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Development and structure/property relationship of new electron accepting polymers based on thieno[2',3':4,5]pyrido[2,3g]thieno[3,2-c]quinoline-4,10-dione for all-polymer solar cells In Hwan Jung, Donglin Zhao, Jaeyoung Jang, Wei Chen, Erik S. Landry, Luyao Lu, Dmitri V. Talapin, and Luping Yu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01928 • Publication Date (Web): 17 Aug 2015 Downloaded from http://pubs.acs.org on August 19, 2015

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Chemistry of Materials

Development and structure/property relationship of new electron accepting polymers based on thieno[2',3':4,5]pyrido[2,3-g]thieno[3,2-c]quinoline-4,10-dione for all-polymer solar cells

In Hwan Jung,†,⊥, # Donglin Zhao,†, # Jaeyoung Jang,†,┬ Wei Chen,‡ Erik S. Landry,‡ Luyao Lu,† Dmitri V. Talapin,†,§ Luping Yu*,† †

Department of Chemistry and The James Franck Institute, The University of Chicago, Chicago, IL 60637, United States ‡ Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, United States § Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, United States ⊥ Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305‐600, Republic of Korea ABSTRACT: Several electron accepting polymers having weak accepting-strong accepting (WA-SA) and strong accepting-strong accepting (SA-SA) monomer alternation were synthesized for studies of structure/property relationship in all-polymer solar cells. Two kinds of cyclic amide monomers, 4,10-bis(2-butyloctyl)-thieno[2',3':5,6]pyrido[3,4-g]thieno-[3,2-c]isoquinoline-5,11-dione (TPTI) and 5,11-bis(2-butyloctyl)-thieno[2',3':4,5]pyrido[2,3-g]thieno[3,2-c]quinoline-4,10-dione (TPTQ) were synthesized as weak accepting monomers (WA). Difluorinated TPTQ (FTPTQ) and well-known perylene diimide (PDI) monomers were synthesized as strong electron accepting monomers (SA). By using 1-chloronaphthalene (CN) as a co-solvent, the morphology of the polymer blended films can be finely tuned to achieve better ordering toward face-on mode and favorable phase separation between electron donor and acceptor, resulting in significant enhancement of short circuit current (Jsc) and fill factor (FF). The fluorination in TPTQ unit reduced the dipole moment of D-A complex and gave a negative effect on a polymer system. PFP showed worse electron accepting property with lower electron mobility than PQP. It is reasoned that the internal polarization plays an important role in the design of electron accepting polymers. As a result, PQP having TPTQ monomer exhibited best photovoltaic performance with power conversion efficiency (PCE) of 3.52 % (Voc = 0.71 V, Jsc = 8.57 mA/cm2, FF = 0.58) at a weight ratio of PTB7-Th:PQP = 1:1, under AM 1.5G.

INTRODUCTION Fullerene derivatives are a key ingredient in bulk heterojunction (BHJ) solar cells as electron acceptors blended with polymeric electron donors.1-4 These compounds, such as phenyl-C61(or C71)-butyric acid methyl ester (PC61BM and PC71BM), exhibit a unique spherical structure and high electron affinity that render them ease in forming continuous charge transport pathways and thus high electron mobility. An extensive research effort has been devoted to understand the structure/property relationship of fullerene as electron acceptor in organic photovoltaic (OPV).5-7 Recently, power conversion efficiency (PCE) of over 10% has been achieved in BHJ OPV solar cells based on fullerenes.8-9 However, current research effort is shifting towards developing electron accepting polymers that can match and replace these fullerenes as electron acceptors. These efforts are important because the fullerenes are expensive compounds and exhibit limited absorption in longer wavelength region of the solar spectrum and thermal instability in the morphology of blend films, which potentially obstruct the commercialization of organic BHJ solar cells.10-14 Surely, a great challenge exists to

develop accepting polymers that match or even surpass the property of fullerene because it is very difficult to mimic the function of fullerenes, such as their good electron transporting property and fast charge separation when an exciton is generated. A relaxed design idea is to develop molecular acceptors with comparable energy levels as those of fullerenes.15,16 Several research groups have achieved certain degree of success in these efforts. All-polymer solar cells showing PCE as high as 6.7% have been reported.17-25 However, it was realized that energy level match alone is not enough to achieve high efficiency in all-polymer solar cells. Previously, we reported alternating electron accepting polymers with the following monomer-comonomer combination: a) Weak donating monomer-strong accepting monomer (WD-SA), b) Weak accepting monomer-strong accepting monomer (WA-SA), c) Strong donating monomerstrong accepting monomer (SD-SA). Several criteria for designing electron accepting polymers for all-polymer solar cells were suggested.26 It was suggested that accepting polymers should exhibit proper energy levels, internal

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polarization and high electron mobility for high OPV performance. To further probe structure-property relationship and search for design principle, we have synthesized a weak accepting monomer, 5,11-bis(2-butyloctyl)-dihydrothieno[2',3':4,5]pyrido[2,3-g]thieno[3,2-c]quinoline-4,10-dione (TPTQ), which is an isomer of previously synthesized cyclic diamide monomer (TPTI).27,28 The TPTQ exhibits a highly planar structure, facilitating intermolecular ordering. The two electron withdrawing carbonyl moieties enhance the electron affinity of the conjugated system, resulting in a weak accepting monomer. For comparing the effect of polarity, a new difluorinated TPTQ (FTPTQ) monomer exhibiting stronger electron accepting properties was synthesized. These monomers allowed us to synthesize polymers with a combination of strong accepting monomer-strong accepting monomer (SA-SA) and weak donating monomer-strong accepting monomer (WD-SA). This paper describes the synthetic approach, molecular structure, charge carrier mobility and morphology of the blended film and their correlation with the photovoltaic J-V characteristics in details. The structure/property relationship established here is valuable for the design of new electron accepting materials.

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evaluated by thermal gravimetric analysis (TGA) under N2 atmosphere. PQP, PFP and PIP exhibited good stability, showing less than 5% weight loss up to 379, 358, and 387 °C, respectively. All of the related physical properties of polymers are summarized in Table 1.

Scheme 1. Synthetic scheme for the electron accepting polymers

RESULTS Synthesis and Structural Characterization: The syntheses and structures of the new monomers (compound TPTQ and FTPTQ) are shown in Scheme S1 in supporting information (SI). Stannylated TPTQ and FTPTQ monomers were synthesized through total five steps starting from di-tertbutyl (2,5-dibromo-1,4-phenylene) dicarbamate with overall yield of 21% and total six steps from 2,5-dibromo-3,6difluorobenzene-1,4-diamine with overall yield of 10%, respectively.26,28 The perylene diimide (PDI) was used as a strong electron accepting co-monomer. PDI dibromide monomer has two isomers; 5,12-dibromo-2,9-bis(2decyltetradecyl)anthra- [2,1,9-def:6,5,10-d'e'f']diisoquinoline1,3,8,10(2H,9H)-tetraone (PDI, shown in Scheme 1):5,13dibromo-2,9-bis(2-decyltetradecyl)anthra[2,1,9-def:6,5,10d'e'f']-diisoquinoline-1,3,8,10(2H,9H)-tetraone = 5:1. The mole ratio is determined by 1H-NMR shown in Figure S5. The accepting polymers, PQP, PFP and PIP, were synthesized through the Stille polycondensation of monomer PDI with stannylated TPTQ, FTPTQ and TPTI, respectively (Scheme 1).29 The number average molecular weights (Mn) of the resulting polymers were determined by using gel permeation chromatography (GPC) and polystyrene as standard and found to be 21,600 g/mol (PDI = 2.83) for PQP, 12,300 g/mol (PDI = 2.74) for PFP, and 18,100 g/mol (PDI = 2.17) for PIP. The PQP is soluble in chloroform and chlorobenzene in R.T., but PFP is soluble in hot chlorinated solvents (e.g., chloroform, chlorobenzene, dichlorobenzene, and etc). The difference in solubility affects their molecular weight as shown in Table 1 because the PFQ polymer precipitated out prematurely during polymerization. The thermal stability of the polymers was

Table 1. Physical properties of the polymers Mna

Mw b

PDI

Td c

θ (°) d

μ (D)e

PQP

21,600

61,000

2.83

379

57.99

1.73

PFP

12,300

33,800

2.74

358

59.59

0.99

PIP 18,100 39,300 2.17 387 60.27 1.58 a. Number average molecular weight, b. Weight average molecular weight, c. Decomposition temperature determined by TGA under N2 based on a 5% weight loss, d. Dihedral angle between the two donor/acceptor units in the polymer backbones and e. dipole moment of the oligomers with three repeating units calculated from the DFT methods using the GAUSSIAN 09 program package.30

Figure 1. Electrochemical bandgap diagram of the synthesized electron accepting polymers.

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Figure 2. (a) U UV-vis absorpttion spectra of the copolymerrs, and (b) Sternn-Volmer plotss of fluorescence quenching oof PTB7-Th (1.04 × 10-5M)) by electron acccepting polym mers in dichloroobenzene soluttion. Table 2. Opticcal properties of o the polymerss abs, soll (nm) a

ɛcoeef (L/mol·cm) b

abs, film (nm)) c

Eg, optd

ems, sol (nm m) a

QY (%) e

ems, film (nnm) c

Ksvv (M-11)

P PQP

427,, 494

53,800

432, 505

1.74

698

1.1

716

5.41 × 103

P PFP

407,, 502

40,000

410, 525

1.86

666

1.5

708

3.83 × 103

P PIP

408, 5007, 622

53,700

413, 509, 6552

1.56

786

0.06

801

–7

a. Dilute solutioon in chloroform m, b. Molar abbsorption coeffiicients from thee absorption maaximum in dilutted solution (1 × 10 M), c. Sppincast film on quuartz plate from m 1 wt.% chloroform solution ffor 30 s at 15000 rpm, d. Band gap g calculated ffrom the absorpption edge in soolid film, e. Relattive fluorescencce quantum yielld calculated froom QY of the flluorescein in ethhanol (0.79).

Electrochem mical prope erties: Thee electrocheemical properties of the synthesizeed monomers and polymers were investigated byy using cyclic voltammetry ((CV) and the reelated cyclic voltam mmograms are shown in SII (Figure S8).. The HOMO and thhe LUMO eneergy levels of the monomerrs and polymers weree calculated from f the oxidaation and reduuction onset potentialls relative to fferrocene (as aan internal stanndard) and are summ marized in Figure 1.31,32 Thee oxidation annd the reduction poteentials of PQP P were determ mined to be 1.226 eV and –0.74 eV, respectively, w which correspoonded to the HO OMO and the LUM MO energy levvels of –5.97 eV and –3.977 eV, respectively. A After introducinng two fluorinee atoms, the HO OMO energy level off PFP was signnificantly decreeased to -6.18 while the LUMO ennergy levels waas almost not aaffected (-3.955 eV), which also cooincide with density functiional theory ((DFT) calculation usiing the B3LYP P functional andd a 3-21G basis set. Optical Prop perties: The aabsorption specctra of the polyymers were recorded both in chlorooform (CF) soluution and in films as shown in Figuure 2(a). The absorption maaxima for PIP were observed at 4008, 507 and 622 nm in chlorooform, and 4133, 509 and 652 nm in the film sstate. PQP exxhibited absorrption maxima at 4277 and 494 nm m in chloroform m, and 432 andd 505 nm in the film m state. PFP shhowed 2 major absorption peaaks at 407 and 502 nnm in CF soluution and 410 and a 525 nm inn film state. The abssorption bandw widths of the polymer film ms are broader and thheir absorption onsets are red--shifted compaaring

O and LUMO diagrams of thhe monomers aand Figgure 3. HOMO pollymers, calculaated with DFT method. to those in soluution because of the strongeer intermolecuular interactions betw ween neighboring molecules in the film staate. Siggnificant differrences can be observed among the polymeers, PIP P, PQP and P PFP, caused bby a small struuctural variatioon. Thhis is most likelly due to the ddifference in eleectronic effect on connjugated backbbones. In PIP, the amide nitroogen is directlyy

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Figure 4. J–V V characteristicss of BHJ photoovoltaic devicees with an activve layer composed of (a) PT TB7:accepting ppolymers and (b) PTB7‐Th: acccepting polymeers, and (c) EQ QE of BHJ phhotovoltaic devvices with an aactive layer coomposed of PT TB7‐Th:acceptiing polymers, under simulated A AM 1.5 G solar irradiation. Table 3. Summaryy of photovoltaiic properties.

a

a

A Active layer

Solvent

Jsc (mA//cm2)

Voc (V)

FF

PCE (PCEmaax) [%]

P PTB7:PIP

CF:DIO (3% vv/v)

4.13 ± 0.02

0.688 ± 0.01

0.34 ± 0.004

0.96 ± 0.01 ((0.97)

P PTB7:PFP

CF:DIO (3% vv/v)

1.06 ± 0.15

0.655 ± 0.01

0.26 ± 0.002

0.18 ± 0.03 ((0.21)

P PTB7:PQP

CF:DIO (3% vv/v)

6.01 ± 0.08

0.688 ± 0.01

0.34 ± 0.004

1.39 ± 0.04 ((1.43)

P PTB7:PQP

CF:CN (6% v/v) v

6.41 ± 0.09

0.688 ± 0.01

0.36 ± 0.005

1.57 ± 0.06 ((1.63)

P PTB7:PQP w with Au rod

CF:CN (6% v/v) v

7.15 ± 0.07

0.688 ± 0.01

0.37 ± 0.004

1.80 ± 0.06 ((1.86)

PT TB7-Th:PFP

CF:CN (6% v/v) v

4.94 ± 0.22

0.699 ± 0.01

0.39 ± 0.003

1.33 ± 0.10 ((1.43)

PT TB7-Th:PQP

CF:CN (6% v/v) v

7.72 ± 0.12

0.700 ± 0.01

0.57 ± 0.008

3.08 ± 0.14 ((3.22)

PT TB7-Th:PQP w with Au rod

CF:CN (6% v/v) v

8.39 ± 0.18

0.700 ± 0.01

0.57 ± 0.008

3.35 ± 0.17 ((3.52)

Onnly the optimizeed recipes were considered for the estimation oof the average P PCE; Average value v from five devices

connected to tthiophene ringgs that make tthem more eleectron rich due to thhe donating prroperties of lonne pair electroon on nitrogen atom ms. That will eenhance the H HOMO energy level more than wheen they are connnected with central benzenee ring. To understandd the optical properties, thee ground-state (S0) geometric struuctures of thee oligomers with w three repeeating units were caalculated with DFT (B3LY YP/3-21G*).33-35 The calculated struuctures of the ccompounds aree shown in Figuure 3, which show thhat all of the poolymer structurres are fairly tw wisted. The dihedral aangles at the junction j of tw wo monomers iin the polymer backkbones are almost a 60 °C C. However, three polymers show wed quite diffferent electronn distribution iin the HOMOs. Thee electron disstribution of P PIP is comppletely localized withiin the TPTI monomer m in the HOMO. Thereefore, the HOMO ennergy level of tthe PIP polymer almost resem mbles that of TPTI m monomer. On thhe other hand, the electron deensity of PFP in HO OMO is delocallized along thee polymer backkbone through both P PDI and FTPTQ Q monomers. In case of PQP P, the HOMO involvves those mostlly from the TP PTQ monomer and a small amount of PDI monoomer. Therefoore, HOMO ennergy level of PQP resembles thatt of TPTQ, buut slightly decrreased due to the PD DI unit. In case of LUMO orbitals, all oof the polymers shoow almost ideentical orbitall distribution only localized at PD DI unit. Thereffore, three polyymers showed same

UMO level of --4.0 eV just likke that of PDII. As a result, tthe LU diffference in the bandgap of polymers is mainnly determined by HO OMO energy leevels of polymeers. T The fluorescennce spectra annd the emissioon quantum yieeld (QY Y) of the polymers were meaasured in a diluuted solution aand aree shown in Figuure S6. The concentration of polymer solutiion forr these measurrements was ccontrolled to aan absorbance of 0.004 at the absoorption maxim ma to prevent the aggregatiion efffect. Fluoresceiin was used ass a reference ffluorescence dyye, whhose QY in eethanol is knoown to be 0.779.36 All of tthe pollymers exhibitted minimal fluorescent f em mission, but P PIP conntaining TPTI derivatives exxhibited lowesst QY of 0.066%. Preeviouly, we sugggested that thhe effective inteernal polarizatiion bettween the two monomers reeduces the fluoorescent quantuum yieeld of the polymers.26 The quantum calcuulation indicatted thaat the repeatinng unit of thhese polymers exhibits stroong internal polarizattion, leading tto small quanttum efficiency in mission. A smalll increase in quuantum efficienncy from PQP P to em PF FP is noticeablle which may reflect the deccrease in internnal pollarization althhough it musst be cautiouus not to ovver em mphasize this pooint. Thhe quenching efficiency of o the PQP and PFP was w 37 invvestigated withh a Stern-Volmeer quenching equation. e I° / I = 1 + Ksv[Q]

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Where I° andd I are the fluuorescence inttensity of PTB B7-Th excited at 6388 nm (the absoorption maximuum of pure PTB B7-Th solution) in thee absence and presence of th he electron acceeptor, respectively. [[Q] is the moolar concentrattion of the eleectron acceptor (calcculated from thhe mass of reepeating unit oof the polymers) andd Ksv is the S Stern-Volmer qquenching connstant, which gives useful u informattion on interm molecular quennching process.38 From m the plots, thhe linear fittedd slope presents Ksv value. As show wn in Figure 2((b), PQP show wed much higher Ksv value of 5.41 × 103 M-1 than PFP (3.83 × 1103 M-1). It indicates that PTB7-Thh/PQP blend is more efficieently quenchinng the fluorescence thhan PTB7-Th/P PFP, which iss consistent witth the expectation thaat PQP is a bettter electron-accceptor. (Figuree S7) Photovoltaic effect: Based on the energyy level values, these polymers are suitable s to servve as electron acceptors in ccouple with many eleectron donatingg polymers. Thhe PV propertiies of the polymers were examineed based on bbulk heterojunnction solar cells with thhe followinng configuration: ITO/PEDOT:P PSS/polymer blends/Ca/Al. b T current-vooltage The (J-V) characterristics of the deevices (under A AM 1.5G conddition, 100 W/cm2) are shown inn Figure 4(a) and (b), andd the corresponding PV parameterrs are summariized in Table 3. 3 The detailed studiees of the J-V ccharacteristics aas a function oof the solvents and thhe weight ratioo between donnor and acceptoor are shown in SI (F Figure S9).

Figure 5. 2D GIWAXS G patteerns of PTB7:P PQP (1:1 w/w)) film from (a) CF, (b) CF:DIO (33% v/v) and (cc) CF:CN (6% % v/v) PQP (1:1 w/w)) film and AFM toppographic imaages of PTB7:P from (d) CF, (ee) CF:DIO (3% % v/v) and (f) C CF:CN (6% v/vv).

These polym mers exhibitedd energy levels that are suitted as electron acceppting polymers. The initial deevices fabricated by the soluutions Poly[[4,8-bbis[(2spin-coating of ethylhexyl)oxyy]benzo[1,2-b:44,5-b']dithiophhene-2,6-diyl][33fluoro-2-[(2-etthylhexyl)carboonyl]thieno[3,44-b]thiopheneddiyl]] (PTB7)/PQP or o PFP in CF (1:1 w/w) exhhibited PCE off 0.76, and 0.03 %, reespectively. It was w found thatt the performannce of PTB7/PQP bllends is very sensitive to thee kinds of addiitives. The PTB7/PQ QP device exhibbited higher PC CE value of 1..43 % (Voc = 0.69 V, Jsc = 6.09 mA A/cm2, FF = 0.334) at a weightt ratio of PTB7/PQP P = 1:1 when 33% v/v of 1,8--diiodooctane ((DIO)

which was furtther increased to waas mixed with chloroform, w 1.663 % (Voc = 0.669 V, Jsc = 6.50 mA/cm2, FF F = 0.36) whenn 1chlloro-naphthalenne (1-CN, 6% %) was mixed w with chloroforrm. Atoomic force m microscopy (A AFM) studiess indicated thhat PT TB7/PQP blennd films preppared from pure p chlorofoorm sollution showed rrather uniform m morphology w with a root-meaansquuare (rms) rouughness of 0.722 nm as shownn in Figure 5((d). Thhe films also exxhibited a miniimal face-on orrdering as show wn in the grazing incidence wide w angle x-ray scatteriing (GIWAXS) pattern (Figure 55(a)). These results indicate relatively homoggeneous topoggraphy and ggood miscibiliity, whhich is not favvored for effeective charge dissociation aand trannsport. When DIO was useed as additivee co-solvent, tthe rouughness of blennd film was inccreased to 1.444 nm (Figure 5(e)) andd GIWAXS paattern (Figure 55(b)) indicated a clearer face--on orddering with a dd-spacing of ~ ~4.0 Å. When 1-CN was used, muuch rougher suurface with an average roughhness of 2.20 nm n waas observed (Fiigure 5(f)) andd face-on orienntation of blendded film ms was further enhanced (F Figure 5(c)). Since 1-CN hass a higgher boiling pooint (263 oC), a slower evapooration rate seems to allow favorabble phase sepaaration betweenn electron donnor andd acceptor andd to form molecular ordering and domain size suiitable for verticcal electron traansport pathwaay.8 It was furthher fouund that the phhotovoltaic perrformance of P PQP device couuld be improved whhen a differennt donor polyymer with bettter LU UMO alignmennt was used. When we introoduced Poly[44,8biss(5-(2-ethylhexxyl)thiophen-2--yl)benzo[1,2-bb;4,5b']ddithiophene-2,6-diyl-alt-(4-(22-ethylhexyl)-33fluuorothieno[3,4--b]thiophene-)--2-carboxylate--2-6-diyl)] (PT TB7-Th) polym mer, a PTB7 deerivative developed in our labb,39 as an electron donating polyymer, devices made from tthe PT TB7-Th:PQP bblend exhibited an enhanced PCE value of 3.222 % (Voc = 0..71 V, Jsc = 7..84 mA/cm2, FF F = 0.58). T The acttive blend hass a weight ratiio 1:1 and waas prepared froom chlloroform soluttion with 6% of 1-chloronapphthalene as coc sollvent. As show wn in Figure 3, the LUMO O energy level of PT TB7-Th is closeer to that of P PQP, showing energy matchiing bettween donor annd acceptor wiith a differencee around 0.37 eeV. Whhen fluorinatedd PFP polymeer was blendedd with PTB7-T Th, sim milar solvent efffect was obserrved. The devvice performannce waas enhanced froom the PCE off 0.19 % made from chlorofoorm sollution to 1.43% % with polymeer blends (1:1 w w/w) spin-coatted froom CF with 6% of 1-CN. The device showed alm most ideentical Voc vallues, but the Jsc value is sm maller than PQ QP sysstem, indicatinng charge generration is not ass efficient as thhat witth PQP system m. Since the onnly difference bbetween PFP aand PQ QP is two fluoorine atoms inn the weak acccepting monom mer uniits, all of the optical properrties are quite similar. The tw wo fluuorine atoms inn PFP reducedd the internal polarization of tthe PF FP polymers, which madee the charge separation leess favvorable. The quantum callculation resullts of molecuular orbbital distributioon seem to be cconsistent withh this observatioon. Thhe net electroniic effect is similar to what wee observed in oour PT TB series of donor polymers.40 This reesult echoes tthe fluuorescent measuurement resultss. T To gain more iinsight into thee structure-propperty correlatioon,

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the carrier m mobility of tthe PTB7-Th::electron acceepting polymers was measured by using sandwicch-type devicees and the steady-sttate space-chharge limited current (SCLC) technique.24 The hole moobilities (μHole) of polymer blend films were eevaluated in vertical hole-only devices with structures of ITO/PEDOT:P PSS/ PTB7-Thh:electron acceepting polymers (1:1 w/w, 6 vol% % 1-CN) (blendd state)/Au, annd the electron mobilities (μElectron) are measuredd with structurres of ITO/ZnO/ PTB B7-Th:electronn accepting poolymers (1:1 w w/w, 6 vol% 1-CN) ((blend state)/LiF/Al. It was found that thee hole mobilities off PTB7-Th/P PQP and PT TB7-Th/PIP films exhibited similar values as 1.12 × 10-4 andd 1.34 × 10-4 cm m2/Vs, respectively. T The PTB7-Th//PFP polymer blends exhibiited a much higher hhole mobility of 8.35 × 10-4 cm m2/Vs. (Figuree 6) In contrast, the electron e mobillity of PTB7-Th/PQP filmss was highest as 1.775 × 10-7 cm2//Vs. As a ressult, PTB7-Th//PQP devices showeed more balannced electron//hole mobility than PTB7-Th/PFP P devices. The better hole/eleectron balance iin the devices contrributes to the higher JSC and FF values in photovoltaic devices d becauuse the extentt of recombinnation processes can bbe minimzed.41

Figure 6. J–V characteeristics of (a)) the electronn-only devices and (bb) the hole-onnly devices off PTB7‐Th/eleectron acceptor blendd films. The mobilities m are ccalculated from m the electron- and hhole-only devicces by fitting thhe J–V curves in the SCLC regime.

Figure 7. TEM M images of (aa) PTB7-Th:PQ QP blends (1:1 w/w) and (b) PTB7--Th:PFP blendds (1:1 w/w) caast from chlorooform with 1-CN (6% % v/v).

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Ass investigated using transmission electtron microscoopy (TE EM), the morpphologies of thhe PTB7-Th:P PFP active layyers shoowed severe pphase-segregatiion with largee domain size as shoown in Figuree 7 (b), but P PTB7-Th:PQP P films exhibitted relatively finer nanoscale m morphology ((Figure 7 (aa)), inddicating PTB7--Th:PQP activve layers exhibbit more efficieent chaarge separationn and transporrt, leading to hhigher JSC valuues thaan PTB7-Th:PF FP devices. GIW WAXS studiess (Figure 8) shhowed that neaat PFP and PQ QP film ms are amorphhous and show no obvious diiffraction featuure. Thhe blended film m spin cast from CF/CN soolution of PTB B7Thh:PQP exhibiteed much stroonger face-on orientation thhan PT TB7-Th:PFP fiilm. An averagge interval of (h00) reflectioons aloong the Qr axis of 0.2723 Å-1 was observed, correspondingg to thee layer spacinng of 23.06 Å for (100) crystal, d(100). A An intermolecular π--stacking distaance of 3.93 Å can be assignned T to the layer spaccing of the (0010) crystal planes, d(010). The bettter face-on ordering of PTB B7-Th:PQP film ms facilitated tthe verrtical electron transport patthway, resultinng in higher Jsc chaaracteristic on PTB7-Th:PQP P devices. It ccan also be notted thaat PTB7-Th:PQ QP films exhibbited stronger face-on orderiing thaan PTB7/PQP films as show wn in Figure 8((e) and (f). Thhus, all of these factoors lead to highhest PCE of 3.22 % for PTB B7Thh/PQP system.

Figgure 8. 2D GIW WAXS patternns of (a) the neeat PFP polym mer, (b)) PTB7-Th:PFP (1:1 w/w, CF:CN C 6% v/vv) blends, (c) tthe neaat PTB7-Th ppolymer, (d) thhe neat PQP polymer and (e) PT TB7-Th:PQP (1:1 w/w, C CF:CN 6% vv/v) blends (f) PT TB7:PQP (1:1 w w/w, CF:CN 6% v/v) blends F Further enhanccement of all-polymer cells can be achievved by introducing pplasmonic effeect, as shown in our previoous woork.42 It was found that the adddition of Au nanorods n into tthe PE EDOT:PSS layeer in (PTB7-Thh/PQP) solar cells increased tthe Jsc value to 8.57 m mA/cm2, and ffurther pushed the PCE valuee to 3.552 % (Voc = 0.771 V, Jsc = 8.577 mA/cm2, FF = 0.58). The J− −V currves of PTB77-Th/PQP deevices with aand without A Au nannorods are shhown in Figurre 4(b) and thheir photovolttaic parrameters are suummarized in T Table 3. Figuree 4(c) showed tthe EQ QE of PTB7-T Th/PQP devicee with Au nannorods increassed duee to the inteensified absorrption within visible regioon, ressulting in 9.8% % enhancement of Jsc in the deevices.

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CONCLUSION We have developed several alternating electron accepting polymers with weak acceptor-strong acceptor (WA-SA) combination. The SA moiety is necessary for the efficient electron transfer as an acceptor. The most promising polymer is PQP, which exhibited a PCE of 3.52 % when coupled with plasmonic effect. The improved device performance was achieved only when 1-chloronaphthalene was used as a cosolvent, which helps to organize polymer films with favored face-on polymer chain assembly and phase separation between donor and acceptor. Despite of similar backbone structures between PQP and PFP, PQP devices showed much better photovoltaic performance. Through the fluorescence quenching experiment, we revealed the PQP has the better binding affinity with a PTB7-Th donor polymer, resulting in better electron accepting characteristic. These results suggest that the proper internal polarization of acceptor polymers, similar to what we observed in our PTB series of donor polymers, and the face-on orientation in an active layer is highly important to optimize the photovoltaic properties in allpolymer solar cells. Although lengthy synthesis is used in obtaining some of the polymers, the insight gained will guide our further development of better electron accepting materials.

EXPERIMENTAL SECTION Materials: All of the chemicals were purchased from Aldrich except for tetrakis(triphenylphosphine)palladium from Strem Chemicals. All reagents purchased commercially were used without further purification except for toluene and tetrahydrofuran (THF), which were dried over sodium/benzophenone. 1H NMR and 13C NMR spectra were recorded on a Bruker DRX-400 spectrometers, with tetramethylsilane as an internal reference. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra were recorded using a Bruker Ultraflextreme MALDITOF/TOF mass spectrometer with dithranol as the matrix. Elemental analysis was performed by Midwest MicroLab. The number- and weight-average molecular weights of the polymers were determined by gel-permeation chromatography (GPC) with a Waters Associates liquid chromatography instrument equipped with a Waters 510 HPLC pump, a Waters 410 differential refractometer, and a Waters 486 tunable absorbance detector. Tetrahydrofuran (THF) was used as the eluent and polystyrene as the standard. Thermogravimetric analysis (TGA) measurement of the polymers was performed using a TA Q600 instrument. UV-vis absorption spectra were measured on a Shimadzu UV-3600. Cyclic voltammetry was performed on an AUTOLAB/PG-STAT12 model system with a three-electrode cell in a 0.1 N Bu4NBF4 solution in acetonitrile at a scane rate of 50 mV/s. A film of each polymer was coated onto a Pt wire electrode by dipping the electrode into a polymer solution in chloroform. All measurements were calibrated against an internal standard of ferrocene (Fc), the ionization potential (IP) value of which is -4.8 eV for the

Fc/Fc+ redox system. AFM images were obtained by using an Asylum MFP-3D AFM. TEM measurements were performed by using a Tecnai F-30 with an accelerating voltage at 300 kV. Grazing Incidence Wide‐Angle X‐ray Scattering (GIWAXS): GIWAXS measurements were performed at the 8ID-E beamline at the Advanced Photon Source (APS), Argonne National Laboratory using x-rays with a wavelength of λ = 1.6868 A and a beam size of ~200 µm (h) and 20 µm (v). To make the results comparable to those of OPV devices, the samples for the measurements were prepared on PEDOT:PSS modified Si substrates under the same conditions as those used for fabrication of solar cell devices. A 2-D PILATUS 1M-F detector was used to capture the scattering patterns and was situated at 208.7 mm from samples. Typical GISAXS patterns were taken at an incidence angle of 0.20°, above the critical angles of neat polymers or polymer blends and below the critical angle of while the qz linecut was achieved by a linecut at qy = 0 Å-1 using the reflected beam center as zero the silicon substrate. Consequently, the entire structure of thin films could be detected. In addition, the qy linecut was obtained from a linecut across the reflection beam center. The background of these linecuts was estimated by fitting an exponential function and the parameters of the scattering peaks were obtained through the best fitting using the Pseudo-Voigt type 1 peak function. Mobility measurement: The hole mobilities (μHole) of polymer blend films were evaluated in vertical hole-only devices with structures of ITO/PEDOT:PSS/ PTB7Th:electron accepting polymers (1:1 w/w, 6 vol% 1-CN) (blend state)/Au, and the electron mobilities (μElectron) are measured with structures of ITO/ZnO/ PTB7-Th:electron accepting polymers (1:1 w/w, 6 vol% 1-CN) (blend state)/LiF/Al. At higher voltages (in the SCLC region), the J-V characteristics can be fitted with the Mott-Gurney equation43: JSCLC = (9/8) εrε0µ(V2/L3) where ε0 is the permittivity of free space (8.85 x 10-14 C V-1 cm-1), εr is the dielectric constant (3.2), μ is the electron mobility, V is the voltage drop across the device, and L is the polymer thickness, V = Vappl - Vr - Vbi, Vappl is the applied voltage to the device, Vr is the voltage drop due to contact resistance and series resistance across the electrodes, Vbi is the built-in voltage due to the difference in work function of the two electrodes. PSC device Fabrication: Organic photovoltaic cells with a device configuration of glass/indium tin oxide (ITO)/poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) /polymer blend films/Ca/Al were prepared. Prior to device fabrication, the ITO substrates were cleaned with detergent and ultrasonicated in deionized water, acetone and isopropanol, and then dried overnight in an oven. The substrate was spincoated by a PEDOT:PSS solution without or with Au nanorod (0.01% in DI water, Nanocs, USA), dried at 100 oC in N2 for 30 min, and then transferred to a glove box for spin-casting of the polymer layer. The solution containing a blended mixture of PTB7 (or PTB7-Th)/acceptors in CF:DIO (3 v/v%) and in CF:CN (6 v/v%) was spin casted by 5000 rpm onto the above substrate. PTB7 (or PTB7-Th)/acceptor films were used directily without annealing process. Subsequently, the device

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was pumped down under vacuum (< 10-6 Torr) and the Ca (20 nm) and Al (80 nm) electrode was deposited by thermal evaporation in the glove box at a chamber pressure of ~5.0×10-7torr. The active area of the solar cell is 3.14 mm2, which is defined by the cathode area. Current density-voltage (J-V) characteristics of the devices under nitrogen were measured using a Keithley 238 Source Measure unit. The photovoltaic properties were characterized under an Air Mass 1.5 Global (AM 1.5G) solar simulator with irradiation intensity of 100 mW/cm2.

ASSOCIATED CONTENT

Supporting Information. 1H and 13C NMR spectra, CV graph, SCLC spectra, photovoltaic properties of the synthesized materials. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected] Present Addresses ⊥

Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 305-600, Republic of Korea ┬ Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea

Author Contributions #these authors contributed equally.

ACKNOWLEDGMENT This work is supported by U. S. National Science Foundation grant (NSF DMR-1263006), Air Force Office of Scientific Research and NSF MRSEC program at the University of Chicago (DMR-0213745), DOE via the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0001059. W. Chen gratefully acknowledges financial support from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number KC020301. Use of the Advanced Photon Source (APS) at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

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(43) Kim, K.-H H.; Kang, H.; K Kim, H. J.; Kim m, P. S.; Yoon, S. C.; Kim, B. Effe fects of Solubbilizing Grouup Modificatioon in

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Fullerene Bis-Addducts on Norm mal and Inverteed Type Polym mer Solar Cells. Chem m. Mater. 20122, 24, 2373.

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