Research ArticleEnhanced Efficiency of PTB7 PC61BM Organic Solar Cells byAdding a Low Efficient Polymer Donor
Joana Farinhas1 Ricardo Oliveira1 Quirina Ferreira1 Jorge Morgado12 and Ana Charas1
1 Instituto de Telecomunicacoes Instituto Superior Tecnico Av Rovisco Pais 1049-001 Lisboa Portugal2Department of Bioengineering Instituto Superior Tecnico Universidade de Lisboa Av Rovisco Pais 1049-001 Lisboa Portugal
Correspondence should be addressed to Jorge Morgado jorgemorgadolxitpt and Ana Charas anacharaslxitpt
Received 16 September 2016 Accepted 18 December 2016 Published 11 January 2017
Academic Editor Yulia Galagan
Copyright copy 2017 Joana Farinhas et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited
Ternary blend polymer solar cells combining two electron-donor polymers poly[48-bis[(2-ethylhexyl)oxy]benzo[12-b45-b1015840]dithiophene-26-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[34-b]thiophenediyl] (PTB7) and poly[25-bis(3-dodecylthiophen-2-yl)thieno[32-b]thiophene] (pBTTT) and [66]-phenyl-C61-butyric acid methyl ester (PC61BM) as electron-acceptor were fabricated The power conversion efficiency of the ternary cells was enhanced by 18 with respect to the referencebinary cells for a blend composition with 25 (wt) of pBTTT in the polymers content The optimized device performance wasrelated to the blend morphology nonrevealing pBTTT aggregates and improved charge extraction within the device
1 Introduction
Polymer photovoltaic cells have demonstrated great potentialas a cost-effective energy solar technology with uniquecharacteristics of light weight and mechanical flexibility Inthe last decade the power conversion efficiency (PCE) ofsuch cells improved remarkably however the highest valuesare still around 10ndash12 [1ndash5] One of the main causes of therelatively low efficiency of polymer solar cells is incompleteharvesting of solar photons which is due to the relativelynarrow absorption spectrum of the polymer donor and thelow absorption of the fullerene acceptor in the visible spectralrange To overcome such limitations the so-called multi-donor solar cells incorporating multiple organic polymerswith different energy gaps and tandem cells [3] have beenexplored However to this respect multidonor solar cells arethe preferred option since they are single cell devices andtherefore require a simpler fabrication process than tandemcells
Typically ternary bulk heterojunction (BHJ) cells com-bine a predominant donor acceptor (D A) system and athird component whose properties enable to pursuit higherefficiency Thus several types of third materials for severaltasks have been effectively tested as high-band gap polymers
to enlarge the photon absorption window of the cell [6] dyesto harvest photons at longer wavelengths [7] additives tofavour the optimal morphology of the BHJ [8] and cross-linkers to stabilize the device performance [9 10]
In this work ternary blends combining the low-band gappolymer donor PTB7 (119864119892 of ca 16 eV) the fullerene acceptorPC61BM and a second polymer donor pBTTT were investi-gated and tested in BHJ solar cells pBTTT is a semicrystallinepolymer exhibiting remarkably high holemobility (120583FET up to1 cm2Vs [11ndash13]) and absorption spectrum complementaryto that of PTB7 For comparison the reported values for thehole mobility of PTB7 in neat film are several orders of mag-nitude lower in the range of 10minus4ndash10minus3 cm2Vs varying withthe method of measurement [14ndash16] These two propertiespotentially lead to a superior charge transport and enlargedphoton harvesting respectively when pBTTT is added tothe PTB7 fullerene active layer Previously reported studiesof photovoltaic (PV) cells based on pBTTT and fullerenesshowed poor performances with maximum PCE values of 1-2 [17ndash19] probably caused by the high energy gap of pBTTT(119864119892 of ca 19 eV) On the contrary PTB7 is a high performingpolymer in solar cells yielding efficiencies as high as 92with the acceptor PC71BM [20] and ca 4ndash6 with PC61BM[21ndash23] A few ternary solar cells based on PTB7 with a
HindawiInternational Journal of PhotoenergyVolume 2017 Article ID 4501758 8 pageshttpsdoiorg10115520174501758
2 International Journal of Photoenergy
second donor polymer and PC71BM showed efficiencies thatsurpassed the reference binary devices [24ndash29] Neverthelessonly one work is reported on ternary systems based onPTB7 and PC61BM this being with poly(3-hexyl)thiophene(P3HT) as the second polymer and exhibiting only 4 inefficiency [30]
Here five ternary blend compositions were tested main-taining the polymers PC61BM weight ratio as 1 2 and vary-ing the relative polymer content PTB7 pBTTT from 1 9 to9 1 by weight Reference binary blend cells PTB7 PC61BMand pBTTT PC61BM were also fabricated and tested Theeffect of pBTTT addition on the morphology of the cellsrsquoactive layer was investigated by Atomic Force Microscopy(AFM) and fluorescence studies were made to assess the roleof pBTTT
2 Experimental
21Materials andDevice Fabrication pBTTT andPTB7werepurchased fromOssila Ltd PC61BM (9955) was purchasedfrom Solenne BV The devices were prepared on ITO-coated(100 nm thick) glass substrates (ITO indium-tin-oxide)previously cleaned sequentially with distilled water and anonionic detergent distilled water acetone and isopropylalcohol under ultrasounds The ITO surface was submittedto UV-oxygen plasma for 3 minutes prior to spin castingpoly(34-ethylenedioxythiophene) polystyrene sulfonic acid(PEDOT PSS) from aqueous dispersion (Clevios P VPAI4083 from Heraeus) The glassITOPEDOT PSS substrateswere then dried over a hot plate at 125∘C for 10minThe blend solutions for the active layers of the cells wereprepared by mixing adequate volumes of separate solutionscontaining the polymers and PC61BM in 12-dichlorobenzene(DCB) in order to achieve polymers PC61BM mass ratioof 1 2 Five blend compositions with total concentration of40mgsdotmLminus1 containing both polymers and PC61BM wereprepared where PTB7 pBTTT weight ratio was varied from9 1 to 1 9 The starting solutions of the pristine materialshad the same concentration and were stirred for 3 h at75∘C The blend solutions were further stirred for 3 h at75∘C before their deposition by spin coating (1800 rpm 45 s)on either PEDOT PSS-coated quartz substrates (for UV-Visabsorption measurements) or ITOPEDOT PSS substratesFor comparison purposes the binary blends solutions weredeposited under the same conditions The thicknesses of theactive layers varied from ca 80 nm (for binary blend filmsof PTB7 PC61BM and ternary blend films with 50 75and 90 of PTB7) to ca 130 nm (for binary blend films ofpBTTT PC61BM and ternary blend films with 80 and 90of pBTTT) Following the deposition of the active blends LiF(15 nm) and Al (100 nm) were thermally evaporated on topunder a base pressure of 2 times 10minus6mbar defining a device areaof 024 cm2
22 Measurements AFM studies were performed on a NanoObserver from Concept Scientific Instruments (Les UlisFrance) operating in noncontact mode with cantilevershaving a resonance frequency between 200 and 400 kHz
and silicon probes with tip radius smaller than 10 nm Allimages were obtained with 256 times 256 pixels resolutionand processed using Gwyddion (version 226) softwareUV-Vis absorption spectra were recorded on a Cecil 7200spectrophotometer Film thicknesses were measured with aDektak 6M profilometer Fluorescence spectra were acquiredusing a SPEX Fluorolog 212I collecting the emission at aright angle arrangement in the RS mode All fluorescencespectra were corrected for the wavelength response of theinstrumental systemThe current-voltage (119868-119881) curves of thecells were measured under inert atmosphere (N2) using aK2400 source-measure unit The curves under illuminationwere measured with a solar simulator with 100mWcm2AM15G illumination (Oriel Sol 3A 69920Newport) At least16 devices of each conditionwere preparedThe light intensityof the solar simulator was verified using a calibrated solar cellExternal quantum efficiency (EQE) spectra were obtainedunder short-circuit conditions using a homemade systemwith a halogen lamp as light source
3 Results and Discussion
31 Photophysical Properties Figure 1 shows the UV-Visibleabsorption and photoluminescence spectra of spin cast filmsof PTB7 pBTTT and PC61BM on quartz substrates and therelative position of their HOMO and LUMO levels [17 20 31]with respect to the work functions of the electrodes of thefabricated cells
The absorption spectrum of pBTTT shows a band whichis blue-shifted (maximum at 535 nm) with respect to themain absorption band of PTB7 (maximum at 675 nm) thuspotentially providing complementary photon absorption forthe ternary cells comprising the two polymers Since theemission of pBTTT overlaps the absorption band of PTB7pBTTT may also sensitize PTB7 via excited state energytransfer The energy diagram for the ternary cells shows alaquocascaderaquo of energy levels where several pathways for thecharge transfer are energetically favourable the photoexcitedpBTTT may transfer electrons to both fullerene (PC61BM)and PTB7 and the photoexcited PTB7 can transfer electronsto PC61BM Thus in other words both polymers can act aselectron-donors with respect to PC61BM contributing to thecharge generation at the pBTTTPC61BMandPTB7PC61BMinterfaces However it is not clear if the small mismatchof 02 eV between the LUMO energies of the two polymersis sufficient to promote the dissociation of the excitonsgenerated within pBTTT at its interface with PTB7
We have carried out photoluminescence studies on filmsof the ternary blends and also on films of blends composedof the two polymers only The films were always in thesame position in the spectrofluorometer cavity to minimizeorientation effects Films of the PTB7 pBTTTblendswithoutPC61BM were excited at the pBTTT absorption maximum(ca 540 nm) The obtained fluorescence spectra showed noevidence of pBTTT emission being only observed PTB7fluorescence (see Figure S1 in Supplementary Material avail-able online at httpsdoiorg10115520174501758) Figure 2shows the PL spectrum when only 10 of PTB7 is presentThe absence of pBTTT emission indicates that excited state
International Journal of Photoenergy 3
S
SOR
ORS
S F
ORO
n
PTB7
S
S
S
S
n
pBTTTR = -CH2CH(C2H5)C4H9
C16H33
C16H33
(a)
Nor
mal
PL
400 500 600 700 800300Wavelength (nm)
pBTTT
PC61BMPC61BM PL PTB7
pBTTT PL
PTB7 PL
00
02
04
06
08
10
12
Nor
mal
abs
orba
nce
00
02
04
06
08
10
12
(b)
LiFAl
ITOPEDOT PSS
PTB7pBTTT LUMO
HOMO
E (eV)eminus
eminus eminus
eminusminus31 eV
minus36 eV
minus391 eVminus37 eV
minus51 eV minus51 eVminus52 eV
minus593 eV
PC61BM
h+
h+
(c)
Figure 1 (a) Chemical structures of the two polymers PTB7 and pBTTT (b) Normalized UV-Visible absorption spectra (filled symbols) offilms of PC61BM pBTTT and PTB7 and corresponding PL spectra (open symbols) recorded upon excitation at 330 nm (PC61BM) 535 nm(pBTTT) and 675 nm (PTB7) and (c) energy diagram of the ternary blend cells
energy transfer andor charge transfer from pBTTT to PTB7as previewed by the optical properties and the frontierenergy levels of the polymers should effectively contributeto the decay of excitons photogenerated in pBTTT althoughthe relative importance of the two mechanisms cannot beinferred from these studies It should be mentioned thatPTB7 is also absorbed at 540 nm thus the recorded emissionresults from the PTB7 direct excitation and possible energytransfer from pBTTT We also observe that when PC61BM isadded (reproducing the composition of the solar cells activelayer) the PTB7 emission is quenched which should be likelyattributed to exciton dissociation at PTB7PC61BM interface
32 Photovoltaic Cells The photovoltaic cells had the ITOPEDOT PSSactive layerLiFAl general structure where theactive layer is made of either the reference binary blendsPTB7 PC61BM and pBTTT PC61BM or the ternary layersbased on PTB7 pBTTT PC61BM The cells performanceparameters and the representative J-V curves obtained underillumination conditions (119875in = 100mWsdotcmminus2) are shown inTable 1 and Figure 3 respectively (J-V curvesmeasured underdark conditions are shown in SupplementaryMaterial FigureS2)
Regarding the pBTTT PC61BM binary blend devicesthe found PCE of 057 and 119881OC of 05 V little surpass thereported values (ca 05 and 045V resp) while 119869SC andFF are similar [17] It should be noted that according toreported works [18 19] the blend composition 1 2 is notthe most efficient one for this system being the highestperformances achieved for the 1 4 ratio using either PC61BMor PC71BM This enhancement in cells performance withhigher PCBM content should be related to the presenceof relatively pure PCBM phases which provide continuousconductive pathways for the electrons [19] In this work the1 4 blend composition was also tested resulting in deviceswith PCE of 107 119881OC of 051 V 119869SC of 474mAsdotcmminus2 andFF of 044 Nevertheless the control devices were based on1 2 blends in order to keep the mass ratio of D A constantin both binary and ternary devices Taking PTB7 PC61BMas the reference system we find that the partial replacementof PTB7 by pBTTT leads to a decrease in PCE with theexception of the composition 075 025 (PTB7 pBTTT)which leads to the best performing cell The reached PCEvalue (473) corresponds to an improvement of ca 18over the binary cells The maximum PCE obtained 472surpasses the reported values for ternary systems based onPTB7 and PC61BM [30] We also observe that on the other
4 International Journal of Photoenergy
Table 1 Performance parameters of representative curves and maxima PCE values of the OPV cells
hand all ternary cells show higher PCE than the cell basedon pBTTT PC61BM
Comparing the parameters characterizing the best per-forming ternary cell with those of the binary cell ofPTB7 PC61BM we find a slight increase of the short-circuitcurrentHowever it is the fill factor that improves significantly(from 040 to 050) This enlarged FF indicates that chargetransport andor charge collectionwere improved in the cellsIn fact the calculated values for 119877sh and 119877119904 for such cell arethemost favourable within the series that is119877sh is maximumand 119877119904 is minimum thus indicating that recombinationpathways for the charges are minimized and charge transporttowards the electrodes is facilitated respectively The similarvalues of 119869SC for the two most efficient cells the ternarycell with 075 025 2 composition and the binary cell ofPTB7 PC61BM being 1317mAsdotcmminus2 and 1325mAsdotcmminus2respectively suggest that photon-to-charge conversion effi-ciency was only a little improved Devices with a content ofpBTTT higher than 25 showed the lowest currents and thepoorest performances probably due to energy losses causedby the high energy band gap of pBTTT and its consequentlittle contribution to photon absorption Along the series119881OC variesmonotonically with the blend composition (exceptfor the 02 08 2 case with a 119881OC of 054 that is slightlylower than the 055V for the 010 090 2) The variationof 119881OC with the blend composition in ternary blend cellshas been rationalised in terms of an alloy model in whichthe donoracceptor interface and corresponding interfaceband gap (charge transfer state) display a material averagedelectronic structure due to the delocalized nature of the oneelectron states [32]
The external quantum efficiency (EQE) spectra of thecells and the UV-Visible absorption spectra obtained for therespective active layers are shown in Figure 4
We observe that the cells with the pBTTT binary blendand with the ternary blends with 90 and 80wt of pBTTTexhibit EQE spectra with a maximum at ca 550 nm and withvibronic structure that mimics the UV-Visible absorptionspectra of the respective blends Such vibronic structure is notevidenced in the neat pBTTT film absorption spectrum (asshown in Figure 2) being likely attributed to ordering effectsinduced by the intermixing with the fullerene In particularseveral studies have demonstrated that pBTTT and PC61BMin blend films form cocrystals of approximately equal content
Figure 2 Photoluminescence spectra of films of the PTB7 pBTTT(01 09) blend and of the same blend upon addition of PC61BMdeposited over quartz substrates following excitation at the maxi-mum of absorption of pBTTT For comparison the spectrum of thebare quartz is also shown
of polymer and fullerene which result in vibronic resolutionin the absorption spectra [33 34] The presence of suchcocrystals in the 1 2 binary blend may justify the lowerefficiency of the corresponding cells in comparison withthe tested 1 4 pBTTT PC61BM cells since less percolationpathways for electrons should be formedThe ternary deviceswith the highest efficiency (075 025 2) do not seem togain significantly from the presence of pBTTT since itsEQE spectrum is very close to that of the binary cells withPTB7 PC61BM This result is in agreement with the similarvalues of 119869SC obtained for such cells In fact it is the fill factorthat mainly causes the PCE enhancement with respect to thebinary PTB7 PC61BM cell when pBTTT is 25 of the totalof polymers
In view of the energy band diagram shown in Figure 1cells with active layers composed of the two polymers only
International Journal of Photoenergy 5Cu
rren
t den
sity
(mAmiddotcm
2)
02 04 06 0800Voltage (V)
minus12
minus8
minus4
0
PTB7 pBTTT PCBM(0 1 2)(01 09 2)(02 08 2)(05 05 2)
(075 025 2)(09 01 2)(1 0 2)
Figure 3 Current density-voltage (J-V) curves under AM 15Gillumination (100mWsdotcmminus2) of PV cells with active layers ofPTB7 PC61BM (1 2) pBTTT PC61BM (1 2) and five ternaryblend active layers with different PTB7 pBTTT weight ratios
(without PC61BM) were also fabricated in order to evaluatethe role of PC61BM in the cells and the importance ofcharge transfer at pBTTTPTB7 interfacesHence three typesof devices with the ITOPEDOT PSSactive layerLiFAlstructure were fabricated with 01 09 05 05 or 09 01(ww) of PTB7 pBTTT blendsThe J-V curves of the devicesmeasured under the same conditions of illumination thanthose used for the ternary blend cells exhibited 119869SC valuesrather low at the order of 10minus2mAcm2 (shown in Figure S3 inSupplementaryMaterial)This confirms that PC61BM shouldbe the main acceptor in the fabricated ternary blend cells andalso suggests that if exciton dissociation between pBTTT andPTB7 interfaces exists it should be an inefficient process
33 Morphologic Characterization of Active Blends Figure 5shows the AFM topography and phase images of the ref-erence binary blends and of the most efficient cell withthe ternary 075 025 2 (PTBT pBTTT PC61BM) blendThe AFM images for the other ternary cells are shown inSupplementary Material (Figure S4) The variation of thesurface roughness 119877rms for all the seven films as a functionof the blend composition is represented in Figure 5(d)
The binary PTB7 PC61BM blend shows a smooth andrelatively homogeneous surface without evidencing phasesegregated domains at the surface However the topographyimage of the binary pBTTT PC61BM blend reveals a morestructured surface and amuch higher 119877rms (ca 137 nm) thussuggesting the presence of segregated domains of one of thematerials Since the corresponding phase image is relativelywell homogeneous such domains should exist within thebulk of the blend film The AFM images of the ternary
blend films with the highest contents of pBTTT (90 and80) show also similar domains (see Figure S3) The averagedimensions of such segregated domains can be estimatedfromprofile lines acquired from theAFMtopographic images(Figure S5 in Supplementary Material) These are at theorder of ca 20 nm thus indicating a lower degree of mixingbetween the polymer pBTTT and PCBM in comparison withthe PTB7 PC61BM binary blend films where as mentionedthe smooth filmsrsquo surface does not evidence aggregates
The 119877rms calculated for the various films as a functionof the pBTTT content shows that the 119877rms values increasewith the pBTTT content although a slightly lower value isfound when pBTTT is the sole polymer within the blend Wetherefore suggest that aggregates of pBTTT have a negativeeffect on the pBTTT contribution to charge extraction thisleading to the lower fill factors found for the cells with higherpBTTT content On the other hand the addition of pBTTT inlow contents (up to 50) leading to rather smooth surfacesshould be related to an increase of the fill factors due toa positive contribution of pBTTT to hole extraction (sincepBTTT is a good hole transport polymer) within the activeblends When pBTTT becomes the dominant polymer FFdecreases a behavior that is accompanied by an increaseof the surface roughness These lower FF values obtainedwhen pBTTT is 90 or 80 may be related to the presence ofPTB7 in low contents that disrupts the favourable (to chargetransport) pBTTT PCBM phase containing cocrystals andPCBM extended phases The above-mentioned observationof vibronic structure on the pBTTT absorption and EQE isalso consistent with such ordering in the blends when pBTTTis the dominant polymer
4 Conclusions
Ternary BHJ solar cells based on PTB7 pBTTT PC61BMwith improved PCE were demonstrated The cells charac-teristics showed an enhancement in PCE for the ternaryblends with 25 of pBTTT (wt in polymers) of ca 18comparing with reference binary cells yielding a PCE of472The observed improvement was related to the optimalbalance between the two polymers providing higher FFvalues and the absence of aggregates of pBTTT within theternary blends Also according to PL steady state studiesenergy andor charge transfer processes from excited pBTTTto PTB7 should occur in the active layers of the ternary blendcells
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
This work was supported by Fundacao para a Ciencia eTecnologia (FCT) under Projects M-ERANET00012012and UIDEEA500082013
6 International Journal of Photoenergy
400 500 600 700 800300Wavelength (nm)
04
08Ab
sorb
ance
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(a)
400 500 600 700 800300Wavelength (nm)
0
20
40
EQE
()
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(b)
Figure 4 (a) UV-Visible absorption spectra of D A binary blends and ternary blends deposited onto quartzPEDOT PSS substrates (b)EQE spectra of the corresponding solar cells
Topography 214 nm
0
Phase
(a) PTB7 PC61BM (1 2)
89 nm
0
Topography Phase
(b) pBTTT PC61BM (1 2)
342 nm
0
Topography Phase
(c) PTB7 pBTTT PC61BM (075 025 2)
20 40 60 80 1000 pBTTT
00
03
06
09
12
15
Rrm
s(n
m)
(d)
Figure 5 AFM topographic and phase images (1 times 1 120583m2) of (a) PTB7 PC61BM (1 2) (b) pBTTT PC61BM (1 2) (c) ternary blend filmsof PTB7 pBTTT PC61BM with 25wt of pBTTT replacing PTB7 All the films were prepared onto glassITOPEDOT PSS substrates (d)119877rms of the various films as a function of pBTTT content (wt)
International Journal of Photoenergy 7
References
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
second donor polymer and PC71BM showed efficiencies thatsurpassed the reference binary devices [24ndash29] Neverthelessonly one work is reported on ternary systems based onPTB7 and PC61BM this being with poly(3-hexyl)thiophene(P3HT) as the second polymer and exhibiting only 4 inefficiency [30]
Here five ternary blend compositions were tested main-taining the polymers PC61BM weight ratio as 1 2 and vary-ing the relative polymer content PTB7 pBTTT from 1 9 to9 1 by weight Reference binary blend cells PTB7 PC61BMand pBTTT PC61BM were also fabricated and tested Theeffect of pBTTT addition on the morphology of the cellsrsquoactive layer was investigated by Atomic Force Microscopy(AFM) and fluorescence studies were made to assess the roleof pBTTT
2 Experimental
21Materials andDevice Fabrication pBTTT andPTB7werepurchased fromOssila Ltd PC61BM (9955) was purchasedfrom Solenne BV The devices were prepared on ITO-coated(100 nm thick) glass substrates (ITO indium-tin-oxide)previously cleaned sequentially with distilled water and anonionic detergent distilled water acetone and isopropylalcohol under ultrasounds The ITO surface was submittedto UV-oxygen plasma for 3 minutes prior to spin castingpoly(34-ethylenedioxythiophene) polystyrene sulfonic acid(PEDOT PSS) from aqueous dispersion (Clevios P VPAI4083 from Heraeus) The glassITOPEDOT PSS substrateswere then dried over a hot plate at 125∘C for 10minThe blend solutions for the active layers of the cells wereprepared by mixing adequate volumes of separate solutionscontaining the polymers and PC61BM in 12-dichlorobenzene(DCB) in order to achieve polymers PC61BM mass ratioof 1 2 Five blend compositions with total concentration of40mgsdotmLminus1 containing both polymers and PC61BM wereprepared where PTB7 pBTTT weight ratio was varied from9 1 to 1 9 The starting solutions of the pristine materialshad the same concentration and were stirred for 3 h at75∘C The blend solutions were further stirred for 3 h at75∘C before their deposition by spin coating (1800 rpm 45 s)on either PEDOT PSS-coated quartz substrates (for UV-Visabsorption measurements) or ITOPEDOT PSS substratesFor comparison purposes the binary blends solutions weredeposited under the same conditions The thicknesses of theactive layers varied from ca 80 nm (for binary blend filmsof PTB7 PC61BM and ternary blend films with 50 75and 90 of PTB7) to ca 130 nm (for binary blend films ofpBTTT PC61BM and ternary blend films with 80 and 90of pBTTT) Following the deposition of the active blends LiF(15 nm) and Al (100 nm) were thermally evaporated on topunder a base pressure of 2 times 10minus6mbar defining a device areaof 024 cm2
22 Measurements AFM studies were performed on a NanoObserver from Concept Scientific Instruments (Les UlisFrance) operating in noncontact mode with cantilevershaving a resonance frequency between 200 and 400 kHz
and silicon probes with tip radius smaller than 10 nm Allimages were obtained with 256 times 256 pixels resolutionand processed using Gwyddion (version 226) softwareUV-Vis absorption spectra were recorded on a Cecil 7200spectrophotometer Film thicknesses were measured with aDektak 6M profilometer Fluorescence spectra were acquiredusing a SPEX Fluorolog 212I collecting the emission at aright angle arrangement in the RS mode All fluorescencespectra were corrected for the wavelength response of theinstrumental systemThe current-voltage (119868-119881) curves of thecells were measured under inert atmosphere (N2) using aK2400 source-measure unit The curves under illuminationwere measured with a solar simulator with 100mWcm2AM15G illumination (Oriel Sol 3A 69920Newport) At least16 devices of each conditionwere preparedThe light intensityof the solar simulator was verified using a calibrated solar cellExternal quantum efficiency (EQE) spectra were obtainedunder short-circuit conditions using a homemade systemwith a halogen lamp as light source
3 Results and Discussion
31 Photophysical Properties Figure 1 shows the UV-Visibleabsorption and photoluminescence spectra of spin cast filmsof PTB7 pBTTT and PC61BM on quartz substrates and therelative position of their HOMO and LUMO levels [17 20 31]with respect to the work functions of the electrodes of thefabricated cells
The absorption spectrum of pBTTT shows a band whichis blue-shifted (maximum at 535 nm) with respect to themain absorption band of PTB7 (maximum at 675 nm) thuspotentially providing complementary photon absorption forthe ternary cells comprising the two polymers Since theemission of pBTTT overlaps the absorption band of PTB7pBTTT may also sensitize PTB7 via excited state energytransfer The energy diagram for the ternary cells shows alaquocascaderaquo of energy levels where several pathways for thecharge transfer are energetically favourable the photoexcitedpBTTT may transfer electrons to both fullerene (PC61BM)and PTB7 and the photoexcited PTB7 can transfer electronsto PC61BM Thus in other words both polymers can act aselectron-donors with respect to PC61BM contributing to thecharge generation at the pBTTTPC61BMandPTB7PC61BMinterfaces However it is not clear if the small mismatchof 02 eV between the LUMO energies of the two polymersis sufficient to promote the dissociation of the excitonsgenerated within pBTTT at its interface with PTB7
We have carried out photoluminescence studies on filmsof the ternary blends and also on films of blends composedof the two polymers only The films were always in thesame position in the spectrofluorometer cavity to minimizeorientation effects Films of the PTB7 pBTTTblendswithoutPC61BM were excited at the pBTTT absorption maximum(ca 540 nm) The obtained fluorescence spectra showed noevidence of pBTTT emission being only observed PTB7fluorescence (see Figure S1 in Supplementary Material avail-able online at httpsdoiorg10115520174501758) Figure 2shows the PL spectrum when only 10 of PTB7 is presentThe absence of pBTTT emission indicates that excited state
International Journal of Photoenergy 3
S
SOR
ORS
S F
ORO
n
PTB7
S
S
S
S
n
pBTTTR = -CH2CH(C2H5)C4H9
C16H33
C16H33
(a)
Nor
mal
PL
400 500 600 700 800300Wavelength (nm)
pBTTT
PC61BMPC61BM PL PTB7
pBTTT PL
PTB7 PL
00
02
04
06
08
10
12
Nor
mal
abs
orba
nce
00
02
04
06
08
10
12
(b)
LiFAl
ITOPEDOT PSS
PTB7pBTTT LUMO
HOMO
E (eV)eminus
eminus eminus
eminusminus31 eV
minus36 eV
minus391 eVminus37 eV
minus51 eV minus51 eVminus52 eV
minus593 eV
PC61BM
h+
h+
(c)
Figure 1 (a) Chemical structures of the two polymers PTB7 and pBTTT (b) Normalized UV-Visible absorption spectra (filled symbols) offilms of PC61BM pBTTT and PTB7 and corresponding PL spectra (open symbols) recorded upon excitation at 330 nm (PC61BM) 535 nm(pBTTT) and 675 nm (PTB7) and (c) energy diagram of the ternary blend cells
energy transfer andor charge transfer from pBTTT to PTB7as previewed by the optical properties and the frontierenergy levels of the polymers should effectively contributeto the decay of excitons photogenerated in pBTTT althoughthe relative importance of the two mechanisms cannot beinferred from these studies It should be mentioned thatPTB7 is also absorbed at 540 nm thus the recorded emissionresults from the PTB7 direct excitation and possible energytransfer from pBTTT We also observe that when PC61BM isadded (reproducing the composition of the solar cells activelayer) the PTB7 emission is quenched which should be likelyattributed to exciton dissociation at PTB7PC61BM interface
32 Photovoltaic Cells The photovoltaic cells had the ITOPEDOT PSSactive layerLiFAl general structure where theactive layer is made of either the reference binary blendsPTB7 PC61BM and pBTTT PC61BM or the ternary layersbased on PTB7 pBTTT PC61BM The cells performanceparameters and the representative J-V curves obtained underillumination conditions (119875in = 100mWsdotcmminus2) are shown inTable 1 and Figure 3 respectively (J-V curvesmeasured underdark conditions are shown in SupplementaryMaterial FigureS2)
Regarding the pBTTT PC61BM binary blend devicesthe found PCE of 057 and 119881OC of 05 V little surpass thereported values (ca 05 and 045V resp) while 119869SC andFF are similar [17] It should be noted that according toreported works [18 19] the blend composition 1 2 is notthe most efficient one for this system being the highestperformances achieved for the 1 4 ratio using either PC61BMor PC71BM This enhancement in cells performance withhigher PCBM content should be related to the presenceof relatively pure PCBM phases which provide continuousconductive pathways for the electrons [19] In this work the1 4 blend composition was also tested resulting in deviceswith PCE of 107 119881OC of 051 V 119869SC of 474mAsdotcmminus2 andFF of 044 Nevertheless the control devices were based on1 2 blends in order to keep the mass ratio of D A constantin both binary and ternary devices Taking PTB7 PC61BMas the reference system we find that the partial replacementof PTB7 by pBTTT leads to a decrease in PCE with theexception of the composition 075 025 (PTB7 pBTTT)which leads to the best performing cell The reached PCEvalue (473) corresponds to an improvement of ca 18over the binary cells The maximum PCE obtained 472surpasses the reported values for ternary systems based onPTB7 and PC61BM [30] We also observe that on the other
4 International Journal of Photoenergy
Table 1 Performance parameters of representative curves and maxima PCE values of the OPV cells
hand all ternary cells show higher PCE than the cell basedon pBTTT PC61BM
Comparing the parameters characterizing the best per-forming ternary cell with those of the binary cell ofPTB7 PC61BM we find a slight increase of the short-circuitcurrentHowever it is the fill factor that improves significantly(from 040 to 050) This enlarged FF indicates that chargetransport andor charge collectionwere improved in the cellsIn fact the calculated values for 119877sh and 119877119904 for such cell arethemost favourable within the series that is119877sh is maximumand 119877119904 is minimum thus indicating that recombinationpathways for the charges are minimized and charge transporttowards the electrodes is facilitated respectively The similarvalues of 119869SC for the two most efficient cells the ternarycell with 075 025 2 composition and the binary cell ofPTB7 PC61BM being 1317mAsdotcmminus2 and 1325mAsdotcmminus2respectively suggest that photon-to-charge conversion effi-ciency was only a little improved Devices with a content ofpBTTT higher than 25 showed the lowest currents and thepoorest performances probably due to energy losses causedby the high energy band gap of pBTTT and its consequentlittle contribution to photon absorption Along the series119881OC variesmonotonically with the blend composition (exceptfor the 02 08 2 case with a 119881OC of 054 that is slightlylower than the 055V for the 010 090 2) The variationof 119881OC with the blend composition in ternary blend cellshas been rationalised in terms of an alloy model in whichthe donoracceptor interface and corresponding interfaceband gap (charge transfer state) display a material averagedelectronic structure due to the delocalized nature of the oneelectron states [32]
The external quantum efficiency (EQE) spectra of thecells and the UV-Visible absorption spectra obtained for therespective active layers are shown in Figure 4
We observe that the cells with the pBTTT binary blendand with the ternary blends with 90 and 80wt of pBTTTexhibit EQE spectra with a maximum at ca 550 nm and withvibronic structure that mimics the UV-Visible absorptionspectra of the respective blends Such vibronic structure is notevidenced in the neat pBTTT film absorption spectrum (asshown in Figure 2) being likely attributed to ordering effectsinduced by the intermixing with the fullerene In particularseveral studies have demonstrated that pBTTT and PC61BMin blend films form cocrystals of approximately equal content
Figure 2 Photoluminescence spectra of films of the PTB7 pBTTT(01 09) blend and of the same blend upon addition of PC61BMdeposited over quartz substrates following excitation at the maxi-mum of absorption of pBTTT For comparison the spectrum of thebare quartz is also shown
of polymer and fullerene which result in vibronic resolutionin the absorption spectra [33 34] The presence of suchcocrystals in the 1 2 binary blend may justify the lowerefficiency of the corresponding cells in comparison withthe tested 1 4 pBTTT PC61BM cells since less percolationpathways for electrons should be formedThe ternary deviceswith the highest efficiency (075 025 2) do not seem togain significantly from the presence of pBTTT since itsEQE spectrum is very close to that of the binary cells withPTB7 PC61BM This result is in agreement with the similarvalues of 119869SC obtained for such cells In fact it is the fill factorthat mainly causes the PCE enhancement with respect to thebinary PTB7 PC61BM cell when pBTTT is 25 of the totalof polymers
In view of the energy band diagram shown in Figure 1cells with active layers composed of the two polymers only
International Journal of Photoenergy 5Cu
rren
t den
sity
(mAmiddotcm
2)
02 04 06 0800Voltage (V)
minus12
minus8
minus4
0
PTB7 pBTTT PCBM(0 1 2)(01 09 2)(02 08 2)(05 05 2)
(075 025 2)(09 01 2)(1 0 2)
Figure 3 Current density-voltage (J-V) curves under AM 15Gillumination (100mWsdotcmminus2) of PV cells with active layers ofPTB7 PC61BM (1 2) pBTTT PC61BM (1 2) and five ternaryblend active layers with different PTB7 pBTTT weight ratios
(without PC61BM) were also fabricated in order to evaluatethe role of PC61BM in the cells and the importance ofcharge transfer at pBTTTPTB7 interfacesHence three typesof devices with the ITOPEDOT PSSactive layerLiFAlstructure were fabricated with 01 09 05 05 or 09 01(ww) of PTB7 pBTTT blendsThe J-V curves of the devicesmeasured under the same conditions of illumination thanthose used for the ternary blend cells exhibited 119869SC valuesrather low at the order of 10minus2mAcm2 (shown in Figure S3 inSupplementaryMaterial)This confirms that PC61BM shouldbe the main acceptor in the fabricated ternary blend cells andalso suggests that if exciton dissociation between pBTTT andPTB7 interfaces exists it should be an inefficient process
33 Morphologic Characterization of Active Blends Figure 5shows the AFM topography and phase images of the ref-erence binary blends and of the most efficient cell withthe ternary 075 025 2 (PTBT pBTTT PC61BM) blendThe AFM images for the other ternary cells are shown inSupplementary Material (Figure S4) The variation of thesurface roughness 119877rms for all the seven films as a functionof the blend composition is represented in Figure 5(d)
The binary PTB7 PC61BM blend shows a smooth andrelatively homogeneous surface without evidencing phasesegregated domains at the surface However the topographyimage of the binary pBTTT PC61BM blend reveals a morestructured surface and amuch higher 119877rms (ca 137 nm) thussuggesting the presence of segregated domains of one of thematerials Since the corresponding phase image is relativelywell homogeneous such domains should exist within thebulk of the blend film The AFM images of the ternary
blend films with the highest contents of pBTTT (90 and80) show also similar domains (see Figure S3) The averagedimensions of such segregated domains can be estimatedfromprofile lines acquired from theAFMtopographic images(Figure S5 in Supplementary Material) These are at theorder of ca 20 nm thus indicating a lower degree of mixingbetween the polymer pBTTT and PCBM in comparison withthe PTB7 PC61BM binary blend films where as mentionedthe smooth filmsrsquo surface does not evidence aggregates
The 119877rms calculated for the various films as a functionof the pBTTT content shows that the 119877rms values increasewith the pBTTT content although a slightly lower value isfound when pBTTT is the sole polymer within the blend Wetherefore suggest that aggregates of pBTTT have a negativeeffect on the pBTTT contribution to charge extraction thisleading to the lower fill factors found for the cells with higherpBTTT content On the other hand the addition of pBTTT inlow contents (up to 50) leading to rather smooth surfacesshould be related to an increase of the fill factors due toa positive contribution of pBTTT to hole extraction (sincepBTTT is a good hole transport polymer) within the activeblends When pBTTT becomes the dominant polymer FFdecreases a behavior that is accompanied by an increaseof the surface roughness These lower FF values obtainedwhen pBTTT is 90 or 80 may be related to the presence ofPTB7 in low contents that disrupts the favourable (to chargetransport) pBTTT PCBM phase containing cocrystals andPCBM extended phases The above-mentioned observationof vibronic structure on the pBTTT absorption and EQE isalso consistent with such ordering in the blends when pBTTTis the dominant polymer
4 Conclusions
Ternary BHJ solar cells based on PTB7 pBTTT PC61BMwith improved PCE were demonstrated The cells charac-teristics showed an enhancement in PCE for the ternaryblends with 25 of pBTTT (wt in polymers) of ca 18comparing with reference binary cells yielding a PCE of472The observed improvement was related to the optimalbalance between the two polymers providing higher FFvalues and the absence of aggregates of pBTTT within theternary blends Also according to PL steady state studiesenergy andor charge transfer processes from excited pBTTTto PTB7 should occur in the active layers of the ternary blendcells
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
This work was supported by Fundacao para a Ciencia eTecnologia (FCT) under Projects M-ERANET00012012and UIDEEA500082013
6 International Journal of Photoenergy
400 500 600 700 800300Wavelength (nm)
04
08Ab
sorb
ance
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(a)
400 500 600 700 800300Wavelength (nm)
0
20
40
EQE
()
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(b)
Figure 4 (a) UV-Visible absorption spectra of D A binary blends and ternary blends deposited onto quartzPEDOT PSS substrates (b)EQE spectra of the corresponding solar cells
Topography 214 nm
0
Phase
(a) PTB7 PC61BM (1 2)
89 nm
0
Topography Phase
(b) pBTTT PC61BM (1 2)
342 nm
0
Topography Phase
(c) PTB7 pBTTT PC61BM (075 025 2)
20 40 60 80 1000 pBTTT
00
03
06
09
12
15
Rrm
s(n
m)
(d)
Figure 5 AFM topographic and phase images (1 times 1 120583m2) of (a) PTB7 PC61BM (1 2) (b) pBTTT PC61BM (1 2) (c) ternary blend filmsof PTB7 pBTTT PC61BM with 25wt of pBTTT replacing PTB7 All the films were prepared onto glassITOPEDOT PSS substrates (d)119877rms of the various films as a function of pBTTT content (wt)
International Journal of Photoenergy 7
References
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
Figure 1 (a) Chemical structures of the two polymers PTB7 and pBTTT (b) Normalized UV-Visible absorption spectra (filled symbols) offilms of PC61BM pBTTT and PTB7 and corresponding PL spectra (open symbols) recorded upon excitation at 330 nm (PC61BM) 535 nm(pBTTT) and 675 nm (PTB7) and (c) energy diagram of the ternary blend cells
energy transfer andor charge transfer from pBTTT to PTB7as previewed by the optical properties and the frontierenergy levels of the polymers should effectively contributeto the decay of excitons photogenerated in pBTTT althoughthe relative importance of the two mechanisms cannot beinferred from these studies It should be mentioned thatPTB7 is also absorbed at 540 nm thus the recorded emissionresults from the PTB7 direct excitation and possible energytransfer from pBTTT We also observe that when PC61BM isadded (reproducing the composition of the solar cells activelayer) the PTB7 emission is quenched which should be likelyattributed to exciton dissociation at PTB7PC61BM interface
32 Photovoltaic Cells The photovoltaic cells had the ITOPEDOT PSSactive layerLiFAl general structure where theactive layer is made of either the reference binary blendsPTB7 PC61BM and pBTTT PC61BM or the ternary layersbased on PTB7 pBTTT PC61BM The cells performanceparameters and the representative J-V curves obtained underillumination conditions (119875in = 100mWsdotcmminus2) are shown inTable 1 and Figure 3 respectively (J-V curvesmeasured underdark conditions are shown in SupplementaryMaterial FigureS2)
Regarding the pBTTT PC61BM binary blend devicesthe found PCE of 057 and 119881OC of 05 V little surpass thereported values (ca 05 and 045V resp) while 119869SC andFF are similar [17] It should be noted that according toreported works [18 19] the blend composition 1 2 is notthe most efficient one for this system being the highestperformances achieved for the 1 4 ratio using either PC61BMor PC71BM This enhancement in cells performance withhigher PCBM content should be related to the presenceof relatively pure PCBM phases which provide continuousconductive pathways for the electrons [19] In this work the1 4 blend composition was also tested resulting in deviceswith PCE of 107 119881OC of 051 V 119869SC of 474mAsdotcmminus2 andFF of 044 Nevertheless the control devices were based on1 2 blends in order to keep the mass ratio of D A constantin both binary and ternary devices Taking PTB7 PC61BMas the reference system we find that the partial replacementof PTB7 by pBTTT leads to a decrease in PCE with theexception of the composition 075 025 (PTB7 pBTTT)which leads to the best performing cell The reached PCEvalue (473) corresponds to an improvement of ca 18over the binary cells The maximum PCE obtained 472surpasses the reported values for ternary systems based onPTB7 and PC61BM [30] We also observe that on the other
4 International Journal of Photoenergy
Table 1 Performance parameters of representative curves and maxima PCE values of the OPV cells
hand all ternary cells show higher PCE than the cell basedon pBTTT PC61BM
Comparing the parameters characterizing the best per-forming ternary cell with those of the binary cell ofPTB7 PC61BM we find a slight increase of the short-circuitcurrentHowever it is the fill factor that improves significantly(from 040 to 050) This enlarged FF indicates that chargetransport andor charge collectionwere improved in the cellsIn fact the calculated values for 119877sh and 119877119904 for such cell arethemost favourable within the series that is119877sh is maximumand 119877119904 is minimum thus indicating that recombinationpathways for the charges are minimized and charge transporttowards the electrodes is facilitated respectively The similarvalues of 119869SC for the two most efficient cells the ternarycell with 075 025 2 composition and the binary cell ofPTB7 PC61BM being 1317mAsdotcmminus2 and 1325mAsdotcmminus2respectively suggest that photon-to-charge conversion effi-ciency was only a little improved Devices with a content ofpBTTT higher than 25 showed the lowest currents and thepoorest performances probably due to energy losses causedby the high energy band gap of pBTTT and its consequentlittle contribution to photon absorption Along the series119881OC variesmonotonically with the blend composition (exceptfor the 02 08 2 case with a 119881OC of 054 that is slightlylower than the 055V for the 010 090 2) The variationof 119881OC with the blend composition in ternary blend cellshas been rationalised in terms of an alloy model in whichthe donoracceptor interface and corresponding interfaceband gap (charge transfer state) display a material averagedelectronic structure due to the delocalized nature of the oneelectron states [32]
The external quantum efficiency (EQE) spectra of thecells and the UV-Visible absorption spectra obtained for therespective active layers are shown in Figure 4
We observe that the cells with the pBTTT binary blendand with the ternary blends with 90 and 80wt of pBTTTexhibit EQE spectra with a maximum at ca 550 nm and withvibronic structure that mimics the UV-Visible absorptionspectra of the respective blends Such vibronic structure is notevidenced in the neat pBTTT film absorption spectrum (asshown in Figure 2) being likely attributed to ordering effectsinduced by the intermixing with the fullerene In particularseveral studies have demonstrated that pBTTT and PC61BMin blend films form cocrystals of approximately equal content
Figure 2 Photoluminescence spectra of films of the PTB7 pBTTT(01 09) blend and of the same blend upon addition of PC61BMdeposited over quartz substrates following excitation at the maxi-mum of absorption of pBTTT For comparison the spectrum of thebare quartz is also shown
of polymer and fullerene which result in vibronic resolutionin the absorption spectra [33 34] The presence of suchcocrystals in the 1 2 binary blend may justify the lowerefficiency of the corresponding cells in comparison withthe tested 1 4 pBTTT PC61BM cells since less percolationpathways for electrons should be formedThe ternary deviceswith the highest efficiency (075 025 2) do not seem togain significantly from the presence of pBTTT since itsEQE spectrum is very close to that of the binary cells withPTB7 PC61BM This result is in agreement with the similarvalues of 119869SC obtained for such cells In fact it is the fill factorthat mainly causes the PCE enhancement with respect to thebinary PTB7 PC61BM cell when pBTTT is 25 of the totalof polymers
In view of the energy band diagram shown in Figure 1cells with active layers composed of the two polymers only
International Journal of Photoenergy 5Cu
rren
t den
sity
(mAmiddotcm
2)
02 04 06 0800Voltage (V)
minus12
minus8
minus4
0
PTB7 pBTTT PCBM(0 1 2)(01 09 2)(02 08 2)(05 05 2)
(075 025 2)(09 01 2)(1 0 2)
Figure 3 Current density-voltage (J-V) curves under AM 15Gillumination (100mWsdotcmminus2) of PV cells with active layers ofPTB7 PC61BM (1 2) pBTTT PC61BM (1 2) and five ternaryblend active layers with different PTB7 pBTTT weight ratios
(without PC61BM) were also fabricated in order to evaluatethe role of PC61BM in the cells and the importance ofcharge transfer at pBTTTPTB7 interfacesHence three typesof devices with the ITOPEDOT PSSactive layerLiFAlstructure were fabricated with 01 09 05 05 or 09 01(ww) of PTB7 pBTTT blendsThe J-V curves of the devicesmeasured under the same conditions of illumination thanthose used for the ternary blend cells exhibited 119869SC valuesrather low at the order of 10minus2mAcm2 (shown in Figure S3 inSupplementaryMaterial)This confirms that PC61BM shouldbe the main acceptor in the fabricated ternary blend cells andalso suggests that if exciton dissociation between pBTTT andPTB7 interfaces exists it should be an inefficient process
33 Morphologic Characterization of Active Blends Figure 5shows the AFM topography and phase images of the ref-erence binary blends and of the most efficient cell withthe ternary 075 025 2 (PTBT pBTTT PC61BM) blendThe AFM images for the other ternary cells are shown inSupplementary Material (Figure S4) The variation of thesurface roughness 119877rms for all the seven films as a functionof the blend composition is represented in Figure 5(d)
The binary PTB7 PC61BM blend shows a smooth andrelatively homogeneous surface without evidencing phasesegregated domains at the surface However the topographyimage of the binary pBTTT PC61BM blend reveals a morestructured surface and amuch higher 119877rms (ca 137 nm) thussuggesting the presence of segregated domains of one of thematerials Since the corresponding phase image is relativelywell homogeneous such domains should exist within thebulk of the blend film The AFM images of the ternary
blend films with the highest contents of pBTTT (90 and80) show also similar domains (see Figure S3) The averagedimensions of such segregated domains can be estimatedfromprofile lines acquired from theAFMtopographic images(Figure S5 in Supplementary Material) These are at theorder of ca 20 nm thus indicating a lower degree of mixingbetween the polymer pBTTT and PCBM in comparison withthe PTB7 PC61BM binary blend films where as mentionedthe smooth filmsrsquo surface does not evidence aggregates
The 119877rms calculated for the various films as a functionof the pBTTT content shows that the 119877rms values increasewith the pBTTT content although a slightly lower value isfound when pBTTT is the sole polymer within the blend Wetherefore suggest that aggregates of pBTTT have a negativeeffect on the pBTTT contribution to charge extraction thisleading to the lower fill factors found for the cells with higherpBTTT content On the other hand the addition of pBTTT inlow contents (up to 50) leading to rather smooth surfacesshould be related to an increase of the fill factors due toa positive contribution of pBTTT to hole extraction (sincepBTTT is a good hole transport polymer) within the activeblends When pBTTT becomes the dominant polymer FFdecreases a behavior that is accompanied by an increaseof the surface roughness These lower FF values obtainedwhen pBTTT is 90 or 80 may be related to the presence ofPTB7 in low contents that disrupts the favourable (to chargetransport) pBTTT PCBM phase containing cocrystals andPCBM extended phases The above-mentioned observationof vibronic structure on the pBTTT absorption and EQE isalso consistent with such ordering in the blends when pBTTTis the dominant polymer
4 Conclusions
Ternary BHJ solar cells based on PTB7 pBTTT PC61BMwith improved PCE were demonstrated The cells charac-teristics showed an enhancement in PCE for the ternaryblends with 25 of pBTTT (wt in polymers) of ca 18comparing with reference binary cells yielding a PCE of472The observed improvement was related to the optimalbalance between the two polymers providing higher FFvalues and the absence of aggregates of pBTTT within theternary blends Also according to PL steady state studiesenergy andor charge transfer processes from excited pBTTTto PTB7 should occur in the active layers of the ternary blendcells
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
This work was supported by Fundacao para a Ciencia eTecnologia (FCT) under Projects M-ERANET00012012and UIDEEA500082013
6 International Journal of Photoenergy
400 500 600 700 800300Wavelength (nm)
04
08Ab
sorb
ance
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(a)
400 500 600 700 800300Wavelength (nm)
0
20
40
EQE
()
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(b)
Figure 4 (a) UV-Visible absorption spectra of D A binary blends and ternary blends deposited onto quartzPEDOT PSS substrates (b)EQE spectra of the corresponding solar cells
Topography 214 nm
0
Phase
(a) PTB7 PC61BM (1 2)
89 nm
0
Topography Phase
(b) pBTTT PC61BM (1 2)
342 nm
0
Topography Phase
(c) PTB7 pBTTT PC61BM (075 025 2)
20 40 60 80 1000 pBTTT
00
03
06
09
12
15
Rrm
s(n
m)
(d)
Figure 5 AFM topographic and phase images (1 times 1 120583m2) of (a) PTB7 PC61BM (1 2) (b) pBTTT PC61BM (1 2) (c) ternary blend filmsof PTB7 pBTTT PC61BM with 25wt of pBTTT replacing PTB7 All the films were prepared onto glassITOPEDOT PSS substrates (d)119877rms of the various films as a function of pBTTT content (wt)
International Journal of Photoenergy 7
References
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
hand all ternary cells show higher PCE than the cell basedon pBTTT PC61BM
Comparing the parameters characterizing the best per-forming ternary cell with those of the binary cell ofPTB7 PC61BM we find a slight increase of the short-circuitcurrentHowever it is the fill factor that improves significantly(from 040 to 050) This enlarged FF indicates that chargetransport andor charge collectionwere improved in the cellsIn fact the calculated values for 119877sh and 119877119904 for such cell arethemost favourable within the series that is119877sh is maximumand 119877119904 is minimum thus indicating that recombinationpathways for the charges are minimized and charge transporttowards the electrodes is facilitated respectively The similarvalues of 119869SC for the two most efficient cells the ternarycell with 075 025 2 composition and the binary cell ofPTB7 PC61BM being 1317mAsdotcmminus2 and 1325mAsdotcmminus2respectively suggest that photon-to-charge conversion effi-ciency was only a little improved Devices with a content ofpBTTT higher than 25 showed the lowest currents and thepoorest performances probably due to energy losses causedby the high energy band gap of pBTTT and its consequentlittle contribution to photon absorption Along the series119881OC variesmonotonically with the blend composition (exceptfor the 02 08 2 case with a 119881OC of 054 that is slightlylower than the 055V for the 010 090 2) The variationof 119881OC with the blend composition in ternary blend cellshas been rationalised in terms of an alloy model in whichthe donoracceptor interface and corresponding interfaceband gap (charge transfer state) display a material averagedelectronic structure due to the delocalized nature of the oneelectron states [32]
The external quantum efficiency (EQE) spectra of thecells and the UV-Visible absorption spectra obtained for therespective active layers are shown in Figure 4
We observe that the cells with the pBTTT binary blendand with the ternary blends with 90 and 80wt of pBTTTexhibit EQE spectra with a maximum at ca 550 nm and withvibronic structure that mimics the UV-Visible absorptionspectra of the respective blends Such vibronic structure is notevidenced in the neat pBTTT film absorption spectrum (asshown in Figure 2) being likely attributed to ordering effectsinduced by the intermixing with the fullerene In particularseveral studies have demonstrated that pBTTT and PC61BMin blend films form cocrystals of approximately equal content
Figure 2 Photoluminescence spectra of films of the PTB7 pBTTT(01 09) blend and of the same blend upon addition of PC61BMdeposited over quartz substrates following excitation at the maxi-mum of absorption of pBTTT For comparison the spectrum of thebare quartz is also shown
of polymer and fullerene which result in vibronic resolutionin the absorption spectra [33 34] The presence of suchcocrystals in the 1 2 binary blend may justify the lowerefficiency of the corresponding cells in comparison withthe tested 1 4 pBTTT PC61BM cells since less percolationpathways for electrons should be formedThe ternary deviceswith the highest efficiency (075 025 2) do not seem togain significantly from the presence of pBTTT since itsEQE spectrum is very close to that of the binary cells withPTB7 PC61BM This result is in agreement with the similarvalues of 119869SC obtained for such cells In fact it is the fill factorthat mainly causes the PCE enhancement with respect to thebinary PTB7 PC61BM cell when pBTTT is 25 of the totalof polymers
In view of the energy band diagram shown in Figure 1cells with active layers composed of the two polymers only
International Journal of Photoenergy 5Cu
rren
t den
sity
(mAmiddotcm
2)
02 04 06 0800Voltage (V)
minus12
minus8
minus4
0
PTB7 pBTTT PCBM(0 1 2)(01 09 2)(02 08 2)(05 05 2)
(075 025 2)(09 01 2)(1 0 2)
Figure 3 Current density-voltage (J-V) curves under AM 15Gillumination (100mWsdotcmminus2) of PV cells with active layers ofPTB7 PC61BM (1 2) pBTTT PC61BM (1 2) and five ternaryblend active layers with different PTB7 pBTTT weight ratios
(without PC61BM) were also fabricated in order to evaluatethe role of PC61BM in the cells and the importance ofcharge transfer at pBTTTPTB7 interfacesHence three typesof devices with the ITOPEDOT PSSactive layerLiFAlstructure were fabricated with 01 09 05 05 or 09 01(ww) of PTB7 pBTTT blendsThe J-V curves of the devicesmeasured under the same conditions of illumination thanthose used for the ternary blend cells exhibited 119869SC valuesrather low at the order of 10minus2mAcm2 (shown in Figure S3 inSupplementaryMaterial)This confirms that PC61BM shouldbe the main acceptor in the fabricated ternary blend cells andalso suggests that if exciton dissociation between pBTTT andPTB7 interfaces exists it should be an inefficient process
33 Morphologic Characterization of Active Blends Figure 5shows the AFM topography and phase images of the ref-erence binary blends and of the most efficient cell withthe ternary 075 025 2 (PTBT pBTTT PC61BM) blendThe AFM images for the other ternary cells are shown inSupplementary Material (Figure S4) The variation of thesurface roughness 119877rms for all the seven films as a functionof the blend composition is represented in Figure 5(d)
The binary PTB7 PC61BM blend shows a smooth andrelatively homogeneous surface without evidencing phasesegregated domains at the surface However the topographyimage of the binary pBTTT PC61BM blend reveals a morestructured surface and amuch higher 119877rms (ca 137 nm) thussuggesting the presence of segregated domains of one of thematerials Since the corresponding phase image is relativelywell homogeneous such domains should exist within thebulk of the blend film The AFM images of the ternary
blend films with the highest contents of pBTTT (90 and80) show also similar domains (see Figure S3) The averagedimensions of such segregated domains can be estimatedfromprofile lines acquired from theAFMtopographic images(Figure S5 in Supplementary Material) These are at theorder of ca 20 nm thus indicating a lower degree of mixingbetween the polymer pBTTT and PCBM in comparison withthe PTB7 PC61BM binary blend films where as mentionedthe smooth filmsrsquo surface does not evidence aggregates
The 119877rms calculated for the various films as a functionof the pBTTT content shows that the 119877rms values increasewith the pBTTT content although a slightly lower value isfound when pBTTT is the sole polymer within the blend Wetherefore suggest that aggregates of pBTTT have a negativeeffect on the pBTTT contribution to charge extraction thisleading to the lower fill factors found for the cells with higherpBTTT content On the other hand the addition of pBTTT inlow contents (up to 50) leading to rather smooth surfacesshould be related to an increase of the fill factors due toa positive contribution of pBTTT to hole extraction (sincepBTTT is a good hole transport polymer) within the activeblends When pBTTT becomes the dominant polymer FFdecreases a behavior that is accompanied by an increaseof the surface roughness These lower FF values obtainedwhen pBTTT is 90 or 80 may be related to the presence ofPTB7 in low contents that disrupts the favourable (to chargetransport) pBTTT PCBM phase containing cocrystals andPCBM extended phases The above-mentioned observationof vibronic structure on the pBTTT absorption and EQE isalso consistent with such ordering in the blends when pBTTTis the dominant polymer
4 Conclusions
Ternary BHJ solar cells based on PTB7 pBTTT PC61BMwith improved PCE were demonstrated The cells charac-teristics showed an enhancement in PCE for the ternaryblends with 25 of pBTTT (wt in polymers) of ca 18comparing with reference binary cells yielding a PCE of472The observed improvement was related to the optimalbalance between the two polymers providing higher FFvalues and the absence of aggregates of pBTTT within theternary blends Also according to PL steady state studiesenergy andor charge transfer processes from excited pBTTTto PTB7 should occur in the active layers of the ternary blendcells
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
This work was supported by Fundacao para a Ciencia eTecnologia (FCT) under Projects M-ERANET00012012and UIDEEA500082013
6 International Journal of Photoenergy
400 500 600 700 800300Wavelength (nm)
04
08Ab
sorb
ance
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(a)
400 500 600 700 800300Wavelength (nm)
0
20
40
EQE
()
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(b)
Figure 4 (a) UV-Visible absorption spectra of D A binary blends and ternary blends deposited onto quartzPEDOT PSS substrates (b)EQE spectra of the corresponding solar cells
Topography 214 nm
0
Phase
(a) PTB7 PC61BM (1 2)
89 nm
0
Topography Phase
(b) pBTTT PC61BM (1 2)
342 nm
0
Topography Phase
(c) PTB7 pBTTT PC61BM (075 025 2)
20 40 60 80 1000 pBTTT
00
03
06
09
12
15
Rrm
s(n
m)
(d)
Figure 5 AFM topographic and phase images (1 times 1 120583m2) of (a) PTB7 PC61BM (1 2) (b) pBTTT PC61BM (1 2) (c) ternary blend filmsof PTB7 pBTTT PC61BM with 25wt of pBTTT replacing PTB7 All the films were prepared onto glassITOPEDOT PSS substrates (d)119877rms of the various films as a function of pBTTT content (wt)
International Journal of Photoenergy 7
References
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
Figure 3 Current density-voltage (J-V) curves under AM 15Gillumination (100mWsdotcmminus2) of PV cells with active layers ofPTB7 PC61BM (1 2) pBTTT PC61BM (1 2) and five ternaryblend active layers with different PTB7 pBTTT weight ratios
(without PC61BM) were also fabricated in order to evaluatethe role of PC61BM in the cells and the importance ofcharge transfer at pBTTTPTB7 interfacesHence three typesof devices with the ITOPEDOT PSSactive layerLiFAlstructure were fabricated with 01 09 05 05 or 09 01(ww) of PTB7 pBTTT blendsThe J-V curves of the devicesmeasured under the same conditions of illumination thanthose used for the ternary blend cells exhibited 119869SC valuesrather low at the order of 10minus2mAcm2 (shown in Figure S3 inSupplementaryMaterial)This confirms that PC61BM shouldbe the main acceptor in the fabricated ternary blend cells andalso suggests that if exciton dissociation between pBTTT andPTB7 interfaces exists it should be an inefficient process
33 Morphologic Characterization of Active Blends Figure 5shows the AFM topography and phase images of the ref-erence binary blends and of the most efficient cell withthe ternary 075 025 2 (PTBT pBTTT PC61BM) blendThe AFM images for the other ternary cells are shown inSupplementary Material (Figure S4) The variation of thesurface roughness 119877rms for all the seven films as a functionof the blend composition is represented in Figure 5(d)
The binary PTB7 PC61BM blend shows a smooth andrelatively homogeneous surface without evidencing phasesegregated domains at the surface However the topographyimage of the binary pBTTT PC61BM blend reveals a morestructured surface and amuch higher 119877rms (ca 137 nm) thussuggesting the presence of segregated domains of one of thematerials Since the corresponding phase image is relativelywell homogeneous such domains should exist within thebulk of the blend film The AFM images of the ternary
blend films with the highest contents of pBTTT (90 and80) show also similar domains (see Figure S3) The averagedimensions of such segregated domains can be estimatedfromprofile lines acquired from theAFMtopographic images(Figure S5 in Supplementary Material) These are at theorder of ca 20 nm thus indicating a lower degree of mixingbetween the polymer pBTTT and PCBM in comparison withthe PTB7 PC61BM binary blend films where as mentionedthe smooth filmsrsquo surface does not evidence aggregates
The 119877rms calculated for the various films as a functionof the pBTTT content shows that the 119877rms values increasewith the pBTTT content although a slightly lower value isfound when pBTTT is the sole polymer within the blend Wetherefore suggest that aggregates of pBTTT have a negativeeffect on the pBTTT contribution to charge extraction thisleading to the lower fill factors found for the cells with higherpBTTT content On the other hand the addition of pBTTT inlow contents (up to 50) leading to rather smooth surfacesshould be related to an increase of the fill factors due toa positive contribution of pBTTT to hole extraction (sincepBTTT is a good hole transport polymer) within the activeblends When pBTTT becomes the dominant polymer FFdecreases a behavior that is accompanied by an increaseof the surface roughness These lower FF values obtainedwhen pBTTT is 90 or 80 may be related to the presence ofPTB7 in low contents that disrupts the favourable (to chargetransport) pBTTT PCBM phase containing cocrystals andPCBM extended phases The above-mentioned observationof vibronic structure on the pBTTT absorption and EQE isalso consistent with such ordering in the blends when pBTTTis the dominant polymer
4 Conclusions
Ternary BHJ solar cells based on PTB7 pBTTT PC61BMwith improved PCE were demonstrated The cells charac-teristics showed an enhancement in PCE for the ternaryblends with 25 of pBTTT (wt in polymers) of ca 18comparing with reference binary cells yielding a PCE of472The observed improvement was related to the optimalbalance between the two polymers providing higher FFvalues and the absence of aggregates of pBTTT within theternary blends Also according to PL steady state studiesenergy andor charge transfer processes from excited pBTTTto PTB7 should occur in the active layers of the ternary blendcells
Competing Interests
The authors declare that they have no competing interests
Acknowledgments
This work was supported by Fundacao para a Ciencia eTecnologia (FCT) under Projects M-ERANET00012012and UIDEEA500082013
6 International Journal of Photoenergy
400 500 600 700 800300Wavelength (nm)
04
08Ab
sorb
ance
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(a)
400 500 600 700 800300Wavelength (nm)
0
20
40
EQE
()
(0 1 2)(01 09 2)(02 08 2)(05 05 2)
PTB7 pBTTT PC61BM(075 025 2)(09 01 2)(1 0 2)
(b)
Figure 4 (a) UV-Visible absorption spectra of D A binary blends and ternary blends deposited onto quartzPEDOT PSS substrates (b)EQE spectra of the corresponding solar cells
Topography 214 nm
0
Phase
(a) PTB7 PC61BM (1 2)
89 nm
0
Topography Phase
(b) pBTTT PC61BM (1 2)
342 nm
0
Topography Phase
(c) PTB7 pBTTT PC61BM (075 025 2)
20 40 60 80 1000 pBTTT
00
03
06
09
12
15
Rrm
s(n
m)
(d)
Figure 5 AFM topographic and phase images (1 times 1 120583m2) of (a) PTB7 PC61BM (1 2) (b) pBTTT PC61BM (1 2) (c) ternary blend filmsof PTB7 pBTTT PC61BM with 25wt of pBTTT replacing PTB7 All the films were prepared onto glassITOPEDOT PSS substrates (d)119877rms of the various films as a function of pBTTT content (wt)
International Journal of Photoenergy 7
References
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
Figure 4 (a) UV-Visible absorption spectra of D A binary blends and ternary blends deposited onto quartzPEDOT PSS substrates (b)EQE spectra of the corresponding solar cells
Topography 214 nm
0
Phase
(a) PTB7 PC61BM (1 2)
89 nm
0
Topography Phase
(b) pBTTT PC61BM (1 2)
342 nm
0
Topography Phase
(c) PTB7 pBTTT PC61BM (075 025 2)
20 40 60 80 1000 pBTTT
00
03
06
09
12
15
Rrm
s(n
m)
(d)
Figure 5 AFM topographic and phase images (1 times 1 120583m2) of (a) PTB7 PC61BM (1 2) (b) pBTTT PC61BM (1 2) (c) ternary blend filmsof PTB7 pBTTT PC61BM with 25wt of pBTTT replacing PTB7 All the films were prepared onto glassITOPEDOT PSS substrates (d)119877rms of the various films as a function of pBTTT content (wt)
International Journal of Photoenergy 7
References
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
[1] I Etxebarria J Ajuria and R Pacios ldquoPolymer fullerene solarcells materials processing issues and cell layouts to reachpower conversion efficiency over 10 a reviewrdquo Journal ofPhotonics for Energy vol 5 no 1 Article ID 057214 2015
[2] J-D Chen C Cui Y-Q Li et al ldquoSingle-junction polymer solarcells exceeding 10 power conversion effi ciencyrdquo AdvancedMaterials vol 27 no 6 pp 1035ndash1041 2015
[3] A R B M Yusoff D Kim H P Kim F K Shneider W J DaSilva and J Jang ldquoA high efficiency solution processed polymerinverted triple-junction solar cell exhibiting a power conversionefficiency of 1183rdquo Energy and Environmental Science vol 8no 1 pp 303ndash316 2015
[4] Y Yang Z Zhang H Bin et al ldquoSide-chain isomerization on ann-type organic semiconductor ITIC acceptormakes 1177highefficiency polymer solar cellsrdquo Journal of the American ChemicalSociety vol 138 no 45 pp 15011ndash15018 2016
[5] H Bin L Gao Z Zhang et al ldquo114 efficiency non-fullerenepolymer solar cells with trialkylsilyl substituted 2D-conjugatedpolymer as donorrdquo Nature Communications vol 7 Article ID13651 2016
[6] K Yao Y-X Xu F Li X Wang and L Zhou ldquoA simple anduniversal method to increase light absorption in ternary blendpolymer solar cells based on ladder-type polymersrdquo AdvancedOptical Materials vol 3 no 3 pp 321ndash327 2015
[7] S Honda T Nogami H Ohkita H Benten and S ItoldquoImprovement of the light-harvesting efficiency in poly-merfullerene bulk heterojunction solar cells by interfacial dyemodificationrdquo ACS Applied Materials and Interfaces vol 1 no4 pp 804ndash810 2009
[8] Z Sun K Xiao J K Keum et al ldquoPS-b-P3HT copolymersas P3HTPCBM interfacial compatibilizers for high efficiencyphotovoltaicsrdquo Advanced Materials vol 23 no 46 pp 5529ndash5535 2011
[9] S Miyanishi K Tajima and K Hashimoto ldquoMorphologicalstabilization of polymer photovoltaic cells by using cross-linkable poly(3-(5-hexenyl)thiophene)rdquo Macromolecules vol42 no 5 pp 1610ndash1618 2009
[10] L Derue O Dautel A Tournebize et al ldquoThermal stabilisationof polymer-fullerene bulk heterojunction morphology for effi-cient photovoltaic solar cellsrdquo Advanced Materials vol 26 no33 pp 5831ndash5838 2014
[11] B H Hamadani D J Gundlach I McCulloch and M HeeneyldquoUndoped polythiophene field-effect transistors with mobilityof 1 cm2 Vminus1 sminus1rdquo Applied Physics Letters vol 91 no 24 ArticleID 243512 2007
[12] I Mcculloch M Heeney M L Chabinyc et al ldquoSemicon-ducting thienothiophene copolymers design synthesis mor-phology and performance in thin-film organic transistorsrdquoAdvanced Materials vol 21 no 10-11 pp 1091ndash1109 2009
[13] I McCulloch M Heeney C Bailey et al ldquoLiquid-crystallinesemiconducting polymers with high charge-carrier mobilityrdquoNature Materials vol 5 no 4 pp 328ndash333 2006
[14] B Ebenhoch S A Thomson K Genevicius G Juska and ID Samuel ldquoCharge carrier mobility of the organic photovoltaicmaterials PTB7 and PC71BM and its influence on deviceperformancerdquo Organic Electronics vol 22 pp 62ndash68 2015
[15] N ZhouH Lin S J Lou et al ldquoMorphology-performance rela-tionships in high-efficiency all-polymer solar cellsrdquo AdvancedEnergy Materials vol 4 no 3 Article ID 1300785 2014
[16] R L Uy S C Price andW You ldquoStructure-property optimiza-tions in donor polymers via electronics substituents and sidechains toward high efficiency solar cellsrdquoMacromolecular RapidCommunications vol 33 no 14 pp 1162ndash1177 2012
[17] J E Parmer A C Mayer B E Hardin et al ldquoOrganic bulkheterojunction solar cells using poly(25-bis(3- tetradecyllthi-ophen-2-yl)thieno[32- b] thiophene)rdquo Applied Physics Lettersvol 92 no 11 Article ID 113309 2008
[18] N C Cates R Gysel Z Beiley et al ldquoTuning the properties ofpolymer bulk heterojunction solar cells by adjusting fullerenesize to control intercalationrdquo Nano Letters vol 9 no 12 pp4153ndash4157 2009
[19] D W Gehrig I A Howard S Sweetnam T M Burke M DMcGehee and F Laquai ldquoThe impact of donor-acceptor phaseseparation on the charge carrier dynamics in pBTTTPCBMphotovoltaic blendsrdquo Macromolecular Rapid Communicationsvol 36 no 11 pp 1054ndash1060 2015
[20] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012
[21] J C Aguirre S A Hawks A S Ferreira et al ldquoSequential pro-cessing for organic photovoltaics design rules for morphologycontrol by tailored semi-orthogonal solvent blendsrdquo AdvancedEnergy Materials vol 5 no 11 Article ID 1402020 2015
[22] W Chen T Xu F He et al ldquoHierarchical nanomorphologiespromote exciton dissociation in polymerfullerene bulk hetero-junction solar cellsrdquo Nano Letters vol 11 no 9 pp 3707ndash37132011
[23] E A A Arbab B Taleatu and G T Mola ldquoEnvironmentalstability of PTB7PCBM bulk heterojunction solar cellrdquo Journalof Modern Optics vol 61 no 21 pp 1749ndash1753 2014
[24] Y Yang Michael W Chen L Dou et al ldquoHigh-performancemultiple-donor bulk heterojunction solar cellsrdquoNature Photon-ics vol 9 no 3 pp 190ndash198 2015
[25] L Lu T Xu W Chen E S Landry and L Yu ldquoTernary blendpolymer solar cells with enhanced power conversion efficiencyrdquoNature Photonics vol 8 no 9 pp 716ndash722 2014
[26] Y Xiao H Wang S Zhou et al ldquoEfficient ternary bulk het-erojunction solar cells with PCDTBT as hole-cascade materialrdquoNano Energy vol 19 pp 476ndash485 2016
[27] M Zhang F Zhang J Wang Q An and Q Sun ldquoEfficientternary polymer solar cells with a parallel-linkage structurerdquoJournal of Materials Chemistry C vol 3 no 45 pp 11930ndash119362015
[28] S Liu P You J Li et al ldquoEnhanced efficiency of polymer solarcells by adding a high-mobility conjugated polymerrdquoEnergy andEnvironmental Science vol 8 no 5 pp 1463ndash1470 2015
[29] S Zhang L Zuo J Chen et al ldquoImproved photon-to-electronresponse of ternary blend organic solar cells with a low bandgap polymer sensitizer and interfacial modificationrdquo Journal ofMaterials Chemistry A vol 4 no 5 pp 1702ndash1707 2016
[30] Y Ohori S Fujii H Kataura and Y Nishioka ldquoImprovement ofbulk heterojunction organic solar cells based on PTB7PC61BMwith small amounts of P3HTrdquo Japanese Journal of AppliedPhysics vol 54 no 4S pp 1ndash8 2015
[31] Y He G Zhao B Peng and Y Li ldquoHigh-yield synthesisand electrochemical and photovoltaic properties of indene-C70bisadductrdquo Advanced Functional Materials vol 20 no 19 pp3383ndash3389 2010
8 International Journal of Photoenergy
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
[32] R A Street D Davies P P Khlyabich B Burkhart and BC Thompson ldquoOrigin of the tunable open-circuit voltage internary blend bulk heterojunction organic solar cellsrdquo Journalof the American Chemical Society vol 135 no 3 pp 986ndash9892013
[33] M Scarongella J De Jonghe-Risse E Buchaca-Domingo et alldquoA close look at charge generation in polymer fullerene blendswith microstructure controlrdquo Journal of the American ChemicalSociety vol 137 no 8 pp 2908ndash2918 2015
[34] N C Miller E Cho M J N Junk et al ldquoUse of X-ray diffrac-tion molecular simulations and spectroscopy to determine themolecular packing in a polymer-fullerene bimolecular crystalrdquoAdvanced Materials vol 24 no 45 pp 6071ndash6079 2012
![L‑BCpositMerials fPtocatalysis Ptovoltaics · HEL Holeextractinglayer PC 71 BM [6,6]-Phenl-Cy 71-butyricacidmethylester PTB7 Poly[[4,8-bis[(2-elheyhxyt l)oxy]benzo[1,2-b:4,5-b′]](https://static.documents.pub/doc/80x56/60b1126bd18fb34aeb7c355f/labcpositmerials-fptocatalysis-ptovoltaics-hel-holeextractinglayer-pc-71-bm-66-phenl-cy.jpg)
