Effect of Halogenation in Isoindigo-Based Polymers on the Phase

Publication Date (Web): August 14, 2015 ... Fluorinated and chlorinated isoindigo-based polymers, F-IIDT and Cl-IIDT, were ... Chlorination of Side Ch...
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Effect of Halogenation in Isoindigo-Based Polymers on the Phase Separation and Molecular Orientation of Bulk Heterojunction Solar Cells Yu-Qing Zheng, Zhi Wang, Jin-Hu Dou, Shi-Ding Zhang, Xu-Yi Luo, Ze-Fan Yao, Jie-Yu Wang,* and Jian Pei* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, Center of Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: Fluorinated and chlorinated isoindigo-based polymers, F-IIDT and Cl-IIDT, were developed as the donor materials for photovoltaic cells. Although showing similar frontier orbital energy levels, both polymers exhibited distinct solar cell device performance, for example, power conversion efficiency of 4.60% for Cl-IIDT and 1.19% for F-IIDT. The investigation of photophysical properties demonstrated that the strong preaggregation of F-IIDT was formed in solution due to increased noncovalent interactions between the backbones of the polymer. Therefore, large crystal domains in blended thin films with no preferred crystallographic orientation were obtained. On the other hand, the introduction of chlorine atoms increased the torsional angle of the polymer backbone, resulting in reduced crystallization tendency. Films with less crystallinity and face-on polymer orientation contributed to better device performance of Cl-IIDT. Our research proved that the effects of halogenation are far more than the tuning of frontier energy levels. Phase separation in blended films, including size of the crystallized domains and molecular packing orientation, is also greatly influenced by halogenation.

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INTRODUCTION

materials pack into aggregates through weak noncovalent interactions between adjacent molecules.12 Although individually these interactions are much weaker than covalent interactions, they jointly exert significant influence over molecular stacking and thus the performance of BHJ solar cells. Therefore, subtle changes in noncovalent interactions induced by the chemical structure modification can lead to distinct molecular packing and thus different device performance. Halogenation of the molecular backbones is an effective method of structure manipulation to finely tune energy levels and phase separation simultaneously.13 It has been a long history that fluorination was applied in enriching the diversity

In recent years, donor (D)−acceptor (A) copolymers have gained their popularity as donor materials in bulk heterojunction (BHJ) organic photovoltaics (OPVs) due to their narrow band gaps and strong intermolecular interaction to form proper phase separation.1,2 To obtain desirable device performance, molecular properties, including frontier energy levels, band gaps, and film forming ability, should be elaborately controlled.3,4 For example, narrow band gaps and low highest occupied molecular orbital (HOMO) levels are two basic design principles for donor materials as to obtain broad absorption features and high open-circuit voltage (Voc).5−7 Moreover, the morphology of the active layer is critical to obtain high efficiency.8,9 In an ideal BHJ active layer, cocontinuous donor and acceptor domains with sizes closely related to the diffusion length of excitons should be targeted.10,11 Different from inorganic materials, organic © XXXX American Chemical Society

Received: May 19, 2015 Revised: August 4, 2015

A

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Scheme 1. Chemical Structures of F-IIDT, Cl-IIDT, and IIDT and Their Corresponding Possible Monomer Structuresa

a

Monomer structures of (a) F-IIDT and (b) Cl-IIDT.

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of organic semiconductors.14−20 Fluorinated materials generally showed better device performance in organic field-effect transistors (OFETs) and in OPVs because of their lower HOMO levels, more planar backbones, and more ordered packing mode than their unsubstituted counterparts.21−25 Apart from the F atom, the second most electrophilic atom in halogen is chlorine. Previous studies in small molecules show that chlorinated materials have comparable or even deeper energy levels than fluorinated ones because of chlorine’s ability to accommodate more electron density than fluorine.26 Large atom size of chlorine always induces contorted π-plane, which has been proved to realize slipped π-stacked arrangement in single crystals.27 Therefore, chlorine-substituted organic semiconductors showed unique performance in OFETs and OSCs other than fluorine-substituted ones.27−29 However, studies on the properties of chlorinated organic semiconductors in OSCs are far less than those of fluorinated materials.30 There is even no study that elucidates the detailed differences between fluorinated and chlorinated materials as donors in BHJ solar cells. Herein, the isoindigo unit is selected as core structure to study the effect of halogenation on the properties of donor materials in BHJ solar cells, since isoindigo derivatives have been widely proved to be one of the most promising organic semiconductors both in OFETs and in OPVs.23,31−39 Through effective synthetic methods, two isoindigo polymers, substituted by fluorine (F-IIDT) and chlorine (Cl-IIDT), were developed. Multiple characterization methods demonstrate that chlorinated polymers with reduced polymer backbone planarity show proper phase separation with acceptors. Reduced crystallization tendency of Cl-IIDT leads to much higher device performance of 4.60% than fluorinated ones with only 1.19% of power conversion efficiency (PCE). Our results indicate that halogenation is a promising modification method to finely tune the film microstructure for BHJ solar cells.

EXPERIMENTAL SECTION

General Procedures and Experimental Details. Chemical reagents and 6,6-diphenyl C71 butyric acid methyl ester (PC71BM) were purchased and used as received. Toluene and tetrahydrofuran (THF) were distilled from sodium before using. All air- and watersensitive reactions were performed under nitrogen or argon atmosphere. 1H and 13C NMR spectra were recorded on a Bruker ARX-400 (400 MHz). All chemical shifts were reported in parts per million (ppm). TMS (0 ppm) was the reference for 1H NMR chemical shifts, and CDCl3 (77.00 ppm) was the reference for 13C NMR chemical shifts. Mass spectra were recorded on a Bruker BIFLEX III mass spectrometer. Elemental analyses were performed on a German Vario EL III elemental analyzer. Differential scanning calorimetry (DSC) analyses were performed using a METTLER TOLEDO Instrument DSC822 calorimeter. Thermal gravity analyses (TGA) were performed on a TA Instruments Q600 analyzer. Gel permeation chromatography (GPC) was carried out using Polymer Laboratories PL-GPC220 at 150 °C with 1,2,4-tricholorobenzene (TCB) as eluent. Absorption spectra were recorded on a PerkinElmer Lambda 750 UV−vis spectrometer. Cyclic voltammetry (CV) was tested on BASI Epsilon workstation. Thin film CV measurements were recorded in acetonitrile with 0.1 M n-Bu4NPF6 as supporting electrolyte, glassy carbon electrode as working electrode, and platinum wire as counter electrode. All potentials were represented versus Ag/AgCl (saturated) as reference electrode (scan rate: 50 mV s−1). Photoelectron spectra (PES) were tested on an AC-2 photoelectron spectrometer (RikenKeiki Co.). The X-ray scattering data were recorded at beamline BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 1.24 Å. BL14B1 is a beamline based on bending magnet, and a Si (111) double crystal monochromator was employed to monochromatize the beam. The size of the focus spot is about 0.5 mm, and the end station is equipped with a Huber 5021 diffractometer. A NaI scintillation detector was used for data collection. Atomic force microscopy (AFM) studies were performed with a Nanoscope IIIa microscope (Extended Multimode) at tapping mode under ambient conditions using a silicon nitride cantilever (Budget Sensors Tap300Al) with a resonant frequency around 300 kHz. B

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Macromolecules Table 1. Optical and Electrochemical Properties of the Polymers polymers

Mn (kDa)/PDIa

Td (oC)

b λsol max (nm)

λfilm max (nm)

c Eopt g (eV)

EHOMO (eV)d

ELUMO (eV)d

e ECV g (eV)

EPES HOMO (eV)

F-IIDT Cl-IIDT IIDT

39.7/2.30 30.1/2.44 41.5/2.67

382 399 389

696 640 630

691, 753 682 639

1.45 1.50 1.65

−5.51 −5.53 −5.63

−3.92 −3.85 −3.63

1.59 1.68 2.00

−5.27 −5.37 −5.26

Molecular weights of all polymers were evaluated by GPC using TCB as eluent at 150 °C. b10−5 M in chloroform. cEstimated from the onset of thin-film absorption. dCyclic voltammetry determined with Fc/Fc+ (EHOMO = −4.80 eV) as external reference. eECV g = ELUMO − EHOMO.

a

Figure 1. Comparison between the normalized UV−vis absorption spectra of (a) F-IIDT and (b) Cl-IIDT in CHCl3 (1 × 10−5 M), in thin films spin-coated from CHCl3 solution (1 mg mL−1), and in annealed films (at 180 °C for 30 min). Molecular models of (c) F-IIDT and (d) Cl-IIDT fragments. Device Fabrication and Characterization. ITO-coated glass substrates were cleaned with acetone, detergent, distilled water, and isopropanol. Then substrates were dried in a vacuum oven at 80 °C followed by 15 min oxygen plasma treatment. ZnO precursor was prepared according to previous work by spin-coating onto the ITOcoated glass substrates at a speed of 4000 rpm for 30 s. Then, the substrates were annealed at 140 °C for 1 h and were cooled to room temperature under vacuum. Solutions of donor and acceptor were prepared by premixing powders of two materials in o-dichlorobenzene (ODCB) with the polymer concentration at 10 mg mL−1. 1,8Diiodoctane (DIO) was used as additive in blended solutions. All the active layers were prepared by spin-coating in the glovebox. 15 nm of MoO3 and 65 nm of Ag were deposited onto the films by thermal evaporation (device active area: 15 mm2). All the devices were encapsulated in the glovebox and tested under ambient conditions. The PCE was tested under AM 1.5G irradiation with the intensity of 100 mW cm−2 (Newport Solar Simulator 94021A) calibrated by a NREL certified standard silicon cell (4 cm2). J−V curves were recorded with a Keithley 2636A semiconductor analyzer.

IIDT, Cl-IIDT, and IIDT may exhibit three different conformers varying with the orientation of flanked thiophenes (Scheme 1a,b and Figure S2). Geometries of nine conformers were optimized at the B3LYP/6-311G(d,p) level. The calculation results reveal that the lowest energy for F-IIDT (Figure S3a) and for Cl-IIDT (Figure S3b) are assigned to conformation C-1 and C-4, in which fluorine and chlorine atoms face toward the β-hydrogen on flanked thiophenes. Photophysical and Electrochemical Properties. UV− vis absorption spectra of F-IIDT (Figure 1a) and Cl-IIDT (Figure 1b) were measured both in solution and in thin films. Both polymers represent typical dual band absorption features.22 The optical band gap calculated from the wavelength of onset for F-IIDT is 1.45 eV, smaller than that of Cl-IIDT of 1.50 eV and that of IIDT of 1.65 eV (Figure S4a). This indicates that the incorporation of electron-withdrawing halogen on the acceptor part of conjugated polymers effectively lowers the gap and expands the absorption spectra to the longwavelength region. Compared with Cl-IIDT and IIDT, F-IIDT shows obviously red-shifted features both in solution and in thin film (Figure S4b). Computational analysis shows that the dihedral angle of polymer backbones of F-IIDT is about 11°, and the distance between F−H (2.27 Å) is significantly shorter than the sum of their van der Waals radii (2.56 Å), indicating the existence of strong interactions between F−H (Figure 1c).40,41 In stark contrast, chlorine has much larger atom size, making the backbone of Cl-IIDT less planar with phenyl− thienyl torsional angle as large as 40°, which is in consistent with absorption features (Figure 1d). The calculated conformation is also verified by the single crystal structures of two similar compounds (Figure S5).42,43 The 0−0 vibrational peak of F-IIDT and IIDT is invisible in absorption spectra in solution, while it increases as shoulder peak at 753 nm for FIIDT and at 687 nm for IIDT in thin film, which may be attributed to increased polymer backbone planarity in solid state. In the absorption features of Cl-IIDT in thin film, only the 0−1 vibrational peak is observed with obviously bathochromic shift by 42 nm compared with absorption peaks in

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RESULTS AND DISCUSSION Design Strategy, Theoretical Analysis, and Synthesis. To study the effect of halogenation on energy levels and microstructures in BHJ solar cells, two polymers based on isoindigo were designed by incorporating fluorine and chlorine atoms on backbones (Scheme 1). The parent polymer IIDT was also synthesized for better comparison, and the corresponding properties of IIDT are summarized in the Supporting Information. The synthetic route to the desired monomers and polymers are shown in the Supporting Information. In order to exclude the impact of molecular weight, careful control of the polymerization process was realized to achieve very close number-average molecular weights (Mn) and polydispersity (PDI) for all the polymers (Table 1). All the polymers show good thermal stability with decomposition temperatures of over 380 °C (Figure S1a−c). No obvious phase transition process is observed in DSC analyses for all the polymers (Figure S1d−f). Scheme 1 illustrates the chemical structures of F-IIDT, Cl-IIDT, and IIDT and their different conformations. The backbones of FC

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Figure 2. (a) Cyclic voltammograms of F-IIDT (blue lines) and Cl-IIDT (red lines) in thin films drop-casted on glassy carbon electrode (scan rate 50 mV s−1). Calculated molecular orbitals of the trimers of (b) F-IIDT and (c) Cl-IIDT (B3LYP/6-311G(d,p)).

hole mobility of two polymers under ambient conditions. Both polymers show typical transfer and output curves of hole transporting materials (Figure 3). The hole mobility of F-IIDT

solution, implying that the polymer backbones may become more planar in the solid states. After the thin films were annealed at 180 °C for 30 min, a blue-shift of absorption maximum λmax is observed for F-IIDT and IIDT, while the absorption spectrum of Cl-IIDT is almost identical. To get information on HOMO and LUMO energy levels, a CV test was performed, and energy levels of frontier orbitals were calculated from the onset of oxidative and reductive peaks. As shown in Figure 2a, the CV curves of both polymer-cast films reveal quasi-reversible oxidation and reduction behaviors, and both polymers show much stronger oxidative peaks than reductive ones. Both polymers show much lower LUMO level than parent polymer of −3.63 eV because fluorine and chlorine are incorporated on acceptor part and LUMO mainly localizes on it (Figure 2b,c and Figure S6). The LUMO level of F-IIDT (−3.92 eV) is lower than that of Cl-IIDT (−3.85 eV) by nearly 0.07 eV, which results from the stronger electrophilicity of fluorine.44 On the other hand, the HOMO levels of both polymers show the opposite trend. The HOMO level of FIIDT is −5.51 eV, slightly higher than that of Cl-IIDT by 0.02 eV, which is consistent with HOMO levels obtained by extrapolating the calculated HOMO levels for oligomers (n = 1, 2, 3) (Figure S7). The HOMO levels of F-IIDT and Cl-IIDT were calculated to be −4.96 and −5.13 eV. Although fluorine has the largest Pauling electronegativity of 4.0, the dipole moment of the C−Cl bond is slightly larger than that of C−F bond.13 The MO diagrams in Figure S8 reveal that for HOMOs there is increased electron density over Cl atoms compared to F atoms. This is probably due to the fact that the empty 3d orbitals on Cl can accept π-electrons from conjugated core, while there is no empty orbital with proper energy level on F for such delocalization.26 The relatively lower LUMO level and the relatively higher HOMO level of F-IIDT than those of ClIIDT lead to narrower band gap of F-IIDT, in accordance with the data calculated from UV−vis absorption spectra. The HOMO levels of two polymers obtained from PES are −5.27 eV for F-IIDT and −5.37 eV for Cl-IIDT, consistent with data obtained from CV tests (Figure S9). Data of photophysical and electrochemical properties of all the polymers are summarized in Table 1. Fabrication and Performance of OFETs. To evaluate the carrier mobility of both polymers, top-contact, bottom-gate devices were fabricated by spin-coating polymer solutions (3 mg/mL in ODCB) onto octadecyltrichlorosilane (OTS)modified SiO2 (300 nm)/n++-Si substrates. We measured the

Figure 3. Transfer (left) and output (right) characteristics of the devices measured under ambient conditions. (a, b) F-IIDT and (c, d) Cl-IIDT (films spin-coated from ODCB solutions, 3 mg/mL) at VDS = −100 V (transfer curves) and VGS changes from 0 to −100 V (output curves) (L = 60 μm, W = 2400 μm, capacitance Ci = 11 nF cm−2) after thermal annealing at 180 °C for 10 min.

is about 0.75 cm2 V−1 s−1, far surpassing that of Cl-IIDT, which is only 0.10 cm2 V−1 s−1. In OFET devices, the direction of hole transporting is along the surface of substrates, while in OPV devices, the carrier transporting direction is perpendicular to the substrates. Therefore, these two types of devices favor different polymer packing conformation. The space charge limited current (SCLC) method was also used to evaluate the exact carrier mobility of both polymers in OPV devices. Hole-only mobilities of both polymers were measured by SCLC method with device configuration of ITO/PEDOT:PSS/ polymers:PC71BM/MoO3/Ag. The thin films fabrication process is the same as OPV devices. Hole mobilities were calculated to be 2.07 × 10−5 cm2 V−1 s−1 for F-IIDT and 1.06 × D

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Macromolecules 10−5 cm2 V−1 s−1 for Cl-IIDT (Figure S10). In both FET and SCLC methods, F-IIDT showed higher hole mobility than ClIIDT, which might be resulted from the more planar polymer backbone and increased intermolecular interactions.28 Fabrication and Performance of BHJ Solar Cells. The configuration of the photovoltaic device is ITO/ZnO/ polymer:PC71BM/MoO3/Ag. We chose inverted configuration of OPV devices because of the possible improved device stability brought about by high-work-function metals.45,46 To find the best condition for each polymer, the photovoltaic properties were systematically screened by varying D/A ratio and spin-coating speeds. The ratio of D/A (w/w) was adjusted from 1:1 to 1:3, and the separated results for three polymers are summarized in Table S1. The optimized condition was 1:1.5 (D/A, w/w) in ODCB with 2.5% DIO as additives for all the polymers. F-IIDT-based devices showed lower PCE of 1.19% with moderate Jsc of 4.26 mA cm−2, Voc of 0.61 V, and FF of 0.46. To further improve the device performance of three polymers, careful device fabrication process was conducted and devices with different film thickness were fabricated. The device based on Cl-IIDT/PC71BM with 93 nm thick film showed the highest PCE of 4.60% (Table S2 and Figure S11). IIDT as donor material blended with PC71BM showed lower PCE of 3.24%. Although three polymers possess similar HOMO levels (−5.3 eV), the Voc of F-IIDT-based devices (∼0.63 V) are much lower than that of Cl-IIDT- and IIDT-based ones of 0.75 V (Figure 4 and Figure S12). This is likely due to the nonexact

penetrating network formed by donor and acceptor materials in blended thin films.10 Therefore, the morphology and microstructure, especially the phase separation condition, exert decisive influences on the device performance of an OPV device. In order to gain a thorough understanding of the cause for the difference in device performance of F-IIDT and ClIIDT, AFM and grazing incident wide-angle X-ray scattering (GIWAXS) were used to analyze film morphology and microstructure of each polymer film. All the films were prepared under the same condition as the device fabrication. As can be calculated from AFM height images (Figure 5a,b and Figure S14a), the surface of blended thin film of Cl-IIDT/ PC71BM and IIDT/PC71BM is rather smooth with tiny roughness of 5.4 and 4.9 nm, while F-IIDT/PC71BM formed rough thin films with calculated roughness as high as 48.0 nm. The rough surface indicates the strong crystallization tendency of polymer grains, which is harmful to the formation of effective phase separation between polymer and PC71BM.48,49 Besides, such inhomogeneous surface is unfavorable to carrier collection process since active layer and electrodes are unable to form conformal contact.50 On the other hand, the smooth film surface of Cl-IIDT/PC71BM and IIDT/PC71BM not only benefits effective charge collection but also indicates reduced crystallization tendency of chlorinated polymers.51 To better elucidate the microstructure and molecular packing in the blended thin films, GIWAXS was conducted for both pure polymer films and blended thin films. In pristine films, both polymers showed typical edge-on packing mode with intense out-of-plane diffraction peaks, indexed as (h00) (Figure 5c,d).52 The thin film of F-IIDT showed obvious three diffraction peaks along qz, while thin film of Cl-IIDT only showed two diffraction peaks, indicating that F-IIDT has more ordered lamellar packing than Cl-IIDT.53−55 The lamellar distances calculated from 1D out-of-plane plots (Figure S13) showed that F-IIDT (∼23.1 nm) had denser lamellar packing than Cl-IIDT (∼25.6 nm). The sizes of crystalline grains were obtained from the Scherrer equation.56 The coherence length (Lc) of pristine F-IIDT film is calculated to be 71.2 Å, larger than that of pristine film of Cl-IIDT of 62.1 Å, implying stronger crystallization tendency of F-IIDT than Cl-IIDT. The increased coherence length of F-IIDT can be attributed to the enhanced π−π interaction between polymer backbones, while for Cl-IIDT, contorted backbones caused by the large size of chlorine inhibited efficient packing of polymers. The more ordered and denser packing accounts for much higher hole mobility of F-IIDT than that of Cl-IIDT in OFET devices.22,28 After blended with PC71BM, both polymers exhibited remarkable change in orientation, from edge-on in neat films to face-on polymer backbone orientation in blended films (Figure 5e,f). The intensity distribution of lamellar diffraction peaks (h00) of F-IIDT/PC71BM blend in reciprocal (qxy−qz) space showed a ring of uniform intensity. For Cl-IIDT/ PC71BM and IIDT/PC71BM, (100) diffraction peaks contained arcs of intensity along qz and qxy direction (Figure S14b). For π−π stacking diffraction peaks, indexed as (010), it appeared

Figure 4. Current density−voltage (J−V) curves of the OSCs based on F-IIDT/PC71BM (black line) and Cl-IIDT/PC71BM (red line), tested under the illumination of AM1.5G, 100 mW cm−2.

linear proportionality between HOMO energy level and Voc, and morphology also exerts a great influence.47 The main reason that accounts for the dramatic difference in device performance of three polymers is Jsc, as Cl-IIDT devices showed Jsc of 10.00 mA cm−2, more than twice the Jsc of FIIDT-based devices (Figure 4). The performances of OFET and OPV devices of F-IIDT and Cl-IIDT are summarized in Table 2. Film Morphology and Microstructural Analysis. All the essential working processes of OPVs happen in the inter-

Table 2. Characterization of OFET and OPV Devices of F-IIDT and Cl-IIDT polymers F-IIDT Cl-IIDT a

μh (cm2 V−1 s−1)a 0.75 (0.69 ± 0.06) 0.10 (0.08 ± 0.02)

Vth (V) −24.14 −18.45

μh (cm2 V−1 s−1)b −5

Jsc (mA cm−2)

Voc (V)

FF

PCE

4.26 10.00

0.63 0.75

0.44 0.61

1.19% (0.96 ± 0.23%) 4.60% (4.35 ± 0.25%)

−5

2.07 × 10 ((1.98 ± 0.09) × 10 ) 1.06 × 10−5 ((0.95 ± 0.11) × 10−5)

Hole mobility measured in OFET devices. bHole mobility measured by SCLC method. E

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Figure 5. Tapping-mode AFM height images of (a) F-IIDT/PC71BM and (b) Cl-IIDT/PC71BM. 2D-GIWAXS patterns of (c) pure F-IIDT film, (d) pure Cl-IIDT film, (e) F-IIDT/PC71BM blended film, and (f) Cl-IIDT/PC71BM blended film.

also with no directivity for F-IIDT blended films, while it was only observed along qz direction for Cl-IIDT blended films and along qxy direction for IIDT blended films (Figure S14c,d). These phenomena indicate that in F-IIDT blended films the packing direction of polymer backbone is isotropic with no preferred crystallographic orientation, as shown in Figure 6b.57 In IIDT blended films, polymer backbones mainly adopting edge-on packing orientation (Figure S15). As for Cl-IIDT blended films, face-on packing is preferred, inferred from the intense out-of-plane (010) diffraction peak. The stacked ClIIDT polymer backbone planes are mainly parallel to the

substrate (Figure 6c). In stark contrast, most other donor materials, such as poly-3-hexylthiophene (P3HT) and IIDT in this work, have strong preference for edge-on orientation in blended films.58 Generally speaking, in a BHJ organic solar cell device, the optimal packing mode for blended films should be interpenetrated pure phase of each component with proper sizes (∼10−20 nm) with the charge transportation path perpendicular to the substrates for effective charge collection process.24 The face-on packing mode of Cl-IIDT in blended films proves that such perpendicular transport path has been realized, which, to some extent, may contribute to the better device performance of Cl-IIDT than F-IIDT and IIDT. The distinct packing orientation of three polymers with the same parent structure might be rationalized by the effects of preaggregates.59 For IIDT without any chemical modification, it may form larger preaggregation in solution than Cl-IIDT due to free from steric effect caused by large-size atom. The aggregation of IIDT with short-range order is inclined to adopt in-plane π stacking to minimize surface free energy, thus inducing disordered polymers to pack with preferred edge-on orientation in blended thin films during solution evaporation process. The inclusion of fluorine atoms results in the formation of relatively large polymer preaggregation in solution, which can be inferred from the absorption spectra.60 Such preaggregates soon precipitate onto the substrates without further adjustment process.61 As for Cl-IIDT, polymers with weakened intermolecular interactions may interact with PC71BM better in solution, and the slow precipitation process provides sufficient time for polymer and fullerene to pack with kinetically favorable face-on orientation. The diffraction hollows in two blended thin films with q of ∼1.34 Å−1 obtained from the 1D line (cut from 2D-GIWAXS patterns) (Figure 6a and Figure S14c,d) corresponds to the formation of pure PC71BM grains.58 This indicates that both donor and acceptor materials crystallize in blended films. PC71BM grains in F-IIDT and ClIIDT blended films show the same coherence length of 28.3 Å calculated from the Scherrer equation. Both polymers show the same lamellar distance of 23.22 Å in blended films. The π−π stacking distance for F-IIDT (4.14 nm) in blended films is far less than that of Cl-IIDT (4.25 nm).

Figure 6. (a) Integrated out-of-plane GIWAXS 2D patterns of FIIDT/PC71BM (black line) and Cl-IIDT/PC71BM (red line) blended films. Models of polymer packing orientation in blended films of (b) F-IIDT/ PC71BM and (c) Cl-IIDT/ PC71BM. F

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The coherence length of F-IIDT calculated from the full width at half-maximum (fwhm) of (100) in blended films is 122.1 Å, much larger than that of Cl-IIDT of 73.0 Å. Both polymers show larger Lc than commonly studied highperformance PTB7:PCBM system with ∼20 Å,62 which might be the reason for the moderate device performance of these two isoindigo polymer-based devices. Improved backbone planarity and increased interchain interaction in F-IIDT are probably responsible for the strong crystallization tendency of fluorinated polymers and thereby larger crystal domains in blended films. This impedes efficient D−A miscibility and is harmful to the formation of penetrated phase separation between donor (polymers) and acceptor (PC71BM) materials, therefore resulting in insufficient charge separation and low short-current density of F-IIDT-based devices. The incorporation of large-size chlorine atoms not only provides similar HOMO levels as fluorinated ones but also induces torsional backbone and moderates the strong intermolecular interactions between isoindigo-based polymers. This enables the acquisition of phase separation with proper coherence length and crystallized domains with face-on orientated polymer chains, which are essential to obtaining of high device performance in organic BHJ solar cells.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program (2013CB933501) from the Ministry of Science and Technology and National Natural Science Foundation of China. The authors thank beamlines BL14B1 and BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.

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CONCLUSION Two isoindigo-based polymers (F-IIDT and Cl-IIDT) have been developed as donor materials in BHJ solar cells to understand the effects of halogenation on microstructure of blended films. Through careful scrunity of UV−vis absorption spectra, theoretical calculation, AFM, and GIWAXS, we demonstrate that improved backbone planarity and increased noncovalent interactions brough about by fluorine atoms may impede the miscibility between polymer and fullerene. The coarse crystallization tendency of F-IIDT leads to the formation of highly crystallized films with no preferred polymer orientation and thus poor device performance. Thanks to the incoporation of large-atomic-size chlorine onto polymer backbones, distorted polymer backbone and moderate interchain interaction are obtained. Because of the reduced crystallization tendency, Cl-IIDT mixed with PC71BM formed films with less crystallinity and polymer chains are parallel to the substrate in blended films, thus realizing relatively better device performance. Overall, these findings indicate that halogenation can have strong influence on crystallization tendency as well as molecular orientation in blended films. Therefore, halogenation is a powerful strategy to modulate and balance different factors in D−A conjugated polymers, and its effects are far beyond frontier orbital enegy level modulation.

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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.5b01074. PES, TGA, DSC, and DFT calculation of F-IIDT, ClIIDT, and IIDT and GIWAXS profiles of IIDT:PC71BM blends (PDF)

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*E-mail: [email protected] (J.P.). *E-mail: [email protected] (J.Y.W.). G

DOI: 10.1021/acs.macromol.5b01074 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.5b01074 Macromolecules XXXX, XXX, XXX−XXX