Energetics of Excited States in the Conjugated Polymer Poly(3-hexylthiophene)

0.27 MB·English

by  Carsten Deibel

EnergeticsofExcitedStatesintheConjugatedPolymerPoly(3-hexylthiophene)
Carsten Deibel,a Daniel Mack, and Julien Gorenflot
ExperimentalPhysicsVI,Julius-Maximilians-UniversityofWu¨rzburg, 97074Wu¨rzburg, Germany
Achim Scho¨ll, Stefan Krause, and Friedrich Reinert
Gemeinschaftslabor fu¨r Nanoanalytik, Forschungszentrum Karlsruhe, 76021 Karlsruhe,
and Julius-Maximilians-University of Wu¨rzburg, 97074 Wu¨rzburg, Germany
Daniel Rauh and Vladimir Dyakonov
ExperimentalPhysicsVI,Julius-Maximilians-UniversityofWu¨rzburg,97074Wu¨rzburg,Germanyand
Bavarian Centre for Applied Energy Research (ZAE Bayern), 97074 Wu¨rzburg, Germany
(Dated:January13,2010)
There is an enormous potential in applying conjugated polymers in novel organic opto-electronic devices
such as light emitting diodes and solar cells. Although prototypes and first products exist, a comprehensive
understandingofthefundamentalprocessesandenergeticsinvolvedduringphotoexcitationisstilllackingand
0 limitsfurtherdeviceoptimisations.Herewereportonauniqueanalysisoftheexcitedstatesinvolvedincharge
1 generationbyphotoexcitation. Onthemodelsystempoly(3-hexylthiophene)(P3HT),wedemonstratethegen-
0 eralapplicabilityofournovelapproach. Fromphotoemissionspectroscopyofoccupiedandunoccupiedstates
2
wedeterminethetransportgapto2.6eV,whichweshowtobeinagreementwiththeonsetofphotoconductivity

n byspectrallyresolvedphotocurrentmeasurements. Forphotogeneratedsingletexcitonattheabsorptionedge,
a 0.7eVofexcessenergyarerequiredtoovercomethebindingenergy;theintermediatechargetransferstateis
J situatedonly0.3eVabovethesingletexciton. Ourresultsgivedirectevidenceofenergylevelsinvolvedinthe

3 photogenerationandchargetransportwithinconjugatedpolymers.
1
PACSnumbers:71.23.Cq;79.60.Ht;73.50.Pz;78.55.Qr;73.61.Ph

] Keywords:organicsemiconductors;polymers;energylevels;photoelectronspectroscopy;photoluminescence;photoconduc-
i tivity
c
s
-
rl I. INTRODUCTION energyinconjugatedpolymers.4 Theexcitonbindingenergy
t hasbeenmeasuredforarangeofconjugatedpolymers,butthe
m
In bulk heterojunction solar cells,1,2 which have already single particle gap has not been determined.11–13 The reason
.
t show about 6% power conversion efficiency,3 the light is for the non-comprehensive determination of energy levels in
a
m mainlyabsorbedintheconjugatedpolymer,generatingsinglet previousworksisthedifficultytofindasuitable,complemen-
excitons, which are strongly bound due to the weak screen- tarychoiceofexperimentaltechniquesrelevanttophotoexci-
d- inginorganicsemiconductors.4 Theexcitondissociationcan tationandthegenerationoffreecharges.
n beinducedbyacceptormolecules,oftenfullerenederivatives, Inthisarticlewereportonanovelapproachwhichiscapa-
o anddependsonthedifferencebetweentheenergyofthesin- bleofprovidingthisdatabyapplyingseveralcomplementary
c gletexcitonandtheenergyofthedonor–acceptorpolaronpair. experimental techniques. The investigation was performed
[
If the latter state is energetically favourable, the excitons are systematically by photoelectron spectroscopy (PES), inverse
1 dissociated by an ultrafast and very efficient electron trans- photoelectron spectroscopy (IPES), optical absorption, field-
v ferfromthedonorpolymer.5Unfortunately,theenergydiffer- induced photoluminescence quenching (PL(F)), and wave-
0 encebetweenexcitonandpolaronpairstateislost,leadingto length sensitive external quantum efficiency measurements
3 aloweringoftheopencircuitvoltage,animportantparameter (EQE)onthesameorcomparablesamplesofthemodelsys-
3
determining the photovoltaic power conversion efficiency.6,7 tempoly(3-hexylthiophene)(P3HT).Bythisuniquecombina-
2
. This example illustrates clearly why the energy levels of the tion,wecanprovideconclusiveenergylevelsforthepositive
1 utilizedmaterialsareofcrucialimportanceforthefundamen- and negative polarons, the transport gap, the absorption gap,
0
tal understanding and, consequently, for the systematic opti- theexcitonbindingenergy,aswellasavalueforthepolaron
0
1 mizationoforganicsolarcells. pairbindingenergy.
: Themagnitudeofthebindingenergiesofexcitedstatesin Fig. 1 schematically shows the processes occuring during
v organic semiconductors, in particular for polaron pairs, is a photoexcitation and charge transport for a typical polymeric
i
X controversial topic.4,8 Lately, the notion that the binding en- semiconductor, as well as the respective energy levels. If
r ergyoftheprimaryexcitationcanbeovercomebythethermal charge is transported, the molecule will adapt to the (single)
a energy9isbecominglessaccepted,10beingreplacedbyacon- positiveornegativechargeintheHOMOorLUMOlevel,re-
sensusthatbindingenergiesaremuchlargerthanthethermal spectively, resulting in positive or negative polaronic levels
(HOMO+ andLUMO−). Theyareseparatedbythetransport
gap, also called single particle gap E . In contrast, the ab-
g,s
sorption of a photon results in the generation of a correlated
[email protected] electron–holepair,asingletexciton. Theseparationofthere-
2
no polaron polaron pair,
charge levels singlet exciton P3HT
(+ or -) (+ and -)
LUMO UPS IPES
LUMO-
PP1
S1 u.]
b.
r
a
Eg Eg,s Eg,PP Eg,abs s [
EF nt
S0 u
(i) Co
(ii) PP0
HOMO+ E =2.6 eV
g,s
HOMO
band transport optical
gap gap gap EHOMO+= ELUMO-=
-0.75 eV 1.85 eV
(i) ΔES→PP = Eg,PP - Eg,abs } ΔES→P = Eg,s - Eg,abs
(ii) ΔEPP→P = Eg,s - Eg,PP -4 -2 0 2 4
Binding Energy [eV]
FIG. 1. (Color Online) Energy levels in a polymer in the ground
state (a), during charge transport (b), and after photoexcitation (c).
Thedetailsaredescribedinthetext. FIG. 2. (Color Online) UPS (black) and IPES (grey) spectra of a
70nmthickP3HTfilmonITO/glass. Measureddataareplottedas
points, the straight lines are 5 point averages. The transport levels
spective excitonic levels S0 and S1 is the optical gap E . were derived from the band onsets as indicated by the lines. The
g,abs
During exciton dissociation, a strongly bound polaron pair transportgapis2.6eV.ThemolecularstructureofP3HTisshownin
(PP) with the levels PP0 and PP1 and the energy separation theinset.
E can be formed. While the ground state band gap E
g,PP g
andtheunchargedHOMO–LUMOlevelscangenerallynotbe
glass/ITO/(optionalPMMA)/P3HT/(optionalPMMA)/Al.
determined directly—particularly the common electrochemi-
All UV photoelectron and inverse photoelectron spec-
cal methods being only sensitive to singly charged levels—a
troscopy measurements (UPS and IPES, respectively) were
combinationoftechniquescanshedlightontheenergeticsin-
performedinaUHVsystem,describedindetailelsewhere.14
volvedinthegenerationandseparationofcharge. Theoptical Thebasepressurewasbelow2·10−10 mbarandthesamples
gap can be measured by optical absorption. The position of
were at room temperature. The He-I line of a gas discharge
thepolaronictransportlevels(relativetotheFermilevelE )
F lampwasusedforUPS.TheIPESdetector(aGeiger–Mu¨ller
andthetransportgapcanbeaccessedbyphotoelectronspec-
detectorwithSrF windowfilledwithArandI )wasusedin
2 2
troscopy. In addition, photoluminescence quenching and ex-
the isochromatic mode with a fixed photon energy of about
ternal quantum efficiency measurements can provide the en-
9.5eV.Theelectronbeamwasdefocussedoverthesampleto
ergies necessary to transfer the excitonic into bound or free
minimizeradiationdamage,anddataacquisitionwasstopped
polaronstates,respectively.
immediately when changes in the IPES spectra occurred be-
tweentwoscans.
The field-induced photoluminescence was performed on
II. EXPERIMENTALMETHODS
samples held in a helium cold finger cryostat under vacuum.
The excitation source was a mechanically modulated solid
Thin films were processed by spin-coating a solution of
state laser at a wavelength 532 nm with a power of 30 mW.
20mg/mlofregioregularP3HTinchlorobenzeneonoxygen-
The photoluminescence was measured using a Cornerstone
plasmatreatedindiumtinoxide(ITO)coatedglasssubstrates.
monochromator with a silicon diode and a liquid nitrogen
The resulting layer thickness was 70 nm and 110 nm, as de-
cooledInSb-detector,andrecordedbyalock-inamplifier.The
termined by a surface profiler (Veeco). All samples were
electric field was applied by a Keithley 237 source measure
thermally treated at 120◦C for 10 min. For diodes, 80 nm
unit. Aftereachvoltagestepfieldandlightwereswitchedoff
thickAlcathodeswerethermallyevaporated. TheP3HTwas
for4minutes,whichwefoundwastheminimumtimelagto
purchased from Rieke Metals. According to the manufac-
avoidcharging.
turer, it has a regioregularity of between 90 and 93%, and
External quantum efficiency spectra were recorded by a
typically contains less than 0.08% Ni, 0.5% Zn and 0.7%
homemade, lock-in based setup equipped with a calibrated
Br as residues from the synthesis. For field-induced photo-
referencesiliconphotodiode. A300WXenonlampwasap-
luminescence measurements, we additionally deposited two
pliedaslightsource.
dielectric layers of 200 nm thickness by spincoating a so-
lution of polymethylmethacrylat (PMMA), in order to avoid
charge carrier injection. Thus, the device configuration was
3
Wavelength [nm]
1.0
experiment
1000 700 500 8x10-2 P3HT
2.0 0.6 0.8
P3HT Braun-Onsager model
ce (PL) [arb.u.] 1.5 APEbLQsEorption 0.4 bA ,]%[ EQE Q(F) 64 EEEEEBBBBB ===== 00000.....3345637200 eeeeeVVVVV 00..64 regasnO-nuarBF(Q
n 1.0 s )
e o
uminesc 0.5 0.2 a[ noitpr 2 0.2
ol br 0 0.0
ot u. 2 4 6 2 4 6 2 4 6
Ph ]. 106 107 108 109
F [V/m]
0.0 0.0
0.5 1.0 1.5 2.0 2.5 3.0
FIG. 4. (Color Online) The photoluminescence quenching yield
Energy [eV] Q(F)=(PL(0)−PL(F))/PL(0) of a P3HT diode with dielectric
layers. Athighfields,itbecomesapparentthatnotallluminescing
FIG.3. (ColorOnline)Absorption(solidline), photoluminescence singletexcitonscanbequenchedbythefieldsattainablewiththeex-
(dotted line) and external quantum efficiency (EQE) of P3HT thin perimentalsetup.
films. Thethinlinesshowhowtheonsetsoftherespectivespectra
weredetermined. Theonsetofthespectrallyresolvedphotocurrent,
theEQE,startsjustabovetheabsorptiononsetof1.86eV.Themajor photon-conversionyield,i.e.,thefractionofgeneratedandex-
photocurrentcontribution,however,isseenattheenergyof2.58eV, tractedchargesperincidentphotonindependenceonthepho-
whichcorrespondstothetransportgapextractedinFig.2.
tonenergy. ThelowenergyregionofsuchanEQEspectrum
of the P3HT sample is presented in Fig. 3, measured under
short circuit conditions. It shows a clear—although weak—
III. EXPERIMENTALRESULTS
onset of the photocurrent already at the absorption edge of
1.85 eV, with a quantum efficiency of about 0.1%. This im-
The combined UPS and IPES spectra of P3HT are pre- pliesthatafractionofthesingletexcitonsisalreadydissoci-
sented in Fig. 2. The band edges can be derived from the ated without excess energy. Synthesis residues such as Ni—
onset values of the HOMO and LUMO signals, as indicated typically below 100 parts per million relative to P3HT—or
bythelinesusedforevaluation.15Theresultingvalues,deter- dissociation at the electrodes can be responsible for this low
minedand givenrelativetothe Fermilevel EF, are-0.75 eV energy photocurrent, and would explain its low magnitude.
for the HOMO and 1.85 eV for the LUMO. They can be as- Apartfromthisrelativelyweaksignal,whichisthusprobably
sociatedwiththerespectiverelaxedionicstates,whichcorre- duetoextrinsiceffects,thephotocurrentstronglyincreasesat
spondtotherearrangementoftheelectronicsystemduetothe photonenergiesaboveapprox. 2.6eV,andthusequalsthein-
additional positive (negative) charge generated by (inverse) trinsictransportgapasdeterminedbyourphotoemissionex-
photoemission.16,17 Inconsequence, thesevaluesprovidethe periments.
positionoftheenergylevelsrelevantforchargetransport.The The electric field dependence of the P3HT luminescence
corresponding transport gap is the energetic separation be- peak of 1.65 eV is shown in Fig. 4. The spectral shape
tween the respective transport levels and can be determined of the PL is field independent, only the magnitude changes
to 2.6 eV. Note that this is significantly different from other (not shown). The field-induced photoluminescence is a suit-
data in literature,18 where the band energies are determined able technique to determine the P3HT singlet exciton bind-
notfromtheonsetvaluesbutthemaximaofthesignalpeaks. ing energy. We point out that the magnitude of the binding
InFig.3theabsorptionandphotoluminescencespectraare energy cannot be extracted directly. Therefore, we related
shown for a 110 nm thick P3HT film on ITO. The absorp- our field-dependent PL quenching measurements to the es-
tion onset is at 1.85 eV and the maximum of the first singlet tablishedOnsager–Braun19,20 (OB)theoryforcalculatingthe
excitontransition1Bu isatabout2.1eV.ByapplyingaGaus- separationyieldoftheboundstate,whichconsidersthefinite
sianfittothehighestenergyphotoluminescencepeakatabout lifetimeoftheexcitedstate. Althoughthisempiricaltheory21
1.85eV,wecandeterminethelinewidthtoσ=68meV.This was originally derived for ion pair dissociation, is has been
distributionalsodescribestheabsorptionedgeverywell,and found suitable for calculating the exciton binding energy.4,8
is assigned to the variance of the gaussian density of states Within this framework, the dissociation probability P(F) is
distribution. TheStokesshiftbetweenthe0–0absorptionand givenby22
emissionbandisabout250meV.
These measurements are complemented by EQE spectra. kd(F) κd(F)
P(F)= = . (1)
This experimental technique shows the spectrally resolved k (F)+k κ (F)+(µτ )−1
d f d f
4
F istheelectricfield,τ =k−1theexcitonlifetime,µthesum summarized in Tab. I. A central result is the finding that the
f f
ofelectronandholemobilities. Thefielddependentdissocia- transport gap E of 2.6 eV directly corresponds to the en-
g,s
tionratek (F)=µκ (F)is ergyneededforachievinganefficientphotocurrent. Wepoint
d d
√ out that the two independent methods involved in determin-
k (F)= 3γ exp(cid:18)−Eb(cid:19)J1(cid:0)√2 −2b(cid:1) (2) ingthissingleparticlegap—photoemissionspectroscopyand
d 4πr3 kT −2b external quantum efficiency—rely on very different princi-
S
ples. The former is very surface sensitive and is performed
where γ=qµ/εε is the Langevin recombination factor,23 r on thin films, whereas the latter relies on charge generation
0 S
istheinitialexcitonradius,E =e2/(4πεε r )istheexciton and transport in the bulk of a device. The nevertheless same
b 0 S
bindingenergy,kT thethermalenergy,J theBesselfunction resulting energy for the transport gap is consistent with for-
1
of order one, and b=e3F/(8πεε0(kT)2). F is the electric mer work.25 The difference between Eg,s and the absorption
field,eistheelementarycharge,andεε0 theeffectivedielec- onsetEg,abs=1.85eVcanbeattributedtotheexcitonbinding
tricconstantoftheorganicsemiconductorblend. energy∆ES→P(cid:39)0.7eV.Sakuraietal.12 investigatedpoly(3-
UsingtheOBmodelwithµτ=5·10−16m2/V,ε=3.4anda octylthiophene) and found the 1Ag polaron pair state 0.5 eV
maximumquenchingof9%,wecouldachieveareasonablefit above the 1B exciton by electroabsorption and two-photon
u
toourexperiment,yieldingE =0.42eVasshowninFig.4. absorption. Usingelectroabsorption,Liessetal.11determined
b
Thediscrepancybetweentheshoulderatlowfieldsintheex- 0.6eVforthetransitionfromthe1B excitonstatetothemA
u g
perimentaldataandthefitcanbeexplainedbythereasonable stateofanotherpoly(3-alkylthiophene),pointingoutthatthis
assumption that the exciton binding energies are distributed might be lower limit for the continuum band threshold. Van
over a finite energy range. We attribute the different magni- derHorstetal. calculated0.61eVforthelattertransition.13
tudesofexperimentalandmodelledthefielddependentpho- Wenotethatourexperimentalapproach,incontrasttoSakurai
toluminescencequenchingtohigherenergytransistionsfrom etal.,Liessetal.andvanderHorstetal.,probesthetransition
singletexcitontopolaronpair,whicharebeyondourmeasure- fromsingletexcitonstofreepolarons,thusgivingtheexciton
mentrange. bindingenergyrelevantforapplicationssuchasphotovoltaics.
UsingtheEinsteinrelation, theµτproductcanalsobeex- The apparently lower exciton binding energy found by
pressedintermsofadiffusionlengthL , PL(F)measurements,0.42eVinsteadof0.7eV,canbeunder-
D
stoodasfollows. InordertoquenchthePL,itissufficientto
(cid:114)
√ kT separatetheconstituentsoftheexcitonpartly,i.e.,generating
L = Dτ= µτ, (3)
D e a—still bound—polaron pair. Therefore, not the full energy
togenerateafreepolaronfromtheexcitonhastobeinvested.
D being the diffusion coefficient. For µτ=5·10−16 m2/V, Inotherwords, thePLquenchingexperimentdeterminesthe
L becomes 3.5 nm. Using the singlet exciton lifetime as energy of the transition from singlet exciton to polaron pair,
D
determined recently for regioregular P3HT,24 τ=0.4 ns, we ∆E ≥0.42eV.Notethatfromthelevellingoffofthefield
S→PP
determine a mobility of the strongly bound charges of about dependentPLshowninFig.4itisclearthatonlyafractionof
1.3·10−6 m2/Vsfromtheabovementionedµτproductused the singlet exciton radiative decay is suppressed. This effect
todescribethedatainFig.4. is probably related to the semicrystalline regions in P3HT,26
leading to self-quenching of PL even at zero field. Also, the
excitonbindingenergy,andspecificallytheexcitontopolaron
IV. DISCUSSION pair transition, is probably distributed in energy. The exper-
imentally determined value of 0.42 eV thus presents a lower
limitfortheexciton-to-polaron-pairtransitionenergy. Itcor-
respondswellwiththedensityfunctionaltheorycalculations
TABLEI.Experimentallydeterminedenergylevelsofregioregular of van der Horst13 for polythiophene, where 0.45 eV were
P3HT.ThecorrespondingenergiesareshownandexplainedinFig.1.
determined for this transition, and the photophysical experi-
mentsofSakuraietal.12onpoly(3-alkylthiophene)(0.55eV).
Energy [eV] Method
Theremainingenergydifferencebetweenpolaronpairand
Eg,s 2.6 PES/IPES freepolaron,whichisnecessarytocreateaphotocurrent,can
LUMO−relativetoEF 1.85 PES/IPES be expressed as ∆ES→P−∆ES→PP. Experimentally, we find
HOMO+relativetoEF −0.75 PES/IPES that≤0.3eVareneededforthistransition. Thisenergydif-
E 1.85 absorption ference corresponds to a Coulomb binding energy. Assum-
g,abs
ingadielectricconstantε=3.4,anelectron–holedistanceof
Eg,s 2.58 EQE
about1.4nmcanbederived,whichissimilartothed-spacing
E 1.88 EQE
g,abs of 1.6 nm found in X-ray diffraction.27 The latter can be at-
∆ES→PP ≥0.42 fielddependentPL tributed to twice the hexyl side chain length of P3HT, repre-
∆ES→P ≈0.7 PES/IPESvs.absorption,EQE sentingthestackingofdifferentmolecularunits.
Wepointoutthatourresultssuggestthattheinterpretation
ofO¨sterbackaetal.28concerningpolarontransitionsobserved
The experimentally determined energy levels of regioreg- inP3HTbyphotoinducedabsorptionspectroscopymightneed
ular P3HT, schematically shown and explained in Fig. 1, are
5
to be reconsidered. In Ref.28, two peaks assigned to local- experimentsthatitisequivalenttotheonsetofphotogenera-
ized polarons, P and P , are reported to absorb at 0.45 eV tion. Bycomparingthisdatatoopticalabsorption,theexciton
1 2
and1.3eVinregioregularandregiorandomP3HT.Twoother bindingenergyisdeterminedforregioregularP3HTtoabout
peaks assigned to delocalized polarons, DP and DP , are 0.7eV.Moreover,fromacomparisontofielddependentpho-
1 2
only observed in regioregular P3HT at 60 meV resp. 1.8 eV. toluminescence measurements, we can provide a lower limit
O¨sterbacka et al. explain their findings on basis of charged of0.42eVforthetransitionfromasingletexcitontoabound
HOMO and LUMO levels moving closer together into the polaronpairandapolaronpairbindingenergyofabout0.3eV.
HOMO–LUMObandgapduetothepolaronicrelaxation.The Asthemethodoffieldinducedphotoluminescencequenching
two polaron signals are then explained by optical transitions wasusedcommonlytodeterminesingletexcitonbindingen-
fromtheHOMOtothechargedHOMO,correspondingtoP , ergies,butactuallyyieldsthepolaronpairbindingenergy,pre-
1
and from there to the charged LUMO, corresponding to P . viousexperimentalresultsshouldbereconsidered. Allinall,
2
In this interpretation, the HOMO–LUMO gap should corre- weprovideacomprehensiveinsightintotheenergylevelsand
spond to 2×P +P =2.1 eV resp. to 2×DP +DP =1.92 eV. bindingenergiesofexcitedstatesinthefrequentlyusedcom-
1 2 1 2
Boththeseenergydifferencesaremuchsmallerthanthetrans- pound P3HT. We emphasize that the presented experimental
portgap—whichwedeterminedtobe2.6eVwithtwoinde- approachisapplicabletoconjugatedpolymersingeneral,and
pendenttechniques—letalonetheHOMO–LUMOgap.How- willbeusedtoexpandourfindingsinviewofacomprehen-
ever,onbasisofourdatawecurrentlycannotpresentaconcise siveunderstandingofthisclassofmaterials.
alternativeexplanation.
ACKNOWLEDGMENTS
V. CONCLUSIONS
DiscussionswithK.Kanaiareacknowledged. Thecurrent
In conclusion, we have presented an experimental ap- workissupportedbytheBundesministeriumfu¨rBildungund
proach, which is capable of providing the position of all en- ForschungintheframeworkoftheGREKOSproject(contract
ergy levels relevant during photoexcitation and charge trans- no. 03SF0356B) and by the DFG (GK 1221). J.G.’s work
port within a conjugated polymer. For the first time, we is funded by the solarNtype project (contract no. 035533-2)
determine the single particle gap of the model polymer withinthe6thframeworkprogrammeoftheEU.V.D.’swork
poly(3-hexylthiophene) (P3HT) relevant for charge transport at the ZAE Bayern is financed by the Bavarian Ministry of
to 2.6 eV by PES/IPES measurements, and show with EQE EconomicAffairs,Infrastructure,TransportandTechnology.
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