Chlorination of Low-Band-Gap Polymers: Toward High-Performance

Article pubs.acs.org/cm

Chlorination of Low-Band-Gap Polymers: Toward High-Performance Polymer Solar Cells Daize Mo,† Huan Wang,† Hui Chen,† Shiwei Qu,† Pengjie Chao,† Zhen Yang,† Leilei Tian,‡ Yu-An Su,§ Yu Gao,⊥ Bing Yang,⊥ Wei Chen,*,§,∥ and Feng He*,† †

Department of Chemistry, South University of Science and Technology of China, Shenzhen, 518055, People’s Republic of China Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, 518055, People’s Republic of China § Materials Science Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois 60439, United States ∥ Institute for Molecular Engineering, The University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, United States ⊥ State Key Laboratory of Supramolecular Structure and Materials College of Chemistry Jilin University, Changchun 130012, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Halogenation is an effective way to tune the energy levels of organic semiconducting materials. To date, fluorination of organic semiconducting materials to fabricate polymer solar cells (PSCs) has been used far more than chlorination; however, fluorine exchange reactions suffer from low yields and the resulting fluorinated polymer always comes with a higher price, which will greatly hinder their commercial applications. Herein, we designed and synthesized a series of chlorinated donor−acceptor (D-A) type polymers, in which benzo[1,2-b:4,5-b]dithiophene and chlorinated benzothiadiazole units are connected by thiophene π-bridges with an asymmetric alkyl chain. These chlorinated polymers showed deep highest occupied molecular orbital (HOMO) energy levels, which promoted the efficiency of their corresponding PSCs by increasing the device open circuit voltage. The asymmetric alkyl chain on the thiophene moieties gave the final polymer sufficient solubility for solution processing and strong π−π stacking in films allowed for high mobility. Although the introduction of a large Cl atom increased the torsion angle of the polymer backbone, the chlorinated polymers maintained a favorable backbone orientation in blend films for efficient PSC application. These factors contributed to respectable device performances from thickfilm devices, which showed PCEs as high as 9.11% for a 250-nm-thick active layer. These results demonstrate that chlorination is a promising method to fine-tune the energy levels of conjugated polymers, and chlorinated benzothiadiazole may be a versatile building block in materials for efficient solar energy conversion.

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INTRODUCTION Polymer solar cells (PSCs) have attracted considerable attention from both academic and industrial research communities in the past decade, because of their low cost, light weight, ease of solution processing, and the ability to tune various structural features.1−6 To date, power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) solar cells have exceeded 11%.7 The tremendous efficiency boost of PSCs has greatly benefited from new materials design and device engineering, including the use of donor−acceptor (D-A) active materials,8−10 morphological control of films with solvent additives,11 the addition of interfacial layers to improve charge transport and collection,12,13 inverted architectures with enhanced stabilities,14,15 and tandem cells for more-efficient light harvesting.16 In addition, parallel advances in the design of conjugated polymers by atomic substitution,17−19 functional group substitution,20−22 aromatic ring fusion, side-chain optimization,23−25 and end-group functionalization26,27 have © 2017 American Chemical Society

also been developed. Among these methods, atomic substitution, particularly replacement by fluorine, has attracted considerable attention in the past few years. This approach lowers the highest occupied molecular orbital (HOMO) energy levels of polymers by inducing a small steric effect, which enhances the open-circuit voltage (Voc) of the corresponding photovoltaic devices.28,29 Although fluorinating conjugated polymers can enhance device performance, the low yields of fluorine exchange, photochemical stability of the resulting polymers, and their severe aggregation are some problems that limit the use of fluorination methods. 30−32 Moreover, fluorination of monomers usually involves difficult reaction steps and requires hazardous fluorination reagents. Consequently, it is necessary to develop alternative methods, such as Received: November 11, 2016 Revised: February 23, 2017 Published: March 8, 2017 2819

DOI: 10.1021/acs.chemmater.6b04828 Chem. Mater. 2017, 29, 2819−2830

Article

Chemistry of Materials

the bridging unit. For the acceptor moiety of those D-A polymers, we chose a BT unit with one Cl atom and an asymmetric thiophene at both ends to give suitable solubility and promote intermolecular interactions. As a result, both the lowest unoccupied molecular orbital (LUMO) and HOMO levels of these polymers were lowered, which resulted in an increase in the open circuit voltage of their devices. The photovoltaic performances of the polymers in inverted cells were characterized and compared in parallel. The PSC device based on a 250 nm blend film of PBDTHD-ClBTDD and PC71BM demonstrated an open-circuit potential (Voc) of 0.76 V, a short-circuit charge density (Jsc) of 16.79 mA cm−2, and a fill factor (FF) of 71.69%, yielding an overall PCE of 9.11%. This value is one of the highest reported PCEs among chlorinated PSCs. The chlorine-substituted polymers exhibited improved mobility and favorable backbone orientation in blend films. These factors enabled good device performances, with PCEs over 8.30% in a series of thick-film devices with an active layer ranging from 180 nm to 320 nm in our experiments. These results indicate that chlorination is a promising method to fine-tune the energy levels of conjugated polymers and also a promising approach for high-efficiency PSCs with affordable and inexpensive raw materials. Such materials will be especially applicable in roll-to-roll printing processes that feature relatively thick active layers.

chlorination, for use in high-throughput roll-to-roll production of organic solar cells. In contrast to fluorination, chlorination has been investigated in organic semiconductors, because of the simpler synthesis and the high efficiency with which molecule energy levels can be adjusted.33 However, there are very few examples of chlorinated molecules being used in organic optoelectronic applications, particularly for polymer solar cells. Bao and collaborators reported that, despite the relatively low electronegativity of Cl atoms,34 chlorinated molecules typically perform as well or better than their fluorinated analogues, both in terms of electron mobility and their ambient stability inorganic field electronic transistor (OFET) devices. Pei and collaborators synthesized fluorinated and chlorinated isoindigo-based polymers, and applied them to photovoltaic conversion. The chlorinated polymers exhibited superior solar energy conversion with a PCE of 4.60%. This efficiency was much higher than that of similar fluorinated polymers, which showed a PCE only 1.19%.35 Overall, these findings indicate that chlorination is a viable route toward developing high-performance materials for PSCs and OFETs. On the other hand, the benzothiadiazole (BT)-based conjugated polymers are an important group of organic semiconductors, because of their easy preparation, excellent stability, and unique electro-optical characteristics. Many efforts have been devoted to designing and synthesizing new conjugated D-A copolymers containing the BT unit36,37 and its derivatives, especially the difluorobenzothiadiazole unit. Although BT monomers and their fluorinated derivatives have been widely investigated and show great promise, there have been far fewer studies into chlorination of BT-based polymers. In this study, we designed and synthesized a series of chlorinated BT-benzo[1,2-b:4,5-b′]dithiophene (BDT) polymers (PBDTClBT; see Chart 1). The polymers featured a facile

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RESULTS AND DISCUSSION Synthesis and Characterization. As shown in Scheme 1, the polymers were synthesized via the following route: Scheme 1. Synthetic Routes for Monomers and Polymers

Chart 1. Chemical Structures of PBDTHD-ClBTDD, PBDTBO-ClBTDD, PBDTHD-ClBTEH, and PBDTBOClBTEH

(a) 5-Chlorobenzo[c][1,2,5]thiadizole (2) was chosen as the π-electron-deficient component, which was synthesized according to a previous report.38 (b) 4,7-Dibromo-5-chlorobenzo[c][1,2,5]thiadizole (3) was synthesized by bromination with liquid bromine and HBr at 128 °C for 3.5 days. (c) 4,7-Dibromo-5-chlorobenzo[c][1,2,5] was first coupled with 2-(tributylstannyl)thiophene via a Stille coupling reaction to give compound 4 in 72% yield.

chemical synthesis and showed good performance for solar energy conversion in devices. We synthesized a series of D-A copolymers (PBDTClBT) containing 2-alkylthiophene-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT) donor units and asymmetric chlorinated BT acceptor units, connected by 3-alkyl substituted thiophene and nonalkyl substituted thiophene as 2820


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Figure 1. Normalized absorption spectra of PBDTHD-ClBTEH, PBDTBO-ClBTEH, PBDTBO-ClBTDD, and PBDTHD-ClBTDD in (a) CHCl3 solutions and (b) films on quartz cast from CHCl3 solution.

Table 1. Optical and Electrochemical Properties of Polymers polymer

Eox oneset [V]

Ered oneset [V]

ionization potential [eV]

electron affinity [eV]

Eec g [eV]

λsol max [nm]

λfilm max [nm]

λfilm onset [nm]

Eopt g [eV]

PBDTBO-ClBTDD PBDTBO-ClBTEH PBDTHD-ClBTEH PBDTHD-ClBTDD

1.07 1.15 1.13 1.13

−0.69 −0.67 −0.69 −0.69

−5.47 −5.50 −5.53 −5.53

−3.70 −3.73 −3.71 −3.71

1.77 1.82 1.82 1.82

578.0 574.5 575.0 623.5

605.0 602.0 611.5 622.5

730.0 734.0 727.0 737.0

1.70 1.69 1.71 1.68

ClBTEH, and PBDTBO-ClBTEH were determined to be 36.69, 32.85, 21.79, and 17.50 kDa, respectively, with polydispersity indices (PDI) of 3.10, 2.69, 2.69, and 2.23, respectively. Thermal properties of the copolymers were determined by thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating rate of 10 °C min−1. The onset decomposition temperatures (the temperature corresponding to 5% weight loss) of the four polymers were determined at 423.8, 387.7, 423.8, and 412.0 °C, respectively (see Figure S2 in the Supporting Information), which demonstrated that the thermal stabilities of this series of polymers were good enough for photovoltaic applications. The absorption spectra of the four polymers in chloroform solution (1 mg mL−1) and in solid films are shown in Figure 1. The optical data are summarized in Table 1, including the absorption peak wavelengths (λmax), absorption edge wavelengths (λonset), and the optical band gaps (Eopt g ). In dilute chloroform, all the polymers showed similar three feature absorption bands: the first band centered around 354 nm; the second band at 360−480 nm can be attributed to a localized π−π* transition; and the third band covered a broad range of 480−700 nm in the long wavelength region, which can be attributed to intramolecular charge transfer (ICT) between the donor and the acceptor units of the polymers. The absorption spectra of the polymers in solid films showed clear red-shifts of 27−36 nm for all the peak values and an additional weak absorption shoulder appeared at ∼658−669 nm, caused by intermolecular interactions. These differences indicated that aggregates or ordered assemblies were formed in the solid state through the intermolecular π−π stacking interactions.39 Notably, the absorption spectra of PBDTHD-ClBTDD revealed a stronger vibrionic shoulder than the other polymers in both solution and solid film measurements, which was likely due to the high molecular weight and increased intermolecular interactions of the PBDTHD-ClBTDD polymer. This pronounced shoulder suggested stronger π−π stacking for PBDTHD-ClBTDD, which may be beneficial for charge carrier mobility in the resulting polymer films. The optical band gaps (Eopt g ) determined from the absorption onset of the polymer

(d) Product 4 from the previous step was further coupled with tributyl(4-(2-butyloctyl)thiophen-2-yl)stannane or tributyl(4-dodeylthiophen-2-yl)stannane, two different side chains, to produce the precursors (5a and 5b) in satisfactory yields (typically >70%). (e) Subsequently, the electron-deficient monomers were obtained by brominating the precursors with Nbromosuccimide (NBS) in yields of >84% (6a and 6b). 1 H and 13C NMR spectra of these electron-deficient monomers are provided in the Supporting Information (Figures S12−S18). (f) Finally, we synthesized the target polymers via a Stille coupling polycondensation reaction between the electron-rich bis(trimethyltin) BDT monomer and the electron-deficient dibrominated BT monomers in chlorobenzene, using Pd2(dba)3 and P(o-tol)3 as catalysts. Four types of polymer were synthesized: PBDTHD-ClBTDD, PBDTBO-ClBTDD, PBDTHD-ClBTEH, and PBDTBOClBTEH. Purifications were carefully performed by precipitation and Soxhlet extraction with methanol, acetone, dichloromethane, and chloroform. When a Br atom of compound 3 is substituted by a thienyl unit, the resulting compound 4 can form in two different isomers. Thus, the structure of compound 4 was confirmed by single-crystal X-ray diffraction (XRD) (see Figure S1 in the Supporting Information). The structures of 3 and 4 were determined by comparison of their 1H NMR spectra (see Figure S11 in the Supporting Information). From Figure S1 in the Supporting Information, we concluded that only one molecular structure existed in compound 4. This result was further confirmed by single-crystal XRD studies. Figure S1a shows that a thienyl unit existed far from the Cl atom at the BT unit. Compound 4 featured a short π−π stacking distance of 3.36 Å between the thiophene and BT planes, and the C−S···N noncovalent bond length was 3.45 Å (Figure S1b). The synthesized polymers showed good solubility in common organic solvents, such as chloroform, chlorobenzene, and dichlorobenzene. The number-average molecular weights (Mn) of PBDTHD-ClBTDD, PBDTBO-ClBTDD, PBDTHD2821


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Figure 2. (a) Cyclic voltammograms of PBDTHD-ClBTDD, PBDTBO-ClBTDD, PBDTHD-ClBTEH, and PBDTBO-ClBTEH in the thin films drop-casted on glassy carbon electrodes at a scan rate of 100 mV/s; (b) energy level diagrams for PBDTBO-ClBTDD, PBDTBO-ClBTEH, PBDTHD-ClBTEH, PBDTHD-ClBTDD, and PC71BM.

films were 1.70, 1.69, 1.71, and 1.68 eV, respectively. These values were all lower than those of typical nonhalogenated analogues.40 Cyclic voltammetry was used to investigate the electrochemical properties of the conjugated polymers, as shown in Figure 2a. The onset oxidation/onset reduction potentials (Eox/ Ered) of PBDTHD-ClBTDD, PBDTBO-ClBTDD, PBDTHDClBTEH, and PBDTBO-ClBTEH were 1.07/−0.69, 1.15/− 0.67, 1.13/−0.69, and 1.13/−0.69 V vs. Ag/Ag+, respectively. The corresponding ionization potentials (IPs) were estimated as −5.47, −5.50, −5.53, and −5.53 eV (Figure 2b), respectively. These results indicated that the alkyl substituent had little influence on the molecular energy levels of the polymers, but longer and bulker side chains lowered the IP to some degree. These factors will definitely help to increase the Voc value of the corresponding donor polymers. Notably, because of the substitution of electron-deficient Cl atom, the IPs of the polymers were all decreased to a relatively low level, compared with that of the non-chlorine-substituted materials.41−43 The Voc value of PSCs is related to the difference between the electron affinities (EAs) of the acceptor materials, such as PCBM, and the IPs of the donor polymer. Thus, a lower IP of the chlorinated polymer is desirable for achieving a higher Voc value in the corresponding BHJ polymer solar cells.41 Computational Stimulation. To provide further insights into the molecular energy levels and molecular conformations of the polymers, theoretical calculations were performed with density functional theory (DFT) at the ωB97X-D/6-31+G(d,p) level.44 To simply the calculations, oligomers of two repeating units of the corresponding polymers were used, as shown in Figure 3. Alkyl groups were replaced by methyl groups to reduce the computational demands. As shown in Figure 3, the dihedral angle between the BDT and the thiophene units was 20.05°, which indicated no significant steric hindrance between those components. However, the dihedral angles between the BT unit and the thiophene (near the Cl atom) were 53.13°, and the opposite dihedral angle was 18.31°. The difference between these two angles indicated the stronger steric hindrance between the BT and thiophene unit close to the Cl atom, which attributed to the chlorine substitution at the 5-position of the BT unit. The Cl atom is much larger than a proton and thus causes greater steric crowding. We found that the IPs of the polymers had similar distributions, delocalized over BDT and

Figure 3. (a) Optimized geometries and theoretical results of electronic distributions on frontier orbitals of the dimer of (b) PBDTClBT:HOMO and (c) LUMO. To simplify the calculations, alkyl chains were replaced by methyl groups. The S atoms are marked in yellow, N atoms are shown in blue, Cl atoms are marked in green, C atoms are shown in gray, and H atoms are marked in white.

the thiophene unit, close to the BDT unit. The electron affinities (EAs) were mostly concentrated on the benzothiadiazole-based acceptor group. These calculations provided further evidence for the formation of a well-defined D-A structure and for intramolecular charge transfer in the materials.45 2822


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the high hole mobility of the films and favorable morphology. Nanostructure films with optimized morphology can promote exciton dissociation and reduce charge recombination, leading to higher FF values in PSCs.47 Increasing the film thickness can decrease the Jsc value, because of the relatively low charge carrier mobility of conjugated polymers. Thus, we investigated the effects of the active layer thicknesses on device performances. The performances of PBDTHD-ClBTDD/PC71BMbased devices, having different active layer thicknesses, are summarized in Table 3. The Jsc values increased as the active layer thickness increased from 100 nm to 320 nm, while the Voc values remained constant at 0.76−0.77 V, and the FF values were maintained at >64%. Notably, PBDTHD-ClBTDD exhibited unusually high efficiency, considering the thickness of the active layers: PCE values of >8.30% were obtained for film thicknesses of 180−320 nm. Such thick films would be particularly useful in roll-to-roll printing applications. As shown in Figure S3 in the Supporting Information, the continuously increasing Jsc values, along with increasing the active layer thickness up to 320 nm, can be attributed to the stronger absorption of the blend thick films. However, despite an increased absorption, when the thickness was beyond 250 nm, the overall power conversion efficiency of the device started to decrease, because of the increasing series resistance of the solar cell devices.48,49 The decreased FF values of the devices stemmed from the produced charge recombination in the blend film, because of the increased carrier drift length.50,51 Thus, the highest PCE of 9.11% was observed at an active layer thickness of 250 nm, with an average PCE of 9.01% reported in this work. Both of these values are among the highest PCEs reported for chlorinated PSCs. The accuracy of the photovoltaic measurement was further confirmed by external quantum efficiency (EQE) measurements of the devices. Figure 5 shows the EQE curves of the PSCs fabricated under the same optimized conditions as those used for the J−V measurements. All of the polymers exhibited a broad response in the range of 300−800 nm. The PBDTHDClBTDD-based devices showed more effective solar energy harvesting capabilities over the entire wavelength range. Consequently, the EQE values for these devices were higher than those of devices based on the other three polymers. This result was in accordance with the high Jsc values measured for the PBDTHD-ClBTDD-based devices. The Jsc values obtained from the EQE integration and J−V measurements were close (within 5% error). For example, the calculated Jsc value of the device based on PBDTHD-ClBTDD was 16.06 mA cm−2, which was ∼4% lower than the value obtained from the J−V curve. The EQE results indicated that the photovoltaic results were reliable. High charge carrier mobility and balanced chargecarrier transport are very important for high-performance photovoltaic materials in PSCs. As discussed above, hole mobilities of the blend films prepared under the optimal

Photovoltaic Properties. To investigate the photovoltaic properties of the four chlorinated polymers, inverted PSCs devices with the configuration ITO/ZnO/polymer:PC71BM/ MoO3/Ag were fabricated. The performances of the devices were then characterized. We chose an inverted device configuration because the high-work-function metal anode allows for improved device stabilities. The active layers, polymer:PC71BM (1:1.5, w/w) (12 mg mL−1), were spincoated from chlorobenzene solutions. Approximately 3% (v/v) 1,8-diiodooctane (DIO) was added as an additive to optimize the morphology of the active layer and improve the photovoltaic performance.46 Figure 4 shows the current

Figure 4. Current density−voltage (J−V) curves of the PSCs based on PBDTBO-ClBTDD/PC 71 BM, PBDTBO-ClBTEH/PC 71 BM, PBDTHD-ClBTEH/PC71BM, and PBDTHD-ClBTDD/PC71BM, tested under AM1.5G illumination, 100 mW cm−2.

density−voltage characteristics of PSCs based on blends of the polymers and PC71BM under AM 1.5G (100 mW cm−2) illumination. Table 2 lists the characteristics of inverted PSC devices, with the corresponding active layer thickness optimized for each of the four polymers. The best performance device was achieved with PBDTHD-ClBTDD, which showed a PCE as high as 9.11% with Voc = 0.76 V, Jsc = 16.79 mA cm−2, and FF = 71.69%. The thickness of the active layer of the device was 250 nm. The PCE of PBDTHD-ClBTDD was ∼31% higher than that of PBDTHD-ClBTEH and ∼44% higher than that of PBDTBO-ClBTDD. The Jsc values of the PBDTHDClBTDD-based devices were considerably higher, which may be attributed to the balanced intermolecular interactions by the difference of the side chains. The PBDTHD-ClBTDD-based device also showed the highest hole mobility of 9.3 × 10−4 cm2 V−1 s−1 in these four polymers; the PBDTBO-ClBTDD-based device showed a hole mobility of 2.6 × 10−5 cm2 V−1 s−1, which may be attributed to the relatively low molecular weight of this polymer. Fine tuning the side chains on the backbones of the polymers can greatly improve the charge carrier mobility of the polymer films as well as the PCE of its PSCs. The high FF of the PBDTHD-ClBTDD-based devices can also be attributed to

Table 2. Photovoltaic Properties of the PSCs Based on the Polymers under AM 1.5G Illumination, 100 mW cm−2 thickness [nm] PBDTHD-ClBTDD PBDTHD-ClBTEH PBDTBO-ClBTEH PBDTBO-ClBTDD a

250 160 140 120

Jsc [mA cm−2]

Voc [V] 0.76 0.79 0.78 0.68

(0.76 (0.79 (0.78 (0.68

± ± ± ±

0.01) 0.01) 0.01) 0.01)

16.79 13.33 10.94 11.69

(16.65 (13.02 (10.71 (11.20

± ± ± ±

0.14) 0.37) 0.37) 0.56)

PCEmax (PCEa) [%]

FF [%] 71.69 63.12 64.36 62.30

(70.82 (62.11 (62.45 (60.54

± ± ± ±

1.11) 1.23) 1.86) 1.41)

9.11 6.88 5.46 4.95

(9.01 (6.51 (5.29 (4.72

± ± ± ±

0.09) 0.33) 0.19) 0.17)

mobility [cm2 V−1 s−1] 9.3 1.3 3.9 2.6

× × × ×

10−4 10−4 10−4 10−5

The efficiency value was calculated from 25 devices. 2823


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Table 3. Photovoltaic Properties of PBDTHD-ClBTDD/PC71BM-Based BHJ Solar Cells Fabricated with Different Active-Layer Thickness 100 140 180 220 250 280 320 a

Jsc [mA cm−2]

Voc [V]

thickness (nm) 0.77 0.77 0.76 0.76 0.76 0.76 0.76

(0.77 (0.77 (0.76 (0.76 (0.76 (0.76 (0.76

± ± ± ± ± ± ±

0.01) 0.01) 0.01) 0.01) 0.01) 0.01) 0.01)

12.58 14.67 15.14 16.33 16.79 17.19 17.68

(12.25 (14.22 (14.90 (16.02 (16.65 (16.95 (17.39

± ± ± ± ± ± ±

PCEmax (PCEa) [%]

FF [%]

0.40) 0.42) 0.35) 0.29) 0.14) 0.33) 0.31)

74.37 71.68 71.90 70.53 71.69 68.22 64.08

(72.52 (70.51 (70.89 (69.56 (70.82 (67.12 (63.50

± ± ± ± ± ± ±

1.77) 1.23) 1.09) 1.30) 1.11) 1.05) 0.67)

7.26 7.95 8.32 8.77 9.11 8.94 8.69

(7.01 (7.89 (8.22 (8.59 (9.01 (8.77 (8.53

± ± ± ± ± ± ±

0.21) 0.06) 0.08) 0.19) 0.09) 0.16) 0.14)

The efficiency value was calculated from 25 devices.

patterns exhibited several scattering peaks in the q range of 0.2−0.8 Å−1 with an approximate ratio of 1:31/2:2:71/2. These results indicated that the polymer chains were packed in a distorted hexagonal lattice. From the in-plane GIWAXS linecuts (Figure 6e), the scattering peaks at qy ≈ 0.263, 0.280, 0.240, and 0.242 Å−1 were assigned to the (10) reflection of the twodimensional (2D) hexagonal lattice and correspond to the lattice parameters of a hexagonal array, L0, of 23.9, 22.4, 26.2, and 25.9 Å, respectively. The spacing of lamellar polymer chains, dhk, was deduced from the following general relation: 1 dhk 2

=

4 ⎛ h2 + hk + k 2 ⎞ ⎟ ⎜ 3⎝ L0 2 ⎠

The values of 20.7, 19.4, 22.7, and 22.4 Å for all four polymers were determined, and these values were consistent with the lengths of their respective alkyl side chains. In addition, arc-like scattering was observed along the out-of-plane direction (qz) at ∼1.7 Å−1 (Figure 6f) for all four polymers. This result indicated that a face-on orientation of the polymer chains dominated the structures of the blended films, with an almost identical π−π stacking distance of 3.7 Å. The incorporation of longer alkyl side chains into these polymers did not affect the intermolecular π−π interactions in the blend films, but increased the solubility of the polymers in solution. Both the compact π−π stacking and face-on molecular orientation facilitated the interchain π-electron transport in these conjugated polymer films,53,54 and improved the solar energy conversion in their corresponding devices. In particular, these structural factors contributed to the high power conversion efficiency of 9.11% in the PBDTHD-ClBTDD-based device. The morphology of polymer:PC71BM (1:1.5) blend active layers greatly influences the photovoltaic performance of the PSCs. An ideal BHJ device morphology should feature an interpenetrating nanoscale network between the donor and acceptor materials to enable a large interfacial area for exciton dissociation and a continuous percolating paths for hole and electron transport to the corresponding electrodes.55 Here, the surface morphologies of the polymer:PC71BM blend films were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). As shown in Figure 7, the rootmean-square roughness (RMS) value of the atomic force microscopy (AFM) topographic images for the PBDTHDClBTCDD:PC71BM blend film was determined to be 2.39 nm, and the RMS value of the PBDTBO-ClBTDD:PC71BM blend film with a short side chain increased to 3.10 nm. TEM images of the four blend films are shown in Figure S9 in the Supporting Information, and all show uniform morphologies with appropriate domain sizes. Photoluminescence (PL) spectra were measured to study the exciton quenching in the

Figure 5. EQE spectra of the PSCs based on polymer:PC71BM blends.

conditions were measured by the space-charge-limited current (SCLC) method with a structure of ITO/PEDOT:PSS/ polymer:PC71BM (1:1.5, w/w)/MoO3/Ag. The results are listed in Table 2, as well as Figures S4−S7 in the Supporting Information. The thin-film fabrication process is exactly the same, according to the fabrication of PSC devices. The hole mobilities of the PBDTHD-ClBTDD, PBDTBO-ClBTDD, PBDTHD-ClBTEH, and PBDTBO-ClBTEH devices were determined to be 9.3 × 10−4, 2.6 × 10−5, 1.3 × 10−4, and 3.9 × 10−4 cm2 V−1 s−1, respectively. PBDTHD-ClBTDD showed a higher hole mobility than the other three polymers, which might have resulted from increased intermolecular interactions of the side chains. A high hole mobility is favorable for attaining high current densities and can improve the PCEs of the PSCs. Thus, PBDTHD-ClBTDD showed the highest PCE of these four polymers, benefiting from the broader optical response and improved hole mobility. In addition to PCE, the stability of the device was another critical issue that affected the commercial realization of PSCs.52 Figure S8 in the Supporting Information shows the PCE decay of a PBDTHDClBTDD:PC71BM-based device stored in a nitrogen-filled chamber, as a function of time. The PCE of the device could maintain a relatively decent value (∼80% of its initial efficiency) within 9 days (∼216 h), the acceptor replacement of the PC71BM might further enhance the device stability fabricated from this chlorinated donor polymer. Grazing incidence wide-angle X-ray scattering (GIWAXS) is an efficient approach to investigate the crystallinity and molecular orientation of semiconducting polymers in films. Figure 6 shows 2D GIWAXS patterns and corresponding linecuts of the films of four conjugated polymers: PBDTHDClBTEH, PBDTBO-ClBTEH, PBDTBO-ClBTDD, and PBDTHD-ClBTDD, blended with PC71BM. All four GIWAXS 2824


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Figure 6. Two-dimensional GIWAXS patterns of the blend films of (a) PBDTHD-ClBTEH:PC71BM/CB, (b) PBDTBO-ClBTEH:PC71BM/CB, (c) PBDTBO-ClBTDD:PC71BM/CB, and (d) PBDTHD-ClBTDD:PC71BM/CB, and their corresponding (e) in-plane and (f) out-of-plane linecuts. Note: GIWAXS linecuts have been shifted vertically, for the sake of clarity.

chlorinated polymer films; Figure S10 in the Supporting Information showed the PL spectra of four polymers and their blend films with PC71BM. The polymers showed an obvious PL emission, but the fluorescence intensities of the four blends were strongly quenched (>99%), when compared with their pristine films, which indicated the efficient charge-transfer process in the blend films of those polymers.

synthesized and characterized. We used a chlorinated benzothiadiazole unit to lower the HOMO levels of the polymer, while maintaining the optical band gap of the polymer at ∼1.70 eV. Among these polymers, PBDTHD-ClBTCDD, with an optimized side-chain distribution, exhibited the highest hole mobility and π−π stacking in blended film. Respectable performance of up to 9.11% was achieved from a 250-nm-thick PBDTHD-ClBTCDD:PC71BM film that was used as the active layer in the device. Although chlorine substitution slightly reduced the backbone planarity of the polymer films, chlorination is a promising method to finely tune the energy levels of conjugated polymers and improve device performance.

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CONCLUSION In summary, four novel polymers with chlorinated benzothiadiazole as the acceptor unit and alkylthienyl-substituted benzo[1,2-b:4,5-b′]dithiophene as the donor unit were 2825


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Figure 7. AFM topography images of (a) PBDTHD-ClBTDD, (b) PBDTBO-ClBTEH, (c) PBDTHD-ClBTEH, and (d) PBDTBO-ClBTDD. HRMS: m/z calcd for [M + H]+ C6H3ClN2S+, 170.6194; found 170.9775. Anal. Calcd for C6H3ClN2S (%): C, 42.24; H, 1.77; N, 16.42; S, 18.79. Found (%): C, 41.17; H, 1.79; N, 16.81; S, 18.35. Synthesis of Compound 3. 5-Chloro-1,2-diaminobenzene (4.16 g, 24.4 mmol) and HBr (10.0 mL) were added into a 250.0 mL roundbottom flask and heated to reflux. After slowly adding a solution of 27 mL Br2 (480 mmol), the reaction mixture was refluxed for 3.5 d at 128 °C and cooled to room temperature. The reaction mixture then was washed by saturated Na2SO3 solution and saturated NaHCO3 solution subsequently, and extracted with CHCl3 (3 × 100 mL). The organic layer was dried over anhydrous Na2SO4, concentrated under pressure to leave a crude residue, and further purified by column chromatography (petroleum ether−ethyl acetate (20:1, v/v)) to provide 7 as a light yellow solid (7.88 g, 98.5%). 1H NMR (400 MHz, CDCl3) δ: 7.97 (s, 1H). 13C NMR (100 MHz, CDCl3) δ: 153.4, 151.3, 136.7, 133.6, 113.9, 113.6. ESI-TOF-HRMS: m/z calcd for [M + H]+ C6HBr2ClN2S+, 328.7995; found 328.7968. Anal. Calcd for C6HBr2ClN2S (%): C, 21.94; H, 0.31; N, 8.53; S, 9.76. Found (%): C, 22.28; H, 0.28; N, 8.48; S, 10.04. Synthesis of Compound 4. 4,7-Dibromo-5-chlorobenzo[c][1,2,5]thiadizole (5.03 g, 15.31 mmol), 2-(tributylstannyl) thiophene (5.71 g, 15.31 mmol), and Pd(PPh3)4(2.2 g, 1.92 mmol) were added to a 250.0 mL round-bottom flask. The flask was blanked by argon three times and 200 mL of a toluene/DMF (4:1) solution was syringed at once; the reaction was stirred at 120 °C for 2 days and then allowed to cool to room temperature. The mixture was poured into saturated aqueous brine and then extracted with dichloromethane. The organic layer was washed twice with water, dried over anhydrous Na2SO4 and purified by silica column chromatography to yield yellow solid product (3.74 g, 73.8%). 1H NMR (400 MHz, CDCl3) δ: 8.13 (d, 1H, J = 4 Hz), 7.94 (s, 1H), 7.56 (d, 1H, J = 4 Hz), 1.43 (t, 1H, J = 6 Hz). 13C NMR (100 MHz, CDCl3) δ: 153.4, 149.9, 137.0, 136.9, 128.8, 128.2, 126.7, 126.5, 111.8. ESI-TOF-HRMS: m/z calcd for [M + H]+ C10H4BrClN2S2+, 331.6392; found 330.8761. Anal. Calcd for C10H4BrClN2S2 (%): C, 36.22; H, 1.22; N, 8.45; S, 19.34. Found (%): C, 35.85; H, 1.24; N, 8.40; S, 19.02. Synthesis of Compound 5a. Compound 4 (0.44 g, 1.35 mmol), 4(tributylstannyl)-3-dodeylthiophene (1.20 g, 1.69 mmol), and Pd-

The structural simplicity and synthetic accessibility of chlorinated benzothiadiazole-based materials, as well as their deeper energy levels, make these versatile and promising building blocks for constructing conjugated polymers. These components should be widely useful in future studies of polymer solar cells (PSCs) and other organic electronic devices.

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EXPERIMENTAL SECTION

Materials. All reagents and chemicals were purchased from commercial sources and used without further purification, unless stated otherwise. 3-Dodecylthiophene, 3-(2-ethylhexyl)thiophene, 4chloro-1,2-diaminobenzene, thionyl chloride (SOCl 2 ), 2(tributylstannyl)thiophene, N-bromosccinimide (NBS), tetra-kis(triphenylphosphine)palladium(0) (Pd(PPh3)4), Tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3), trio-tolylphopine (P(o-tol)3), n-butyllithium (n-BuLi), tri-n-butyltin chloride ((nBu)3SnCl), pyridine, bromine (Br2), and hydrobromic acid (HBr) were purchased from commercial sources and used as received. Poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios P VP AI 4083) was purchased from H.C. Stark and passed through a 0.45 μm PVDF syringe filter before spin-coating. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) was obtained from Nano-C. Tetrahydrofuran (THF) and toluene were distilled under nitrogen protection from Na/benzophenone before reaction. Monomer Synthesis. Synthesis of Compound 2. 5-Chloro-1,2diaminobenzene (7.12 g, 50 mmol), CHCl3 (500 mL), and pyridine (12.1 mL, 150 mmol) was added in a two-necked flask and the combination was stirred for ∼0.5 h until the 5-chloro-1,2diaminobenzene was dissolved completely. Then 7.3 mL of SOCl2 (100 mmol) was slowly added into the solution. It then was stirred for another 5 h; after the mixture was cooled, the resulted solution was extracted using CHCl3 and dried by using anhydrous sodium sulfate. After the solvent was removed by rotary evaporation, the crude product was purified by column chromatography to yield the white solid product (7.59 g, 89.1%). 1H NMR (400 MHz, CDCl3) δ: 8.06 (s, 1H), 7.97 (d, 1H, J = 8 Hz), 7.58 (d, 1H, J = 4 Hz). 13C NMR (100 MHz, CDCl3) δ: 153.4, 151.5, 136.8, 133.4, 113.8, 113.6. ESI-TOF2826


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7.65; N, 2.10; S, 16.85. The other three polymers were synthesized in a similar manner. PBDTBO-ClBTDD. Yield: 74.7%. GPC: Mw = 88.49 kg mol−1, Mn = 32.85 kg mol−1, PDI = 2.69. 1H NMR (400 MHz, CDCl3): δ 8.00 (b, 1H), 7.87 (b, 1H), 7.68 (b, 2H), 7.29 (b, 2H), 6.87 (b, 2H), 2.95 (b, 4H), 1.71 (b, 3H), 1.33 (b, 48H), 0.85 (b, 12 H). Anal. Calcd for C68H87ClN2S7 (%): C, 68.50; H, 7.35; N, 2.35; S, 18.82. Found (%): C, 68.03; H, 7.20; N, 2.34; S, 18.42. PBDTHD-ClBTEH. Yield: 55.7%. GPC: Mw = 58.57 kg mol−1, Mn = 21.79 kg mol−1, PDI = 2.69. 1H NMR (400 MHz, CDCl3): δ 8.02 (b, 1H), 7.90 (b, 1H), 7.73 (b, 2H), 7.31 (b, 2H), 6.88 (b, 2H), 2.86 (b, 4H), 2.60 (b, 1H), 1.74 (b, 3H), 1.34 (b, 52H), 0.81 (b, 14 H). Anal. Calcd for C72H95ClN2S7 (%): C, 68.27; H, 7.67; N, 2.24; S, 17.98. Found (%): C, 67.74; H, 7.52; N, 2.14; S, 17.79. PBDTBO-ClBTEH. Yield: 64.6%. GPC: Mw = 38.94 kg mol−1, Mn = 17.50 kg mol−1, PDI = 2.23. 1H NMR (400 MHz, CDCl3): δ 8.00 (b, 1H), 7.87 (b, 1H), 7.70 (b, 2H), 7.29 (b, 2H), 7.29 (b, 2H), 2.82 (b, 4H), 2.65 (b, 1H), 1.69 (b, 3H), 1.24 (b, 43H), 0.80 (b, 14 H). Anal. Calcd for C64H79ClN2S7 (%): C, 67.65; H, 7.01; N, 2.47; S, 19.75. Found (%): C, 67.64; H, 6.72; N, 2.24; S, 19.49. Measurement and Characterization. 1H and 13C NMR spectra were recorded on Bruker Avance-400 spectrometers. All chemical shifts (δ) are reported in units of ppm with tetramethylsilane (TMS) as the internal standard. The following abbreviations were used for signal multiplicities: s, singlet; d, doublet; t triplet; m, multiplet. Electrospray ionization−high-resolution mass spectrometry (ESIHRMS) experiments were conducted on an applied Q EXACTIVE mass spectrometry system. The elemental analyses (C, H, N, and S) of the new compounds and the polymers were measured by using a FLASH EA1112 elemental analyzer. The molecular weight and molecular weight distribution of the synthesized polymers were measured via gel permeation chromatography (GPC) at 40 °C on a Malvern Viscotek 270 max system equipped with a UV detector, using polystyrene (Aldrich) as the standard and THF as the eluent. The optical absorption spectra of solution and thin film were measured with a UV-vis-IR spectrophotometer (Shimadzu, Model UV3600). The thin films of the polymers were spin-coated from their solutions in chloromethane, and the film absorption spectra were measured. Cyclic voltammetry was performed on a Model CHI 660E potentiostat/ galvanostat (Shanghai Chenhua Instrumental Co., Ltd. China) to determine the IP and EA of the polymers, in an acetonitrile solution of 0.1 mol L−1 n-Bu4NPF6 at a potential scan rate of 100 mV s−1 with an Ag/Ag+ reference electrode and a platinum wire counter electrode under an argon atmosphere. Polymer films were deposited from chloroform solutions on a glass carbon working electrode (2 mm in diameter). The redox potential of ferrocene/ferrocene+ (Fc/Fc+) under the same conditions is located at 0.044 V, which is assumed to have an absolute energy level of −4.8 eV to vacuum. The IP and EA were calculated by the following equation:

(PPh3)4 (0.195 g, 0.17 mmol) were added to a 100.0 mL roundbottom flask. The next procedure was similar to the procedure used for 7-bromo-5-chloro-4-(thiophen-2-yl)benzo[c][1,2,5]thiadizole. Yield: 57.1%. 1H NMR (400 MHz, CDCl3) δ: 8.16 (m, 1H), 7.99 (s, 1H), 7.64 (d, 1H, J = 4 Hz), 7.54 (d, 1H, J = 4 Hz), 7.25 (d, 1H, J = 4 Hz), 7.21 (d, 1H, J = 4 Hz), 1.43 (t, 1H, J = 6 Hz), 2.74 (t, 2H, J = 6 Hz), 1.73 (m, 2H), 1.29 (m, 18H), 0.89 (t, 3H, J = 8 Hz) . 13C NMR (100 MHz, CDCl3) δ: 155.0, 150.9, 142.9, 137.8, 134.1, 133.6, 132.5, 128.5, 128.1, 127.8, 126.0, 124.2, 122.9, 31.9, 30.5, 29.7, 29.6, 29.5, 29.4, 22.7, 14.1. ESI-TOF-HRMS: m/z calcd for [M + H] + C 22 H 23 ClN 2 S 3 + , 447.0794; found 447.0782. Anal. Calcd for C22H23ClN2S3 (%): C, 59.10; H, 5.19; N, 6.27; S, 21.52. Found (%): C, 58.94; H, 4.94; N, 6.35; S, 21.25. Synthesis of Compound 5b. Synthesis of 5b was carried out in a similar manner to that of 5a.1H NMR (400 MHz, CDCl3) δ: 8.15 (m, 1H), 7.98 (s, 1H), 7.63 (s, 1H), 7.54 (d, 1H, J = 4 Hz), 7.25 (d, 1H, J = 4 Hz), 7.19 (d, 1H, J = 4 Hz), 2.67 (t, 1H, J = 8 Hz), 1.65 (d, 2H, J = 4 Hz), 1.34 (m, 12H), 0.94 (m, 6H) . 13C NMR (100 MHz, CDCl3) δ: 155.0, 150.9, 141.6, 137.7, 133.9, 133.6, 133.0, 128.4, 128.1, 127.8, 126.2, 124.3, 123.8, 118.7, 40.4, 34.7, 32.5, 28.8, 25.8, 23.1, 14.1, 11.1. ESI-TOF-HRMS: m/z calcd for [M + H]+ C26H31ClN2S3+, 503.1857; found 503.1406. Anal. Calcd for C26H31ClN2S3 (%): C, 62.06; H, 6.21; N, 5.57; S, 19.12. Found (%): C, 61.76; H, 5.94; N, 5.75; S, 18.79. Synthesis of Compound 6a. To a solution of compound 5a (2.19 g, 4.35 mmol) in THF (150 mL) was added N-bromosuccinimide (NBS, 1.70 g, 9.57 mmol) in a one-neck round flask. The mixture was then stirred overnight at room temperature. After the solvent was distilled, the residue was purified by silica gel column chromatography, using PE-DCM (v/v, 4/1) as an eluent, to yield 6a as a red solid (2.48 g, 86.5%). 1H NMR (400 MHz, CDCl3) δ: 7.88 (s, 1H), 7.82 (d, 1H, J = 4 Hz), 7.63 (s, 1H), 7.19 (d, 1H, J = 4 Hz), 2.68 (t, 2H, J = 8 Hz), 1.69 (m, 2H), 1.41 (m, 20H), 0.90 (m, 18H), 0.90 (t, 3H, J = 6 Hz). 13 C NMR (100 MHz, CDCl3) δ: 154.4, 150.6, 141.9, 138.8, 134.0, 133.4, 132.4, 130.8, 128.2, 127.5, 125.4, 123.4, 116.1, 112.9, 31.9, 29.7, 29.6, 29.4, 29.3, 22.7, 14.1. ESI-TOF-HRMS: m/z calcd for [M + H]+ C22H21Br2ClN2S3+, 604.8715; found, 604.8964. Anal. Calcd for C22H21Br2ClN2S3 (%): C, 43.68; H, 3.50; N, 4.63; S, 15.90. Found (%): C, 44.63; H, 3.61; N, 4.45; S, 15.68. Synthesis of Compound 6b. Synthesis of 6b was carried out in a similar manner to that of 6a. 1H NMR (400 MHz, CDCl3) δ: 7.89 (s, 1H), 7.82 (d, 1H, J = 4 Hz), 7.61 (s, 1H), 7.19 (d, 1H, J = 4 Hz), 2.63 (t, 2H, J = 8 Hz), 1.72 (t, 2H, J = 6 Hz), 1.38 (m, 8H), 0.93 (m, 6H). 13 C NMR (100 MHz, CDCl3) δ: 155.0, 150.9, 141.6, 137.7, 133.9, 133.6, 133.0, 128.4, 128.1, 127.8, 126.2, 124.3, 123.8, 118.7, 40.4, 34.7, 32.5, 28.8, 25.8, 23.1, 14.1, 11.1. ESI-TOF-HRMS: m/z calcd for [M + H]+ C26H29Br2ClN2S3+, 660.9779; found, 660.9587. Anal. Calcd for C26H29Br2ClN2S3 (%): C, 47.24; H, 4.42; N, 4.24; S, 14.55. Found (%): C, 47.55; H, 4.33; N, 4.36; S, 14.78. Synthesis of Polymers. Synthesis of PBDTHD-ClBTDD. (4,8Bis(5-(2-hexyldecyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene2,6-diyl)bis(trimethylstannane) (550.7 mg, 0.487 mmol) was weighed into a 50-mL one-neck round-bottom flask. Compound 5a (322.4 mg, 0.487 mmol), Pd2dba3 (17.86 mg, 0.019 mmol), and P(o-tol)3 (23.75 mg, 0.078 mmol) were added. The flask was subjected to three successive vacuum cycles, followed by refilling with argon. Anhydrous chlorobenzene (36.58 mL) then was added via a syringe. The polymerization was carried out at 130 °C for 18 h under argon protection. The mixture then was cooled to room temperature, and the polymer was precipitated via the addition of 200 mL of methanol, filtered through a Soxhlet thimble, and then subjected to Soxhlet extraction with methanol, hexane, acetone, dichloromethane, and chloroform. The polymer solution was condensed to ∼5 mL and slowly poured into methanol (200 mL). The precipitate was collected and dried under vacuum overnight to yield PBDTHD-ClBTDD (607.2 mg, 69.6%). GPC: Mw = 113.8 kg mol−1, Mn = 36.69 kg mol−1, PDI = 3.10. 1H NMR (400 MHz, CDCl3): δ 8.00 (b, 1H), 7.87 (b, 2H), 7.71 (b, 2H), 7.31 (b, 2H), 6.88 (b, 2H), 2.84 (b, 4H), 1.71 (b, 4H), 1.33 (b, 60 H), 0.78 (b, 10 H). Anal. Calcd for C76H103ClN2S7 (%): C, 69.97; H, 7.96; N, 2.15; S, 17.21. Found (%): C, 68.85; H,

IP (eV) = − (φox + 4.74)

EA (eV) = − (φred + 4.74) where φox is the onset oxidation potential vs Ag/Ag+ and φred is the onset reduction potential vs Ag/Ag+. DFT calculations were carried out at the ωB97X-D/6-31+G(d,p) level in the Gaussian 09 package.44 Tapping mode atomic force microscopy (TM-AFM) images were taken on a NanoScopeIIIa controller (Veeco Metrology Group/Digital Instruments, Santa Barbara, CA), using built-in software (version V6.13R1) to capture images. The thickness of the blend films was determined by a Dektak 6 M surface profilometer. All J−V curves were captured under an AAA solar simulator (SAN-EI) calibrated by a standard single-crystal Si photovoltaic cell (certificated by the National Institute of Metrology). All EQE data were gained through the measurement of solar cell spectral response measurement system (Model QE-R3011, Enli Technology, Ltd., Taiwan). The hole mobility of the active layers was measured by the space charge limited current (SCLC) method. The device structure for hole-only is ITO/ PEDOT:PSS/polymer:PC71BM/MoO3/Ag. The processing conditions used for the active layers were the optimized ones. Charge 2827


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Chemistry of Materials mobility was extracted by fitting the current density−voltage curves, recorded under dark conditions, with the Mott−Gurney equation. Hole-only devices were fabricated with the device structure ITO/ PEDOT:PSS/polymer:PC71BM (1:1.5, w/w)/MoO3/Ag. The mobility was determined by fitting the dark current to the model of a single carrier SCLC, which is described by the equation

ORCID

Leilei Tian: 0000-0001-6695-4614 Wei Chen: 0000-0001-8906-4278 Feng He: 0000-0002-8596-1366 Author Contributions

⎛ V2 ⎞ 9 J = ε0εrμh ⎜ 3 ⎟ 8 ⎝d ⎠

The manuscript was written through contributions of all authors. Notes

where J is the current, μh the zero-field mobility, ε0 the permittivity of free space, εr the relative permittivity of the material, d the thickness of the active layer, and V the effective voltage. The effective voltage can be obtained by subtracting the built-in voltage (Vbi) and the voltage drop (Vs) from the substrate’s series resistance from the applied voltage (Vappl):

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.H. gratefully acknowledges financial support from the South University of Science and Technology of China (No. FRGSUSTC1501A-18), The Recruitment Program of Global Youth Experts of China, the National Basic Research Program of China (No. 2013CB834805), the Shenzhen fundamental research programs (No. JCYJ20150630145302237), the Shenzhen Key Lab funding (No. ZDSYS201505291525382), and the Shenzhen peacock program (No. KQTD20140630110339343). W.C. gratefully acknowledges financial support from the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division. We also thank Dr. Joseph Strzalka and Dr. Zhang Jiang for the assistance with GIWAXS measurements. Use of the Advanced Photon Source (APS) at the Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357.

V = Vappl − Vbi − Vs The hole-mobility can be calculated from the slope of the J1/2 vs V curves. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) Measurements. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were performed at the 8ID-E beamline at the Advanced Photon Source (APS), Argonne National Laboratory, using X-rays with a wavelength of λ = 1.1385 Å and a beam size of 200 μm (h) and 20 μm (v). A 2-D PILATUS 1M-F detector was used to capture the scattering patterns and was situated at 208.7 mm from samples. Typical GIWAXS patterns were taken at an incidence angle of 0.13°, above the critical angles of conjugated polymer or polymer:PC71BM blends and below the critical angle of the silicon substrate. Consequently, the entire structure of thin films could be detected. In addition, the qy scan was obtained from a linecut across the reflection beam center, while the qz scan was achieved by a linecut at qy = 0 Å−1, using the reflected beam center as zero. Device Fabrication and Testing. The fabrication and measurement methods of PSC devices are as follows. After a thorough cleaning of the indium-doped tin oxide (ITO)-coated glass substrate with detergent, deionized water, acetone, and isopropyl alcohol under ultrasonication for 15 min each, samples were subsequently dried in an oven overnight at 80 °C under vacuum. The ITO glass substrate was UV-ozone for 15 min and then the sol−gel-derived ZnO films were spin-coated onto the ITO substrates, followed by thermal treatment at 200 °C for 30 min. The concentration of the polymer:PC71BM blend solution in this study for spin-coating was 12 mg mL−1, and CB with 3% DIO was used as the processing solvent. The additive (DIO) was added into solution 30 min before the spin-coating process. The blend solution was stirred at 80 °C in the glovebox for overnight. The active layer was spin-coated at 800 rpm for 30 s to get the blend film. A 10 nm MoO3 anode interlayer and a 100 nm Ag metal electrode were subsequently evaporated through a shadow mask to define the active area of the devices. The integrated device structure is ITO/ZnO/ polymers:PC71BM/MoO3/Ag. The current−voltage (J−V) curves were measured under 100 mW cm−2 standard AM 1.5G spectrum (calibrated by 2 cm × 2 cm monocrystalline silicon solar cell with a KG-3 filter).

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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.6b04828. Detailed experimental procedures, characterization of monomers and polymers, and additional figures (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] (F. He). *[email protected] (W. Chen). 2828


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