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Article Cite This: Macromolecules 2019, 52, 738−746

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Synergistic Effects of Terpolymer Regioregularity on the Performance of All-Polymer Solar Cells Sang Woo Kim,† Honggi Kim,‡ Jin-Woo Lee,† Changyeon Lee,† Bogyu Lim,§ Jaechol Lee,§ Youngu Lee,*,‡ and Bumjoon J. Kim*,†

Macromolecules 2019.52:738-746. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/28/19. For personal use only.

†

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ Department of Energy Science and Engineering, Daegu Gyeongbuk Institute of Science and Technology (DGIST), Daegu 42988, Republic of Korea § Future Technology Research Center, Corporate R&D, LG Chem R&D Campus Seoul, LG Science Park, 30 Magokjungang 10-ro, Gangseo-gu, Seoul 07796, Republic of Korea S Supporting Information *

ABSTRACT: Random terpolymers with three different monomer units can provide broader light absorption than the most widely used donor−acceptor (D−A) alternating copolymers, but their electrical properties are often sacrificed by the randomly distributed monomers in the polymeric backbone that prevent efficient intermolecular π−π interactions. Here, we report the development of a regioregular terpolymer and demonstrate its importance in enhancing the power conversion efficiency (PCE) of all-polymer solar cells (all-PSCs). To investigate the impact of the monomer sequence and regioregularity in the terpolymer, we designed and synthesized two terpolymers (Ra-(D1−A−D2−A) random terpolymer and RR-(D1−A−D2−A) regioregular terpolymer) consisting of two electron-donating benzodithiophene (BDT) units with different side chains and one electron-withdrawing fluorinated thieno[3,4-b]thiophene (TT-F) unit. As a reference polymer, we also synthesized the D1−A alternating copolymer. The RR-(D1−A−D2−A) film exhibited stronger π−π stacking and a larger crystallite size than the D1−A and Ra-(D1−A−D2−A) films, resulting in 1 order of magnitude higher hole mobility than those of the other polymers. When blended with the P(NDI2HD−DTAN) polymer acceptor, the RR-(D1−A−D2−A)-based all-PSC yielded an outstanding PCE of 6.13%, which was superior to those of the D1−A-based all-PSCs (4.81%) and Ra-(D1−A−D2− A)-based all-PSCs (4.93%). These findings indicate that the synthesis of the regioregular terpolymer is a promising design strategy for the development of high-performance all-PSCs with improved optical and electrical properties.

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INTRODUCTION All-polymer solar cells (all-PSCs), composed of a polymer donor (PD) and polymer acceptor (PA) in their active layers, have proven their great potential over conventional fullerenebased PSCs in terms of complementary light absorption, facile energy-level tunability, and superior stabilities against thermal, photo, and mechanical stresses.1−10 Recently, the development of all-PSCs has been rapidly driven by rational molecular design of the PD or PA and device engineering, enhancing the power conversion efficiencies (PCEs) over 10%.11,12 In particular, PD’s based on benzodithiophene (BDT) with twodimensional conjugated side chains have been widely explored for the fabrication of efficient all-PSCs because of their broad light absorption range, high hole mobility, preferential face-on orientation, and good compatibility with efficient naphthalene diimide (NDI)-based PA’s.13−18 Motivated by these attractive features, we considered that it would be worthwhile to optimize the molecular structure of the BDT-based PD to further improve the optical, electrical, structural, and, thereby, photovoltaic properties of all-PSCs. © 2019 American Chemical Society

Among the various strategies for rational molecular design, the design of terpolymer with three different electroactive moieties is an effective approach through which to finely control light absorption, the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy levels, crystalline behavior, and solubility/miscibility, by varying the polymer composition.19−26 For example, controlling the side chains of terpolymers could optimize the polymer packing structure and provide favorable blend morphologies of the all-PSCs, which could further improve the photovoltaic property by overcoming the limitations of when only one type of side chain architecture is used.27,28 Recently, Sun and co-workers reported that a PSC based on the terpolymer PB55 with differing alkyl side chain lengths (ethylhexyl:butyloctyl side chain ratio = 5:5) showed better device performance than PSCs based on the alternating Received: October 31, 2018 Revised: December 19, 2018 Published: January 8, 2019 738

DOI: 10.1021/acs.macromol.8b02337 Macromolecules 2019, 52, 738−746

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Figure 1. (a) Device structure of all-PSCs. Chemical structures of (b) the polymer donors and (c) polymer acceptor used in this study.

Scheme 1. Synthesis of the (a) D1−A Alternating Copolymer, (b) Ra-(D1−A−D2−A) Random Terpolymer, and (c) RR-(D1− A−D2−A) Regioregular Terpolymer Used in This Study

junction (BHJ) morphology.29 However, despite its effectiveness, the performance improvement via the random terpolymer

copolymers PB01 (with only ethylhexyl) or PB10 (with only butyloctyl) due to the formation of a better bulk-hetero739

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Macromolecules Table 1. Characteristics of the Polymer Donors and Acceptors in This Study polymer

εfilma (×104 cm−1)

Egopt a (eV)

Mnb (kg mol−1)

Mwb (kg mol−1)

Đb (Mw/Mn)

HOMO (eV)

LUMO (eV)

D1−A Ra-(D1−A−D2−A) RR-(D1−A−D2−A) P(NDI2HD−DTAN)

8.8 8.7 9.5 4.8

1.57 1.57 1.58 1.64

37.5 31.9 38.8 55.5

92.0 75.0 88.3 114.8

2.5 2.4 2.3 2.1

−5.43c −5.38c −5.38c −5.59d

−3.68c −3.63c −3.65c −3.95c

Measured from UV−vis absorption spectra of thin films. bDetermined by high temperature GPC. cMeasured by CV. dCalculated from ELUMO = EHOMO + Egopt. a

Figure 2. (a) The absorption spectra and (b) energy level diagram of the polymer donors and polymer acceptor.

approach is often restricted by the weak intermolecular π−π stacking interactions and disrupted molecular packing of the resulting polymers due to the randomly distributed monomers, which is one of the major limiting factors for increasing the electrical properties and short-circuit current (Jsc) of the solar cells.30−32 To address these issues associated with the randomness of terpolymer units, regioregular terpolymers having regular monomer sequences and orientations of asymmetric repeating units need to be designed.33−36 One example was successfully demonstrated in our previous work, in which the regioregular terpolymer PDTSTTBDT (consisting of two electron-donating units, dithienosilole (DTS) and BDT, and one electron-withdrawing unit of thienothiophene (TT)) exhibited improved light absorption, more favorable molecular ordering, and higher charge carrier mobility compared with random PDTSTTBDT, resulting in a significant increase in the PCE from 1.11 to 6.14%.35 However, the use of regioregular terpolymer in the all-PSC system has been very limited and the effect of the monomer sequence of the terpolymer on the performance of the all-PSC has not been reported, to the best of our knowledge. Herein, we report the development of a regioregular terpolymer donor with two electron-donating units with different side chains and one electron-withdrawing unit and demonstrate significant PCE enhancements in the all-PSC. To elucidate the impact of the monomer sequence and regioregularity, we synthesized two different D1−A−D2−A terpolymers consisting of a 2-ethylhexyl-thio-substituted BDT unit (D1), a 2-ethylhexyl-substituted BDT unit (D2), and a 2ethylhexyl-4,6-dibromo-3-fluorothieno[3,4-b]-thiophene-2-carboxylate (F-TT) unit (A); however, whereas one polymer has a random sequence between D1−A and D2−A (Ra-(D1−A− D2−A)), the other has a regular sequence (RR-(D1−A−D2− A)) with controlled regioregularity. We also synthesized the D1−A alternating copolymer on the basis of D1 and TT-F as a reference. We investigated the optical, structural, and electrical properties of these three polymers and examined their electrical and photovoltaic characteristics in organic field-effect

transistor (OFET) and all-PSC devices. Importantly, grazing incidence X-ray scattering (GIXS) results showed that RR(D1−A−D2−A) had significantly stronger face-on π−π stacking interactions than the other polymers in the films. Thus, the hole mobility for the RR-(D1−A−D2−A) polymer was greatly improved, which was an order of magnitude higher than those of the D1−A and Ra-(D1−A−D2−A). The RR(D1−A−D2−A) polymer-based all-PSC, blended with poly[[N,N′-bis(2-hexyldecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-(E)-2,3-bis(thiophen-2-yl)acrylonitrile] P(NDI2HD−DTAN) as PA, had a high PCE of 6.13%, which was superior to those of the (D1−A)- and Ra(D1−A−D2−A)-based all-PSCs. This improvement was attributed to the higher hole mobility, well-balanced hole/ electron mobility, better light absorption, and reduced charge recombination, which resulted in improved Jsc and fill factor (FF) values. Therefore, we successfully demonstrate the importance of the design of the regioregular terpolymer with simultaneously enhanced optical and electrical properties that can lead to efficient all-PSCs.

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RESULTS AND DISCUSSION The molecular structures of the D1−A alternating copolymer, random terpolymer (Ra-(D1−A−D2−A)), and regioregular terpolymer (RR-(D1−A−D2−A)) are shown in Figure 1. To synthesize the three PD’s, we prepared two donor units with different side chains (2,6-bis(trimethyltin)-4,8-bis(5-((2ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (alkylthio-substituted BDT) and 2,6-bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (alkyl-substituted BDT) as the D1 and D2 units, respectively), as well as the acceptor unit, 2-ethylhexyl-4,6dibromo-3-fluorothieno[3,4-b]-thiophene-2-carboxylate (FTT). The PD’s were synthesized analogously, using a direct reaction procedure based on the conventional Stille coupling reaction. However, for the synthesis of RR-(D1−A−D2−A), a stepwise synthetic procedure was employed (Scheme 1). First, the A−D2−A intermediate was synthesized, and then, D1 was 740

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Figure 3. (a) 2D-GIXS images of D1−A, Ra-(D1−A−D2−A), and RR-(D1−A−D2−A) pristine films. Line cuts of GIXS images: (b) in-plane and (c) out-of-plane. The inset graph is the enlarged view of the π−π stacking peak.

attached to it. The detailed syntheses of the monomers and polymers are described in the Supporting Information. The number average molecular weight (Mn) and dispersity (Đ) of the synthesized PD’s were measured by high-temperature gelpermeation chromatography (HT-GPC) analysis at 150 °C in 1,2,4-trichlorobenzene as an eluent relative to polystyrene standards. All three PD’s had similar Mn (32−39 kg mol−1) and Đ (2.3−2.5) values, which minimizes the effects of molecular weight on the properties of the polymers. Table 1 summarizes the molecular weights and optical and electrochemical properties of the polymers. The UV−vis absorption spectra of pristine thin films of the PD’s and PA (P(NDI2HD−DTAN)) are shown in Figure 2a. All of the PD’s exhibited strong absorptions in the visible range (500−750 nm), which is attributed to the intramolecular charge transfer between the electron-donating and electronwithdrawing units. While all three PD’s had high absorption coefficients (>8.7 × 104 cm−1), the absorption coefficient of the regioregular RR-(D1−A−D2−A) polymer was approximately 10% higher than those of the D1−A and Ra-(D1−A− D2−A) polymers. The enhanced peak intensity, particularly in the vibronic regime, suggests that the improved absorption ability can be mainly attributed to the stronger interchain interactions in the regioregular polymer, which would improve light harvesting in the all-PSCs.37−39 The electrochemical properties of the PD’s were examined by cyclic voltammetry (CV) (Figure S1). As depicted in the energy diagram in Figure 2b, the HOMO energy levels were determined to be −5.43 eV for D1−A and −5.38 eV for both Ra-(D1−A−D2−A) and RR-(D1−A−D2−A). The respective LUMO values were measured to be −3.68 eV for D1−A, −3.63 eV for Ra-(D1−A−D2−A), and −3.65 eV for RR-(D1−A−D2− A). On the basis of the electrochemical results, both Ra-(D1− A−D2−A) and RR-(D1−A−D2−A) showed slightly up-shifted HOMO energy levels compared with that of D1−A, because their alkyl side chains have a stronger electron-donating ability than the alkylthio side chains in D1−A.40,41

To investigate the effects of the side chain modification and structural regularity on the packing structures of the polymers, pristine thin films of the three PD’s were investigated by GIXS measurements. Figure 3 shows the 2D GIXS images and their line-cut plots. All of the PD’s exhibited distinct (100) reflection peaks in the in-plane direction, corresponding to different lamellar spacings (d100) of 24.6 Å for D1−A, 23.5 Å for Ra(D1−A−D2−A), and 23.7 Å for RR-(D1−A−D2−A) (Table S1). Interestingly, both Ra-(D1−A−D2−A) and RR-(D1−A− D2−A) had lamellar distances of ∼1 Å shorter than D1−A because the alkyl side chains in the D1−A−D2−A terpolymers are shorter than the alkylthio chains with the additional sulfur spacer. The crystallization behavior of the polymers can be compared by estimating the crystal correlation lengths (CCLs) of the polymer crystallites, which were calculated from the full widths at half-maximum of the (100) peaks in the in-plane direction using the Scherrer equation.42,43 The CCL100 value (88.5 Å) of the RR-(D1−A−D2−A) polymer in the films is higher than those of D1−A (83.7 Å) and Ra-(D1−A−D2−A) (78.5 Å), suggesting that the RR-(D1−A−D2−A) polymer film has relatively larger crystallites. Moreover, the relative intensity of the (100) diffraction peak in the in-plane direction for the RR-(D1−A−D2−A) film is stronger than those of both D1−A and Ra-(D1−A−D2−A), indicating a greater degree of order in the crystalline structures (Figure 3b).44,45 The difference in the crystalline behavior is also evident in the pattern of π−π stacking (010) that represents the intermolecular packing between the polymers. While the π−π stackings (010) of all of the PD’s exhibit similar distances of 3.9 Å with preferential faceon orientations along the out-of-plane direction, the intensity of the π−π stacking peak for the RR-(D1−A−D2−A) film is significantly higher than those of the D1−A and Ra-(D1−A− D2−A) films (Figure 3a,c). These results demonstrate that the enhanced structural regularity can effectively enhance crystallinity, resulting in the stronger interchain π−π interactions and the better vertical π-overlap between the RR-(D1−A−D2−A) polymer backbones in the film.46−50 This feature is important 741

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Macromolecules Table 2. OFET and SCLC Performances of D1−A, Ra-(D1−A−D2−A), and RR-(D1−A−D2−A) Pristine Films polymer D1−A Ra-(D1−A−D2−A) RR-(D1−A−D2−A)

μh,FET(max) (cm2 V−1 s−1) −4

6.3 × 10 2.2 × 10−4 4.3 × 10−3

μh,FET(avg)a (cm2 V−1 s−1)

Vth (V)

−4

5.1 × 10 1.9 × 10−4 2.9 × 10−3

−3.3 2.1 4.7

Ion/Ioff

μh,SCLC(max) (cm2 V−1 s−1)

4

>10 >104 >104

μh,SCLC(avg)b (cm2 V−1 s−1)

−5

6.0 × 10−5 2.7 × 10−5 1.9 × 10−4

7.1 × 10 3.9 × 10−5 3.1 × 10−4

a

The average values were determined from measurements of at least 20 devices. bThe average values were obtained from at least 5 SCLC devices.

Figure 4. (a) J−V curves and (b) EQE responses of the D1−A-, Ra-(D1−A−D2−A)-, and RR-(D1−A−D2−A)-based all-PSC devices.

Table 3. Photovoltaic Performance and SCLC Mobilities of the D1−A-, Ra-(D1−A−D2−A)-, and RR-(D1−A−D2−A)-Based All-PSC Devices active layer D1−A:PA Ra-(D1−A−D2−A):PA RR-(D1−A−D2−A):PA

Voc (V) 0.74 ± 0.01 0.71 ± 0.01 0.73 ± 0.00

Jsc (mA cm−2) 12.22 ± 0.37 11.53 ± 0.70 13.59 ± 0.35

FF 0.51 ± 0.01 0.58 ± 0.02 0.60 ± 0.01

PCEavga (PCEmax) (%) 4.55 ± 0.18 (4.81) 4.72 ± 0.15 (4.93) 5.93 ± 0.10 (6.13)

μhb (cm2 V−1 s−1) −5

8.4 × 10 6.1 × 10−5 2.6 × 10−4

μeb (cm2 V−1 s−1) 2.7 × 10−5 2.6 × 10−5 9.8 × 10−5

a Average values were obtained from at least 10 devices. The deviation values were obtained using the standard deviation formula. bAverage values were determined by the SCLC method.

electron mobility and good compatibility with BDT-based PD’s.57 To optimize device performance, we varied the active layer blend conditions, including the blend ratio, polymer concentration, and solvent additive. The optimal blend ratio of PD:PA was determined to be 1.3:1.0 (w/w), and the blend films were spin-coated from a chloroform (CF) solution with 2.5 vol % diphenyl ether (DPE) to produce an active layer thickness of ∼90 nm. Device fabrication is detailed in the Supporting Information. The current density−voltage (J−V) curves of the all-PSC devices under AM1.5G illumination (100 mW cm−2) are shown in Figure 4, and the corresponding photovoltaic parameters are summarized in Table 3. Interestingly, the RR(D1−A−D2−A)-based all-PSCs had a PCEmax of 6.13%, which outperformed those of the all-PSCs based on D1−A (PCEmax = 4.81%) and Ra-(D1−A−D2−A) (PCEmax = 4.93%). Although the Voc values for the three PD-based all-PSCs were barely changed, the Jsc value for the RR-(D1−A−D2−A)-based allPSC (13.59 mA cm−2) was much higher than that of the random-terpolymer-based all-PSC (11.53 mA cm−2). And, the FF value for the RR-(D1−A−D2−A)-based all-PSC was also enhanced. These features are mainly attributed to the enhanced light absorption ability (i.e., higher absorption coefficient, as shown in Figure 2a) and higher hole mobility. The external quantum efficiencies (EQEs) of the optimal devices were measured and are compared each other, as presented in Figure 4b. The RR-(D1−A−D2−A)-based all-PSC exhibits a higher EQE value than the other PD’s in the range 600−800 nm, which is consistent with the vibronic features of its UV−vis spectrum.

to enhance charge transport properties, which will be discussed in the following section. Next, we fabricated OFET devices with a bottom-gate topcontact configuration to examine the electrical properties of the three PD’s. Detailed procedures are described in the Supporting Information. The current−voltage (I−V) characteristics of the OFET devices show typical p-channel operation (Figure S2), and the electrical performances of the OFET devices are summarized in Table 2. The OFET hole mobility (μh,FET(max)) values were measured to be 6.3 × 10−4 cm2 V−1 s−1 for D1−A, 2.2 × 10−4 cm2 V−1 s−1 for Ra-(D1−A−D2−A), and 4.3 × 10−3 cm2 V−1 s−1 for RR-(D1−A−D2−A). Therefore, controlling the monomer sequence and regioregularity of Ra(D1−A−D2−A) increases the hole mobility by 20 times (from Ra-(D1−A−D2−A) to RR-(D1−A−D2−A)). These results correlate well with the GIXS results showing the intensified crystalline features for the RR-(D1−A−D2−A) PD. Next, we compared the charge transport properties of the PD’s in the vertical direction relative to the film by measuring the hole mobility (μh,SCLC) using the space-charge limited current (SCLC) method (Table 2 and Figure S3).51,52 Again, we found the increase of μh,SCLC (by 1 order of magnitude) from 3.9 × 10−5 (Ra-(D1−A−D2−A)) to 3.1 × 10−4 cm2 V−1 s−1 (RR(D1−A−D2−A)). These enhancements in the charge carrier mobility will affect the Jsc and PCE values in the corresponding solar cells.53−56 To evaluate the potential of the PD polymers in the all-PSCs, we fabricated inverted-type solar devices with an indium tin oxide (ITO)/ZnO/active layer/MoO3/Ag architecture. P(NDI2HD−DTAN) was chosen as PA because of its high 742

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between the polymers in the pristine films, because the (100) and (010) reflection peaks of the PD’s and PA are overlapped and the peak intensity of the semicrystalline PA is relatively larger than those of the amorphous PD’s.45 Nevertheless, the RR-(D1−A−D2−A):PA blend film exhibits a relatively stronger (010) peak intensity for π−π stacking as well as (100) peak intensity for the lamellar stacking than the other blend films. For example, the RR-(D1−A−D2−A) blend film shows a larger CCL100 value of 72.2 Å compared with those of the D1−A (60.4 Å) and Ra-(D1−A−D2−A) (61.0 Å) blend films (Table S2). From these results, it is suggested that the regioregular terpolymer is beneficial for efficient charge transport and suppressed charge recombination, thus leading to the increases in the Jsc, FF, and PCE.

To better understand the performance of the all-PSCs, we measured the SCLC hole and electron mobilities with device configurations of ITO/PEDOT:PSS/active layer/Au (holeonly) and ITO/ZnO/active layer/LiF/Al (electron-only). The hole and electron mobilities of the RR-(D1−A−D2−A):PA blend film (2.6 × 10−4 and 9.8 × 10−5 cm2 V−1 s−1, respectively) were higher than those of D1−A:PA and Ra-(D1− A−D2−A):PA. This trend is consistent with the hole mobilities in the pristine film. In addition, to investigate the charge recombination behavior, the three PD-based all-PSCs were evaluated in terms of their J−V characteristics as a function of light intensity (P).58−60 Figure S4 shows the log−log plot of the Jsc and log−linear plot of the Voc values. The Jsc values exhibit a power-law dependence on the light intensity (Jsc ∝ Pα); the obtained α value is closer to 1, which indicates that bimolecular recombination is more suppressed.58 The fitted slope of the RR-(D1−A−D2−A)-based all-PSC exhibited a slightly higher α value of 0.94 than the slopes of the other blend systems. Next, the plot of Voc linearly depends on the light intensity, with a slope of kBT q−1, where kB is the Boltzmann constant, T is the temperature in Kelvin, and q is the elementary charge.58 The S value of the RR-(D1−A−D2− A):PA blend (1.08) is lower than those of the D1−A:PA blend (1.33) and Ra-(D1−A−D2−A):PA blend (1.33), suggesting that monomolecular recombinations in the RR-(D1−A−D2− A):PA blend are significantly more inhibited. These beneficial features in the RR-(D1−A−D2−A)-based all-PSC device can be mainly attributed to the better charge transport capability and stronger crystalline behavior, resulting in the enhanced Jsc and FF values.61−67 The surface morphology and packing structures of the PD:PA blend films were examined by atomic force microscopy (AFM) and GIXS, respectively. All PD:PA blend films were prepared under optimized device conditions. As shown in Figure 5, the

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CONCLUSIONS In summary, we have described the design of a regioregular terpolymer (RR-(D1−A−D2−A)) and investigated how its packing structure and light absorption properties can affect its electrical and photovoltaic properties through all-PSC and OFET devices, respectively. The RR-(D1−A−D2−A) showed stronger intermolecular π−π stacking interactions, tighter lamellar stacking, and a larger crystallite size, leading to a significantly higher hole mobility in the OFET and SCLC devices. Importantly, compared with D1−A and Ra-(D1−A− D2−A), the best performance in the all-PSC devices was obtained from the RR-(D1−A−D2−A):P(NDI2HD−DTAN) (6.13%), attributed to the high charge transport and enhanced light absorption coefficient. Overall, our results highlight the synergistic effects of the regioregular terpolymer approach with structural regularity control and side chain engineering on the development of high-performance all-PSCs.

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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.8b02337. Materials and methods, detailed experimental procedures, and additional characterization data (PDF)

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Figure 5. AFM height images of (a) D1−A:P(NDI2HD−DTAN), (b) Ra-(D1−A−D2−A):P(NDI2HD−DTAN), and (c) RR-(D1−A−D2− A):P(NDI2HD−DTAN) blend films.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

root-mean-square roughness (RMS) values of the polymer blend films are 0.98, 0.92, and 1.12 nm for D1−A:PA, Ra-(D1− A−D2−A):PA, and RR-(D1−A−D2−A):PA, respectively. Thus, all three blend films (D1−A:PA, Ra-(D1−A−D2−A):PA, and RR-(D1−A−D2−A):PA) showed similar, finely separated nanoscale domains. Next, we investigated the packing structures and crystal orientations in the blend films using GIXS measurements (Figure S5). Since both PD’s and P(NDI2HD−DTAN) pristine films favor a face-on orientation, all of the blend films preferentially adopt a face-on orientation, as evidenced by the π−π stacking peaks in the out-of-plane direction (Figures S5 and S6).57 This feature suggests that face-to-face alignment between the PD’s and P(NDI2HD− DTAN) acceptor in the blend can sigificantly reduce charge recombination.68−70 The GIXS results for the three blend films seem not much different in comparison with the difference

Bumjoon J. Kim: 0000-0001-7783-9689 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea (2017M3A7B8065584, 2016R1E1A1A02921128, and 2015M1A2A2056216). We thank Dr. Hongseok Yun for helpful discussion.

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REFERENCES

(1) Zhou, N.; Facchetti, A. Naphthalenediimide (NDI) Polymers for All-Polymer Photovoltaics. Mater. Today 2018, 21, 377−390. (2) Long, X.; Ding, Z.; Dou, C.; Zhang, J.; Liu, J.; Wang, L. Polymer Acceptor Based on Double B←N Bridged Bipyridine (BNBP) Unit

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