Materials Design via Optimized Intramolecular Noncovalent

Article pubs.acs.org/cm

Materials Design via Optimized Intramolecular Noncovalent Interactions for High-Performance Organic Semiconductors Xiaojie Guo,†,∥ Qiaogan Liao,†,∥ Eric F. Manley,‡,¶ Zishan Wu,†,⊥ Yulun Wang,† Weida Wang,† Tingbin Yang,† Young-Eun Shin,§ Xing Cheng,† Yongye Liang,† Lin X. Chen,‡,¶ Kang-Jun Baeg,*,§ Tobin J. Marks,*,‡ and Xugang Guo*,† †

Shenzhen Key Laboratory for Printed Organic Electronics, Department of Materials Science and Engineering, South University of Science and Technology of China (SUSTC), No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡ Department of Chemistry and the Materials Research Center, the Argonne-Northwestern Solar Energy Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States § Department of Graphic Arts Information Engineering, Pukyong National University, 365 Sinseon-ro, Nam-gu, Busan 48547, Republic of Korea ¶ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, United States S Supporting Information *

ABSTRACT: We report the design, synthesis, and implemention in semiconducting polymers of a novel head-to-head linkage containing the TRTOR (3-alkyl-3′-alkoxy-2,2′-bithiophene) donor subunit having a single strategically optimized, planarizing noncovalent S···O interaction. Diverse complementary thermal, optical, electrochemical, X-ray scattering, electrical, photovoltaic, and electron microscopic characterization techniques are applied to establish structure−property correlations in a TRTOR-based polymer series. In comparison to monomers having double S···O interactions, replacing one alkoxy substituent with a less electron-donating alkyl one yields TRTOR-based polymers with significantly depressed (0.2−0.3 eV) HOMOs. Furthermore, the weaker single S···O interaction and greater TRTOR steric encumberance enhances materials processability without sacrificing backbone planarity. From another perspective, TRTOR has comparable electronic properties to ring-fused 5Hdithieno[3,2-b:2′,3′-d]pyran (DTP) subunits, but a centrosymmetric geometry which promotes a more compact and ordered structure than bulkier, axisymmetric DTP. Compared to monosubstituted TTOR (3-alkoxy-2,2′-bithiophene), alkylation at the TRTOR bithiophene 3-position enhances conjugation and polymer crystallinity with contracted π−π stacking. Grazing incidence wide-angle X-ray scattering (GIWAXS) data reveal that the greater steric hindrance and the weaker single S···O interaction are not detrimental to close packing and high crystallinity. As a proof of materials design, copolymerizing TRTOR with phthalimides yields copolymers with promising thin-film transistor mobility as high as 0.42 cm2/(V·s) and 6.3% power conversion efficiency in polymer solar cells, the highest of any phthalimide copolymers reported to date. The depressed TRTOR HOMOs imbue these polymers with substantially increased Ion/Ioff ratios and Voc’s versus analogous subunits with multiple electron donating, planarizing alkoxy substituents. Implementing a head-to-head linkage with an alkyl/alkoxy substitution pattern and a single S···O interaction is a promising strategy for organic electronics materials design.

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INTRODUCTION Rationally tailored polymer semiconductors offer great potential for fabricating diverse opto-electrical devices using low-capital solution-based processing techniques, such as coating and printing.1−3 The solution processability enables fabrication of cost-effective, large area, and mechanically flexible devices, such as organic thin-film transistors (OTFTs) and polymer solar cells (PSCs).2,4−6 In order to achieve optimal performance while maintaining straightforward, scalable device fabrication, the semiconducting materials should possess well© XXXX American Chemical Society

tuned bandgaps, energetically optimized frontier molecular orbitals (FMOs), a favorable film morphology, and good solubility.4,7−15 However, designing and realizing macromolecules which synergistically integrate these performance enhancing parameters without sacrificing processability represents a great challenge. To achieve such processability, polymer Received: February 29, 2016 Revised: March 4, 2016

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DOI: 10.1021/acs.chemmater.6b00850 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Design strategies for polymer semiconductors to achieve enhanced planarity, conjugation, and mobility by introducing: (a) spacer groups, (b) conformational locking via covalent bonds, (c) conformational locking via noncovalent double S···O interactions, and (d) conformational locking via a noncovalent single S···O interaction (this work).

semiconductors are typically functionalized with solubilizing alkyl side-chain substituents. However, the alkylation patterns must be strategically located to minimize steric hindrance; hence, head-to-head (HH) linkages should be avoided in semiconducting polymer design to minimize accompanying backbone torsion, which reduces conjugation along the polymer backbone, compromises film crystallinity/order, and diminishes charge carrier mobility.16−20 To achieve high degrees of macromolecular backbone planarity for enhanced charge carrier delocalization, the backbone must be designed to energetically favor planar conformations versus twisted alternatives. Although in principal conjugated backbones should energetically favor planar conformations, adverse steric interactions, mainly from side substituents, often prevent realization of this ideal case. Two materials design strategies are widely employed in organic electronics: (1) reducing steric hindrance by inserting spacers (or bridges) along the chain17,21 and (2) conformation locking via covalent bonds.22,23 The strategy of inserting spacers (Figure 1a), mainly unsubstituted thiophene (or thiazole) derivatives, has yielded great success in high-performance semiconductors such as PBTTT and PQT.21,24−28 However, spacer incorporation requires additional steps in the monomer synthesis29−31 and could also dilute the concentration of key building blocks, typically the acceptor subunits, in the polymeric backbones, risking suboptimal optoelectronic properties for the resulting semiconductors.32−35 Moreover, such spacers are typically nonalkylated, which could reduce polymer solubilities.17,36,37 Conformational locking (Figure 1b) with covalent bonds has also provided great success as a planarizing design strategy; however, the sp3 hybridization of the bridging atoms, such as C, Si, and Ge, leads to out-of-plane substituent orientation, thereby enlarging intermolecular stacking distances, reducing interchain π−π orbital overlap, and lowering carrier mobilities.22,23,38 Therefore, polymer semiconductors containing cyclopentadithiophene or dithienosilole/germole frequently have limited OTFT mobilities and suboptimal fill factors (FFs; < 70%) in bulk heterojunction (BHJ) PSCs.39−42 Moreover, the covalent bond fused arenes can lead to reduced materials solubility of the resulting polymers.43 Poly(3,4-ethylenedioxythiophene) (PEDOT) is a widely used conducting polymer with high doped state conductivity,44,45 which is partially attributed to substantial backbone planarity.46 From the crystal structure of 2,2-bis(3,4-ethylenedioxythiophene), the distance between the (thienyl)sulfur and (3,4-ethylenedioxy)oxygen atoms (S···O) is 2.96 Å, substantially below the sum of the S and O van der Waals radii (3.25 Å).47 This intramolecular noncovalent S···O interaction promotes a planar backbone conformation and charge carrier delocalization.42,48,49 Inspired by the success of PEDOT, we first reported a HH linkage containing donor subunit, 3,3′-dialkoxy-2,2′-bithiophene (BTOR, Figure 2b),50 via employing a dual strategy of reducing the steric bulk of the side-chain fragment closest to the conjugated system by

Figure 2. Chemical structures, optimized geometries, and energy levels of the frontier molecular orbitals of (a) 3,3′-dialkyl bithiophene (BTR); (b) 3,3′-dialkoxy bithiophene (BTOR); (c) 5H-dithieno[3,2b:2′,3′-d]pyran (DTP); (d) 3-alkoxy-2,2′-bithiophene (TTOR); (e) 3alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR). Calculations carried out at the DFT//B3LYP/6-31G** level. Alkyl substituents truncated here to simplify the calculations.

replacing a CH2 fragment with a less bulky O group and utilizing the intramolecular noncovalent (thienyl)S···(alkoxy)O interaction51−58 to help planarize the backbone. Incorporating BTOR subunits into polymers affords semiconductors (Figure 1c) with small bandgaps, close intermolecular packing, a substantial hole mobility of ∼0.2 cm2/(V·s), and excellent solubility from the high density of alkyl substituents.59 The results demonstrate that strategically utilizing alkoxy side chains and employing the intramolecular noncovalent S···O interaction is an effective strategy for organic electronic materials design.48,52,53 Despite the attractions of the above planarization strategy, the electron-rich BTOR structure significantly elevates the resulting polymer HOMOs, and such semiconductors suffer from unsatisfactory air stability in OTFTs60 and small opencircuit voltages (Voc’s < 0.6 V) in PSCs, severely limiting BTOR materials applicability.61−63 Note that Yoshimura and coworkers developed an electron-rich 5H-dithieno[3,2-b:2′,3′d]pyran (DTP, Figure 2c),64 which has greater electrondonating capacity than cyclopentadithiophene.65 When copolymerized with difluorobenzothiadiazole, the resulting polymer semiconductor has a smaller bandgap than the cyclopentadithiophene counterpart but maintains a favorable Voc of 0.7 V. The PSCs show promising power conversion efficiencies (PCEs) of 8.0%64 and 10.6%66 in single junction and tandem cells, respectively. However, the DTP synthesis is tedious and PSC performance is limited by the small FF (63%) reflecting the low DTP polymer carrier mobility, likely attributable to the B


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emcumbrance in combination with a noncovalent S···O interaction to lock the polymer backbone toward planarity render TRTOR a promising building block for polymer semiconductor construction. Materials structure−property− device performance correlations are established here, offering useful insights into organic electronics materials design. On the basis of the correlations, more promising device performance should be readily obtained by optimizing the polymer structures using other acceptor subunits.

out-of-plane orientation of the solubilizing substituents and the asymmetric DTP structure. Inspired by the promising device performance of the BTOR and DTP-based copolymers, here we report the design and synthesis of the novel 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR) donor subunit (Figure 2e) and that its incorporation into copolymers affords semiconductors (Figure 1d) with good materials processability, high degrees of backbone planarity, appropriately positioned FMOs, and ordered film morphologies. To reduce BTOR electron-donating characteristics, an oxygen is removed from an alkoxy substituent to afford the optimized TRTOR building block, which bears a single alkoxyand alkyl-chain substituent and, hence, a single S···O interaction. From another perspective, TRTOR is a 2H-pyran ring opened DTP; hence, TRTOR should have electronic properties comparable to those of DTP. Density functional theory (DFT) computation demonstrates a planar backbone conformation for 3-alkoxy-2,2′-bithiophene (TTOR) subunit (Figure 2d), which is functionalized with a single solubilizing alkoxy chain. Introducing an extra alkyl chain on the 3-position of thiophene should not be detrimental to the TRTOR backbone planarity. Indeed, this is supported by the DFT computation, which indicates that a TRTOR-containing HH linkage maintains a planar conformation enabled by the single planarizing alkoxy side chain (Figure 2e).48,52 In comparison to TTOR, TRTOR is a more promising building block since it contains more solubilizing substituent chains and has a more symmetrical structure. In contrast, the asymmetric single-chain functionalized bithiophene-based polymer exhibits poor PSC performance.33 Regarding energetic considerations, TRTOR has a low-lying HOMO at −4.92 eV, 0.25 eV below that of BTOR (−4.67 eV) and comparable to that of DTP (−4.97 eV; Figure 2). Therefore, TRTOR-based copolymers should have lower-lying HOMOs versus the BTOR-based counterparts, which will benefit both OTFT and PSC device performance. Furthermore, the single TRTOR S···O interaction should afford the copolymers with enhanced processability versus the BTOR counterparts due to the weaker noncovalent interactions in the former. Compared to DTP, TRTOR should have more compact stacking due to the elimination of out-of-plane substituents and its centrosymmetric geometry, which is advantageous over the axisymmetric geometry of DTP.67 Thus, TRTOR should be a promising building block for polymer semiconductor construction due to its planar conformation, solubilizing groups, appropriately placed HOMO energy, and centrosymmetric geometry. To verify the above materials design strategy, phthalimide10,59,68 was chosen as the in-chain acceptor block to construct a semiconducting copolymer series. Although it is not the optimal acceptor subunit for constructing state-of-the-art polymer semiconductors, phthalimide was chosen here for its easy materials accessibility, effectiveness for polymerization, and better elucidation of the materials structure−property−device performance correlations based on previous work.59−61,63 On the basis of complementary thermal, optical, electrochemical, X-ray scattering, electrical, photovoltaic, and electron microscopic characterization data, it will be seen that the resulting phthalimide-TRTOR polymers exhibit promising device performance in both ogranic thin-film transistors and polymer solar cells, with the PCE (6.3%) of the phthalimide-TRTOR polymer being among the highest reported to date for phthalimide-based polymers.10 These results demonstrate that a single planarizing alkoxy substituent to reduce steric

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RESULTS AND DISCUSSION Materials Synthesis. The synthesis of the key TRTOR building block is straightforward, and Scheme 1 depicts the

Scheme 1. Synthesis of TRTOR Monomer and TRTORBased Polymers P5a

a

The BTR, BTOR, DTP, and TTOR-based polymers P1−P4 were also synthesized for comparison. The intramolecular noncovalent S··· O interactions are indicated by red dotted lines. Reagents/conditions: (i) NBS, chloroform, HOAc; (ii) n-BuLi, isopropoxyboronic acid pinacol ester, THF; (iii) ROH, PTSA, toluene, 110 °C; (iv) NBS, DMF; (v) Pd(PPh3)4, K2CO3, THF, H2O; (vi) n-BuLi, Me3SnCl, THF; (vii) Pd2(dba)3, P(o-tolyl)3, toluene, microwave, 140 °C.

synthetic approach. More detailed monomer and polymer synthetic and characterization information is reported in the Supporting Information. Dioxaborolane (369) and brominated thiophene (6 50 ) were synthesized following published procedures. Suzuki coupling between 3 and 6 affords monomer precursor 7 in good yield (>60%), with subsequent lithiation and quenching with Me3SnCl providing monomer 8 in good purity. After further purification via recrystallization from isopropanol, 8 was copolymerized with the imide-functionalized arene, dibromophthalimide,59,68 under a typical Stille polymerC


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Chemistry of Materials Table 1. Molecular Weights, Optical Absorption, and Electrochemical Data for Polymer Series P1−P5 polymer

Mn (kDa)/PDIa

λmax sol (nm)b

λmax film (nm)c

λshoulder film (nm)c

λonset film (nm)

EHOMO (eV)d

ELUMO (eV)e

Egopt (eV)f

P1 P2 P3 P4 P5a P5b P5c

9.6/1.9 38.3/2.5 8.1/3.1 7.8/1.3 29.3/2.1 35.3/1.5 18.5/1.9

390 565 515 513 474 485 472

410 612 533 556 570 579 582

NA 674 NA 608 630 644 641

513 752 642 653 670 691 685

−5.66 −4.93 −5.34 −5.28 −5.19 −5.18 −5.25

−3.24 −3.29 −3.41 −3.38 −3.34 −3.39 −3.44

2.42 1.65 1.93 1.90 1.85 1.79 1.81

a GPC versus polystyrene standards, trichlorobenzene as eluent, at 170 °C. bSolution absorption spectra (1 × 10−5 M in chloroform). cThin film absorption spectra of pristine film cast from 5 mg/mL CHCl3 solution. dEHOMO = −(Eoxonset + 4.80) eV, and Eoxonset determined electrochemically using Fc/Fc+ internal standard. eELUMO = EHOMO + Egopt. fOptical bandgap estimated from absorption edge of as-cast thin film.

Figure 3. (a) Optical absorption spectra of polymer films of P1−P5. (b) Cyclic voltammograms of P1−P5 films in 0.1 M (n-Bu)4N·PF6 acetonitrile solution with the Fc/Fc+ redox couple as the internal standard.

ization protocol using microwaves for heating (Scheme 1). For better comparison and elucidation of the structure−property correlations of TRTOR-based polymers P5, BTR (P1), BTOR (P2), and DTP (P3) and TTOR-based (P4) polymer, analogues were also synthesized. After polymerizations, the polymer chains were end-capped with monofunctionalized thiophene.70 Product polymers were collected by precipitation in methanol and were then subjected to purification via Soxhlet extractions using different solvent sequences, depending on the polymer solubility. The identity and purity of the product polymers are supported by 1H NMR and elemental analysis. All the polymers exhibit good solubility in common organic solvents for device fabrication. Molecular weights were measured by gel permeation chromatography (GPC) versus polystyrene standards. Number-average molecular weight, Mn, and polydispersity index (PDI) data are summarized in Table 1. It is instructive to compare the polymer solubilities. P1 is most soluble due to its twisted backbone induced by the headto-head linkage. Due to the single S···O interaction in TRTOR, P5a shows greatly enhanced solubilities versus P2, which has stronger BTOR double S···O interactions.59 Hence, polymers P5b and P5c with shorter alkyl chains are also synthesized and exibit good solubility at 25 °C, while P2 analogues having the same side chains are intractable. Therefore, enhanced materials processability can be achieved by optimizing the intramolecular S···O interactions. For polymer P4, a large branched 2hexyldecyl chain is attached to the phthalimide for achieving good solubility since the TTOR subunit only contains one solubilizing side chain. Polymer Optical and Electrochemical Properties. Absorption spectra of all polymer films are shown in Figure 3, and relevant absorption data and optical bandgaps are summarized in Table 1. In solution, all polymers show

featureless absorption profiles (Figure S1), which indicate a lack of aggregation and ordering in chloroform. From solution to film state, the polymers show bathochromic shifts due to the increased backbone planarity and enhanced aggregation.71 Among all polymers, P2 shows the most bathochromically shifted absorption and the smallest bandgap of 1.65 eV due to the strong BTOR electron-donating ability. In comparison to P1, the insertion of the oxygen atom between the thiophene and alkyl side chain leads to a dramatic red shift (202 nm) of the absorption maximum and an ∼0.8 eV smaller optical bandgap for P2 in the film state. This substantial bandgap narrowing in P2 is attributed to the stronger alkoxy substituent electron-donating ability (versus that of an alkyl substituent), more planar polymer backbone due to the reduced steric hindrance by replacing a CH2 with a less bulky O, and the intramolecular noncovalent S···O interaction. Polymer P3 containing the 5H-dithieno[3,2-b:2′,3′-d]pyran (DTP)64 electron donor subunit has a bandgap of 1.93 eV, which lies between that of P1 (2.42 eV) and P2 (1.65 eV). In comparison to the P2 bandgap, the larger P3 bandgap (1.93 eV) is attributed to the weaker electron donating ability of DTP which contains one electron donating OR and one C versus two ORs in BTOR. Considering the comparable electronic properties of DTP and TRTOR (Figure 2) with each having one electron donating OR and one C, P5a exhibits red-shifted absorption and a smaller bandgap (1.85 eV) versus that of P3 (1.93 eV), likely reflecting the more compact intermolecular stacking due to the absence of out-of-plane substituents and greater phthalimide acceptor-thiophene donor coplanarity in P5a, supported by DFT analysis (Figure S3). Therefore, TRTOR should be a more effective subunit for creating narrow bandgap polymers than DTP. D


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Figure 4. DSC thermograms of polymers P1−P5 at a temperature ramp rate of 10 °C/min under N2. The top lines are from the first cooling run, and the bottom lines are from the second heating run.

It is instructive to compare the optical absorption spectra of P4 and P5a, which both contain a single electron-donating alkoxy substituent. In spite of potentially enhanced steric repulsion by introducing the solubilizing alkyl chain on TRTOR vs TTOR, P5a shows a red-shifted absorption maximum and a slightly smaller bandgap than P4. The smaller P5a bandgap is reasonably attributed to the weak electrondonating ability of the alkyl chain, but the effect of polymer molecular weight (Mn) should not be ruled out.72 Importantly, these results show that the TRTOR alkyl chain is not detrimental to backbone planarity or molecule packing; otherwise, the P5a bandgap would be greater than that of P4. Due to the weaker conformational locking strength of the single TRTOR S···O noncovalent interaction vs the double S··· O interaction in BTOR, polymers P5b and P5c containing smaller solubilizing groups are also synthesized (Scheme 1) and have good solubility, which will be beneficial to P5 performance in PSCs due to the greater absorption coefficients. As the solubilizing substituents contract, P5b and P5c have red-shifted absorptions and smaller bandgaps of 1.79 and 1.81 eV, respectively, accompanied by a distinctive absorption shoulder at ∼640 nm. From the solution to film state, TRTOR polymers P5 show the largest redshift (∼100 nm) of the absorption maximum versus others in the series (18−47 nm), indicating the highest degree of polymer backbone conformational change. Such changes are attributed to the strong TRTOR solubilizing ability, the weak single S···O conformational locking, and the high degree of film state backbone planarity. Of all the present polymers, only those having S···O interactions show optical absorption shoulders typical of ordered film structures.17,73,74 From the optical absorption profiles and DFT results, it can be concluded that the steric benefit and the S···O interaction of the single alkoxy side chain are effective to force the polymer backbone toward planarity. Clearly, addition of the extra 3-position bithiophene alkyl substituent is not detrimental to polymer packing in TRTORbased polymers. The electrochemical properties of polymers P1−P5 were investigated using cyclic voltammetry vs a ferrocene/ferrocium (Fc/Fc+) internal standard (Figure 3b). Within this series, P1 does not show an obvious oxidation peak, likely due to the weak BTR electron donor and limited backbone conjugation of the head-to-head linkage. In contrast, polymers P2−P5 exhibit distinctive oxidation peaks. Going from P1 to P2, the O insertion affords the highest lying HOMO (−4.93 eV) in the series for polymer P2, in good agreement with the DFT

computation (Figure 2) and reflecting the strong BTOR subunit electron donating ability. P3 has a low-lying HOMO of −5.34 eV due to the moderate DTP electron donating ability64 and torsional polymer backbone.21,75 In comparison to P3, polymer P4 has a slightly higher-lying HOMO at −5.28 eV, attributable to the higher degree of P4 conjugation from the more planar backbone and compact packing.76 In comparison to BTOR-based polymer P2, replacement of one alkoxy substituent by an alkyl substituent lowers the P5a HOMO (−5.19 eV) by 0.26 eV vs that of P2 (−4.93 eV). Polymer P5c with a branched N-2-ethylhexyl group has a slightly lower HOMO (−5.25 eV) than P5a (−5.19 eV) and P5b (−5.18 eV) with linear N-alkyl substituents.76 In comparison to P4, polymers P5a−c have slightly higher HOMOs, likely due to the weak electron-donating P5 alkyl substituent. From the electrochemical data, the removal of one oxygen in BTOR effectively lowers the TRTOR polymer HOMOs by a large margin (∼0.3 eV), which should enhance Ion/Ioff ratios and device stability in OTFTs as well as increase Voc’s in PSCs (vide infra).77−79 Polymer Thermal Properties. Polymer thermal analysis was carried out by differential scanning calorimetry (DSC, Figure 4). The BTOR-based polymer P2 shows an irreversible endotherm around 340 °C, which is likely attributed to the thermal decomposition of polymer according to the thermogravimetric analysis (Figure S2).59 For the DTP-based polymer P3, DSC reveals a featureless thermogram, indicating amorphous character or low crystallinity, while the TRTORbased polymers P5a−c exhibit distinct thermal transitions, indicating higher degrees of crystallinity vs BTOR-based polymer P2 and DTP-based polymer P3.17 Therefore, in comparison to BTOR-based polymer P2, the removal of one O is not detrimental to TRTOR-based P5 crystallinity. Compared to DTP-based P3, DTP ring-opening affords enhanced crystallinity for TRTOR-based polymer P5, attributable to eliminating out-of-plane alkyl substituents and the centrosymmetric TRTOR geometry. As the solubilizing substituents contract, the transition temperatures rise and the peaks sharpen from P5a to P5b.34,73 Note also that, in spite of its twisted backbone, BTR-based polymer P1 has a distinct thermal transition at a low temperature of ∼200 °C, consistent with some ordering (vide infra). Organic Thin-Film Transistors and Polymer Solar Cells. Top-gate/bottom-contact organic thin-film transistors (OTFTs) with poly(methyl methacrylate) (PMMA) gate dielectrics and gold source/drain electrodes were fabricated E


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Table 2. Organic Thin-Film Transistor Performance and Photovoltaic Response Properties of Polymer Semiconductors P1−P5

a

polymer

μa (cm2/(V·s))

Ion/Ioff

Vt (V)

polymer:PC71BMb

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

P1 P2 P3 P4 P5a P5b P5c

NA 1.45 NA NA 0.18 0.043 0.030

NA 103−4 NA NA 105−6 105−6 105

NA −19 NA NA −33 −30 −29

1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.6

0.26 11.84 2.67 0.49 11.81 12.50 11.81

0.49 0.45 0.95 0.65 0.72 0.71 0.77

37.5 63.5 57.4 33.5 66.1 71.4 68.1

0.05 3.38 1.46 0.11 5.62 6.31 6.17

Average performance of ≥5 devices. bDevice area = 0.16 cm2.

Figure 5. Transfer curves of P2 and P5 thin-film transistors at (a) the linear (Vd = −10 V) and (b) saturation (Vd = −80 V) regions; (c) J−V characteristics of the optimized inverted polymer solar cells of P1−P5 under simulated AM 1.5 G illumination (100 mW cm−2).

Figure 6. In-plane (qxy) and out-of-plane (qz) line cuts of the GIWAXS images for the indicated neat polymer films cast on octyldecyltrichlorosilane (OTS)-modified Si substrates. Spectra are split in order to better view the (100) lamellar interactions (left) and the (010) π−π interactions (right).

dielectric compatibility without charge-trapping sites, and favorable self-encapsulation by the upper dielectric layer and gate electrode. Nevertheless, despite the substantial P2 μh, the TFT performance is plagued by high off-currents (10−8−10−7 A), hence a small on/off-current ratio (Ion/Ioff) of 103−4 in the linear regime at a drain voltage (Vd) of −10 V (Figure 5a) due to the high-lying P2 HOMO. New TRTOR-based polymer P5a has an average μh of 0.18 cm2/(V·s) with small off-currents, 10−12−10−11 A, and a high Ion/Ioff of 105−6 (Table 2); the maximum P5a mobility measured is 0.42 cm2/(V·s) in topgate/bottom-contact OTFTs. The substantial μh reflects the high degree of backbone planarity, close π−π stacking distance, and ordered film morphology. In comparison to BTOR-based polymer P2, P5a has a > 1000× smaller off-current (10−12− 10−11 A) and ∼100× higher Ion/Ioff (105−6) in the linear region (Figure 5a), primarily ascribable to the lower-lying P5a HOMO. Due to the enhanced solubility, P5b and P5c with

to characterize the charge transport properties of phthalimide copolymers P1−P5.80 Device performance data are compiled in Table 2, and representative transfer curves (the drain current, Id versus gate voltage, Vg) of the P2 and P5 OTFTs are shown in Figure 5. BTR-based polymer P1 is inactive in OTFTs, which is attributed to the very low-lying HOMO (−5.66 eV), twisted backbone, limited conjugation, and low degree of crystallinity.81 Under optimized conditions, P2 has a high hole mobility (μh) of 1.45 cm2/(V·s), almost 10× that in bottom-gate/bottomcontact OTFTs (0.17 cm2/(V·s)).59 The high P2 mobility can be partially ascribed to facile hole injection from the gold electrode to the high-lying HOMO, since the bottom-contact gold electrode has lower work function of ca. −4.5 to −4.7 eV. Versus bottom-gate OTFTs, the performance enhancement in the top-gate OTFTs likely reflects the lower contact resistance due to reduced current crowding effects in staggered contact structures,82 as well as the excellent active channel-polymer F


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Figure 7. GIWAXS images of polymer blends with PC71BM, processed with 3% ODT additive.

large Voc is ascribed to the twisted polymer backbone,75 the small Jsc to the suboptimal blend film morphology (vide infra), and the negligible carrier mobility measured in both OTFT/inplane (Table 2) and space-charge limited current (SCLC)/outof-plane techniques (Table S3). Note that TRTOR-based P5a shows a promising PCE of 5.62% with a Jsc = 11.81 mA/cm2, Voc = 0.72 V, and FF = 66%. Due to the weaker intramolecular S···O interaction in TRTOR versus the double S···O interactions in BTOR, more compact solubilizing substituents can be used and the resulting P5b cells have a further enhanced PCE of 6.31% with a Jsc = 12.50 mA/cm2, Voc = 0.71 V, and FF = 71.4%. The Jsc’s integrated from external quantum efficiency (EQE, Figure S5) data vs an AM1.5 reference spectrum are within ±5% of those from the J−V data, showing good internal consistency. During the course of device optimization, high FFs of 70−75% were routinely obtained from P5b PSCs under various fabrication conditions, primarily attributable to the high hole mobility of P5b with μh,SCLC = 1.98 × 10−3 cm2/(V·s) (Table S3) due to absence of out-of-plane substituents and greater film order/crystallinity,24,86 while P3 PSCs have a lower FF (57.4%), which is partially attributed to the low SCLC mobility (2.93 × 10−5 cm2/(V·s)). In comparison to P2, TRTOR-based P5 exhibits greatly increased PSC performance in Voc, EQE, and FF. Using branched 2-ethylhexyl substituted P5c as the donor further increases Voc to 0.77 V. Therefore, TRTOR is an effective subunit for creating semiconducting copolymers with large PSC Voc’s. In comparison to P2, the Voc enhancement is attributed to the depressed HOMO achieved by replacing one alkoxy with an alkyl substituent, and in comparison to P3, the substantially enhanced Jsc and FF are ascribed to the smaller P5 bandgap, closer packing, and higher mobility. Compared to P4, placing an alkyl chain on the bithiophene 3-position greatly enhances P5 PSC response. Note that P5 performance is far higher than for any phthalimide-based polymer reported to date,10 which demonstrates the great potential of TRTOR subunits for highperformance OTFTs and PSCs. Film Morphology: Device Performance Correlations. GIWAXS was carried out at beamline 8-ID-E of the Advanced Photon Source to examine polymer film microstructure and morphology. The polymer films were prepared neat on octyldecyltrichlorosilane (OTS)-modified Si substrates, blended with PC71BM, and blended with PC71BM processed with 3% (v/v) ODT as the processing additive on bare Si. 2D images for the blend films processed with ODT are shown in Figure 7, and 2D images for the neat films and the blend films

short alkyl substituents are also sufficiently soluble for OTFT fabrication, and the resulting OTFTs have appreciable μh’s of 0.043 and 0.030 cm2/(V·s), respectively. The enhanced P5a mobility with longer alkyl substituents may be due to the compressed π−π stacking and more ordered microstructure from the side-chain crystallization49 as revealed by grazing incidence wide-angle X-ray scattering (GIWAXS, vide infra). In comparison to P2, P5a films have lower mobility, likely due to the lower-lying HOMO and hence a larger charge injection barrier. Although extensive film growth optimizations were carried out with DTP-based polymer P3, this material exhibits negligible transistor response, in accord with the out-of-plane alkyl-chain orientation, twisted backbone, and amorphous film morphology revealed by the GIWAXS (Figure 6). Therefore, opening the DTP ring affords greatly enhanced μh for the TRTOR-based polymer P5a vs that of DTP-based P2. TTORbased polymer P4 also shows negligible OTFT mobility, which agrees with the poor film-forming properties and limited crystallinity revealed by GIWAXS (vide infra). Therefore, introducing an alkyl chain in the bithiophene 3-position greatly increases the hole mobility of TRTOR-based polymer P5a vs that of TTOR-based polymer P4 due to the more symmetric TRTOR structure and more crystalline P5a film morphology. The OTFT data indicate that TRTOR is a promising building block for high-mobility polymer semiconductors. Inverted PSCs having a device structure of ITO/ZnO/ polymer:PC71BM/MoO3/Ag were fabricated to investigate the PSC performance of polymers P1−P5,83 and current density− voltage (J−V) plots are illustrated in Figure 5c, with relevant performance parameters collected in Table 2. During device optimization, it was found that the processing additive, 1,8octanedithiol (ODT), greatly enhances PSC performance by promoting nanoscale phase separation and bicontinuous interpenetrating network formation (vide infra).84,85 Among all polymers, P1 and P4 show negligible PSC response with PCE ≤ 0.1% (Table 2), due to their nonideal film morphologies, poor film-forming properties, and negligible charge transport capacity. The P2 performance parameters of PCE = 3.38%, Jsc = 11.84 mA/cm2, Voc = 0.45 V, and FF = 63.5% are substantially higher than previous cells with conventional architectures.61 The performance of P2 cells is mainly limited by the small Voc, in good agreement with the high-lying HOMO (Figure 3b), reflecting the electron-rich BTOR character. The DTP-based polymer P3 cells have PCE = 1.46%, Jsc = 2.67 mA/cm2, Voc = 0.95 V, and FF = 57.4%. The G


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For the polymers with intramolecular S···O interactions, P2, P5a, P5b, and P5c exhibit fairly standard lamellar structure with short π−π stacking distances. P2 and P5a have larger (100) dspacings (∼30 Å) versus those (∼22 Å) of P5b and P5c, consistent with their longer side substituents. The π−π stacking distances derived from (010) reflections are 3.61, 3.56, 3.59, 3.66 Å for neat P2, P5a, P5b, and P5c films, respectively. Therefore, replacing BTOR with TRTOR leads to comparable or even slightly smaller π−π stacking distances. In comparison to BTOR, the greater steric hindrance engendered by replacing one alkoxy substituent with a bulkier alkyl substituent and the weaker single S···O conformational locking strength in TRTOR is clearly not detrimental to the packing and crystallinity of polymer P5a. As the phthalimide N-substituents are varied from a linear n-dodecyl chain to a branched 2-ethylhexyl chain, the π−π stacking distance expands by 0.1 Å for P5c, similar to effects observed in BTOR-phthalimide copolymers59 and alkylthieno[3,4-c]pyrrole-4,6-dione-based copolymers.76 The small P5a π−π stacking distance of 3.56 Å correlates with the highest mobility within the P5 series. In comparison to TTOR polymer P4 with a π−π stacking distance of 3.72 Å, the smaller distance in TRTOR polymer P5a indicates that alkylation at the bithiophene 3-position enhances polymer film crystallinity and contracts the π−π stacking distance, consistent with the more symmetrical TRTOR structure and removal of the highly branched phthalimide N-alkyl substituent. The diffraction data also indicate that neat P2, P5a, P5b, and P5c films do not exhibit clear preference for edge-on or face-on orientation in neat films and exhibit lamellar (100) and π−π stacking (010) reflections in both the in-plane (qxy) and out-of-plane (qz) directions (Figure 6). Among TRTOR-based polymers, P5a has the largest (010) correlation lengths (5.8 nm; Table 3), which in combination with its smallest π−π stacking distance (3.56 Å) is consistent with the highest mobility in the P5a−c subseries. For the P1−5 bulk heterojunction blend films, ODT processing generally affords more crystalline morphologies, leading to enhanced carrier mobility in the SCLC regime (Table S3) and enhanced PSC performance. On the basis of the GIWAXS images, ODT-processed P1:PC71BM films show little difference from neat P1 films or PC71BM blends prepared without ODT, exhibiting multiple lamellar interactions, likely resulting from the head-to-head BTR linkage-induced backbone twisting. As for neat P3, the blends exhibit little intermolecular order with no scattering beyond an initial isotropic lamellar peak and a PC71BM ring. The total lack of π−π stacking order and the amorphous character are not surprising given the outof-plane substituents that should disrupt polymer π−π stacking, which is associated with the low PSC PCE (1.46%). P4 blends processed with ODT exhibit substantial crystallinity as evidenced by sharp, intense on-axis diffraction peaks and additional off-axis reflections (Figures 7 and S8). The poor PSC performance appears to result from excessive crystallinity, which can create poorly intermixed domains and grain boundaries that will cause charge recombination.88,89 This is also evident in TEM images of P4:PC71BM films which reveal large crystallites and the relatively low SCLC mobility (10−5 cm2/(V·s)) of these films. The ODT-processed bulk heterojunction blends of polymers P2, P5a, P5b, and P5c with intramolecular S···O interactions all exhibit preferential π-face-on orientations with high degrees of crystallinity (Figure 7), which should enhance PSC charge transport (μh,SCLC = 10−4 to 10−3 cm2/(V·s), Table S3) and

without ODT are shown in the Supporting Information. Inplane (qxy) and out-of-plane (qz) line cuts were taken from all 2D spectra to examine polymer orientation differences in the films. Figure 6 shows the line cuts for the neat polymer films on OTS-modified Si substrate, and the line cuts for the blend films are presented in Figure S8. The d-spacings and crystalline correlation lengths (CCL) from Scherrer analysis87 for polymers P2, P5a, P5b, and P5c are collected in Table 3, and values for polymers P1, P3, and P4 are available in Tables S4 and S5. Table 3. d-Spacings and Crystalline Correlation Lengths (Calculated via Scherrer Analysis) for the Face-on Domains in Polymers P2, P5a, P5b, and P5c

The neat polymer films exhibit a range of differing preferred orientations and relative crystallinities (Figure 6). Polymers P2 and P5 featuring intramolecular S···O interactions exhibit similar lamellar and π−π stacking structures, while polymers P1, P3, and P4 appear to have more disparate stacking structures. Despite head-to-head linkages, BTR-based polymer P1 exhibits in-plane (qxy) peaks that are each consistent with a face-on lamellar structure. There are two different peaks that could be assigned to the (100) peak, at d-spacings of 39.6 and 22.3 Å, and the third d-spacing of 19.6 Å is likely attributable to a second-order (200) reflection. The existence of multiple interaction distances suggests possible backbone twisting, yielding a variety of side-chain interaction lengths. P1 also exhibits a large π−π stacking distance of 3.93 Å (Table S4), in accord with the twisted backbone. On the basis of the diffraction data, DTP-based polymer P3 is almost completely amorphous with only a broad reflection at qxy = ∼0.23 Å−1 (d ≈ 27.1 Å) and no evidence of intermolecular order, which is attributed to its slightly twisted backbone and the out-of-plane substituents. The TTOR-based polymer P4 exhibits a clear edge-on orientation with a lamellar (100) spacing of 23.2 Å and a π−π stacking distance of 3.72 Å. From the π−π stacking distance, it can be seen that the TTOR subunit leads to a more compact organization for P4 vs BTR-based polymer P1, attributable to reduced steric repulsions and planarizing intramolecular S···O interactions. H


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Figure 8. TEM images of P1 (a and h), P2 (b and i), P3 (c and j), P4 (d and k), P5a (e and l), P5b (f and m), and P5c (g and n) bulk heterojunction blend (polymer:PC71BM) films processed without (upper row) and with (bottom row) processing additive, 1,8-octanedithiol (ODT). (Scale bar: 500 nm).

carrier extraction.8,90 Indeed, such morphology combined with good optical absorption characteristics affords impressive PSC performance. As noted previously, substantial TRTOR subunit planarity promoted by alkoxy substitution correlates with significant π-face-on (010) orientation. In the case of P5b blends, Scherrer analysis reveals larger π-face-on (010) domains in P5b vs P5a, P5c, and P2 blends, either with or without ODT. This extended π-periodicity helps explain why P5b blends exhibit the highest PSC performance among the present materials. Overall, the BTOR and TRTOR polymers containing the planarizing alkoxy substituents evidence the highest degrees of face-on π−π stacking in this series. However, despite comparable mobility, blend film crystallinity, and backbone orientation, BTOR-based P2 blends show smaller EQEs (Figure S5) and FFs (Table 2) vs the P5-based blends, consistent with the coarser nanoscale phase separation in P2:PC71BM blends as revealed by TEM images; see below. Thus, replacing an alkoxy with an alkyl substituent promotes better P5 donor:PC71BM acceptor mixing, and the higher Jsc and FF combined with a far larger Voc result in higher P5b PCE (6.31%) vs P2 cells (3.38%). Compared to TTOR-based polymer blends, alkylation at the TRTOR bithiophene 3position affords ordered packing favorable for device performance with standard lamellar and close π−π stacking morphologies and favorable polymer orientation on the electrode. Versus DTP-based polymers, TRTOR-based polymer blends have greatly enhanced Jsc, FF, and PCE due to their more planar backbones, higher degrees of crystallinity with favorable backbone orientation, and higher mobility. TEM was also used here to investigate blend film morphology. For blend films processed without additive ODT, TEM images (Figure 8) show that the PC71BM-rich domains with sizes up to several hundred nm, greater than the typical exciton diffusion lengths (∼20 nm), are embedded in the blend film, leading to poor PSC performance due to inefficient exciton dissociation. In marked contrast, ODT as a processing additive greatly enhances polymer donor−fullerene acceptor mixing and nanoscale phase separation, with significantly smaller domain sizes and bicontinuous interpenetrating networks clearly visible. This results in greater PSC performance metrics. The TEM images show that the TRTORbased P5 blends (Figure 8l−n) have finer domain sizes versus the BTOR-based P2 blends (Figure 8i), affording greater EQEs and FFs for P5 PSCs. AFM topographic images (Figure S9) reveal similar morphology evolution after ODT introduction. The better P5:PC71BM mixing is likely attributable to reduced P5 aggregation due to the weaker TRTOR single S···O

conformational locking strength; hence, optimizing the intramolecular noncovalent interaction is an effective strategy to tune film morphology for performance enhancement.

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CONCLUSIONS A novel 3-alkyl-3′-alkoxy-2,2′-bithiophene electron donor subunit, TRTOR, was designed, synthesized, and incorporated in polymers for OTFTs and PSCs. TRTOR contains a head-tohead linkage, a favorable HOMO energy, good solubility characteristics, well-tailored optoelectronic properties, and a high degree of backbone planarity enabled by the single alkoxy side chain. This reduces substituent steric repulsion near the backbone (versus dialkyl bithiophene in BTR) and leads to an optimized single intramolecular S···O interaction. Using a range of diverse and complementary characterization methods, the materials structure−property−device performance correlations for these new TRTOR-based polymers are established. Compared to the dialkoxy bithiophene BTOR, replacement of one alkoxy chain with a less electron-donating alkyl substituent yields lower-lying TRTOR polymer HOMOs, and such structure modifications also enhance materials processability without sacrificing backbone planarity and macromolecular crystallinity. Versus DTP, TRTOR has comparable electronic properties but with easy materials accessbility, centrosymmetric geometry, and more compact structure. Incorporating the single alkoxy substituent and optimizing the S···O interaction affords TRTOR polymers with low-lying HOMOs, close intermolecular π−π stacking, high degrees of crystallinity, and enhanced materials processability. When incorporated into OTFTs and PSCs, the TRTOR-based polymers show promising device performance, and the PCE (6.3%) of a TRTOR-phthalimide copolymer is the highest among all phthalimide-based copolymers reported to date. The results demonstrate that TRTOR is an effective building block for constructing high-performance polymer semiconductors. Indeed, more promising device performance has been achieved by copolymerizing TRTOR with other acceptor subunits, which will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00850. Experimental details, synthesis, and characterization of monomers and polymers, optical spectra of copolymer I


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(8) Vohra, V.; Kawashima, K.; Kakara, T.; Koganezawa, T.; Osaka, I.; Takimiya, K.; Murata, H. Efficient Inverted Polymer Solar Cells Employing Favourable Molecular Orientation. Nat. Photonics 2015, 9, 403−408. (9) Li, W.; Roelofs, W. S. C.; Wienk, M. M.; Janssen, R. A. J. Enhancing the Photocurrent in Diketopyrrolopyrrole-Based Polymer Solar Cells via Energy Level Control. J. Am. Chem. Soc. 2012, 134, 13787−13795. (10) Guo, X.; Facchetti, A.; Marks, T. J. Imide- and AmideFunctionalized Polymer Semiconductors. Chem. Rev. 2014, 114, 8943−9021. (11) Li, Y. Molecular Design of Photovoltaic Materials for Polymer Solar Cells: Toward Suitable Electronic Energy Levels and Broad Absorption. Acc. Chem. Res. 2012, 45, 723−733. (12) Lu, L.; Zheng, T.; Wu, Q.; Schneider, A. M.; Zhao, D.; Yu, L. Recent Advances in Bulk Heterojunction Polymer Solar Cells. Chem. Rev. 2015, 115, 12666−12731. (13) Lei, T.; Wang, J.-Y.; Pei, J. Design, Synthesis, and Structure− Property Relationships of Isoindigo-Based Conjugated Polymers. Acc. Chem. Res. 2014, 47, 1117−1126. (14) Kawashima, K.; Tamai, Y.; Ohkita, H.; Osaka, I.; Takimiya, K. High-Efficiency Polymer Solar Cells with Small Photon Energy Loss. Nat. Commun. 2015, 6, 10085. (15) Li, W.; Hendriks, K. H.; Furlan, A.; Wienk, M. M.; Janssen, R. A. J. High Quantum Efficiencies in Polymer Solar Cells at Energy Losses below 0.6 eV. J. Am. Chem. Soc. 2015, 137, 2231−2234. (16) Osaka, I.; McCullough, R. D. Advances in Molecular Design and Synthesis of Regioregular Polythiophenes. Acc. Chem. Res. 2008, 41, 1202−1214. (17) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.; Cölle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.; Salleo, A.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. Semiconducting Thienothiophene Copolymers: Design, Synthesis, Morphology, and Performance in Thin-Film Organic Transistors. Adv. Mater. 2009, 21, 1091−1109. (18) McCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.; Jayaraman, M. Self-Orienting Head-to-Tail Poly(3-alkylthiophenes): New Insights On Structure-Property Relationships in Conducting Polymers. J. Am. Chem. Soc. 1993, 115, 4910−4911. (19) Lai, L. F.; Love, J. A.; Sharenko, A.; Coughlin, J. E.; Gupta, V.; Tretiak, S.; Nguyen, T.-Q.; Wong, W.-Y.; Bazan, G. C. Topological Considerations for the Design of Molecular Donors with Multiple Absorbing Units. J. Am. Chem. Soc. 2014, 136, 5591−5594. (20) Koch, F. P. V.; Smith, P.; Heeney, M. Fibonacci’s Route” to Regioregular Oligo(3-hexylthiophene)s. J. Am. Chem. Soc. 2013, 135, 13695−13698. (21) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. High-Performance Semiconducting Polythiophenes for Organic Thin-Film Transistors. J. Am. Chem. Soc. 2004, 126, 3378−3379. (22) Chen, H.-Y.; Hou, J.; Hayden, A. E.; Yang, H.; Houk, K. N.; Yang, Y. Silicon Atom Substitution Enhances Interchain Packing in a Thiophene-Based Polymer System. Adv. Mater. 2010, 22, 371−375. (23) Amb, C. M.; Chen, S.; Graham, K. R.; Subbiah, J.; Small, C. E.; So, F.; Reynolds, J. R. Dithienogermole As a Fused Electron Donor in Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2011, 133, 10062− 10065. (24) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (25) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Liquid-Crystalline Semiconducting Polymers with High ChargeCarrier Mobility. Nat. Mater. 2006, 5, 328−333. (26) Osaka, I.; Sauvé, G.; Zhang, R.; Kowalewski, T.; McCullough, R. D. Novel Thiophene-Thiazolothiazole Copolymers for Organic FieldEffect Transistors. Adv. Mater. 2007, 19, 4160−4165.

solutions, details of OTFTs and PSC fabrication, GIWAXS measurements and data, TGA plots, DFT computational results, OTFT and PSC performance data, EQEs of PSCs, SCLC data, and AFM images of blend films. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.-J.B.). *E-mail: [email protected] (T.J.M.). *E-mail: [email protected] (Xugang Guo). Present Address ⊥

Z.W.: Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut, 06520, USA. Author Contributions ∥

Xiaojie Guo and Q.L. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Xugang Guo is thankful for the financial support from Shenzhen Peacock Plan project (KQTD20140630110339343), the National Science Foundation of China (NSFC) (51573076), the Basic Research fund of Shenzhen City (JCYJ20140714151402769), the Guangdong Natural Science Foundation (2015A030313900), the Shenzhen Key Lab funding (ZDSYS201505291525382), and the AFOSR (FA9550-08-1-0331) at Northwestern University. T.Y. acknowledges the Basic Research fund of Shenzhen City (JCYJ20130401144532130). E.F.M. acknowledges financial support from Argonne-Northwestern Solar Energy Research (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, by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-FG0208ER46536. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357.

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REFERENCES

(1) Forrest, S. R. The Path to Ubiquitous and Low-Cost Organic Electronic Appliances on Plastic. Nature 2004, 428, 911−918. (2) Krebs, F. C. Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394−412. (3) Arias, A. C.; MacKenzie, J. D.; McCulloch, I.; Rivnay, J.; Salleo, A. Materials and Applications for Large Area Electronics: Solution-Based Approaches. Chem. Rev. 2010, 110, 3−24. (4) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (5) Kim, T.; Kim, J.-H.; Kang, T. E.; Lee, C.; Kang, H.; Shin, M.; Wang, C.; Ma, B.; Jeong, U.; Kim, T.-S.; Kim, B. J. Flexible, Highly Efficient All-Polymer Solar Cells. Nat. Commun. 2015, 6, 8547. (6) Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Flexible Organic Transistors and Circuits with Extreme Bending Stability. Nat. Mater. 2010, 9, 1015−1022. (7) Liang, Y.; Yu, L. A New Class of Semiconducting Polymers for Bulk Heterojunction Solar Cells with Exceptionally High Performance. Acc. Chem. Res. 2010, 43, 1227−1236. J


Article

Chemistry of Materials (27) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. S. Poly(2,5bis(2-thienyl)-3,6-dialkylthieno[3,2-b]thiophene)sHigh-Mobility Semiconductors for Thin-Film Transistors. Adv. Mater. 2006, 18, 3029−3032. (28) Chabinyc, M. L.; Toney, M. F.; Kline, R. J.; McCulloch, I.; Heeney, M. X-ray Scattering Study of Thin Films of Poly(2,5-bis(3alkylthiophen-2-yl)thieno[3,2-b]thiophene). J. Am. Chem. Soc. 2007, 129, 3226−3237. (29) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.; Belletête, M.; Durocher, G.; Tao, Y.; Leclerc, M. Toward a Rational Design of Poly(2,7-Carbazole) Derivatives for Solar Cells. J. Am. Chem. Soc. 2008, 130, 732−742. (30) Chen, Z.; Cai, P.; Chen, J.; Liu, X.; Zhang, L.; Lan, L.; Peng, J.; Ma, Y.; Cao, Y. Low Band-Gap Conjugated Polymers with Strong Interchain Aggregation and Very High Hole Mobility Towards Highly Efficient Thick-Film Polymer Solar Cells. Adv. Mater. 2014, 26, 2586− 2591. (31) Chen, G.-Y.; Cheng, Y.-H.; Chou, Y.-J.; Su, M.-S.; Chen, C.-M.; Wei, K.-H. Crystalline Conjugated Polymer Containing Fused 2,5Di(thiophen-2-yl)thieno[2,3-b]thiophene and Thieno[3,4-c]pyrrole4,6-dione Units for Bulk Heterojunction Solar Cells. Chem. Commun. 2011, 47, 5064−5066. (32) Zhou, N.; Guo, X.; Ortiz, R. P.; Harschneck, T.; Manley, E. F.; Lou, S. J.; Hartnett, P. E.; Yu, X.; Horwitz, N. E.; Burrezo, P. M.; Aldrich, T. J.; López Navarrete, J. T.; Wasielewski, M. R.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Marked Consequences of Systematic Oligothiophene Catenation in Thieno[3,4-c]pyrrole-4,6dione and Bithiopheneimide Photovoltaic Copolymers. J. Am. Chem. Soc. 2015, 137, 12565−12579. (33) Hu, H.; Jiang, K.; Yang, G.; Liu, J.; Li, Z.; Lin, H.; Liu, Y.; Zhao, J.; Zhang, J.; Huang, F.; Qu, Y.; Ma, W.; Yan, H. Terthiophene-Based D−A Polymer with an Asymmetric Arrangement of Alkyl Chains That Enables Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 14149−14157. (34) Osaka, I.; Shimawaki, M.; Mori, H.; Doi, I.; Miyazaki, E.; Koganezawa, T.; Takimiya, K. Synthesis, Characterization, and Transistor and Solar Cell Applications of a NaphthobisthiadiazoleBased Semiconducting Polymer. J. Am. Chem. Soc. 2012, 134, 3498− 3507. (35) Ma, Z.; Sun, W.; Himmelberger, S.; Vandewal, K.; Tang, Z.; Bergqvist, J.; Salleo, A.; Andreasen, J. W.; Inganas, O.; Andersson, M. R.; Muller, C.; Zhang, F.; Wang, E. Structure-Property Relationships of Oligothiophene-Isoindigo Polymers for Efficient Bulk-Heterojunction Solar Cells. Energy Environ. Sci. 2014, 7, 361−369. (36) Li, J.; Qin, F.; Li, C. M.; Bao, Q.; Chan-Park, M. B.; Zhang, W.; Qin, J.; Ong, B. S. High-Performance Thin-Film Transistors from Solution-Processed Dithienothiophene Polymer Semiconductor Nanoparticles. Chem. Mater. 2008, 20, 2057−2059. (37) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. High-Mobility Semiconducting Naphthodithiophene Copolymers. J. Am. Chem. Soc. 2010, 132, 5000−5001. (38) Zhong, H.; Li, Z.; Deledalle, F.; Fregoso, E. C.; Shahid, M.; Fei, Z.; Nielsen, C. B.; Yaacobi-Gross, N.; Rossbauer, S.; Anthopoulos, T. D.; Durrant, J. R.; Heeney, M. Fused Dithienogermolodithiophene Low Band Gap Polymers for High-Performance Organic Solar Cells without Processing Additives. J. Am. Chem. Soc. 2013, 135, 2040− 2043. (39) Small, C. E.; Chen, S.; Subbiah, J.; Amb, C. M.; Tsang, S.-W.; Lai, T.-H.; Reynolds, J. R.; So, F. High-Efficiency Inverted Dithienogermole-Thienopyrrolodione-Based Polymer Solar Cells. Nat. Photonics 2012, 6, 115−120. (40) Chu, T.-Y.; Lu, J.; Beaupré, S.; Zhang, Y.; Pouliot, J.-R.; Wakim, S.; Zhou, J.; Leclerc, M.; Li, Z.; Ding, J.; Tao, Y. Bulk Heterojunction Solar Cells Using Thieno[3,4-c]pyrrole-4,6-dione and Dithieno[3,2b:2′,3′-d]silole Copolymer with a Power Conversion Efficiency of 7.3%. J. Am. Chem. Soc. 2011, 133, 4250−4253. (41) Usta, H.; Lu, G.; Facchetti, A.; Marks, T. J. Dithienosilole− and Dibenzosilole−Thiophene Copolymers as Semiconductors for Organic Thin-Film Transistor. J. Am. Chem. Soc. 2006, 128, 9034−9035.

(42) Guo, X.; Zhou, N.; Lou, S. J.; Hennek, J. W.; Ponce Ortiz, R.; Butler, M. R.; Boudreault, P.-L. T.; Strzalka, J.; Morin, P.-O.; Leclerc, M.; López Navarrete, J. T.; Ratner, M. A.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Bithiopheneimide−Dithienosilole/ Dithienogermole Copolymers for Efficient Solar Cells: Information from Structure−Property−Device Performance Correlations and Comparison to Thieno[3,4-c]pyrrole-4,6-dione Analogues. J. Am. Chem. Soc. 2012, 134, 18427−18439. (43) Zhong, H.; Han, Y.; Shaw, J.; Anthopoulos, T. D.; Heeney, M. Fused Ring Cyclopentadithienothiophenes as Novel Building Blocks for High Field Effect Mobility Conjugated Polymers. Macromolecules 2015, 48, 5605−5613. (44) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Poly(3,4-ethylenedioxythiophene) and Its Derivatives: Past, Present, and Future. Adv. Mater. 2000, 12, 481−494. (45) Ouyang, J.; Chu, C. W.; Chen, F. C.; Xu, Q.; Yang, Y. HighConductivity Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) Film and Its Application in Polymer Optoelectronic Devices. Adv. Funct. Mater. 2005, 15, 203−208. (46) Shi, W.; Zhao, T.; Xi, J.; Wang, D.; Shuai, Z. Unravelling Doping Effects on PEDOT at the Molecular Level: From Geometry to Thermoelectric Transport Properties. J. Am. Chem. Soc. 2015, 137, 12929−12938. (47) Raimundo, J.-M.; Blanchard, P.; Frère, P.; Mercier, N.; LedouxRak, I.; Hierle, R.; Roncali, J. Push−Pull Chromophores Based on 2,2′Bi(3,4-ethylenedioxythiophene) (BEDOT) π-Conjugating Spacer. Tetrahedron Lett. 2001, 42, 1507−1510. (48) Pomerantz, M. Planar 2,2′-Bithiophenes with 3,3′- and 3,3′,4,4′Substituents. A Computational Study. Tetrahedron Lett. 2003, 44, 1563−1565. (49) Guo, X.; Ortiz, R. P.; Zheng, Y.; Kim, M.-G.; Zhang, S.; Hu, Y.; Lu, G.; Facchetti, A.; Marks, T. J. Thieno[3,4-c]pyrrole-4,6-dioneBased Polymer Semiconductors: Toward High-Performance, AirStable Organic Thin-Film Transistors. J. Am. Chem. Soc. 2011, 133, 13685−13697. (50) Guo, X.; Watson, M. D. Conjugated Polymers from Naphthalene Bisimide. Org. Lett. 2008, 10, 5333−5336. (51) Irvin, J. A.; Schwendeman, I.; Lee, Y.; Abboud, K. A.; Reynolds, J. R. Low-oxidation-potential Conducting Polymers Derived from 3,4Ethylenedioxythiophene and Dialkoxybenzenes. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 2164−2178. (52) Jackson, N. E.; Savoie, B. M.; Kohlstedt, K. L.; Olvera de la Cruz, M.; Schatz, G. C.; Chen, L. X.; Ratner, M. A. Controlling Conformations of Conjugated Polymers and Small Molecules: The Role of Nonbonding Interactions. J. Am. Chem. Soc. 2013, 135, 10475−10483. (53) Nguyen, T. L.; Choi, H.; Ko, S. J.; Uddin, M. A.; Walker, B.; Yum, S.; Jeong, J. E.; Yun, M. H.; Shin, T. J.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Semi-Crystalline Photovoltaic Polymers with Efficiency Exceeding 9% in a ∼ 300 nm thick conventional single-cell device. Energy Environ. Sci. 2014, 7, 3040−3051. (54) Hergué, N.; Mallet, C.; Savitha, G.; Allain, M.; Frère, P.; Roncali, J. Facile Synthesis of 3-Alkoxy-4-cyanothiophenes As New Building Blocks for Donor−Acceptor Conjugated Systems. Org. Lett. 2011, 13, 1762−1765. (55) Berrouard, P.; Grenier, F.; Pouliot, J.-R.; Gagnon, E.; Tessier, C.; Leclerc, M. Synthesis and Characterization of 5-Octylthieno[3,4c]pyrrole-4,6-dione Derivatives As New Monomers for Conjugated Copolymers. Org. Lett. 2011, 13, 38−41. (56) Huang, H.; Chen, Z.; Ortiz, R. P.; Newman, C.; Usta, H.; Lou, S.; Youn, J.; Noh, Y.-Y.; Baeg, K.-J.; Chen, L. X.; Facchetti, A.; Marks, T. Combining Electron-Neutral Building Blocks with Intramolecular “Conformational Locks” Affords Stable, High-Mobility P- and NChannel Polymer Semiconductors. J. Am. Chem. Soc. 2012, 134, 10966−10973. (57) Kim, H. G.; Kang, B.; Ko, H.; Lee, J.; Shin, J.; Cho, K. Synthetic Tailoring of Solid-State Order in Diketopyrrolopyrrole-Based Copolymers via Intramolecular Noncovalent Interactions. Chem. Mater. 2015, 27, 829−838. K


Article

Chemistry of Materials (58) Kim, F. S.; Guo, X.; Watson, M. D.; Jenekhe, S. A. Highmobility Ambipolar Transistors and High-gain Inverters from a Donor−Acceptor Copolymer Semiconductor. Adv. Mater. 2010, 22, 478−482. (59) Guo, X.; Kim, F. S.; Jenekhe, S. A.; Watson, M. D. PhthalimideBased Polymers for High Performance Organic Thin-Film Transistors. J. Am. Chem. Soc. 2009, 131, 7206−7207. (60) Guo, X.; Quinn, J.; Chen, Z.; Usta, H.; Zheng, Y.; Xia, Y.; Hennek, J. W.; Ortiz, R. P.; Marks, T. J.; Facchetti, A. Dialkoxybithiazole: A New Building Block for Head-to-Head Polymer Semiconductors. J. Am. Chem. Soc. 2013, 135, 1986−1996. (61) Xin, H.; Guo, X.; Kim, F. S.; Ren, G.; Watson, M. D.; Jenekhe, S. A. Efficient Solar Cells Based on a New Phthalimide-Based DonorAcceptor Copolymer Semiconductor: Morphology, Charge-Transport, and Photovoltaic Properties. J. Mater. Chem. 2009, 19, 5303−5310. (62) Guo, X.; Xin, H.; Kim, F. S.; Liyanage, A. D. T.; Jenekhe, S. A.; Watson, M. D. Thieno[3,4-c]pyrrole-4,6-dione-Based Donor−Acceptor Conjugated Polymers for Solar Cells. Macromolecules 2011, 44, 269−277. (63) Xin, H.; Guo, X.; Ren, G.; Watson, M. D.; Jenekhe, S. A. Efficient Phthalimide Copolymer-Based Bulk Heterojunction Solar Cells: How the Processing Additive Influences Nanoscale Morphology and Photovoltaic Properties. Adv. Energy Mater. 2012, 2, 575−582. (64) Dou, L.; Chen, C.-C.; Yoshimura, K.; Ohya, K.; Chang, W.-H.; Gao, J.; Liu, Y.; Richard, E.; Yang, Y. Synthesis of 5H-Dithieno[3,2b:2′,3′-d]pyran as an Electron-Rich Building Block for Donor− Acceptor Type Low-Bandgap Polymers. Macromolecules 2013, 46, 3384−3390. (65) Jung, J. W.; Russell, T. P.; Jo, W. H. A Small Molecule Composed of Dithienopyran and Diketopyrrolopyrrole as Versatile Electron Donor Compatible with Both Fullerene and Nonfullerene Electron Acceptors for High Performance Organic Solar Cells. Chem. Mater. 2015, 27, 4865−4870. (66) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; Yang, Y. A Polymer Tandem Solar Cell with 10.6% Power Conversion Efficiency. Nat. Commun. 2013, 4, 1446. (67) Lei, T.; Cao, Y.; Zhou, X.; Peng, Y.; Bian, J.; Pei, J. Systematic Investigation of Isoindigo-Based Polymeric Field-Effect Transistors: Design Strategy and Impact of Polymer Symmetry and Backbone Curvature. Chem. Mater. 2012, 24, 1762−1770. (68) Dierschke, F.; Jacob, J.; Müllen, K. A Hybrid Polymer of Polyaniline and Phthalimide Dyes. Synth. Met. 2006, 156, 433−443. (69) Kong, H.; Jung, Y. K.; Cho, N. S.; Kang, I.-N.; Park, J.-H.; Cho, S.; Shim, H.-K. New Semiconducting Polymers Containing 3,6Dimethyl(thieno[3,2-b]thiophene or selenopheno[3,2-b]selenophene) for Organic Thin-Film Transistors. Chem. Mater. 2009, 21, 2650− 2660. (70) Park, J. K.; Jo, J.; Seo, J. H.; Moon, J. S.; Park, Y. D.; Lee, K.; Heeger, A. J.; Bazan, G. C. End-Capping Effect of a Narrow Bandgap Conjugated Polymer on Bulk Heterojunction Solar Cells. Adv. Mater. 2011, 23, 2430−2435. (71) Kim, J.; Swager, T. M. Control of Conformational and Interpolymer Effects in Conjugated Polymers. Nature 2001, 411, 1030−1034. (72) Zhang, L.; Colella, N. S.; Liu, F.; Trahan, S.; Baral, J. K.; Winter, H. H.; Mannsfeld, S. C. B.; Briseno, A. L. Synthesis, Electronic Structure, Molecular Packing/Morphology Evolution, and Carrier Mobilities of Pure Oligo-/Poly(alkylthiophenes). J. Am. Chem. Soc. 2013, 135, 844−854. (73) Osaka, I.; Zhang, R.; Sauvé, G.; Smilgies, D.-M.; Kowalewski, T.; McCullough, R. D. High-Lamellar Ordering and Amorphous-Like πNetwork in Short-Chain Thiazolothiazole−Thiophene Copolymers Lead to High Mobilities. J. Am. Chem. Soc. 2009, 131, 2521−2529. (74) Al-Hashimi, M.; Han, Y.; Smith, J.; Bazzi, H. S.; Alqaradawi, S. Y. A.; Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Influence of the Heteroatom on the Optoelectronic Properties and Transistor Performance of Soluble Thiophene-, Selenophene- and Tellurophene-Vinylene Copolymers. Chem. Sci. 2016, 7, 1093−1099.

(75) Ko, S.; Hoke, E. T.; Pandey, L.; Hong, S.; Mondal, R.; Risko, C.; Yi, Y.; Noriega, R.; McGehee, M. D.; Brédas, J.-L.; Salleo, A.; Bao, Z. Controlled Conjugated Backbone Twisting for an Increased OpenCircuit Voltage while Having a High Short-Circuit Current in Poly(hexylthiophene) Derivatives. J. Am. Chem. Soc. 2012, 134, 5222−5232. (76) Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Fréchet, J. M. J. Synthetic Control of Structural Order in N-Alkylthieno[3,4-c]pyrrole-4,6-dione-Based Polymers for Efficient Solar Cells. J. Am. Chem. Soc. 2010, 132, 7595−7597. (77) Zhou, H.; Yang, L.; You, W. Rational Design of High Performance Conjugated Polymers for Organic Solar Cells. Macromolecules 2012, 45, 607−632. (78) Coakley, K. M.; McGehee, M. D. Conjugated Polymer Photovoltaic Cells. Chem. Mater. 2004, 16, 4533−4542. (79) Kim, D. H.; Lee, B.-L.; Moon, H.; Kang, H. M.; Jeong, E. J.; Park, J.-I.; Han, K.-M.; Lee, S.; Yoo, B. W.; Koo, B. W.; Kim, J. Y.; Lee, W. H.; Cho, K.; Becerril, H. A.; Bao, Z. Liquid-Crystalline Semiconducting Copolymers with Intramolecular Donor−Acceptor Building Blocks for High-Stability Polymer Transistors. J. Am. Chem. Soc. 2009, 131, 6124−6132. (80) Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296−1323. (81) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Brédas, J.L.; Ewbank, P. C.; Mann, K. R. Introduction to Organic Thin Film Transistors and Design of n-Channel Organic Semiconductors. Chem. Mater. 2004, 16, 4436−4451. (82) Street, R. A.; Salleo, A. Contact Effects in Polymer Transistors. Appl. Phys. Lett. 2002, 81, 2887−2889. (83) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-TemperatureAnnealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679−1683. (84) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497−500. (85) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim, J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. Processing Additives for Improved Efficiency from Bulk Heterojunction Solar Cells. J. Am. Chem. Soc. 2008, 130, 3619−3623. (86) Li, W.; Hendriks, K. H.; Roelofs, W. S. C.; Kim, Y.; Wienk, M. M.; Janssen, R. A. J. Efficient Small Bandgap Polymer Solar Cells with High Fill Factors for 300 nm Thick Films. Adv. Mater. 2013, 25, 3182−3186. (87) Smilgies, D.-M. Scherrer Grain-Size Analysis Adapted to Grazing-Incidence Scattering with Area Detectors. J. Appl. Crystallogr. 2009, 42, 1030−1034. (88) Woo, C. H.; Thompson, B. C.; Kim, B. J.; Toney, M. F.; Fréchet, J. M. J. The Influence of Poly(3-hexylthiophene) Regioregularity on Fullerene-Composite Solar Cell Performance. J. Am. Chem. Soc. 2008, 130, 16324−16329. (89) Huang, Y.; Kramer, E. J.; Heeger, A. J.; Bazan, G. C. Bulk Heterojunction Solar Cells: Morphology and Performance Relationships. Chem. Rev. 2014, 114, 7006−7043. (90) Guo, X.; Zhou, N.; Lou, S. J.; Smith, J.; Tice, D. B.; Hennek, J. W.; Ortiz, R. P.; Navarrete, J. T. L.; Li, S.; Strzalka, J.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Polymer Solar Cells with Enhanced Fill Factors. Nat. Photonics 2013, 7, 825−833.

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