonic treatment in acetone,detergent,de-ionizedwater,methanolandisopropyl alco-hol sequentially. The ITO surface was further coated with ca. 80nm layer of poly(3,4-ethylene dioxythiophene):poly(styrene) (PEDOT:PSS)byspincoating.Thesubstratewasdriedfor10min at140◦C inair, andthen movedinto thenitrogen-filled glove-boxforspincoatingtheactivelayer.TheP3HT:CdSe–PVKblend waspreparedwith1:1 weightratio (10mgmL−1 P3HT) in 1,2-dichlorobenzene(DCB)astheactivelayer.Thissolutionblendwas spin-coated onto thePEDOT:PSS layerat 800rpm for 30s. The obtainedthicknessfortheblendfilmofP3HT:CdSe–PVKwasca. 100nm.ThedevicewascompletedbydepositingathinAllayeras anelectrodewithanareaof6mm2asdefinedbyamask.
Thefilmsofactivelayerswereannealeddirectlyontopofahot plateintheglovebox,andthetemperaturewasmonitoredbyusing athermocoupletouchingthetopofthesubstrates.Afterremoval fromthehotplate,thesubstrates wereimmediatelyputonto a metalplateattheroomtemperature.Ultraviolet-visible(UV–vis) spectroscopicanalysiswasconductedonaPerkin-ElmerLambda 35UV-Visspectrophotometer.Roomtemperature photolumines-cence(PL)spectrumwasrecordedonaHitachiF-7000fluorescence spectrophotometer.Thefilmtopographyimagesofactivelayers wererecordedwithaDigitalInstrumentsDimension3100atomic forcemicroscope(AFM)intapping modeunderambient condi-tions.TheJ–VcurvesweremeasuredusingaKeithley2400source meter,underilluminationfromasolarsimulator.Theintensityof solarsimulatorwassetwithaprimaryreferencecellanda spec-tralcorrectionfactortogivetheperformanceundertheAM1.5 (100mWcm−2)globalreferencespectrum(IEC60904-9).
3. Resultsanddiscussion
3.1. Effectsofannealingtemperature
Fig.1showsthechemicalstructuresofthematerialsusedinthis study.TheeffectofannealingtemperatureontheUV–vis absorp-tionspectraforthethinfilmsofP3HT:CdSe–PVK(1:1weightratio) spuncastonquartzsubstratesisshowninFig.2.Thesefilmswere annealedundernitrogenatmosphereinsidethegloveboxat atmo-sphericpressure.Theannealingtimewaskept30minforallofthe annealingtemperatures.Theabsorptionspectrashowa consider-ablechangeafterthermalannealingofthefilms.Fortheuntreated
Fig.1. (a)ChemicalstructuresofCdSe–PVKusedintheactivelayer.(b)Schematic representationofatypicaldevicestructureofpolymerbulk-heterojunction photo-voltaicdevicefabricatedinthisstudy.
film,thepeakabsorptionwavelength(max)is516nmwithtwo
shouldersrangingfrom540to553nmand590to605nm, respec-tively.The firsttwo bandsis attributedtothe–*transition, whereasthelast shoulderis duetotheinter-chaininteractions
[23,24].Theabsorptionpeaksatabout344and331nmareascribed tothe–*transitionofcarbazolegroupsthatarependenttothe PVKbackboneofCdSe–PVK[19].
At annealing temperature of 100◦C, the intensities of three bandsincreasewithoutchangeinthepositionofthethreevibronic peaks.Anincreaseintheabsorptionstrengthafterheattreatment normallymeansincreasedpackingoftheP3HTdomains.Thefilm heat-treatedat125◦C showsa similarbehavior. Themaximum absorptionisobservedforthefilmannealedat150◦C,indicating anenhancedconjugationlengthandthemoreorderedstructureof
300 400 500 600 700 800 Absorbance Wavelength (nm) RT 100 C 125 C 150 C 175 C 200 C
Fig.2. UV–visabsorptionspectraofP3HT:CdSe–PVKblendfilmsafterannealingat differenttemperaturesfor30min.
T.-L.Wangetal./MaterialsChemistryandPhysics132 (2012) 131–137 137
performancewasdramaticallyimprovedandthepower conver-sion efficiency of device reached to 0.235% under white light illumination(100mWcm−2).Thethermalannealingcontributed to the enhanced PV cell performance by optimizing both the donor/acceptormorphologyintheBHJactivelayer.Inconclusion, annealingtreatmentonthePVdevicesenhanced3D interpenetrat-ingnetworksintheactivelayer,lightabsorption,andthecarrier mobility,leadingtotheimprovementofthedeviceperformance.
Acknowledgements
WegratefullyacknowledgethesupportoftheNationalScience CouncilofRepublicofChinawithGrantNSC 97-2221-E-390-005-MY2.
References
[1]O.Inganäs,M.Svensson,F.Zhang,A.Gadisa,N.K.Persson,X.Wang,M.R. Ander-sson,Appl.Phys.A79(2004)31.
[2]F.Zhang,W.Mammo,L.M.Andersson,S.Admassie,M.R.Andersson,O.Inganäs, Adv.Mater.18(2006)2169.
[3]A.Gadisa,W.Mammo,L.M.Andersson,S.Admassie,F.Zhang,M.R.Andersson, O.Inganäs,Adv.Funct.Mater.17(2007)3836.
[4]D.J.Milliron,I.Gur,A.P.Alivisatos,MRSBull.30(2005)41. [5] N.C.Greenham,X.Peng,A.P.Alivisatos,Phys.Rev.B54(1996)17628. [6]W.J.E.Beek,M.M.Wienk,R.A.J.Janssen,Adv.Mater.16(2004)1009.
[7]D.Qi,M.Fischbein,M.Drndic,S.Selmic,Appl.Phys.Lett.86(2005)093103. [8]J.Boucle,S.Chyla,M.S.P.Shaffer,J.R.Durrant,D.D.C.Bradley,J.Nelson,Adv.
Funct.Mater.18(2008)622.
[9]S.D.Oosterhout,M.M.Wienk,S.S.vanBravel,R.Thiedmann,L.J.A.Koster,J.Gilot, J.Loos,V.Schmidt,R.A.J.Janssen,Nat.Mater.8(2009)818.
[10]A.J.Nozik,PhysicaE14(2002)115. [11]A.P.Alivisatos,Science271(1996)933.
[12] B.L.Cushing,V.L.Kolesnichenko,C.J.O’Connor,Chem.Rev.104(2004)3893. [13]Y.Yin,A.P.Alivisatos,Nature437(2005)664.
[14] W.U.Huynh,X.Peng,A.P.Alivisatos,Adv.Mater.11(1999)923.
[15]J.Liu,T.Tanaka,K.Sivula,A.P.Alivisatos,J.M.J.Fréchet,J.Am.Chem.Soc.126 (2004)6550.
[16]W.U.Huynh,J.J.Dittmer,A.P.Alivisatos,Science295(2002)2425. [17] B.Sun,E.Marx,N.C.Greenham,Nano.Lett.3(2003)961.
[18]M.D.Heinemann,K.vonMaydell,F.Zutz,J.Kolny-Olesiak,H.Borchert,I.Riedel, J.Parisi,Adv.Funct.Mater.19(2009)3788.
[19]T.L.Wang,C.H.Yang,Y.T.Shieh,A.C.Yeh,Macromol.RapidCommun.30(2009) 1679.
[20]T.L.Wang,C.H.Yang,Y.T.Shieh,A.C.Yeh,Euro.Polym.J.46(2010)634. [21] F.Padinger,R.S.Ritterberger,N.S.Sariciftci,Adv.Funct.Mater.13(2003)85. [22] V.Dyakonov,Appl.Phys.A:Mater.Sci.Process.79(2004)21.
[23]X.Jiang,R.Osterbacka,O.Korovyanko,C.P.An,B.Horowitz,R.A.J.Janssen,Z.V. Vardeny,Adv.Funct.Mater.12(2002)587.
[24]P.J.Brown,D.S.Thomas,A.Kohler,J.S.Wilson,J.S.Kim,C.M.Ramsdale,H. Sir-ringhaus,R.H.Friend,Phys.Rev.B67(2003)064203.
[25]G.Li,V.Shrotriya,Y.Yao,Y.Yang,J.Appl.Phys.98(2005)043704.
[26]G.Li,Y.Yao,H.Yang,V.Shrotriya,G.Yang,Y.Yang,Adv.Funct.Mater.17(2007) 1636.
[27]P.Peumans,S.R.Forrest,Appl.Phys.Lett.79(2001)126.
[28]X.N.Yang,J.Loos,S.C.Veenstra,W.J.H.Verhees,M.M.Wienk,J.M.Kroon,M.A.J. Michels,R.A.J.Janssen,NanoLett.5(2005)579.
