Probing Temperature-Dependent Recombination Kinetics in Polymer:Fullerene Solar Cells by

14 Pages·2017·2.24 MB·English

energies
Article
Probing Temperature-Dependent Recombination
Kinetics in Polymer:Fullerene Solar Cells by Electric
Noise Spectroscopy
GiovanniLandi1,CarloBarone2,* ID,CostantinoMauro2,AntoniettaDeSio3,
GiovanniCarapella2,HeinzChristophNeitzert1andSergioPagano2
1 DipartimentodiIngegneriaIndustriale,UniversitàdiSalerno,I-84084Fisciano,Salerno,Italy;
[email protected](G.L.);[email protected](H.C.N.)
2 DipartimentodiFisica“E.R.Caianiello”andCNR-SPINSalerno,UniversitàdiSalerno,
I-84084Fisciano,Salerno,Italy;[email protected](C.M.);[email protected](G.C.);[email protected](S.P.)
3 InstituteofPhysics,CarlvonOssietzkyUniversityofOldenburg,26111Oldenburg,Germany;
[email protected]
* Correspondence:[email protected];Tel.:[phone]
Received:6September2017;Accepted:21September2017;Published:26September2017
Abstract: The influence of solvent additives on the temperature behavior of both charge carrier
transport and recombination kinetics in bulk heterojunction solar cells has been investigated
by electric noise spectroscopy. The observed differences in charge carrier lifetime and mobility
are attributed to a different film ordering and donor-acceptor phase segregation in the blend.
Themeasuredtemperaturedependenceindicatesthatbimolecularrecombinationisthedominant
lossmechanismintheactivelayer,affectingthedeviceperformance. Blenddevicespreparedwith
ahigh-boiling-pointsolventadditiveshowadecreasedrecombinationrateatthedonor-acceptor
interfaceascomparedtotheonespreparedwiththereferencesolvent. Aclearcorrelationbetween
thedeviceperformanceandthemorphologicalpropertiesisdiscussedintermsofthetemperature
dependenceofthemobility-lifetimeproduct.
Keywords: organicphotovoltaics;electricnoiseprocesses;Langevin-typerecombination
1. Introduction
Organicsolarcellshavefoundnotableinterestthankstopromisingfeaturessuchaslowcost,
flexibility,lightweight,transparency,andlarge-areamanufacturingcompatibility[1]. Thepossibility
tousealow-costprintingandroll-to-rollfabricationonflexiblesubstratesrepresents,fortheorganic
photovoltaics, a technological advancement for large scale applications. However, low power
conversion efficiency and stability, when compared to inorganic technologies, still represent
a limiting factor for the applicability of organic solar cells [1]. In this work, the donor-acceptor
materialsinbulkheterojunction-type(BHJ)devicesisablendofpoly(3-hexylthiophene)(P3HT)and
[6,6]-phenyl-C -butyricacidmethylester(PCBM).Forsuchdevices,powerconversionefficienciesη
61
ofabout5%havebeenreported[2]. Theuseofdifferentcombinationsofdonor-acceptormaterials
basedonlowbandgappolymersandfullerenederivativesleadstoavalueofηfortheorganicsingle
junction solar cell higher than 10% [1]. Recently, thanks to the addition of Fe O nanoparticles as
3 4
dopantwithintheP3HT:PCBM,improvedefficiencyof11%hasbeenreached[3]. Itisworthnoting
thatfororganictandemdevices,ηvaluesexceeding12%havebeenalreadyobserved[4].
A close intermixing between donor and acceptor materials plays a key role to ensure a high
rate of charge carriers and to lower the recombination rate between electrons and holes in the
blend. Thelossmechanismsaremainlyrelatedtorecombinationofthecharge-separatedexcitons[5].
Energies2017,10,1490;doi:10.3390/en10101490 www.mdpi.com/journal/energies
Energies2017,10,1490 2of14
Sincerecombinationandtransportintheactivelayerareinfluencedbythefilmordering[6],inorder
toimprovetheefficiencyoftheBHJsolarcellitisnecessarytocontrolthenanoscalemorphologyof
theblendbynanostructuring.
Inliterature,severalstudiesreporttheinfluenceofthepreparationconditions,suchasdeposition
temperature and solvent additives, on the active layer morphology [7–9]. Different experimental
techniqueshavebeenappliedtoinvestigatethecarrierdynamicsintheBHJsolarcell(e.g.,thetransient
absorptionspectroscopyandthemethodoftimedelayedcollectionfield)[10,11]. Thesetechniques
giveadirectanalysisonthechargerecombinationmechanismsinthepolymer:fullerenesolarcell.
However, they do not provide information on the carrier mobility and, therefore, on the charge
transport,whichareusuallyconsideredinefficientinlowmobilitymaterials,suchastheP3HT:PCBM
blend [12]. In the BHJ solar cell, the charge carrier recombination dynamics is often found to be
bimolecular and is described by Langevin’s theory [13,14]. In this case, the recombination rate is
proportionaltothesumoftheelectronandholemobilities. Todate,thereareonlyfewtechniques,
suchasthechargeextractionwithlinearlyincreasingvoltagemeasurements(CELIV)andtheopen
circuitcorrectedchargecarrierextraction(OTRACE),thatareabletodeterminesimultaneouslythe
carriermobilityandlifetimeinthesolarcells[15]. Itshouldbenotedthatthemobility-lifetimeproduct
µτgivesadirectmeasurementofthecorrelationbetweenrecombinationandtheextractionofcharges.
Low-frequencynoisespectroscopyisapowerfulandinnovativetoolfortheanalysisofelectronic
transport and recombination phenomena, widely used to perform a non-destructive testing of
electronic devices. In order to compare the results from the noise analysis with the data reported
in literature, the most widely investigated pair of P3HT:PCBM materials in BHJ devices has
been considered as a reference system. The study of charge carrier fluctuations has contributed
to detect: unconventional transport processes in mixed-valence manganites [16] and in oxide
heterojunctions [17], nonequilibrium and degradation effects in iron-based superconductors [18],
andtemperature-dependentstructuralmodificationsinpolymer/carbonnanotubescomposites[19–21].
Moreover, noise investigation has been used to characterize defect states and phase-transitions
in crystalline silicon-based and perovskite-based solar cells [22–24]. Additionally, noise analysis
hasalsobeenrecentlyusedtodeterminethechargecarriertransportandthedegradationphenomena
inpolymer:fullerenesolarcells[25,26].
In the present study voltage-noise analysis, combined with structural and photoelectric
measurements, have been performed in polymer:fullerene photovoltaic devices prepared with
solvent additives. A detailed discussion of the experimental results shows that the use of solvent
additivesincreasesthemesoscopicorderandcrystallinitywithintheactivelayer,andinducesaclear
improvement of the charge carrier transport. The temperature dependence of the mobility and
carrierlifetimesuggeststhatinP3HT:PCBMblendstheLangevin-typebimolecularrecombinationloss
representsoneofthemajordevicelimitingfactorsfortheefficientextractionofthechargecarriers.
2. ElectricNoise: GeneralConcepts
Noise, or fluctuations, are spontaneous random variations of physical quantities in time or,
moreprecisely,randomdeviationsofthesequantitiesfromsomemeanvaluesthatareeitherconstant
orvarynonrandomlyintime. Therefore,noiseisastochasticprocessandisdescribedbyarandom
function x(t). The deviation of x(t) from its mean value (cid:104)x(cid:105) is the fluctuation δx(t) = x(t)−(cid:104)x(cid:105).
The kinetics of such a fluctuation, that is how δx(t) evolves in time on average, can be analyzed
throughthecorrelationfunction.
Inthetheoryoffluctuations,animportantclassofrandomprocessesiscomposedbyGaussian
processes for which the correlation function establishes the correlation between the values of the
random process at two different times t and t . This is commonly known as the autocorrelation
1 2
functionΨ (t ,t ),dependingonlyonthedifferencet −t inthecaseofstationarysystems.
x 1 2 1 2
Theautocorrelationisthemainstatisticalfunctionusedtodescribethebasicpropertiesofrandom
datainthetimedomain. Similarinformationinthefrequencydomaincanbeobtainedbythespectral
Energies2017,10,1490 3of14
densityfunctionS (f),whichistheFouriertransformoftheautocorrelationfunction. Moreindetails,
x
fromtheWiener-Khintchinetheorem,itfollowsthatS (f) = Ψ (ω)andtheintegralofthespectral
x x
densityoverallpositivefrequenciesisexactlythevarianceofthenoise(i.e.,thefluctuationsamplitude).
Theprincipalapplicationforapowerspectraldensityfunctionmeasurementofphysicaldata
istoestablishtheirfrequencycomposition. This,inturn,bearsimportantrelationshipstothebasic
characteristicsofthephysicalsysteminvolved. Themostcommontypesoflow-frequencynoisein
condensedmattersystemsare: (I)theJohnsonorthermalnoise,thatistheelectronicnoisegenerated
bythethermalagitationofthechargecarriers(usuallytheelectrons)insideanelectricalconductor
at equilibrium; (II) the shot noise, that is the electronic noise which can be modeled by a Poisson
processandisoriginatedfromthediscretenatureofelectriccharge;(III)thepinkor1/f noise,thatis
the electronic noise characterized by a frequency spectrum which is inversely proportional to the
frequencyofthesignal;and(IV)therandomtelegraphnoise,thatistheelectronicnoiseconsisting
of sudden step-like transitions between two or more discrete voltage or current levels. As a rule,
in uniform conductors the spectral density of 1/f noise is proportional to the square of the mean
2 2
voltage V or the mean current I . This direct proportionality has been verified many times on
various conductors, and Hooge developed a qualitative rule by extrapolating a value of 2×10−3
astheuniversalproportionalitynoisecoefficient. However, thisempiricalHoogerelationshiphas
notageneralphysicalbaseand,especiallyincomplexsystemssuchasphotovoltaicdevices,cannot
be applied (see the experimental results shown in the following section for a direct confirmation).
Adetailedanalysisofelectricnoiseincondensedmattercanbefoundin[27].
3. Results
3.1. ElectricalTransportandStructuralProperties
Thecurrentdensity-voltage(J-V)characteristicsunderilluminationofthesolarcellsprepared
with 1,2-dichlorobenzene (oDCB) reference solvent and oDCB with 1,2,3,4-tetrahydronaphthalene
(oDCB+THN) mixture solvent are shown in Figure 1a. By considering the single diode model of
thesolarcell, thefollowingparametershavebeenextractedforoDCBbaseddevices: shortcircuit
current density J = (5.5±0.3) mA·cm−2, open circuit voltage V = (0.58±0.03) V, fill factor
SC OC
FF = (69±3)%, and a power conversion efficiency η = (2.2±0.1)%. The same parameters
for oDCB+THN are: J = (7.4±0.4) mA·cm−2, V = (0.61±0.03) V, FF = (72±3)%,
SC OC
andη = (3.3±0.2)%. Theincreaseoftheηvalueforthedevicefabricatedwiththesolventmixture
seems to be mainly due to the increase of the J . Differences in the FF and V are within
SC OC
the experimental errors. As can be observed, the oDCB+THN blend has a higher photocurrent,
ascomparedtotheblendpreparedfrompureoDCB.InFigure1btheexternalquantumefficiency
(EQE)spectrumforbothsolarcellsisshown.InagreementwiththeJ-Vcharacteristics,theoDCB+THN
blendshowsahigherintensityoftheEQEspectrumcomparedtothereferenceblendinthewhole
wavelengthrange.Here,thephotocurrentdensityvaluesevaluatedfromtheintegraloftheEQEspectra
are consistent with what estimated from the J-V characteristics. In particular, the resulting values
ofthe J are5.5mA·cm−2 and7.5mA·cm−2 fortheoDCBandoDCB+THNsolvent, respectively.
SC
Additionally, in the lowest EQE signal, which is related to the oDCB blend, a marked drop in the
spectrumbetween350and450nmisobserved.Thisbehaviorhasbeenalreadyreportedinliteraturefor
theP3HT:PCBMblendwhenapoororderingofthepolymerisobtainedduringthefilmformation[28].
It should be noted that the use of the tetralin leads to an increase of the crystalline order in the
nanoscaleinterpenetratingnetworkforboththeacceptoranddonorcomponents. Thefilmordering
stronglyinfluencestheabsorptionspectraoftheP3HT:PCBMlayer. Inparticular,theshoulderlocated
at620nmgivesanindicationoftheP3HTcrystallinityintheblend,fordetailssee[8]andreferences
therein. Forthesampleshereinvestigated,therelativeintensityoftheshoulderat620nmappearsto
bemorepronouncedintheblenddepositedfromoDCB+THN[8]. ThismeansthattheuseofTHN
additiveleadstoabetterphasesegregationofthepolymerandfullerenematerialsintheactiveblend.
Energies2017,10,1490 4of14
Thisfindingsuggeststhatthesolarcellperformanceisstronglyrelatedtothemorphologyoftheactive
layerduetothesolventaddition.
Energies 2017, 10, 1490 4 of 14
Energies 2017, 10, 1490 4 of 14

2

-2A cm)-2cm)002 000..56.6 oo DDooDDCCCCBBBB++TTHHNN
urrent density (ment density (mA ----8642---642 oDCB EQE (a.u.)EQE (a.u.) 00000000....1234....2345
Curr -8 o DoDCCBB+THN 0.1
C oDCB+THN 0.0
-0.50 -0.25 0.00 0.25 0.50 0.75 1.00 300 400 500 600 700
0.0
-0.50 -0.25 0.00VDC0 .(2V5) 0.50 0.75 1.00 300 40w0aveleng5 t0h0 (nm) 600 700
(aV) DC (V) wav(eble) ng th (nm)
(a) (b)
Figure 1. (a) Illuminated current density-voltage characteristics of solar cells with active layers
Figure1.(a)Illuminatedcurrentdensity-voltagecharacteristicsofsolarcellswithactivelayersprepared
Figure 1. (a) Illuminated current density-voltage characteristics of solar cells with active layers
prepared using the solvents oDCB (black line) and oDCB+THN (blue line); (b) External quantum
usingthesolventsoDCB(blackline)andoDCB+THN(blueline); (b)Externalquantumefficiency
prepared using the solvents oDCB (black line) and oDCB+THN (blue line); (b) External quantum
efficiency (EQE) characteristics of solar cells with blends using the solvents oDCB (black line) and
(EQE)characteristicsofsolarcellswithblendsusingthesolventsoDCB(blackline)andoDCB+THN
efficiency (EQE) characteristics of solar cells with blends using the solvents oDCB (black line) and
oDCB+THN (blue line).
(blueline).
oDCB+THN (blue line).
This hypothesis is also confirmed by atomic force microscopy (AFM) analysis. In Figure 2 the
TThhiiss hhyyppootthheessiiss iiss aallssoo ccoonnfifirrmmeedd bbyy aattoommiicc ffoorrccee mmiiccrroossccooppyy ((AAFFMM)) aannaallyyssiiss.. IInn FFiigguurree 22 tthhee
AFM images for the different blends are shown. The average RMS roughness is about 1.1 nm and
AAFFMM iimmaaggeess ffoorr tthhee ddiiffffeerreenntt bblleennddss aarree sshhoowwnn.. TThhee aavveerraaggee RRMMSS rroouugghhnneessss iiss aabboouutt 11..11 nnmm aanndd
4.6 nm for the blend prepared from oDCB and oDCB+THN, respectively [8]. In literature several
44..66 nnmm ffoorr tthhee bblelenndd pprreeppaarreedd frfroommo oDDCCBBa annddo oDDCCBB++TTHHNN,, rreessppeeccttiivveellyy [[88]].. IInn lliitteerraattuurree sseevveerraall
authors report the positive correlation between increased blend surface roughness and increased
aauutthhoorrss rreeppoorrtt tthhee ppoossiittiivvee ccoorrrreellaattiioonn bbeettwweeeenn iinnccrreeaasseedd bblleenndd ssuurrffaaccee rroouugghhnneessss aanndd iinnccrreeaasseedd
solar cell efficiency in bulk heterojunction solar cells [8,29–31]. This is a further evidence of the P3HT
ssoollaarr cceellll eefffificciieennccyy iinn bbuullkk hheetteerroojjuunnccttiioonn ssoollaarr cceellllss [[88,,2299––3311]].. TThhiiss iiss aa ffuurrtthheerr eevviiddeennccee ooff tthhee PP33HHTT
ordering in the active layer, which leads to improvements in the photocurrent and, therefore, also in
oorrddeerriinngg iinn tthhee aaccttiivvee llaayyeerr,, wwhhiicchh lleeaaddss ttoo iimmpprroovveemmeennttss iinn tthhee pphhoottooccuurrrreenntt aanndd,, tthheerreeffoorree,, aallssoo iinn
the power conversion efficiency.
tthhee ppoowweerr ccoonnvveerrssiioonn eefffificciieennccyy..
(a) (b)
(a) (b)
Figure 2. Atomic force microscopy images of the blends prepared using the solvents oDCB (a) and
Figure 2. Atomic force microscopy images of the blends prepared using the solvents oDCB (a) and
oFDigCuBr+eT2H.NA t(obm). icforcemicroscopyimagesoftheblendspreparedusingthesolventsoDCB(a)and
oDCB+THN (b).
oDCB+THN(b).
3.2. Noise Properties
3.2. Noise Properties
3.2. NoiseProperties
The study of charge carrier fluctuation mechanisms is essentially based on the measurement of
The study of charge carrier fluctuation mechanisms is essentially based on the measurement of
the voTlthaegest-uspdeyctorfacl hdaerngseitcya r(cid:1845)r(cid:3023)ie greflnuecrtautaetdio bny mtheec hdaenviicsem. sInis Feigssuernet 3ia tlhlye bfraesqeudeonncyt hdeepmeenadseunrecme oefn t(cid:1845)(cid:3023)o f
the voltage-spectral density (cid:1845) generated by the device. In Figure 3 the frequency dependence of (cid:1845)
isth sehvoowltna gfoer-s ptheec tsroalladr ecnesliltsy fSa(cid:3023)brigceanteedra wtedithb ythteh eredfeerveincec.e IonDFCigBu sreol3vetnhte (fbrelaqcuke nspcyecdtreap)e anndde nwceitho(cid:3023) f
V
is shown for the solar cells fabricated with the reference oDCB solvent (black spectra) and with
oDCB+THN (blue spectra). A similar behavior is observed at two different temperatures, as shown
oDCB+THN (blue spectra). A similar behavior is observed at two different temperatures, as shown
in Figure 3a ((cid:1846)=300 K) and in Figure 3b ((cid:1846) =320 K), for a fixed applied bias current (cid:1835) =20 μA
(cid:3005)(cid:3004)
in Figure 3a ((cid:1846)=300 K) and in Figure 3b ((cid:1846) =320 K), for a fixed applied bias current (cid:1835) =20 μA
and measured in dark conditions. (cid:3005)(cid:3004)
and measured in dark conditions.
Energies2017,10,1490 5of14
S isshownforthesolarcellsfabricatedwiththereferenceoDCBsolvent(blackspectra)andwith
V
oDCB+THN(bluespectra). Asimilarbehaviorisobservedattwodifferenttemperatures,asshownin
Figure3a(T =300K)andinFigure3b(T =320K),forafixedappliedbiascurrent I =20µAand
DC
Emneeragsieus r2e0d17,i n10d, 1a4r9k0 c onditions. 5 of 14
(1) oDCB (1) oDCB
(2) oDCB+THN (2) oDCB+THN
10-12 (2) best fitting curves 10-12 (2) best fitting curves
z) z)
H H
2 (V/V10-14 (1) 2 (V/V10-14 (1)
S S
10-16 10-16
T = 300 K - I = 20 μA T = 320 K - I = 20 μA
DC DC
101 102 103 104 105 101 102 103 104 105
Frequency (Hz) Frequency (Hz)
(a) (b)
Figure 3. Low-frequency voltage-noise spectra of a device fabricated with oDCB as solvent, curves 1,
Figure3.Low-frequencyvoltage-noisespectraofadevicefabricatedwithoDCBassolvent,curves1,
and of a device fabricated with oDCB+THN as solvent, curves 2. Experimental data are measured in
andofadevicefabricatedwithoDCB+THNassolvent,curves2.Experimentaldataaremeasuredin
ddaarrkk ccoonnddiittiioonnss aatt ddiiffffeerreennttt etemmppeeraratuturerse:s:( a()a)T (cid:1846)==303000K ;K(b; )(bT) =(cid:1846)3=2032K0. TKh.e Tdhaes hdeadshceudrv ceusravrees tahreeb tehset
best fit obtained by using Equation (1).
fitobtainedbyusingEquation(1).
TTwwoo ddiissttininccttn noiosiesec ocmompopnoennetnstcsa ncabne ibdee nidtiefinetdif.ieTdh.e Tfihrset fisirtsht eisfl itchkee rflnicokiseer Snfoliicskeer ,(cid:1845)w(cid:3023)(cid:3033)(cid:3039)h(cid:3036)(cid:3030)i(cid:3038)c(cid:3032)h(cid:3045)a, pwpheiacrhs
appears in the low-frequency region and is associated to electrical conductivity flVuctuations [27]. The
inthelow-frequencyregionandisassociatedtoelectricalconductivityfluctuations[27]. Thesecond
second one is the thermal noise (cid:1845)(cid:3047)(cid:3035)(cid:3032)(cid:3045)(cid:3040)(cid:3028)(cid:3039), which characterizes the high-frequency region of the
oneisthethermalnoiseSthermal,wh(cid:3023)ichcharacterizesthehigh-frequencyregionofthespectrumand
spectrum and depends oVn temperature fluctuations of the charge carriers interacting with the
dependsontemperaturefluctuationsofthechargecarriersinteractingwiththepolymericchains[27].
polymeric chains [27]. By modeling the electronic behavior of the cells with a simple RC circuit
BymodelingtheelectronicbehaviorofthecellswithasimpleRCcircuitformedbytherecombination
formed by the recombination resistance (cid:1844) and the chemical capacitance (cid:1829) of the active layer
resistanceR andthechemicalcapacitance(cid:3045)(cid:3032)C(cid:3030) oftheactivelayer[32,33],itisp(cid:3091)ossibletoexpressS .
rec µ V
[32,33], it is possible to express (cid:1845) as [25]:
as[25]: (cid:3023)
S(cid:1845)V(cid:3023)((cid:4666)(cid:1858)f)(cid:4667)==(cid:1845)S(cid:3023)(cid:3033)Vf(cid:3039)l(cid:3036)i(cid:3030)c(cid:3038)ke(cid:3032)r(cid:3045)++S(cid:1845)Vt(cid:3023)(cid:3047)h(cid:3035)e(cid:3032)r(cid:3045)m(cid:3040)al(cid:3028)(cid:3039)==fK(cid:1858)(cid:1837)γ(cid:3082)(1(cid:4666)1R(cid:1844)+r+(cid:3045)e(cid:3032)c(cid:16)(cid:3030)(cid:3436)I(cid:1835)D(cid:3005)(cid:1858)fC(cid:3004)(cid:17)(cid:3440)(cid:4667))(cid:2870)2(cid:2870)2++44(cid:1863)k(cid:3003)B(cid:1846)T(cid:1844)R(cid:3045)r(cid:3032)e(cid:3030)c (1()1 )
(cid:1858)fx
(cid:3051)
In Equation (1), K is the amplitude of the flicker noise component, γ is an exponent close
In Equation (1), (cid:1837) is the amplitude of the flicker noise component, (cid:2011) is an exponent close to
utoniutyn,i t(cid:1858)y, ifsx ai scuat-ocufft -foreffqufreenqcuye antc wyhaitchw ah cichhanagec hfraonmge a fr1o/m(cid:1858) tao 1a/ 1f/(cid:1858)to(cid:2871) ade1p/efn3dednecpee onfd tehnec evooltfatghee-
(cid:3051)
voltage-noisespectrumisobserved,andk istheBoltzmannconstant. Agoodagreementbetween
noise spectrum is observed, and (cid:1863) is thBe Boltzmann constant. A good agreement between the
(cid:3003)
the experimental voltage-spectral density and the model of Equation (1) is shown by the red
experimental voltage-spectral density and the model of Equation (1) is shown by the red dashed
dashed curves in Figure 3. The corresponding values of the fitting parameters at T = 300 K
curves in Figure 3. The corresponding values of the fitting parameters at (cid:1846)=300 K are: (cid:1837) =
(cid:4666)a8re.5:±K0.6=(cid:4667)×(81.05(cid:2879)±(cid:2869)(cid:2869)0,. 6(cid:1858))×=1(cid:4666)01−9191,±f6x(cid:4667)= H(z1,9 9an±d6 )(cid:2011)H=z,(cid:4666)1a.0n0d±γ0.=02(cid:4667)( 1f.o0r0 ±the0 .0tr2a)cefo r1; thwehitlrea:c e(cid:1837)1=; (cid:4666)w1h.4il±e:
0K.1=(cid:4667)×(11.40(cid:2879)±(cid:2877),0 .(cid:1858)1)=×(cid:4666)1103−39(cid:3051),±fx4(cid:4667)= H(1z3, 3an±d4 )(cid:2011)H=z(cid:4666),1a.n0d0±γ =0.0(21(cid:4667).0 f0o±r t0h.e0 2tr)afcoer 2th. eThtrea cceut2-.oTffh ferecquut-eonffcyfr eshquifetsn ctoy
(cid:3051)
shiftstolowerfrequenciesandthevalueofKincreaseswhentheTHNsolventisaddedtothesolution.
lower frequencies and the value of (cid:1837) increases when the THN solvent is added to the solution. It is
Itisworthnotingthatwhenincreasingthetemperature,thenoiseamplituderemainsalmostconstant
worth noting that when increasing the temperature, the noise amplitude remains almost constant
whereasthecut-offfrequencyshiftstohigherfrequencies.
whereas the cut-off frequency shifts to higher frequencies.
Thebestfittingvaluesoftheparameter f canbeusedtoestimatetheeffectivecarrierlifetimeas:
The best fitting values of the parameter x(cid:1858) can be used to estimate the effective carrier lifetime
(cid:3051)
as:
τ = (2πf )−1 = (cid:0)2πR C (cid:1)−1 (2)
x rec µ
(cid:2028)=(cid:4666)2(cid:2024)(cid:1858)(cid:4667)(cid:2879)(cid:2869) =(cid:3435)2(cid:2024)(cid:1844) (cid:1829) (cid:3439)(cid:2879)(cid:2869) (2)
(cid:3051) (cid:3045)(cid:3032)(cid:3030) (cid:3091)
By defining the recombination resistance of electrons and holes as R = (dV /dI ) [34],
rec F DC
By defining the recombination resistance of electrons and holes as (cid:1844) =(cid:4666)(cid:1856)(cid:1848) /(cid:1856)(cid:1835) (cid:4667) [34], from
fromEquation(2)itisstraightforwardtocomputethelifetimedependence(cid:3045)o(cid:3032)(cid:3030)nthefo(cid:3007)rwar(cid:3005)d(cid:3004)biasvoltage
Equation (2) it is straightforward to compute the lifetime dependence on the forward bias voltage (cid:1848) ,
V ,thatisthedcvoltagewithoutthecontributionofthedeviceseriesresistance[35]. Theexperiment(cid:3007)al
F
that is the dc voltage without the contribution of the device series resistance [35]. The experimental
valuesforτareshowninFigure4atdifferenttemperaturesbetween300and330Kandforthetwo
values for (cid:2028) are shown in Figure 4 at different temperatures between 300 and 330 K and for the
typesofsolarcellsinvestigated(oDCBsolventinpinkregionandoDCB+THNsolventinblueregion).
two types of solar cells investigated (oDCB solvent in pink region and oDCB+THN solvent in blue
region). As clearly evident in Figure 4, the use of the THN as solvent leads to an increase of (cid:2028). Indeed,
at 300 K the observed effective lifetime ranges between 1.2 and 0.6 ms for the blend fabricated
with the solvent mixture oDCB+THN. On the other hand, the devices prepared by using the reference
solvent show lower values of (cid:2028), ranging between 0.8 and 0.2 ms. In literature are reported similar
values of the charge carrier lifetimes, measured with alternative experimental techniques such as
impedance spectroscopy and photo-induced absorption [36,37]. Additionally, a decrease of (cid:2028) with
Energies2017,10,1490 6of14
AsclearlyevidentinFigure4,theuseoftheTHNassolventleadstoanincreaseofτ. Indeed,at300K
theobservedeffectivelifetimerangesbetween1.2and0.6msfortheblendfabricatedwiththesolvent
mixtureoDCB+THN.Ontheotherhand,thedevicespreparedbyusingthereferencesolventshow
lowervaluesofτ,rangingbetween0.8and0.2ms.Inliteraturearereportedsimilarvaluesofthecharge
Energies 2017, 10, 1490 6 of 14
carrierlifetimes,measuredwithalternativeexperimentaltechniquessuchasimpedancespectroscopy
and photo-induced absorption [36,37]. Additionally, a decrease of τ with an increase of the bias
an increase of the bias voltage, corresponding to an increase of the charge carriers stored within the
voltage,correspondingtoanincreaseofthechargecarriersstoredwithintheorganicdevice,hasbeen
organic device, has been observed at all the investigated temperatures and for both types of used
observedatalltheinvestigatedtemperaturesandforbothtypesofusedsolvents. Thissuggeststhat
solvents. This suggests that the recombination dynamics of the blend seems to be bimolecular, as the
ethffeecreticvoem libfeintiamtieo n(cid:2028)d dyenpaemnidcss oofnt htheeb clehnadrgsee ecmarsriteor bceonbcimenotlreactuiolnar ,(cid:1866)a s[1th5]e. eTfhfeec tliovweelirf evtaimlueesτ odbetpaeinnedds
onthechargecarrierconcentrationn[15]. Thelowervaluesobtainedforthedevicefabricatedwiththe
for the device fabricated with the reference solvent (full symbols in Figure 4) indicate a strong
rreecfeormenbcineastoiolvne innt t(hfuel bllseynmdb. oClosninveFrisgeulyre, h4i)gihnedri cvaatleueass otrfo (cid:2028)n,g arse ocbosmerbvinedat fioonr tihnet dheevbilceen pdr.eCpoanrevde rwseitlhy,
highervaluesofτ,asobservedforthedevicepreparedwiththeadditionofTHN(opensymbolsin
the addition of THN (open symbols in Figure 4), are indicative for a more efficient charge carrier
Figure4),areindicativeforamoreefficientchargecarriertransportintheactivelayer. Itseemsthatthe
transport in the active layer. It seems that the solvent mixture leads to a reduced charge carrier
solventmixtureleadstoareducedchargecarrierrecombinationrate,thusleadingtoanincreaseofthe
recombination rate, thus leading to an increase of the carrier lifetime. This is in good agreement with
tchaer rmieeralsifuerteimmee.nTtsh iosni sthine egloeocdtriacgarl etermanesnptowrti athndth tehme setarsuucrteumrael nptrsoopnertthieese slehcotwricna lintr Faingsuproerst 1a nanddth 2e.
Istt rius cwtuorrathlp nrootpinergt itehsasth foowr bnoitnh Fthigeu irnevse1staingdat2e.dI tdiesvwicoerst,h thneo ctianrgritehra ltiffeotrimboet dhetchreeainsvese swtiigtaht eindcdreeavsiicnegs,
ttehmecpaerrraieturrleif.e Ttihmies dise car efausrethsewr itehviidncernecaes itnhgatt etmhep edroamtuirnea.nTth irsecisoamfbuirntahteironev liodsesnecse wthiathtitnh ethdeo mdeinvaicnet
creocuoldm bbein Laatinognelvoisns-etsypwei tbhiimnothleecudleavri creeccoomulbdinbaetiLoann [g1e3v].i n-typebimolecularrecombination[13].
1.5 , T = 300 K

, T = 310 K

, T = 320 K

1.2 , T = 330 K

)
s
m0.9
(
τ
oDCB
0.6
0.3
oDCB+THN
0.5 0.6 0.7 0.8 0.9
V (V)
F
FFiigguurree 44.. EEffffeeccttiivvee ccaarrrriieerr lliiffeettiimmee ddeeppeennddeennccee oonn tthhee ffoorrwwaarrdd vvooltltaaggee, ,aat ttetemmppeerraatuturreess bbeetwtweeeenn 330000
aanndd 333300K K.T. Thheee xepxpereirmimenetnatladl adtaataa raered idreircetclytlyex etxratrcatectdedby bnyo nisoeisme emaesausruemreemnetsn.tsF.u Flluslly smybmoblsorlse freerfetor
ttoh ethoeD oCDBCsBo lsvoelvnetn(pt i(npkinrke greiognio),nw), hwilheiolep oepnesny msybmoblsorlse freerfetor ttoh ethoeD oCDBC+BT+HTNHNso slvoelvnetn(tb (lubleuree greiognio)n.).
4. Discussion
4. Discussion
In order to investigate the influence of the solvent on the film ordering in the blend and hence
Inordertoinvestigatetheinfluenceofthesolventonthefilmorderingintheblendandhenceon
on the charge carrier transport, an evaluation of the carrier mobility from voltage-noise
thechargecarriertransport,anevaluationofthecarriermobilityfromvoltage-noisemeasurements
measurements has been made. As reported in literature, the mobility can be computed from the
hasbeenmade. Asreportedinliterature,themobilitycanbecomputedfromtheamplitudeofflicker
amplitude of flicker and thermal noise components as [38]:
andthermalnoisecomponentsas[38]:
(cid:2020) = (cid:1863)k(cid:3003)(cid:1846)T (cid:3033)(cid:3505)(cid:3288)(cid:90)fm(cid:3276)a(cid:3299)x(cid:1845)S(cid:3023)(cid:3033)f(cid:3039)l(cid:3036)i(cid:3030)ck(cid:3038)e(cid:3032)r(cid:3045)((cid:4666)f(cid:1858))(cid:4667)(cid:1856)(cid:1858) (3)
µ =(cid:1857)(cid:2028)(cid:1831)B(cid:2870) (cid:1845)V(cid:3047)(cid:3035)(cid:3032)(cid:3045)(cid:3040)(cid:3028)(cid:3039) df (3)
eτE2(cid:3033)(cid:3288)(cid:3284)(cid:3289) S(cid:3023)Vthermal
fmin
where (cid:1857) is the elementary charge, (cid:2028) is the carrier lifetime as defined in Equation (2), (cid:1831) =(cid:1848) /(cid:1872) is the
(cid:3007)
awphpelrieede ieslethcteriecl efmieeldn ta(bryeicnhga r(cid:1872)g et,hτe isatchtievec arlariyeerr litfheticimkneeasss)d, eafinnde d(cid:4670)(cid:1858)i(cid:3040)n(cid:3036)E(cid:3041),q(cid:1858)u(cid:3040)a(cid:3028)t(cid:3051)i(cid:4671)o nis( 2t)h,eE e=xpVeFr/imtiesntthael
farpepqluieednceyle cbtarincdfiweliddt(hb.e iTnhget tvhaeluaectsi voebltaayienredth ifcrkonmes sE),qaunadtio[fnm in(3,)f maarxe] isshthoewenx pienr imFigenutrael f5r.e qAute naclyl
tbeamndpweriadtuthr.eTs,h ae vcalelaure sreodbutacitnioedn forof m(cid:2020) Eisq ueavtiidoenn(t3 )foarr etshheo cwenllsi nwFiitghu rthee5 .oADtCaBll+tTeHmNpe rsaotluvreenst, a(ocpleeanr
symbols in blue region) upon respect to the reference solvent oDCB (full symbols in pink region).
From this analysis, it is not possible to distinguish between electron and hole transport. However, it
is well-known that in P3HT:PCBM blends the electron mobility is usually higher than the hole
mobility [39,40]. Therefore, the experimental data of Figure 5 can be attributed to the electrons in the
blends. The observed increase of (cid:2020) with temperature and its bias dependence suggest that the
dominant carrier transport mechanism is a thermally assisted hopping process between localized
charge transport sites [13]. This conduction mechanism has been already found in diodes, field effect
Energies2017,10,1490 7of14
reduction of µ is evident for the cells with the oDCB+THN solvent (open symbols in blue region)
upon respect to the reference solvent oDCB (full symbols in pink region). From this analysis, it is
notpossibletodistinguishbetweenelectronandholetransport. However,itiswell-knownthatin
P3HT:PCBMblendstheelectronmobilityisusuallyhigherthantheholemobility[39,40]. Therefore,
the experimental data of Figure 5 can be attributed to the electrons in the blends. The observed
increaseofµwithtemperatureanditsbiasdependencesuggestthatthedominantcarriertransport
mechanism is a thermally assisted hopping process between localized charge transport sites [13].
Energies 2017, 10, 1490 7 of 14
Thisconductionmechanismhasbeenalreadyfoundindiodes,fieldeffecttransistors,andinsolar
ctreallnssbisatsoerds, oannodr giann icsoalnadr icneollrsg abnaisceddi soornd eorregdamnica tearniadl sisnuocrhgaansipc erdoisvosrkditeerse,ds mmaalltemrioallesc usluecsha nads
cpoenrjouvgsaktietdesp, somlyamlle mrso[l4e1c–u4l5es]. and conjugated polymers [41–45].
oDCB
100
)
1
-s oDCB+THN
1
-V
2 30
m
c
3
-0
1
μ ( 10
, T = 300 K

, T = 310 K

, T = 320 K

, T = 330 K

2
150 160 170 180 190
E1/2 (V1/2cm-1/2)

FFiigguurree 55.. EElleeccttrroonn mmoobbiilliittyy ddeeppeennddeennccee,, oobbttaaiinneedd bbyy nnooiissee aannaallyyssiiss,, oonn tthhee ssqquuaarree rroooott ooff tthhee aapppplliieedd
eelleeccttrriicc ffiieelldd.. ThTeh beesbte fsitttifintgti cnugrvceusr v(seoslid(s olilnides)l ianrees d)earriveedde frriovmed thfero Pmootleh-eFrPeonokleel- mFroednkele olfm Eqoudaetlioonf
E(4q)u, abtoitohn f(o4r) ,thbeo tohDfoCrBt hsoelovDenCt B(fsuolll vseynmtb(foulsll) saynmdb tohles )oaDnCdBt+hTeHoNDC sBol+vTeHntN (ospoelvne snytm(obpoelsn).s ymbols).
Figure 5 also shows an exponential dependence of (cid:2020) on (cid:1831)(cid:2869)/(cid:2870), that can be interpreted in terms
Figure5alsoshowsanexponentialdependenceofµonE1/2,thatcanbeinterpretedintermsof
of the Poole-Frenkel model as [46]:
thePoole-Frenkelmodelas[46]:
(cid:1857)(cid:2871)/(cid:2870) (cid:1831) (cid:2869)/(cid:2870) 1
(cid:2020) =(cid:2020)(cid:2868)(cid:1857)(cid:1876)(cid:34)(cid:1868)e(cid:4680)32/2(cid:1863)(cid:18)(cid:3436)(cid:2024)E(cid:2013) (cid:2013)(cid:19)(cid:3440)1/2(cid:1846)1 (cid:4681)(cid:35) (4)
µ = µ exp (cid:3003) (cid:2868) (cid:3045) (cid:3032)(cid:3033)(cid:3033) (4)
0
2k πε ε T
B 0 r eff
In Equation (4), (cid:2013) is the vacuum permittivity, (cid:2013) ≈3 is the relative dielectric constant for
(cid:2868) (cid:3045)
P3HTIn:PECqBuMat i[o3n6](,4 )(cid:2020),(cid:2868)ε0 isis tthhee vzearcou-ufimeldp emrmobititliivtyit,y ,anεrd ≈(cid:1846)(cid:3032)(cid:2879)3(cid:3033)(cid:2869)(cid:3033)is=t(cid:1846)h(cid:2879)e(cid:2869)r−ela(cid:1846)t(cid:3008)(cid:2879)i(cid:3036)v(cid:2869)(cid:3039)(cid:3039)e idsi erleelcattreicd ctoon stthaen tGfoilrl
Pte3mHpTe:PraCtBuMre [a3t6 w],hµichis ththee mzeorboi-lfiiteyl dis minodbeiplietyn,daenndt oTf− t1he= elTe−ct1ri−c fTie−ld1. iTshreel bateesdt ftiottitnhge Gcuilrlvteesm, opbetraaitnuerde
0 eff Gill
afrtowmh iEchquthaetimono b(4il)i twyiitshi n(cid:2020)d(cid:2868)e paenndd e(cid:1846)n(cid:3032)(cid:3033)t(cid:3033)o fatsh efreelee cftirttiicnfige lpda.rTahmeebteersst,fi atrtien sghcouwrvne sin,o Fbitgauinreed 5f arosm soEliqdu laitnioesn.
(A4) gwoiotdh µagraenedmTent baestwfreeeenfi ttthine gmpoadrealm aentder tsh,ea reexspheoriwmnenintaFli gduatrae i5s acslesaorlliyd elivniedse.nAt, gthouosd caognrfeiermmeinngt
0 eff
btheetw veaelinditthye omf tohdee tlhaenodretthiceale xfrpaemriemweonrtka lodf Eatqauiasticolena (r4l)y ienv didesecnrti,btihnugs thcoe nefilercmtriinc gtrtahnespvoarlitd oift yorogfatnhiec
tdheevoirceetsi c[a4l5,f4r7a]m. Ienw poarrktiocuflEaqr,u tahteio cnar(r4i)eri ntrdanesscproibrti nwgitthhien etlheec tbrliecntdra oncscpuorrst boyf poargthawniacyds eavloicnegs n[4e5a,r4e7s]t.
Isnitepsa,r tlioccualtaerd,t hoenc athrrei eprotrlyanmsepro ratnwdi tfhuilnletrheenbe lepnhdasoecsc ufrosr btyhep ahthowlesa yasnadlo enlgecnteroanress, trseitsepse,cltoicvaetleyd, obny
tohveeprcoolymminegr aannd efnuelrlgereetnice bpahrariseers. fAonr tahcecuhoraletes aenstdimelaetciotrno nosf, trhees paevcetrivageley ,vbayluoev oefr ctohme hinogpapninegn beragreriteicr
bfoarr rtiheer. eAlenctarcocnusr acaten ebseti mpeartfioornmoefdt hbey atvakerinagge invtaol uaeccoofutnhte thhoep Gpiilnl genbearrgriye rdfeofirntehde aesle (cid:1831)ct(cid:3008)r(cid:3036)o(cid:3039)(cid:3039)n=sc(cid:1863)a(cid:3003)n(cid:1846)(cid:3008)b(cid:3036)e(cid:3039)(cid:3039)
p[2e6r]f.o rTmhee dtebmyptearkaitnugrei ndteopaecncdoeunnctietsh eofG (cid:2020)il(cid:2868)l eannedr goyf d(cid:1831)e(cid:3008)fi(cid:3036)(cid:3039)n(cid:3039) eadrea rseEpGoirltle=d iknB TFGigilulr[e2 66] .aTnhde atreem rpeleartaetdu rteo
dtheepremndale nacctieivsaotfedµ parnocdeossfeEs witahrienr tehpeo ratcetdivien lFayigeur r[e266]a. nAds avriesirbellea tiend Ftioguthreer 6m, athlea cutisvea otef dthper osocelvsseenst
0 Gill
wmiitxhtiunrteh deuarcitnivge tlhaey edre[p2o6s]i.tAiosnv pishiabslee ipnrFoidguucrees6 a, tnheegulisgeibolfet hcheasnoglvee onft m(cid:1831)(cid:3008)i(cid:3036)x(cid:3039)(cid:3039)t,u oref ldeussri nthgatnh e3%de. pMoosirteio inn
pdheatasielsp, rtohde uecsetismaanteedg lriogoibmle tcehmapnegreatoufreE Gvialll,uoef olef s(cid:1831)s(cid:3008)t(cid:3036)h(cid:3039)(cid:3039)a nis 3(cid:4666)%54. ±M1o(cid:4667)r emineVd eatnadil s(cid:4666),5th6e±e1s(cid:4667)ti mmaetVed forro othme
treemfepreenracteu raenvda lfuoer otfhEe oDiCsB(5+4T±HN1) msoelvVeannt,d r(e5s6p±ect1i)vemlye.V Tfohresthe evraelfuereesn caerea ncdonfosirsttheneto DwCitBh+ tThHoNse
Gill
sroeplvoerntet,dr einsp leitcetriavteulyr.e Tfohre sBeHvJa sluoelasra creellcso, ncshiasrteancttewriiztehdt hboys tehree pfoorrmteadtiionnl iotfe raa tnueraet fPo3rHBTH lJasyoelra rucnedllesr,
the cathode during thermally induced aging [26].
The zero-field mobility (left y-axis in Figure 6), obtained by noise measurements at 300 K of
oDCB based devices, is (cid:2020) =(cid:4666)22±1(cid:4667)×10(cid:2879)(cid:2874) cm2·V−1·s−1 and is in good agreement with the value
(cid:2868)
measured by using charge extraction with linearly increasing voltage measurements in similar
samples [8]. On the other hand, the zero-field mobility for the device prepared by using oDCB+THN
solvent is (cid:2020) =(cid:4666)2.4±0.1(cid:4667)×10(cid:2879)(cid:2874) cm2·V−1·s−1, lower than what found by using oDCB as solvent. This
(cid:2868)
is consistent with the observed increased mesoscopic ordering in the blend. As already reported in
literature, the blend casted from a high-boiling-point solvent, as the oDCB+THN, has a higher
polymer crystallinity in the active layer as compared to the blend prepared with a low-boiling-point
solvent, as the oDCB [8,48]. Indeed, the increase of the film ordering induces a vertical phase
separation between polymer and fullerene materials which causes a diffusion out of the PCBM
Energies 2017, 10, 1490 8 of 14
material within the blend [49]. The formation of a polymer rich phase reduces the charge carrier
transfer and, therefore, also the zero-field electron mobility decreases for the blend prepared with the
mixture solvent. However, it is worth noting that the device prepared by using the oDCB+THN
solvent shows a higher photocurrent value and, as a consequence, a higher conversion efficiency as
compared to the solar cell fabricated with the reference solvent only. In polymer:fullerene solar cells,
the photocurrent is due to the light absorption in the active layer with the subsequent generation of
excitons. The use of THN influences the phase segregation within the blend with a negligible effect
Energies2017,10,1490 8of14
on the optical properties. It has already been verified that for the thicknesses of the blends
investigated in this study (115 nm for oDCB and 130 nm for oDCB+THN) only a minimal difference
icnh athraec stheroirzte cdirbcyuitth ceurforernmt adtieonnsiotyf ais noebattaiPn3eHd T[8l]a. yTehreurenfdoerer, tthhee cdaitfhfeordeencdeu irnin tghet h(cid:1836)erm vaallluyeisn cdaunc ebde
(cid:3020)(cid:3004)
faugnindgam[2e6n].tally related to the recombination and transport mechanisms in the solar cell.
, oDCB
10-4 , oDCB+THN 72
69
-1-1Vs)10-5 66meV)
2m 63 (Gill
c E
μ (010-6 60
57
, zero-field mobility μ
, Gill energy E 0 54
Gill
300 310 320 330
T (K)
Figure 6. Temperature dependence of the zero-field mobility (left y-axis) and of the Gill energy (right
Figure 6. Temperature dependence of the zero-field mobility (left y-axis) and of the Gill energy
y-axis). Full and open symbols correspond to oDCB and oDCB+THN solvent, respectively.
(righty-axis).FullandopensymbolscorrespondtooDCBandoDCB+THNsolvent,respectively.
4.1. Solvent Influence on the Charge Carrier Recombination Process
The zero-field mobility (left y-axis in Figure 6), obtained by noise measurements at 300 K of
oDCBIn bsaesmedicdonevdiuccetso,ri smµate=rial(s2 2th±e 1c)h×ar1g0e− 6carcrmie2r ·Vre−c1o·ms−b1inaantdionis pinrogcoeossd iang rteheem beuntlkw iist hwtehlel
0
dvaelsuceribmeeda sbuyr tehdeb Lyaunsgienvginc htharegoerye x[1tr4a]c. tTiohne wasisthocliianteeadr lryecinocmrebainsiantgiovno rlatateg ecamne baes uerxepmreesnstesdi nbysi m(cid:1844)(cid:3013)ila=r
(cid:2010)sa(cid:4666)m(cid:1866)(cid:1868)p−les(cid:1866)[(cid:3036)(cid:2870)8(cid:4667)],. wOhnerthe e(cid:1866)o tahnedr h(cid:1868)a nadre, tehleecztreoron- afinedld hmoloeb ciolintycefnotrrtahtieodnes,v riecesppercetpivaerleyd, abnydu s(cid:1866)i(cid:3036)n giso tDheC iBn+trTinHsNic
csaorlvrieenr tcoisnµcen=tra(t2i.o4n±. T0h.1e) L×an1g0e−v6icnm re2c·Vom−1b·isn−a1t,iolonw faecrtothr ains dwehfiantedfo ausn: d by using oDCB as solvent.
0
Thisisconsistentwiththeobservedincreasedme(cid:1857)soscopicorderingintheblend.Asalreadyreportedin
literature,theblendcastedfromahigh-boi(cid:2010)lin=g-(cid:2013)po(cid:2013)in(cid:4666)t(cid:2020)s(cid:3041)o+lve(cid:2020)n(cid:3035)t(cid:4667), astheoDCB+THN,hasahigherpolym(5)e r
(cid:3045) (cid:2868)
crystallinityintheactivelayerascomparedtotheblendpreparedwithalow-boiling-pointsolvent,
where (cid:1857) is the elementary charge, (cid:2013) (cid:2013) the effective dielectric constant of the ambipolar
(cid:3045) (cid:2868)
as the oDCB [8,48]. Indeed, the increase of the film ordering induces a vertical phase separation
semiconductor, and (cid:2020) and (cid:2020) are the electron and hole mobilities. By assuming that the
(cid:3041) (cid:3035)
betweenpolymerandfullerenematerialswhichcausesadiffusionoutofthePCBMmaterialwithin
recombination processes at the metal electrodes are negligible and by considering that the holes are
the blend [49]. The formation of a polymer rich phase reduces the charge carrier transfer and,
solely transported in the donor polymer phase and electrons through the fullerene acceptor, a
therefore, also the zero-field electron mobility decreases for the blend prepared with the mixture
bimolecular recombination can only take place at the heterojunction. Since (cid:2020) ≫(cid:2020) , Equation (5) can
(cid:3041) (cid:3035)
solvent. However,it(cid:3032)isworthnotingthatthedevicepreparedbyusingtheoDCB+THNsolventshows
be written as (cid:2010) ≈ (cid:2020), where (cid:2020) ≈(cid:2020) is the higher carrier mobility value (the electron one, shown
ahigherphotocurr(cid:3084)e(cid:3293)n(cid:3084)(cid:3116)tvalueand,asa(cid:3041)consequence,ahigherconversionefficiencyascomparedtothe
in Figure 5). At room temperature, (cid:2010) =(cid:4666)17.6±0.4(cid:4667)×10(cid:2879)(cid:2877) cm3·s−1 and (cid:2010) =(cid:4666)4.3±0.1(cid:4667)×10(cid:2879)(cid:2877)
solarcellfabricatedwiththereferencesolventonly. Inpolymer:fullerenesolarcells,thephotocurrent
cm3·s−1 can be computed for the oDCB and oDCB+THN solvent, respectively. These values are in
isduetothelightabsorptionintheactivelayerwiththesubsequentgenerationofexcitons. Theuse
good agreement with typical recombination factors reported in literature for various organic solar
of THN influences the phase segregation within the blend with a negligible effect on the optical
cells [12,50]. As already evidenced, the use of THN reduces the recombination rate at the hetero-
properties. Ithasalreadybeenverifiedthatforthethicknessesoftheblendsinvestigatedinthisstudy
interface between the donor and the acceptor phases, leading to an increase of the carrier lifetime.
(115nmforoDCBand130nmforoDCB+THN)onlyaminimaldifferenceintheshortcircuitcurrent
Conversely, for the reference solvent a higher mobility increases the probability of finding the
densityisobtained[8]. Therefore,thedifferenceinthe J valuescanbefundamentallyrelatedtothe
SC
opposite charge carrier, thus enhancing the charge recombination. This finding is fully consistent
recombinationandtransportmechanismsinthesolarcell.
with the assumption that the bimolecular recombination loss is one of the major device efficiency
l4i.m1.itSinolgv efnatctIonrflsu ienn ctheeo nP3thHeTC:hPaCrgBeMC abrlreinerdR perceopmabrienda twiointhP droicffeessrent solvents. The decrease of (cid:2028) values
as a function of the temperature is expected by the Langevin theory, because lower temperatures
Insemiconductormaterialsthechargecarrierrecombinationprocessinthebulkiswelldescribed
(cid:0) (cid:1)
bytheLangevintheory[14]. TheassociatedrecombinationratecanbeexpressedbyR = β np−n2 ,
L i

where n and p are electron and hole concentrations, respectively, and n is the intrinsic carrier
i
concentration. TheLangevinrecombinationfactorisdefinedas:
e
β = (µ +µ ) (5)
n h
ε ε
r 0
Energies2017,10,1490 9of14
whereeistheelementarycharge,ε ε theeffectivedielectricconstantoftheambipolarsemiconductor,
r 0
andµ andµ aretheelectronandholemobilities. Byassumingthattherecombinationprocessesat
n h
themetalelectrodesarenegligibleandbyconsideringthattheholesaresolelytransportedinthedonor
polymerphaseandelectronsthroughthefullereneacceptor,abimolecularrecombinationcanonlytake
placeattheheterojunction. Sinceµ (cid:29) µ ,Equation(5)canbewrittenas β ≈ e µ,whereµ ≈ µ
n h εrε0 n
is the higher carrier mobility value (the electron one, shown in Figure 5). At room temperature,
β = (17.6±0.4)×10−9 cm3·s−1 andβ = (4.3±0.1)×10−9 cm3·s−1 canbecomputedfortheoDCB
andoDCB+THNsolvent,respectively. Thesevaluesareingoodagreementwithtypicalrecombination
factors reported in literature for various organic solar cells [12,50]. As already evidenced, the use
ofTHNreducestherecombinationrateatthehetero-interfacebetweenthedonorandtheacceptor
phases,leadingtoanincreaseofthecarrierlifetime. Conversely,forthereferencesolventahigher
mobilityincreasestheprobabilityoffindingtheoppositechargecarrier,thusenhancingthecharge
recombination. Thisfindingisfullyconsistentwiththeassumptionthatthebimolecularrecombination
loss is one of the major device efficiency limiting factors in the P3HT:PCBM blend prepared with
differentsolvents.ThedecreaseofτvaluesasafunctionofthetemperatureisexpectedbytheLangevin
theory,becauselowertemperaturesreducethecarriermobilityand,consequently,therecombination
rateR ofthecarriersislowered[13].
L
4.2. SolventInfluenceontheChargeCarrierExtraction
In order to evaluate the charge carrier extraction from the investigated devices,
themobility-lifetimeproductµτhasbeencalculatedandanalyzedasafunctionofthebiascurrent
injectionlevel.Thisfigureofmeritgivesadirectinformationaboutthecompetitionbetweenthecharge
carriertransportandtherecombinationmechanismsintheactivelayer. Bothprocesseshavealarge
impactonthevoltagebiasdependenceofthephotocurrentandhenceontheefficiencyofthesolar
cell[15]. AsevidentinFigure5,thechargemobilityisdirectlyproportionaltothetemperaturefor
boththeusedsolvents. Onthecontrary,Figure4showsthatthechargecarrierlifetimeisinversely
proportionaltothetemperature. AccordingtotheLangevin-typebimolecularrecombinationtheory,
µτshouldbeindependentofthetemperature[13].
InFigure7,thevaluesofµτ,measuredbetween300and330K,fordifferentbiascurrents,andin
darkcondition,arereportedasArrheniusplotsforthereferenceandthemixturesolvents. Ascanbe
observedinFigure7aforthereferencesolvent,theµτproduct(atlowcurrentinjectionlevels,thatis
20µAand30µA)showsatemperature-independentvalueofabout15×10−6 cm2·V−1,consistentwith
thatreportedinliterature[15]. Byincreasingthecurrent,thechargeamountn,accumulatedinthe
device,increasesandthemobility-lifetimeproductbecomestemperaturedependent. Inparticular,
withabiascurrentof50µA,correspondingtoachargedensityof1016 cm−3withintheactivelayer,
the µτ parameter decreases of one order of magnitude reaching a value of about 10−6 cm2·V−1.
This suggests that the charge extraction process is less efficient for the device prepared with the
referencesolvent. Thisisalsosupportedbythelowervalueof J observedforthisdevice(Figure1a).
SC
Theexistenceofadditionalrecombinationpathwaysattheheterojunctionbetweenthedonorandthe
acceptormaterialsleadstoanincreaseofthecarrierrecombinationdynamics. Inliteratureseveral
studiesreportthattherecombinationdynamicsatthehetero-interfacecanbeinfluencedbythepresence
ofthechargeaccumulationandenergeticdisorder[14,51].
On the other hand, the active layer prepared by using THN as additive shows
anapproximatelytemperature-independentbehavioroftheµτproductwithanaveragevalueofabout
20×10−6cm2·V−1 (similartothatobservedfortheoDCB)inthewholeinvestigatedcurrentrange.
The reduction in carrier mobility results in a longer carrier lifetime, as expected in Langevin-type
bimolecularrecombination,wheretherecombinationrate R isdeterminedbythechargevelocity
L
(i.e.,themobility). Thisresultisconsistentwiththeassumptionthattheuseofthehigh-boiling-point
additiveaffectspositivelythemorphologytherebyincreasingthemesoscopicorderinginthedonor
Energies 2017, 10, 1490 9 of 14
reduce the carrier mobility and, consequently, the recombination rate (cid:1844) of the carriers is lowered
(cid:3013)
[13].
4.2. Solvent Influence on the Charge Carrier Extraction
In order to evaluate the charge carrier extraction from the investigated devices, the mobility-
lifetime product (cid:2020)(cid:2028) has been calculated and analyzed as a function of the bias current injection level.
This figure of merit gives a direct information about the competition between the charge carrier
transport and the recombination mechanisms in the active layer. Both processes have a large impact
on the voltage bias dependence of the photocurrent and hence on the efficiency of the solar cell [15].
As evident in Figure 5, the charge mobility is directly proportional to the temperature for both the
used solvents. On the contrary, Figure 4 shows that the charge carrier lifetime is inversely
proportional to the temperature. According to the Langevin-type bimolecular recombination theory,
(cid:2020)(cid:2028) should be independent of the temperature [13].
In Figure 7, the values of (cid:2020)(cid:2028), measured between 300 and 330 K, for different bias currents, and
in dark condition, are reported as Arrhenius plots for the reference and the mixture solvents. As can
be observed in Figure 7a for the reference solvent, the (cid:2020)(cid:2028) product (at low current injection levels,
that is 20 μA and 30 μA) shows a temperature-independent value of about 15×10(cid:2879)(cid:2874) cm2·V−1,
consistent with that reported in literature [15]. By increasing the current, the charge amount (cid:1866),
accumulated in the device, increases and the mobility-lifetime product becomes temperature
dependent. In particular, with a bias current of 50 μA, corresponding to a charge density of 10(cid:2869)(cid:2874)
cm−3 within the active layer, the (cid:2020)(cid:2028) parameter decreases of one order of magnitude reaching a value
of about 10(cid:2879)(cid:2874) cm2·V−1. This suggests that the charge extraction process is less efficient for the device
prepared with the reference solvent. This is also supported by the lower value of (cid:1836) observed for
Energies2017,10,1490 (cid:3020)(cid:3004) 10of14
this device (Figure 1a). The existence of additional recombination pathways at the heterojunction
between the donor and the acceptor materials leads to an increase of the carrier recombination
adnydnaamcciecps.t oInr plihtearsaetsu.rTeh siesvleeraadl ssttuodaieres dreupctoirotn thoaftt htheer reeccoommbbininaatitoionn, adsyenvaimdeincsc eadt tbhye ehnehtearnoc-eindteprhfaascee
sceagnr bege aitnioflnueinnctehde bbyle tnhdes pprreespeanrceed owf tihthe mchiaxregdes aoclvcuenmtus.lation and energetic disorder [14,51].
oDCB oDCB+THN
10-4 10-4
-1V) -1V)
2⋅m10-5 2⋅m10-5
τ (c τ (c
μ μ
10-6 20 μA 10-6 20 μA
30 μA 30 μA
40 μA 40 μA
50 μA 50 μA
0.0030 0.0031 0.0032 0.0033 0.0034 0.0030 0.0031 0.0032 0.0033 0.0034
1/T (K-1) 1/T (K-1)
(a) (b)
Figure 7. Temperature dependence of the mobility-lifetime products for the blend prepared by using:
Figure7.Temperaturedependenceofthemobility-lifetimeproductsfortheblendpreparedbyusing:
(a) oDCB reference solvent; (b) oDCB+THN solvent.
(a)oDCBreferencesolvent;(b)oDCB+THNsolvent.
On the other hand, the active layer prepared by using THN as additive shows an approximately
5. MaterialsandMethods
temperature-independent behavior of the (cid:2020)(cid:2028) product with an average value of about 20×10(cid:2879)(cid:2874)
cm2·VT−o1 (psriempialarer ttoh ethaactt oivbeselarvyeedrs f,o1r, 2th-dei ochDlCorBo)b ienn tzheen weh(ooDleC inBv,ebsotiilginatge-dp ocuinrtre1n8t0 r◦aCng)ew. aTshue sreedduacsttiohne
hino sctasrorilevre nmt,owbihlietyre aress1u,l2t,s3 ,i4n- teat rlaohnygderr ocnaarpriherth laifleetniem(eT, HasN e,xbpoeilcitnegd- pioni nLta2n0g7ev◦iCn)-twypaes tbhiemaodledciutilvaer
(r1e0c%omobfitnhaetieonnti, rewvhoelruem teh)e. Sroelcuotmiobnisncaotinotna inrainteg o(cid:1844)nl yist hdeehteorsmtsionlevde nbtyo DthCeB cwhaerregeu sveedlotocitpyr e(pi.aer.e, tthhee
(cid:3013)
rmefoebreilnitcye).s Tamhips lreess.uSlitn icse ctohnesissotleunbti wlitiytho tfhPe3 aHsTsuamndptPioCnB tMhaitn ththee utswe oofs othlvee hnitgshi-sbsoliiglihntgly-pdoiifnfet raedndti[t5iv2]e,
oafnfeecetxs ppeoctssitaivneelyff etchteo mntohrephoordloegryin gthoefrethbey bilnecnrdeafislimngs .tPhe3 HmTe(ssouspcoppliiecd ofrrdoemrinMge ricnk tCheh edmoincoarls altndd)
aanccdepPtCoBr Mph(apsuesr.c hTahsiesd lefardoms toS oal ernendeucBtVio)nw oefr ethdei srseocolvmedbininatoioDnC, Baso erviindoenDcCedB +bTyH eNnhiannace1d: 1prhaatsioe
isnegwreegigahtitoann idn stthierr beldenadts7 0pr◦eCpoarveedrn wigithht mbeifxoerde suosleve[8n]t.sT. hewholesamplepreparationtookplacein
anitrogenfilledglovebox. Indiumtinoxide(ITO)coatedglasssubstrates(fromPGO)wereusedasthe

anode. Thesubstrateswerepatterned,cleanedinisopropylalcohol,andexposedtoanoxygenplasma.
Alayerofpoly(3,4-ethylenedioxythiopene)(PEDOT):poly(styrenesulfonate)(PSS)(CleviosPVPAl
4083fromH.C.Starck)wasthenspunontopoftheITOanddriedfor10minat180◦C.Theactivelayer
wasspunontopofthePEDOT:PSSlayerandannealedfor10minat150◦C.Thefilmthicknesseswere
determinedusingaVeecoDektak6MProfilmeter,resultinginvaluesof115nmforoDCBand130nm
foroDCB+THN.ThethicknessofthePEDOT:PSSlayerwasabout60nm. Thecathodewasdeposited
bythermalevaporationofCaandAl. Theactiveareaofthedeviceswas0.5cm2. Theinvestigated
devicestructurewiththefollowinglayersequenceglass/ITO/PEDOT:PSS/P3HT:PCBM/Ca/Alis
showninFigure8.
Atomicforcemicroscopy(AFM)analyseswereperformedinaUHV-STM/AFMfromOmicron
withapressurebelow5×10−10mbartoavoidtheeffectsofoxygenandhumidityonthesurfaceof
the sample. Measurements were realized using a controller from Nanonis. Pt/Ir cantilevers from
thecompanyNanosensorswerecalibratedbeforeeachmeasurementusingagoldsinglecrystalwith
111-orientationasareference[8].
To record the J-V characteristics, the devices were illuminated with a KH Steuernagel solar
simulatorprovidingthestandardreferencespectrumAM1.5G.Areferencesiliconsolarcellprovided
byFraunhoferISE(Freiburg,Germany)wasusedforthecalibration. EQEwasmeasuredwithaXe–Hg
tandemlampusinga2gratingmonochromator. Acalibratedsiliconphotodectorwasusedtomonitor
theincidentphotonflux. Thephotocurrentofthedevicewasrecordedbymeansofalock-inamplifier.
Thetemperature-dependentnoiseinvestigationswerecarriedoutbyusingathermoelectriccooler
Peltier-typedevice,withanoperatingrangefrom280to340K.ALM35sensorincontactwiththe
sampleholderwasusedtomeasurethetemperature,whosestabilization,betterthan1K,wasrealized