Enhanced carrier extraction efficiency in organic solar cells with reduced space-charge limited transport by engineered vertical compositional profile

ContentslistsavailableatSciVerseScienceDirect

Synthetic

Metals

j o u r n al hom ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / s y n m e t

Enhanced

carrier

extraction

efficiency

in

organic

solar

cells

with

reduced

space-charge

limited

transport

by

engineered

vertical

compositional

profile

Ming-Kun

Lee

_,

_Tsung-Han

_Kuo

_,

_Kuo-Wen

_Kong

_,

_Yun-Ru

_Hong

_,

_Jen-Chun

_Wang

_,

Sheng-Fu

Horng

_,

_Hsin-Fei

_Meng

a_{DepartmentofElectricalEngineering,NationalTsingHuaUniversity,Hsinchu300,Taiwan,ROC} b_{InstituteofPhotonicsTechnologies,NationalTsingHuaUniversity,Hsinchu300,Taiwan,ROC}

c_{DepartmentofMaterialsScienceandEngineering,NationalTsingHuaUniversity,Hsinchu300,Taiwan,ROC} d_{InstituteofPhysics,NationalChiaoTungUniversity,Hsinchu300,Taiwan,ROC}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received1May2012

Receivedinrevisedform25August2012 Accepted12September2012

Available online 13 October 2012 Keywords:

Polymersolarcells Space-chargelimitedeffect Fillfactor

Layerthickness Lightbias Quantumefficiency

a

b

s

t

r

a

c

t

Amulti-layeredpolymersolarcell(PSC)withagradedcompositionalprofiletoreducespace-charge limited(SCL)transportwasdesignedandanalyzedindetails.Wefoundthatbyincreasingtheeffective carrierlifetimefortheslowerphoto-carriers,SCLeffectsthatdegradethefillfactorandthepower con-versionefficiencycanbealleviatedinPSCswithathickactivelayer.ThereductionofSCLeffectswas evidencedbyadiminisheddependenceoftheincidentphoton-to-electronconversionefficiency(IPCE) onlightbias,whichalsoindicatestheimportanceoflightbiasincharacterizingtheoperatingIPCEof PSCswiththickactivelayerswhichoperateinSCLregion.Basedonourresults,devicearchitectureto enhanceFFwasproposed.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Polymer solar cells (PSCs) have attracted much attention

aspotential renewable energy sources because of their

advan-tagesof light weight, low fabrication cost, flexibility,and easy

manufacturing[1,2].ConventionalPSCsgenerallycomprisea

bulk-heterojunction(BHJ)withinterpenetratingnetworkofdonor(D)

andacceptor(A)thatgivesmoreinterfaces,atwhichthe

photo-generatedexcitonscanbeefficientlydissociated,yieldingahigh

photo-carriergenerationefficiency.Muchprogresshasbeenmade

and the power conversion efficiency (PCE) of BHJ PSCs based

onpoly(3-hexylthiophene)(P3HT)andfullerenederivative

(6,6)-phenylC61 butyric acid methyl ester (PCBM)was reported to

exceed5%[3–5].

TheconstitutingmaterialsinconventionalBHJdevicesusually

exhibitlowintrinsiccarriermobility.Inaddition,thedisrupting

(discontinuous)natureofdonorandacceptordistributioninBHJ

layerseverelyretardsthecarriertransport. Asaresult,theBHJ

layerswerecharacterizedbylowercharge-carriermobilitiesand

∗ Correspondingauthor.Tel.:+88635715131x42578;fax:+88635752120. E-mailaddresses:[email protected],[email protected] (S.-F.Horng).

lowcharacteristicchargecollectionlengths[6].Accordingtothe

model proposed by Goodmanand Rose[7], when thecharged

carriermobilities areunbalanced and theblendlayerthickness

islargerthanthecharacteristic chargecollectionlength,

space-charge limited (SCL) transport will be the dominant transport

mechanismofthephoto-carriersintheBHJ.Thisleadstoa

one-halfpowerphotocurrentvs.effectivevoltage(Jph–Veff)relationship

aroundthe operating point ofthe BHJsolarcells, reducing the

changeinthephotocurrentwithrespecttoappliedvoltage,and

thefillfactor (FF)aswell.Althoughvariousmethodshavebeen

proposedtoenhancetheP3HTordering,andthustoimprovethe

balanceincarriermobilities[3,4,8,9],theresultedvertical

com-positiongradientofPCBMandP3HTintheBHJisunfavorablefor

thechargeextraction[10].Therefore,thereisatradeoffbetween

theFFandblendlayerthickness,whichisrelatedtoabsorption,

forconventionalBHJsolarcells.Althoughtheuseofhigh

mobil-itymaterials[11]andmodifiedcathodestructurewithinterlayer

[12,13]helpreducetheSCLeffects,theuseofconventionaland

well-knownmaterialsisstilladvantageoustoelucidatetheorigin

ofFFdegradation.Inthispaper,weproposedaschemetoreduce

theSCLeffectinBHJsolarcellsbyintroducingaP3HT-richlayer

betweentheactiveblendlayerandthebottomanode.Wefound

thattheSCLeffectscanindeedbereduced,leadingtoanenhanced

FF.Ourresultsthussuggestthepossibilitytoincreasethe

absorp-tionlayerthicknesswhilemaintainingahighFF.

0379-6779/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.synthmet.2012.09.007

Fig.1. LayerstructurefordeviceP3IandIusedinthisstudy.

AccordingtoGoodmanandRose[7],thephotocurrent(Jph)due

todoubleextractionofelectron–holepairsfromdeviceswith

non-injectingcontactsatloweffectivevoltage(Veff)isgivenby

Jph=gel[1_b+b]





whereg,e,l,s,and



electroniccharge,thelayerthickness,thelifetimeandthe

mobil-ityofslowcarriers,respectively,andb=ss/ffistheratioofthe

productofcarriermobilityandlifetimeofslowcarrierstothatof

fastcarriers.Sincethecontactsforpracticalorganicphotovoltaic

devicesarenotcompletelynon-injecting,wefollowBlometal.to

reducephenomenallythecarrierinjectionbysubtractingthe

mea-suredilluminatedcurrent(Jlight)fromthemeasureddarkcurrent

(Jdark)[14].Thephotocurrent(Jph)andtheeffectivevoltage(Veff)

inEq.(1)arethusgivenbyJph=Jdark−JlightandVeff=V0−V,

respec-tively,whereV0,thecompensationvoltagein[11],istheapplied

voltage(V)atwhichJdarkequalsJlight.

AthigherVeff,orequivalentlyatVlowerthanopen-circuit

volt-age(Voc),theJ–Vcharacteristicexhibitsatransitiontoaone-half

powerlawoftheeffectivevoltageduetoSCLeffects,andthe

tran-sitionvoltageisgivenby

v

4(1−b)ss (2)

AtevenhigherVeff,whichcorrespondstoreverseappliedbias,all

photo-carriersareextractedfromtheactivelayer.Asaturated

pho-tocurrentwhichisindependentofvoltagewillresult.Thesaturated

photocurrentisgivenby

Jph=egl (3)

InorderfortheFFtoincrease,thehalf-powerregionneedstobe

reduced,requiringalargerbinEq.(2).Onewaytoincreasethe

bistoincreasetheeffectivecarrierlifetimefortheslow

photo-carriers. Since the photo-carrier recombination in PSCsfollows

Langevinrecombination,anincreasedeffectivecarrierlifetimecan

beachievedbyreducingtheconcentrationoftheothertypeof

car-riers.Wetherefore addbetweentheactiveblendlayerandthe

bottomanodeaP3HT-richlayer,withinwhichtheelectron

concen-tration,andthustherecombinationaswell,isreduced.Theweight

ratioofP3HTandPCBMfortheP3HT-richlayeris3:1andthatofthe

activelayerforthecontroldeviceis1:1.Thestructuresforthe

con-troldevice(I)andthedevicewithinsertionofP3HT-richlayer(P3I)

areshowninFig.1.ThislayerstructurefordeviceP3Iissimilarto

thatinvestigatedbyLiangetal.[15],albeitwiththeP3HTelectron

blockinglayerreplacedbyaP3HT-richlayerinsertedforreduced

carrierrecombination.However,aswillbeanalyzedindetailsin

whatfollows,ourdevicewilloperatedifferentlyduetothethicker

activelayer.

2. Experiment

The devices (I and P3I) were prepared as follows. Indium

tin oxide (ITO) coated glass was cleaned by acetone and

iso-propanol toremove residual organicmaterials. UVO treatment

formorehydrophilicsurfacefollowed.Holetransportingmaterial,

PEDOT:PSS(AI4083),wasspunonITOsubstrateswithathickness

of45nm.Thedevicewasheatedonahotplateat200◦Cfor5min

inairforadriedfilm.WethenloadedthePEDOT-coatedITO

sub-stratesintoagloveboxinnitrogenambient.FordeviceP3I,amixed

solutionofP3HTandPCBM(P3HT:PCBM=3:1inweight)intoluene

withaP3HTconcentrationof8.5mg/mlwasspunat3000rpmfor

30stoachieveaP3HT-richlayerof70nmthickness.Toluenerather

thanmorecommonlyemployed1,2-dicholobezene(DCB)wasused

inthisworksinceitismorecompatiblewiththefabricationprocess

ofthebufferlayertechnique[16]whichweusedtocircumventthe

interlayermixingproblem.

The devicewasannealed ona hotplateat 140◦Cfor 10min

to remove the residual solvent and to make it more resistant

againsttheinterlayermixing.Following[16],afterspinningthe

1,2-propyleneglycolasbufferliquid,solutionwhichconsistsofP3HT

andPCBMatweightratioof1:1intoluenewithaP3HT

concentra-tionof17mg/ml,wasspunat1000rpmfor30sontheliquidsurface

immediatelytoformamulti-layereddevice(deviceP3I)withatotal

activelayerthicknessof300nm.Thefilmwasannealedagainona

hotplateat140◦Cfor10min.Forthesingle-layeredcontroldevice

(deviceI),whichconsistedofonlyablendinglayerofP3HTand

PCBMwithweightratioof1:1,solutionwhichconsistsofP3HTand

PCBMatweightratioof1:1intoluenewithaP3HTconcentration

of17mg/ml,wasspunat550rpmfor30s,aconditionwhichwas

optimizedtoachieveanequalactivelayerthickness(300nm)for

bothdevices.Aheattreatmentwasthencarriedoutonahotplate

at140◦Cfor20min,whichequalsthetotalannealingtimeforthe

multi-layereddevice.Subsequently,atopmetalelectrode

consist-ingofcalcium(50nm)andsilver(80nm)wasthermallydeposited

underapressureof2.3_×10−6Torr.Theareaofactivedevicewith

thedepositedtopelectrodewas4mm2_.

3. Characterization

TheJ–V characteristicsofthedevices weremeasuredwitha

Keithley2400sourcemeasurementunitusinga100mW/cm2_AM

1.5Gsolarsimulator(San-eiElectric,XES-301S).Theintensityof

theincidentsolarilluminationwascalibratedbyasilicon

photo-diode(HAMAMATSUS1337-BR).Thelightintensitywasreduced

byneutraldensityfilters(Thorlab)toachievedifferent

illumina-tionlevels.Theincidentphoton-to-electronconversionefficiency

(IPCE)wasconductedusingameasurement system(model

QE-R)builtbyEnliTechnologyCo.,Ltd.Wide-spectrumlightsource

waschoppedanddiffractedintoseparatedmonochromatic

nar-rowbands,andeachofwhichwasprojectedontothedeviceunder

testing.Anotherwide-spectrumlightsourcewasusedasoptional

lightbias.Thephotocurrentgeneratedbyincidentmonochromatic

light wasconverted and amplifiedtoanACvoltagebya

trans-impedanceamplifier, whichwasthen measuredby aDAQ card

(NationalInstrument)atthechopperfrequency.

4. Resultsanddiscussion

Theabsorptionspectraforactivelayerofbothdevices(Iand

P3I) are shown in Fig. 2. Because the thickness of the active

Table1

ThedeviceparametersincludingJsc,Voc,FF,PCE,Vmax,JmaxandV0fordevicesP3IandI.

Jsc(mA/cm2) Voc(v) FF(%) PCE(%) Vmax(v) Jmax(mA/cm2) V0(v)

P3I 9.96 0.63 48.8 3.12 0.46 6.78 0.74

I 9.01 0.63 41.6 2.35 0.42 6.59 0.675

Fig.2.TheabsorptionspectrumoftheactivelayerfordeviceP3IandI.

overlapasexpected.Fig.3(a)showsthemeasureddarkcurrent

den-sity(Jdark)andtheilluminatedcurrentdensity(Jlight)asafunctionof

appliedvoltage(V)forbothdevicesundersimulatedAM1.5Glight

illuminationat100mW/cm2_{.Inaddition,thecompensation}

volta-ges(V0)forbothdevicesarelabeled.Thephotovoltaicparameters

forbothdevicesaresummarizedinTable1.FromTable1,the

inser-tionoftheP3HT-richlayerindeviceP3IhasalittleeffectonVoc,as

comparedtodeviceI.ThisisnotsurprisingsinceVocdependseither

ontheelectrodemetalworkfunctionorthedonor/acceptorenergy

levels,whichareallidenticalforbothdevices.However,the

short-circuitcurrentdensityJsc (FF)increasesbyapproximately10.5%

(17.3%)from9.01mA/cm2_{(41.6%)to9.96mA/cm}2_{(48.8%),yielding}

animprovedPCEby32.3%from2.35%to3.12%.Sincethe

absorp-tionremainsunchanged(Fig.2)forbothdevices,theimprovements

shouldarisefromtheenhancedcarrierextractionefficiency.From

Fig.3(a),thedarkcurrentdensityofdeviceP3Iismorethanthatof

deviceIatforwardbias,revealingthatthecarrierinjectionfromthe

electrodeisnotsuppressedfordeviceP3IascomparedtodeviceI.

Inaddition,thedarkcurrentdensityofdeviceP3Ishowsanorderof

magnitudehigherthanthatofdeviceI.Thereasonofthisincreasein

darkcurrentmaybeattributedtothebufferlayerprocessinwhich

high-boilingpoint(188◦C)1,2-propyleneglycolbufferwasusedto

circumventtheinterlayermixingprobleminpreparingdeviceP3I.

Theresidual1,2-propyleneglycolaftertheannealing(at140◦C)

seemstoactaschemicaldopantsandincreasesthereversedark

current.Amorebalancedinjectionwhichenhancesthe

recombina-tionamongtheinjectedcarriersmayalsoplayarole.Thereforethe

improvedcarrierextractionefficiencyfordeviceP3Ishouldnotbe

attributedtosuppressedcarrierinjection,whichcancauseslossin

photocurrentduetotherecombinationbetweenthephoto-carriers

andtheinjectedcarriers.

Forbettercomparison,thedarkcurrentdensity(Jdark)vs.applied

voltage(V)characteristicsforbothdevicesareplottedinsemi-log

scaleinFig.3(b).Itisclearthat,whilethereisonlyslight

differ-encebetweenthecurrentforbothdevicesatforwardbias,thedark

currentofdeviceP3IishigherthanthatofdeviceIbyanorderof

magnitudeatreversebias.Thisisdifferentfromwhatwasobserved

in[15],inwhichthedarkcurrentissuppressedatreversebiasbythe

insertedP3HTelectronblockinglayer.Theabsenceofsuppressed

carrierinjectionfromtheITOanodecanbeunderstoodbynoting

thattheinsertedP3HT-richlayerin ourdevice(P3I)consistsof

PCBM,whichallowsforelectrontransport.OurinsertedP3HT-rich

layer,however,revertstheconventionallyobservedunfavorable

compositionprofile,whichexhibitsahigherconcentrationofPCBM

attheITOside,toafavorableonethatfacilitatescarriercollection.In

addition,aswillbeshowninwhatfollows,theinsertionofa

P3HT-richlayerreducescarrierrecombination.Therefore,increasedJsc

wasobtainedeventhoughtheabsorptionremainedunchanged.

Thephotocurrentdensities (Jph)vs. effectivevoltage(Veff)in

log–logscaleareplottedinFig.4andthemaximumpowerpoints,

VmaxandImaxinTable1,arealsomarked.WealsoincludeinFig.4

auxiliarylineswithslope1(0.5),whichrepresentbalanced(SCL)

transportforphoto-carriers.AsshowninFig.4theSCLdominated

region,whichfollowstheauxiliarylineofslope0.5,isnarrower

fordeviceP3IthanthatfordeviceI.Sincethemaximumpower

pointsarelocatedintheSCLregion,VmaxandImaxfordeviceP3Iare

largerthanthosefordeviceI(Fig.4).TheimprovementinPCEand

FFfordeviceP3IisthusattributedtoreduceSCLeffectswiththe

insertionoftheP3HT-richlayer.Itisalsonotablethatcurrent

satu-rationathigheffectivevoltage,whichrepresentsthephoto-carrier

generationrateinEq.(3)[7],isobservedforbothdevicesandthe

saturationphotocurrentdensitiesare12.5mA/cm2_and12mA/cm2

Fig.3.(a)Theilluminatedcurrentdensity(Jlight)underAM1.5Gat100mW/cm2anddarkcurrentdensity(Jdark)vs.voltage(V)fordeviceP3IandI.Thecompensationvoltage (V0)islabeledfordeviceP3IandI;(b)Jdark–Vcharacteristicsplottedinsemi-logscale.

Fig.4. Thephotocurrentvs.effectivevoltagecharacteristicsinlog–logscale.

fordeviceP3IandI,respectively.Thephoto-carriergenerationrate

exhibitsonly4.17%increasefordeviceP3Iascomparedtodevice

I,andcannotaccountforthe32.8%enhancementinPCE. Thisis

againdifferentfromwhatwasobservedinRef.[15],inwhichthe

improvedPCEresultsmostlyfromincreasedphoto-carrier

genera-tionrate.

TounderstandthedifferencebetweenourresultsandRef.[15],

weestimatethechargecollectionlengthwhichcharacterizesSCL

transport.Typicalreportedvaluesforcarrierlifetime(10␮s[17])

andholemobility(1.5×10−8m2_{/Vs[18])areusedandacharge}

col-lectionlengtharound300nmat0.6VwasobtainedforP3HT:PCBM

BHJPSCs.While thisestimatedvalueisclosetotheactivelayer

thicknessinourdevices,itissignificantlylargerthanthat(175nm)

inRef.[15].ThedeviceinRef.[15]operatesthereforeinthelinear

regionandtheimprovementinPCEresultsmostlyfromincreased

photo-carrier generation rate. On the other hand, our devices

employmuchthickeractivelayersandtheenhancementinPCE

withinsertedP3HT-richlayerarisesfromreducedSCLeffects.

Itisremarkablethatthetransitionfromthebalancedtransport

regiontotheSCLregionisdeterminedbythetransitionvoltageVtr

giveninEq.(2).AnincreasedvalueofbinEq.(2)leadstoalargerVtr,

extendsthelinearregionandthusincreasesthePCEandFF.From

Fig.4,Vtrincreasesby34.8%from0.155to0.209V.Thisincrease

inVtristentativelyattributedtotheincreaseintheeffective

car-rierlifetimeduetoreducedrecombination.Assumedthatf



productofcarriermobilityandlifetimeofthefastcarrierswhich

areelectronsinourcase,remainsunchangedandb1,VtrinEq.(2)

dependslinearlyonb.Therefore,aroughestimateof34.8%increase

inbwiththeinsertionoftheP3HT-richlayercanbeobtained.

TofurtherinvestigatetheeffectsoftheinsertedP3HT-richlayer

onSCLtransport,theshort-circuitcurrentdensitiesunderdifferent

illuminationlevelsweremeasuredandtheresultswereplottedin

log–logscaleinFig.5.AsshowninFig.5,deviceIexhibitsapower

dependenceof0.72_±0.04,whichisclosetothepredictedvalueof

0.75forSCLphotocurrent[7].Ontheotherhand,anexponentof

0.94±0.06wasobtainedfordeviceP3I,indicatingalleviatedSCL

effects.ThisresultisconsistentwiththeobservationinFig.5,

cor-roboratingthattheinsertionofP3HT-richlayerreducesSCLeffects.

TheIPCEofdeviceIandP3Iwithandwithoutaone-sunDClight

biaswereplottedinFig.6(a)and(b).Sincebothdevicesexhibit

nearlyidenticalabsorption(Fig.2),enhancedquantumefficiency

observedindeviceP3I,ascomparedtodeviceI,indicatesimproved

chargecollectionefficiency.Itisalsointerestingtonotethatthe

Fig.5.Theshort-circuitcurrentdensityvs.incidentlightintensityinlog-logscalefor(a)deviceP3Iand(b)deviceI.

Fig.7.(a)ExtractedintrinsicJ–VcurvesfordeviceI(opencircle)andP3I(solidsquare);extractedJ–Vcurvesanditsdecompositionintoalinearandashiftedquadratic componentfor(b)deviceP3Iand(c)deviceI.In(b)and(c)averticaldashedlinewasshowntodenotetheshifted(transition)voltage,largerthanwhichthequadraticSCL currenttakeseffect.

Table2

TheextractedparametersincludingRs,RshandJphfordevicesP3IandI. Rs(cm2) Rsh(cm2) Jph(mA/cm2)

P3I 14.6 407 10.4

I 9.26 312 9.44

IPCEfordeviceIshowsmuchstrongerdependenceontheDClight

biasthan deviceP3I.Thiscanbeunderstoodbynotingthatthe

DClightbiascontributesmorebackgroundcarrierswhichleadto

segregatedchargedistributionnearbothsidesinthedevices.This

chargesegregationcanscreentheelectricfieldinthebulkregion.

FordeviceswithdominantSCLtransport,suchasdeviceI,reduced

electricfieldinthebulkregionleadstodecreasedcollection

effi-ciency,andthusdecreasedIPCE.Ontheotherhand,sincetheSCL

effectsindeviceP3Iarealreadymuchreduced,littlechangeinIPCE

wasobservedwithDClightbias.

Althoughthecontactstotheactivelayerarecrucialtothe

car-rierextractionaswellasinjection[13]andaffectinturnthecarrier

concentrationwithintheactivelayer,SCLeffectsareintrinsically

bulkeffectswithwhichthespacechargemodulatesthebulk

elec-tricfield.Itwouldthereforeberevealingtoremovetheeffectsdue

tothecontacts,whichmanifestmostlyasparasiticseriesresistance,

andtostudytheintrinsiccurrent–voltage(I–V)characteristicofthe

devices.Wefollowedaprocedurewhichweproposedpreviously

forextractingtheintrinsicI–Vcharacteristicsandtheseries

resis-tancewithoutassumingtheI–Vfunctionalform[19].Inshort,the

extractedI–VrepresentsthebestfitbulkI–Vrelationship,assuming

constantOhmic(linear)I–Vcharacteristicforthecontacts.The

pro-cedurerequiresI–Vmeasurementswithtwoslightlydifferentlight

illuminationlevels.Besidestheoneswithacalibratedsolar

simula-toratAM1.5G(100mW/cm2_{),theotherI–Vcurveswereobtained}

withamicroscopecoverglassplacedontopoftheOSCs,leading

toalightattenuationofabout8%.Theroot-mean-squareerroris

calculatedfortherangefromVD=0toopen-circuitvoltage(Voc)

inthemeasuredI–Vcharacteristicwithlesslightillumination.The

detailedcalculationprocedurecanbefoundin[19].

Theextractedparametersincludingseriesresistance(Rs),shunt

resistance(Rsh)andphoto-inducedcurrentdensity(Jph)were

sum-marizedinTable2.Apparently,theRshofdeviceP3Iisincreasedas

comparedtothatofdeviceI,indicatingreducedrecombinationof

photo-carrierswiththeinsertionoftheP3HT-richlayer.

Fig. 7(a) shows the extracted intrinsic J–V curves for both

devices.Asitwasshownin[19]thattheintrinsiccurrentdensity

ofOSCsatlowforwardbiascomprisesofalinearandaquadratic

component,whichweretentativelyattributedtohoppingandSCL,

respectively,wesubtractthelinearcomponentfromtheextracted

J–VandfitthesubtractedJ–Vwithashiftedquadratictermaround

thevoltagethatcorrespondstoVmax;theresultswereshownin

Fig.7(b)and(c)fordeviceP3IandI,respectively.FromFig.7(b)and

(c),itwasclearthatSCLeffectsappearatlowerbiasfordeviceIthan

deviceP3I.Itisalsounderstandablethatbothvoltagesarelarger

thanthoseobtainedpreviouslyfromthemeasuredI–Vcurves,since

anegativevoltagedropdevelopsacrossRswiththeextracted

pho-tocurrentforthemeasuredI–V.

5. Conclusion

Insummary,ourresultsdemonstratedthatbytailoringthe

com-positionalprofileofblendlayerwithaninsertedP3HT-richlayer,

theeffectivecarrierlifetimecanbeincreasedandthedetrimental

SCLeffectscanbealleviated.Withourresults,itisexpectedthat

theadditionofinterlayers,preferablywithhighercarriermobility

forbettercarrierextractionandwithlessheterojunctionand/or

largerbarriertoreducethecarrierconcentrationoftheothertype

toreducerecombination,betweentheactivelayerandthe

elec-trodeswillenhancetheFF.Itisthereforepossibletoincreasethe

FFandthePCEfordeviceswithathickactivelayerwithasuitably

designedverticalcompositionprofileorlayerstructure.Ourresults

alsoshowthatDClightbiasisimportanttocharacterizethe

operat-ingcharacteristicsofPSCswhenSCLeffectsdominatephoto-carrier

carriertransportneartheoperatingpoints.

Acknowledgment

ThisworkwassupportedbytheNationalScienceCouncil(NSC)

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