Enhancing Polymer Photovoltaic Performance via Optimized

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Enhancing Polymer Photovoltaic Performance via Optimized Intramolecular Ester-Based Noncovalent Sulfur···Oxygen Interactions Jianhua Chen,†,§ Qiaogan Liao,† Gang Wang,‡ Zhenglong Yan,† Hang Wang,† Yulun Wang,† Xianhe Zhang,† Yumin Tang,† Antonio Facchetti,‡ Tobin J. Marks,*,‡ and Xugang Guo*,† †

Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics, 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 § Key Laboratory of Functional Polymer Materials and State Key Laboratory of Medicinal Chemical Biology, The Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Institute of Polymer Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China S Supporting Information *

ABSTRACT: Head-to-head (HH) bithiophenes are typically avoided in polymer semiconductors since they engender undesirable steric repulsions, leading to a twisted backbone. While introducing electron-donating alkoxy chains can lead to intramolecular noncovalent S···O interactions, this comes at the cost of elevating the HOMOs and compromising polymer solar cell (PSC) performance. To address the limitation, a novel HH bithiophene featuring an electron-withdrawing ester functionality, 3-alkoxycarbonyl-3′-alkoxy-2,2′-bithiophene (TETOR), is synthesized. Single crystal diffraction reveals a planar TETOR conformation (versus highly twisted diester bithiophene), showing distinctive advantages of incorporating alkoxy on promoting backbone planarity. Compared to firstgeneration 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR), TETOR contains an additional planarizing (thienyl)S···O(carbonyl) interaction. Consequently, TETOR-based polymer (TffBT-TETOR) has greatly lower-lying FMOs, stronger aggregation, closer π-stacking, and better miscibility with fullerenes versus the TRTOR-based counterpart (TffBT-TRTOR). These characteristics are attributed to the additional S···O interaction and electron-withdrawing ester substituent, which enhances backbone planarity, charge transport, and PSC performance. Thus, TffBT-TETOR-based PSCs exhibit an increased PCE of 10.08%, a larger Voc of 0.76 V, and a higher Jsc of 18.30 mA cm−2 than the TffBT-TRTOR-based PSCs. These results demonstrate that optimizing intramolecular noncovalent S···O interactions by incorporating electron-withdrawing ester groups is a powerful strategy for materials invention in organic electronics.

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INTRODUCTION

advantages of broad and intense absoprtion of emerging nonfullerene small molecule acceptors, maximum PCEs of 13−14% have been reported in polymer:non-fullerene BHJ PSCs,15−17 which are slightly higher than that of polymer:fullerene BHJ PSCs with the state-of-the-art PCEs of 10−11%.14,18−25 Among various PSCs, the polymer:non-fullerene cells show superior photovoltaic performance than polymer:fullerene ones, but different types of PSCs have their own strengths and weaknesses and also offer materials platform for fundemental studies. Although single junction BHJ PSCs with promising PCEs have been realized by many groups, the precise design criteria for polymer semiconductors with state-of-the-art

Solution-processable polymer solar cells (PSCs) have attracted substantial attention as a clean and renewable energy source due to their distinctive advantages, such as mechanical flexibility, low-energy payback time, and the fabrication of large-area devices in a cost-effective, high throughput fashion.1−4 For instance, a semiconductor can be readily synthesized from cheap raw materials in two steps, and facile solution-based device fabrication yields power conversion efficiency (PCE) larger than 12%, demonstrating the great potential of PSCs as a renewable energy source.5 Over the past two decades, efforts have been dedicated to developing polymers with improved optoelectronic properties, optimizing device architectures, improving materials processing, and controlling film morphologies to maximize the PCEs of bulk heterojunction (BHJ) PSCs.6−14 Benefiting from the great © XXXX American Chemical Society

Received: January 23, 2018 Revised: May 2, 2018

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DOI: 10.1021/acs.macromol.8b00161 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Chart 1. Molecular Structures of Traditional High Performance Polymer Semiconductors Which Avoid Head-to-Head (HH) Connections in the Backbone

Chart 2. Molecular Structures of Various Head-to-Head (HH) Connections Containing Bithiophenes, Which Feature a Materials Design Strategy of Incorporating Intramolecular Noncovalent S···O Interactions

noncovalent S···O interaction was established by single crystal structure analysis, revealing a S···O distance of 2.84 Å, which is smaller than the sum (3.25 Å) of the S (1.85 Å) and O (1.40 Å) atom van der Waals radii.48 This S···O interaction imbues poly(3,4-ethylenedioxythiophene) (PEDOT),49 one of the most useful conducting polymers, with a high degree of πbackbone planarity and highly delocalized charge carriers, hence high conductivity in the doped state. The 2,2-bis(3,4-ethylenedioxythiophene) single crystal structure reveals short intramolecular noncovalent S···O distances of 2.96 Å, again promoting a planar π-backbone and substantial carrier delocalization.50 This intramolecular noncovalent S···O interaction imbues BTOR with a highly planar backbone and improved materials solubility since two solubilizing groups can be attached to the bithiophene 3- and 3′-positions. Inspired by the high degree of backbone planarity and good solubility of BTOR, we first reported the OTFT and PSC polymer semiconductors with BTOR as the electron donor unit.51−54 As a consequence of the planar backbone, a BTOR and phthalimide-based copolymer shows a substantial hole mobility of ∼0.2 cm2 V−1 s−1,52 demonstrating that HH connected bithiophenes yield organic semiconductors with promising OTFT performance. Nevertheless, despite the highly planar BTOR structure, the introduction of electron-rich alkoxy groups greatly elevates the HOMOs of BTOR-based polymers, leading to poor OTFT air stability and small open-circuit voltages (Vocs) in PSCs.55,56 To downshift the polymer HOMOs, an electron-deficient thiazole core and electron-withdrawing cyano substituents were next introduced, and the resulting building blocks, 4,4′-dialkoxy-5,5′bithiazole (BTzOR)55 and 3,3′-dialkoxy-4,4′-dicyano-2,2′-bithiophene (BTCNOR),27 maintain planar π-backbones but with lower-lying HOMOs than BTOR (Chart 2). As a result, the BTzOR-based polymers exhibit increased OTFT current modulation ratios (Ion/Ioffs) and improved ambient stability without sacrificing mobility (0.06−0.25 cm2 V−1 s−1) versus the

performance remain limited. It is imperative to propose and test new building blocks and materials design strategies for constructing high-performance polymer semiconductors, to enrich the materials library and to better understand structure− property relationships. Alkyl chain substituents of polymer semiconductors play a critical role in PSC active layer processing and performance optimization. To enable solution processability for PSC fabrication, solubilizing substituents are essential; however, they also have a profound impact on the PSC materials physicochemical properties, such as backbone planarity, frontier molecular orbital (FMO) energies, optical absorption, solidstate packing, charge carrier mobility, and hence the final photovoltaic performance.26−32 To achieve the desired solubility and maximize the device performance, the alkyl chain length, branching, and substitution position must be carefully tuned.33,34 For instance, compared to regioregular poly(3-alkylthiophene) (rr-P3HT, Chart 1), regiorandom poly(3-alkylthiophene) (rra-P3HT) films have a twisted backbone, larger bandgap, and amorphous morphology due to the head-to-head (HH) connection in the polymer backbone. When incorporated into devices, rra-P3HT-based organic thin-film transistors (OTFTs) exhibit lower carrier mobilities and the corresponding PSCs have lower PCEs versus rr-P3HT-based OTFTs and PSCs.35−37 Hence, the head-tohead bithiophene connection can be considered as a structural defect and is typically avoided in polymer semiconductors to achieve large OTFT charge carrier mobilities and substantial PSC PCEs, as exemplified by polymers pBTTT,38,39 PQT,40 FBT-Th4(1,4),41 PffBT-T3(1,2),22 and P3TEA42 (Chart 1). In a seeming contradiction to conventional materials design principles, Meille and co-workers reported that an HH dialkoxy-substituted bithiophene, 3,3′-dialkoxy-2,2′-bithiophene (BTOR, Chart 2), has a completely planar structure, explained on the basis of intramolecular noncovalent (thienyl)S···O(alkoxy) interactions (conformational locks).43−47 The B


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Figure 1. Molecular structures of TETOR- and TRTOR-based polymers. The intramolecular noncovalent S···O interactions are marked by dashed lines.

BTOR-based analogues.57 The BTCNOR-based polymers have very low-lying HOMOs (−5.5 to −5.6 eV) due to the strong electron-withdrawing cyano groups58 and hence remarkable PSC Vocs (0.9−1.0 V) which are significantly larger than in polymer analogues without cyano substituents.27,59 Recently, we demonstrated that removing an oxygen atom from one alkoxy substituent affords a novel building block, 3alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR, Chart 2), with improved electrical properties versus the dialkoxy-containing BTOR unit, lower-lying FMOs, and improved solubility.54 The single S···O interaction effectively planarizes the polymer backbone. Thus, TRTOR and the corresponding phthalimidebased copolymers have much lower-lying HOMOs and greatly increased Vocs and PCEs versus BTOR and phthalimide-based analogues. The TRTOR polymers exhibit a promising PCE of 6.3%, the highest among all phthalimide-based copolymers reported to date.54 Because of the limited absorption of phthalimide polymers, the more electron-deficient difluorinated benzothiadiazole (ffBT) moiety was next used to construct polymers with a narrowed bandgap and broadened optical absorption. The PCE of the ffBT-TRTOR polymer (Figure 1) is significantly increased to 9.7% with a remarkable Jsc of 21 mA/cm2, attributed to the high mobility and greatly broadened light absorption to 800 nm.60 The PCE is the highest among all HH bithiophene-based polymers. However, the Voc (0.65 V) is relatively small, consistent with the high-lying HOMO (−5.20 eV). In order to further enhance the photovoltaic performance, TRTOR-based polymers should be carefully modified to lower the HOMOs for achieving enlarged Vocs without sacrificing other favorable structural parameters. We report here the design and synthesis of a new estersubstituted bithiophene building block, 3-alkoxycarbonyl-3′alkoxy-2,2′-bithiophene (TETOR, Chart 2), and the corresponding polymer semiconductors. To reduce the TRTOR electron-rich character, the electron-donating alkyl chain is replaced with an electron-withdrawing ester moiety, which downshifts the HOMOs, thereby increasing the PSC Vocs and improving OTFT air stability.42,61−71 For instance, the ester-

functionalized polythiophenes show downshifted HOMOs (∼− 5.10 eV vs −4.8 eV), which lead to improved Vocs (∼0.8 V vs 0.62 V) in PSCs in comparison with P3HT.71 To date, the low bandgap polymers PTB7 and PTB7-Th yield state-of-the-art photovoltaic performance, mainly due to the electron-withdrawing ester units, increasing the PSC Vocs to 0.76 and 0.84 V, respectively.2,46,72−79 Recently, an esterfunctionalized PB3T polymer was reported to exhibit excellent photovoltaic performance in non-fullerene-based PSCs with PCE up to 11.9%.80 However, most of previous works mainly focused on the positive effect of ester group on downshifting the HOMO level and improving the Vocs. The carbonyl moiety of imide-functionalized thieno[3,4-c]pyrrole-4,6-dione (TPD) can promote π-planarity via intramolecular noncovalent (thienyl)S···O(carbonyl) interactions.81,82 In the present case, we expect the ester functionality carbonyl moiety will promote similar (thienyl)S···O(carbonyl) interactions with the neighboring thiophene unit. Therefore, in addition to (thienyl)S··· O(alkoxy) interactions, replacement of the alkyl substituent with an ester substituent should create additional (thienyl)S··· O(carbonyl) interactions and result in more strongly planarized conformations.81 To the best of our knowledge, the effects of incorporating ester group on simultaneously optimizing the materials optoelectrical properties and promoting backbone coplanarity have never been reported before. Furthermore, introduction of ester groups may enhance polymer donor− fullerene acceptor miscibility in BHJ PSCs owing to the common ester substituents. Therefore, introducing an ester substituent has the potential to yield planar polymer semiconductors with lower-lying HOMOs, increased charge transport, improved film morphology, and hence higher PCEs than analogous TRTOR materials. We initially targeted the new copolymer ffBT-TETOR (Figure 1) containing electron-accepting ffBT and weakly electron-donating TETOR. Compared to ffBT-TRTOR, ffBTTETOR only replaces the alkyl with an electron-withdrawing ester chain. However, ffBT-TETOR could not be prepared in high molecular weight, while related terthiophene-based TffBTC


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Scheme 1. Synthetic Routes to TRTOR- and TETOR-Containing Polymer Semiconductors TffBT-TRTOR and TffBT-TETOR

TETOR polymers can be readily synthesized in good yields and Mns and will be discussed here (Figure 1). For more consistent comparisons, the terthiophene-based TffBT-TRTOR polymer with alkyl and alkoxy substituents is also synthesized and characterized. We report that ester-funtionalized TffBTTETOR has a greatly lowered HOMO and improved charge transport versus TffBT-TRTOR. BHJ PSCs are fabricated with these polymers as the donor and PC71BM as the acceptor. The TffBT-TETOR PSCs exhibit a significantly enhanced PCE of 10.08% and a larger Voc of 0.76 V versus TffBT-TRTOR devices, with PCE of 7.11% and Voc of 0.63 V. The TffBTTETOR PCE is the highest to date of all polymers containing a HH bithiophene connection. These results demonstrate that optimizing intramolecular noncovalent S···O interactions by incorporating electron-withdrawing ester groups is a promising strategy for enhancing polymer photovoltaic performance and offers new insights for π-electron materials design.

unstable in weakly acidic media (even in CH2Cl2) or at temperature >70 °C (Figure S13). Thus, the polymerization is inefficient, and both Mn and polymer yield are unsatisfactory, which prevent device fabrication and optimization. In contrast, it was found that terthiophene-based polymer TffBT-TETOR can be readily synthesized in good yield and decent Mn. For better elucidation of structure−property correlations, the 3-alkyl-3′-alkoxy-2,2′-bithiophene (TRTOR)based analogous polymer TffBT-TRTOR was also synthesized. The synthetic routes to TRTOR and TETOR-based polymers are shown in Scheme 1. The monomers M1 and M2 were successfully synthesized via Stille coupling between brominated compound 1 or 4 with stannylated 2, which is followed by bromination of coupling products 3 and 5, respectively. Then the monomers were copolymerized with 5,6-difluoro-4,7-bis(5(trimethylstannyl)thiophen-2-yl)benzo[c][1,2,5] thiadiazole (6) under typical Stille coupling-based polycondensation with microwave as the heating source to give the corresponding polymers TffBT-TRTOR and TffBT-TETOR. The product polymers were purified by Soxhlet extractions using methanol, acetone, n-hexane, dichloromethane, chloroform, and chlorobenzene as solvents to remove the low molecular weight polymers and catalytic impurities. Finally, the high molecular weight polymers were collected from o-dichlorobenzene fraction as dark blue solids. The details of synthesis are shown in the Supporting Information. DFT Calculations. Density functional theory (DFT)-based calculations on BTOR, BTzOR, BTCNOR, TRTOR, and TETOR were performed at the B3LYP/6-31G(d,p), Gaussian 03W (Figure S1). The computational results reveal that all monomers are highly planar, enabled by intramolecular noncovalent S···O interactions. Among them, TETOR features two different noncovalent S···O interactions, i.e., (thienyl)S···

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RESULTS AND DISCUSSION Synthesis. Two different synthetic routes were employed to synthesize polymer ffBT-TETOR (Figure 1). Unfortunately, the synthetic approaches (Scheme S1) are challenged by unsatisfactory TETOR stannylation since the lithium reagent can react with the ester carbonyl group, generating impurities which are not easily removed. An alternate approach (Scheme S2) is used to synthesize the bis(2-alkyl)5,5′-(5,6difluorobenzo[c][1,2,5]thiadiazole-4,7-diyl)bis (2-bromothiophene-3-carboxyl-ate) (ffBT-TER-Br) and the stannylated comonomer of 5,6-difluoro-4,7-bis(4-alkoxy-5-(trimethyl stannyl)thiophen-2-yl)benzo[c][1,2,5] thiadiazole (ffBT-TORSn) for the Stille coupling-based polycondensation. Although both monomers are obtained in high purity (see Figures S11 and S12 for NMR spectra), the stannylated monomer is D


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Figure 2. Single crystal structures of model compounds TRTOR-Br (a) and TETOR-Br (b). The gray, red, yellow, and brown balls represent C, O, S, and Br atoms, respectively. Hydrogen atoms are omitted for clarity. The intramolecular noncovalent S···O interactions are marked by red dashed lines.

substituent leads to a completely coplanar backbone for the head-to-head linkage, reflecting the distinctive advantage of alkoxy on promoting backbone coplanarity, which shows profound impacts on the polymer optoelectrical property, materials morphology, and device performance. The distance between (thienyl)S and O(alkoxy) in TRTORBr is ∼2.90 Å, and the TETOR-Br features two different S···O interactions. The S···O distance between the (thienyl)S and the O(carbonyl) atoms is 2.63 Å, slightly shorter than the (thienyl)S···O(alkoxy) distance (2.66 Å). The result argues that the (thienyl)S···O(carbonyl) interaction is likely stronger than the (thienyl)S···O(alkoxy) interaction, which is consistent with the DFT calculations. Compared to the S···O distance (2.8−2.9 Å, Chart 2) in the previously reported alkoxysubstituted HH bithiophenes such as PEDOT, BTOR, BTCNOR, and TRTOR, the S···O distance in TETOR monomer is significantly contracted, indicating that ester group likely strengthens the intramolecular noncovalent interactions. The single crystal data also explain the difference in the temperature-dependent UV−vis absorption spectra between the corresponding TffBT-TETOR and TffBTTRTOR polymers (vide inf ra). The additional and stronger (thienyl)S···(carbonyl)O interaction leads to polymer TffBTTETOR with enhanced aggregation in solution. Materials Characterization. The molecular weights (Mns) of copolymers are measured by high temperature gelpermeation chromatography (GPC); both TffBT-TRTOR and TffBT-TETOR show high and comparable Mn of 114.0 and 102.3 kDa with a polydispersity index (PDI) of 1.9, respectively. The comparable Mns and PDIs minimize the effects of polymer molecular weights on film morphology and device performance. Even though bulky alkyl chain substituents have been incorporated into the polymers, both semiconductors show limited solubility in toluene and chloroform at room temperature, which likely reflects the highly planar polymer backbone, strong intermolecular interactions, and high molecular weights. However, both TffBT-TRTOR and TffBTTETOR readily dissolve in hot o-dichlorobenzene (o-DCB) at 100 °C. The polymer thermal properties were investigated by TGA and DSC measurements (Figures S3 and S4). Both polymers show high decomposition temperatures over 400 °C with ∼5% weight loss and no obvious endotherms or

O(carbonyl) and (thienyl)S···O(alkoxy), with calculated S···O distances of 2.66 and 2.68 Å, respectively. Therefore, the distance between (thienyl)S and O(carbonyl) is smaller than that between (thienyl)S and O(alkoxy). Compared to TRTOR (−4.92/−0.86 eV), the new TETOR shows a significantly lower-lying HOMO/LUMO (−5.19/−1.35 eV) due to the strong electron-withdrawing ester group, which is beneficial to solar cell Vocs. The optimized molecular conformations and electron density distributions of TffBT-TRTOR and TffBT-TETOR were also investigated by DFT calculations (Figure S2), and both polymers show a highly planar backbone, which should benefit three-dimensional polymer chain packing and promote ordered film morphology. Furthermore, the electron density mapping suggests that both the HOMO and the LUMO are welldelocalized along the polymer backbone, which should facilitate intramolecular charge carrier delocalization. Attributed to the strong electron-withdrawing character of the ester group, the DFT analysis also reveals that the TffBT-TETOR HOMO is downshifted by ∼0.2 eV versus that of TffBT-TRTOR, in good agreement with the CV measurements (vide inf ra). Single Crystal Structure Analysis. To gain insight into the polymer backbone conformation, physicochemical properties, and film morphology, TRTOR- and TETOR-containing small molecules were synthesized for single crystal growth. Single crystals of dibrominated TRTOR-Br and TETOR-Br with suitable side chains were obtained by slowly diffusing CH3OH into concentrated TRTOR-Br and TETOR-Br chloroform solutions. Single crystal structure analysis (Figure 2) reveals that both TRTOR-Br (CCDC 1568046) and TETOR-Br (CCDC 1568045) are highly planar, reflecting the intramolecular S···O interaction. Hence, the new TETOR building block greatly differs from the previously reported headto-head linkage containing diester bithiophene, which shows a highly twisted backbone on the basis of single crystal structure83 and the theoretical calculation80 with a dihedral angle of 74.8° and 78°, respectively. In order to reduce the steric hindrance, an unsubstituted thiophene was inserted between the diester bithiophene to improve backbone planarity. Even with such π-spacer insertion, the terthiophene unit is also not highly planar, showing a dihedral angle of 17° between two adjacent thiophene units based on the DFT calculation.80 In our approach, by replacing one ester chain with an alkoxy E


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Figure 3. UV−vis absorption spectra of polymer solutions (1 × 10−5 M in o-DCB) and thin films (spin-coated from 5 mg mL−1 o-DCB solution) (a); temperature-dependent UV−vis absorption of TffBT-TRTOR (b) and TffBT-TETOR (c) in solution (1 × 10−5 M in o-DCB). (d) Cyclic voltammograms of polymer thin films measured in 0.1 M (n-Bu)4N+PF6−−acetonitrile solution at scan rate of 20 mV s−1.

exotherms in the temperature range from 50 to 300 °C during thermal cycling. The UV−vis absorption spectra of the polymer solutions and films are shown in Figure 3a, and the relevant data are summarized in Table 1. Both TffBT-TRTOR and TffBT-

behavior of these polymers in solution, temperature-dependent UV−vis absorption measurements were carried out in odichlorobenzene (o-DCB) solutions (Figures 3b and 3c). The aggregation of TffBT-TRTOR is greatly suppressed at 100 °C, as evidenced by the substantial blue-shift (137 nm) of the absorption maximum (λmax) and the disappearance of the aforementioned absorption shoulder. However, TffBT-TETOR maintains a certain degree of aggregation at this temperature, showing distinctive shoulder at long wavelength and minimal shift of the absorption onset. The results indicate that TffBTTETOR has stronger aggregation tendencies than TffBTTRTOR in solution, which likely reflects the enhanced intramolecular interactions originating from the additional S··· O unit between the carbonyl oxygen and the neighboring thiophene sulfur, evident in the single crystal structure analysis discussed above. The optical bandgaps (Egopts) of these polymers were estimated from the absorption onset (λonset) of the polymer films. TffBT-TRTOR and TffBT-TETOR have narrow optical bandgaps of 1.46 and 1.51 eV, respectively. In comparison to the well-known tertathiophene-based polymer analogue PffBT-T4 with a bandgap of ∼1.65 eV,14,22 both TffBT-TRTOR and TffBT-TETOR show red-shifted absorption and smaller bandgaps, which should be beneficial to achieving larger Jscs in PSCs. Cyclic voltammetry (CV) is employed to investigate the electrochemical properties of TffBT-TRTOR and TffBTTETOR (Figure 3d). The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are calculated from the oxidation onsets and reduction onsets, respectively, using the Fc/Fc+ couple as the internal standard. As shown in Table 1, the TffBT-TRTOR and

Table 1. Molecular Weight and Optical Property Data for TffBT-TRTOR and TffBT-TETOR polymer TffBTTRTOR TffBTTETOR

Mn [kDa]

PDI

λmaxa [nm]

λonseta [nm]

EHOMOb [eV]

ELUMOc [eV]

Egd [eV]

114.0

1.9

712

846

−5.09

−3.12

1.46

102.3

1.9

694

817

−5.23

−3.18

1.51

For polymer films cast from 1,2-dichlorobenzene (o-DCB) solution. EHOMO = −(Eoxonset + 4.80) eV, and Eoxonset determined electrochemically using Fc/Fc+ internal standard. cELUMO = −(Eredonset + 4.80) eV, and Eredonset determined electrochemically using Fc/Fc+ internal standard. dOptical bandgap estimated from absorption onset of as-cast polymer film: Eg = 1240/λonset (eV). a b

TETOR exhibit broad absorption in the 500−800 nm range. Compared to TffBT-TRTOR, the absorption of TffBTTETOR is ∼20 nm blue-shifted. This phenomenon mainly originates from the electron-withdrawing capacity of the ester group, which offsets the electron-donating alkoxy chain, making TETOR a more electron neutral unit. Both polymers exhibit absorption shoulders both in solution and as thin films. When going from solution to the solid state, TffBT-TRTOR and TffBT-TETOR show small red-shifts of 5 and 25 nm, respectively, indicating strong aggregation in both solution and thin films. In order to further investigate the aggregation F


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Table 2. Photovoltaic Parameters of TffBT-TETOR and TffBT-TRTOR Based Bulk Heterojunction Polymer Solar Cells Containing Polymer:PC71BM as the Active Layer polymer TffBT-TRTOR TffBT-TETOR

μh,OTFTa [cm2 V−1 s−1] 0.077 0.131

Voc [V] 0.63 0.76

Jsc [mA cm−2] 16.34 18.30

FF [%] 68.6 72.4

PCEb [%] 7.11 (6.83) 10.08 (9.62)

μh,sclcc [cm2 V−1 s−1] −4

6.27 × 10 5.70 × 10−4

μe,sclcc [cm2 V−1 s−1]

μh/μe

4.6 × 10−5 3.6 × 10−4

13.6 1.58

a The μhs of annealed neat films were measured from BG/TC type OTFTs in the saturated regime. bThe average PCE of about 60 TffBT-TRTOR and TffBT-TETOR-based devices shown in parentheses. cThe μh,sclc and μe,sclc measured from the typical SCLC method.

Figure 4. Typical J−V curves of optimized TffBT-TRTOR- and TffBT-TETOR-based polymer solar cells (a) and their corresponding external quantum efficiency spectra (b).

TffBT-TETOR HOMO energies are estimated to be −5.09 and −5.23 eV, respectively. In comparison to that of TffBTTRTOR, the relatively lower-lying HOMO of TffBT-TETOR should be beneficial to achieving larger Vocs in PSCs. The LUMO energies are calculated to be −3.12 and −3.18 eV for TffBT-TRTOR and TffBT-TETOR, respectively. The CV results clearly indicate that introducing ester group effectively downshifts the HOMO and LUMO levels of these polymer semiconductors. Polymer Charge Transport Properties. Charge transport properties of polymers are critical for device performance in PSCs, which were first investigated here by fabricating bottomgate/top-contact (BG/TC) OTFTs with device architectures, Si/SiO2/SAM/polymer/Au (Table S1). Both TffBT-TRTOR and TffBT-TETOR exhibit hole-dominant transport characteristics, and output and transfer curves of optimized films are illustrated in Figure S5. During device optimization, it is found that thermal annealing promotes hole transport, and TffBTTETOR typically exhibits higher mobility than TffBT-TRTOR. After film annealing at 180 and 210 °C, the hole mobilities (μhs) of the TffBT-TRTOR and TffBT-TETOR films are increased by 8−10 times versus the as-cast films, reaching 6.9 × 10−2 and 8.8 × 10−2 cm2 V−1 s−1 in the linear regime, respectively. In the saturated regime, the optimal μhs of polymers TffBT-TRTOR and TffBT-TETOR are 7.7 × 10−2 and 1.3 × 10−1 cm2 V−1 s−1, respectively. To better understand the transport properties of TffBTTRTOR and TffBT-TETOR, the neat polymer films were characterized by grazing incidence wide-angle X-ray scattering (GIWAXS). The two-dimensional GIWAXS images and the corresponding linecuts are presented in Figure S6. Both TffBTTRTOR and TffBT-TETOR films exhibit a low crystallinity with only (100) diffraction peaks, corresponding to a lamellar structure separated by alkyl chain, and (010) peaks along the qz-axis, corresponding predominantly face-on π-stacking of the polymer backbones. The face-on orientation may not be optimal for charge transport in OTFT devices but should be

beneficial to solar cell performance. After thermal annealing at the optimized temperatures (180 and 210 °C), TffBT-TRTOR and TffBT-TETOR films exhibit enhanced (100) and (010) peak intensities, indicating increased ordering, in agreement with the increased hole mobilities. Calculated from the (010) reflection, TffBT-TETOR shows a closer π-stacking distance of 3.57 Å compared to 3.62 Å in TffBT-TRTOR, consistent with the higher hole mobility of the TETOR-based polymer versus the TRTOR-based counterpart, TffBT-TRTOR. Polymer Solar Cell Performance. Conventional BHJPSCs with device architecture of ITO/PEDOT:PSS/polymers: PC71BM/electron transport layer (ETL)/Al were fabricated using TffBT-TRTOR or TffBT-TETOR as the polymer donor and the fullerene derivative PC71BM as the electron acceptor material. The device fabrication conditions were systematically optimized by varying the solvent, annealing temperature, processing additive, polymer:PC71BM (D:A) ratio, and electron transport layer (ETL), and the device performance parameters under various conditions are summarized in the Supporting Information (Tables S2−S10). It is found that the optimal conditions for both TffBT-TRTOR- and TffBT-TETOR-based PSCs are nearly identical. Specifically, the optimized D:A ratio is 1:1.5, and the PSCs are fabricated using hot ODCB:CB (1:1) as the solvent and chloronaphthalene (CN, 1 vol %) as the processing additive. Moreover, blend thermal annealing shows positive effects, and the optimal annealing temperature is 120 and 150 °C for TffBT-TRTOR- and TffBT-TETOR-based PSCs, respectively. The optimized ETL layer is LiF and perylene diimide functionalized with amino N-oxide (PDINO) for TffBT-TRTOR- and TffBT-TETOR-based PSCs, respectively. The optimized photovoltaic performance data and corresponding current density−voltage (J−V) curves are shown in Table 2 and Figure 4a, respectively. The TffBTTRTOR-based PSCs show an average PCE of 6.83% and a maximum PCE of 7.11% with a Voc of 0.63 V, a Jsc of 16.34 mA cm−2, and a FF of 68.6%. Compared to TffBT-TRTOR, TffBTTETOR exhibits a greatly increased average PCE of 9.62% and G


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TffBT-TETOR-based blend films is slightly increased over that of as-cast blend films due to the use of the CN processing additive combined with thermal annealing, resulting in improved film morphology for charge carrier transport. The optimized TffBT-TETOR-based blend film exhibits comparable μh,sclc (5.70 × 10−4 vs 6.27 × 10−4 cm2 V−1 s−1) and significantly improved μe,sclc (3.60 × 10−4 vs 4.60 × 10−5 cm2 V−1 s−1) over that of the optimized TffBT-TRTOR blend films. As a consequence, the optimized TffBT-TETOR-based blend films exhibit a more balanced μh/μe (1.58) ratio than that of optimized TffBT-TRTOR blend films (13.6), leading to increased Jsc and FF for TffBT-TETOR-based PSCs versus TffBT-TRTOR-based ones. The external quantum efficiencies (EQEs) of the optimized TffBT-TRTOR- and TffBT-TETOR-based PSCs are shown in Figure 4b. Both devices exhibit broad photoresponse from 300 to 800 nm, consistent with the UV absorption range of the blend films under the optimized device conditions (Figure S8). Noting that compared to TffBT-TRTOR, TffBT-TETORbased PSCs show higher EQE values, especially in the long wavelength from 550 to 800 nm, which is attributed to differences in polymer absorption. These results demonstrate that more efficient photocurrent generation occurs in the TffBT-TETOR blends than in the TffBT-TRTOR blends. The integrated current densities are 16.34 and 18.27 mA cm−2 for the TffBT-TRTOR and TffBT-TETOR-based solar cells, respectively, and are within 1% error compared with the Jsc values from J−V curves. Film Morphology and Correlations with Device Performance. The blend film morphologies under various fabrication conditions were investigated by AFM, TEM, and GIWAXS. As shown in Figure 6 and Figure S9, the AFM images indicate that all films are smooth with low rms roughness (σRMS) values of ∼1.0 to 1.6 nm. Compared to the TffBT-TRTOR-based films, the σRMS of TffBT-TETOR-based films is slightly lower, indicating that TffBT-TETOR likely possesses better miscibility with PC71BM due to the common ester chains. Furthermore, the σRMS values for both optimized TffBT-TRTOR and TffBT-TETOR blend films are slightly increased versus as-cast blend films, which indicates that thermal anneal and addition of CN show positive effect on

a maximum PCE of 10.08% with a Voc of 0.76 V, a Jsc of 18.30 mA cm−2, and a FF of 72.4%. The higher performance of TffBT-TETOR is mainly attributed to the great increased Voc and Jsc, which originate from the lower-lying HOMO and more efficient photocurrent generation ability. In addition, TffBTTETOR cells show a larger FF, which is likely attributed to the more favorable blend morphology, resulting in more efficient exciton generation, separation, and balanced and higher charge carrier mobility. During the device fabrication, it was found that the PSC performance of TffBT-TRTOR and TffBT-TETOR is highly reproducible. As shown in Figure 5, 60 TffBT-TRTOR-

Figure 5. Histogram of the PCE counts for 60 TffBT-TRTOR- and TffBT-TETOR-based polymer solar cells.

and TffBT-TETOR-based individual devices using different batches were fabricated under optimized conditions, and both TffBT-TRTOR- and TffBT-TETOR-based PSCs exhibit good reproducibility with an average PCE of 6.83% and 9.62%, respectively. The hole (μh,sclc) and electron (μe,sclc) mobilities of the ascast and optimized blend films were characterized using the space-charge limited current (SCLC) method with a device structure of ITO/PEDOT:PSS/polymer:PC71BM/MoO3/Ag for the hole-only device and ITO/ZnO/polymer:PC71BM/ PFN/Al for electron-only devices (Figure S7 and Table S11). It is found that the μe,sclc of optimized TffBT-TRTOR- and

Figure 6. Tapping-mode AFM height images (upper row) and TEM images (bottom row) of (a, e) TffBT-TRTOR:PC71BM as-cast film; (b, f) TffBT-TRTOR:PC71BM blend film under 120 °C thermal annealing and using 1 vol % CN; (c, g) TffBT-TETOR:PC71BM as-cast film; (d, h) TffBT-TETOR:PC71BM blend film under 150 °C thermal annealing and using 1 vol % CN. The AFM image scale is 5 × 5 μm, and the TEM image scale is 2 × 2 μm. H


Article

Macromolecules

Figure 7. 2D-GIWAXS images of (a) TffBT-TRTOR:PC71BM as-cast film, (b) TffBT-TRTOR:PC71BM blend film under 120 °C thermal annealing and using 1 vol % CN, (c) TffBT-TETOR:PC71BM as-cast film, and (d) TffBT-TETOR:PC71BM blend film under 150 °C thermal annealing and using 1 vol % CN.

molecular organization.84,85 The increased molecular organization of each polymer:PC71BM blend results in improved charge transport and greatly improved PCEs. In Figure 6, the TEM images of the optimized blend films exhibit distinctive interpenetrating networks with well-developed fiber-like aggregates. However, the TffBT-TRTOR blend films show larger PC71BM aggregates (shown as dark dots) than TffBTTETOR blend films, which may originate from the relatively poor miscibility with PC71BM. The large isolated aggregates are known to be unfavorable for exciton dissociation and charge carrier transport, resulting in smaller μe, lower FF, and a reduced PCE as a consequence.86,87 Compared to the as-cast films, the optimized blend films under thermal annealing and using 1vol % CN show more distinctive bicontinuous interpenetrating networks and suppressed PC71BM aggregates, especially for TffBT-TETOR blend films. The more appropriate phase separation scale and the fiber-like bicontinuous network nanostructure of the optimized TffBT-TETOR blend film are beneficial to exciton dissociation and charge transport, resulting in the increased device FF and PCE. To further understand the photovoltaic performance of the PSCs, the as-cast and optimized blend films were characterized by GIWAXS; the two-dimensional GIWAXS images are shown in Figure 7, and the corresponding in-plane and out-of-plane linecuts are presented in Figure S10. Both TffBTTRTOR:PC71BM and TffBT-TETOR:PC71BM as-cast blend films exhibit weaker (100) and (010) peaks in the qz direction corresponding to low crystallinity of both films, which results in poor photovoltaic performance. However, after the thermal annealing and addition of 1 vol % CN processing additive, more distinctive (100) and (010) peaks are observed, indicating enhanced film crystallinity for both polymer blends, which is consistent with the increased photovoltaic performance. The (010) peaks of the optimized TffBT-TRTOR:PC71BM and TffBT-TETOR:PC71BM blend films are located at 1.76 and 1.79 Å−1, corresponding to πstacking distances of 3.55 and 3.51 Å, respectively. These results demonstrate a closer π-stacking of the polymer chains in the TffBT-TETOR:PC71BM blend films versus the TffBTTRTOR:PC71BM films, which should enhace charge transport. The closer π-stacking distance in combination with the improved phase separation principally accounts for the improved Jsc and FF metrics in TffBT-TETOR:PC71BM-based solar cells versus the TffBT-TRTOR:PC71BM counterparts.

TETOR features both an electron-donating alkoxy chain and an electron-withdrawing ester substituent at the bithiophene 3and 3′-positions. The ester group offsets the electron-donating alkoxy substituent, making TETOR electron-neutral and an attractive unit in the new polymer (TffBT-TETOR), which shows the improvement of electrical property by replacement of one alkoxy chain with an ester group. In comparison to the highly twisted diester bithiophene, the single crystal structure reveals that TETOR is completely coplanar, demonstrating the distinctive advantages of incorporating alkoxy chain. Comparison of TffBT-TETOR properties to those of the corresponding TRTOR analogue, TffBT-TRTOR, reveal that due to the intramolecular noncovalent TRTOR and TETOR S···O interactions, both TffBT-TRTOR and TffBT-TETOR have highly planar π-backbones, broad solar region optical absorption, narrow bandgaps, strong aggregation, and good film ordering. The strong electron-withdrawing ester group and the additional (thienyl)S···O(carbonyl) interaction enhance TffBT-TETOR aggregation, lower the HOMO, contract πstacking distances, and increase carrier mobility versus esterfree TffBT-TRTOR. The TETOR ester group plausibly increases miscibility with the ester-functionalized PC71BM acceptor, affording better blend film morphology. Compared to TffBT-TRTOR-based solar cells with PCE of 7.11%, Voc of 0.63 V, Jsc of 16.34 mA cm−2, and FF = 68.6%, the TffBTTETOR-based PSCs shows a greatly increased PCE of 10.08% with an increased Voc of 0.76 V, a Jsc of 18.30 mA cm−2, and a FF of 72.4%. The TffBT-TETOR PSC performance enhancement is attributed to the simultaneously increased Voc, Jsc, and FF, reflecting the significant benefits of introducing the electron-withdrawing ester substituent. To the best of our knowledge, the TffBT-TETOR PCE reported here is the highest among all head-to-head bithiophene-containing polymers reported to date. TETOR is clearly an excellent building block for high-performance organic semiconductors, and optimizing intramolecular noncovalent planarizing interactions by incorporating electron-withdrawing ester groups offers a new strategy for materials design in organic electronics.

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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.macromol.8b00161. Detailed experimental procedures including synthesis, characterization, OTFT and photovoltaic devices fabrication, and the detailed data of TGA, DSC, DFT calculation, OTFT, OPV, SCLC, AFM, and GIWAXS (PDF)

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CONCLUSIONS A novel head-to-head ester-functionalized bithiophene, 3alkoxycarbonyl-3′-alkoxy-2,2′ -bithiophene (TETOR), was designed and incorporated in polymer semiconductors. I


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X-ray crystal structure of TETOR-Br (CIF) X-ray crystal structure of TRTOR-Br (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.J.M.). *E-mail: [email protected] (X.G.). ORCID

Antonio Facchetti: 0000-0002-8175-7958 Tobin J. Marks: 0000-0001-8771-0141 Xugang Guo: 0000-0001-6193-637X Author Contributions

J.C. and Q.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS X.G. thanks the National Science Foundation of China (21774055 and 51573076), the Shenzhen Peacock Plan Project (KQTD20140630110339343), the Shenzhen Key Lab funding (ZDSYS201505291525382), the Shenzhen Basic Research Fund (JCYJ20160530185244662), the Guangdong Natural Science Foundation (2015A030313900), and the South University of Science and Technology of China (FRGSUSTC1501A-72). J.C. thanks the SUSTC Presidential Postdoctoral Fellowship. T.J.M. and G.W. (materials characterization) acknowledge financial support from the ArgonneNorthwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award DE-SC0001059. Use of the Advanced Photon Source, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract DE-AC02-06CH11357. We also acknowledge the financial support from AFOSR Grant FA9550-15-1-0044 on materials design (T.J.M. and A.F.).

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