A Fluorinated Polythiophene Derivative with Stabilized Backbone

Apr 8, 2016 - Here, taking a polythiophene derivative (PBDD4T) as a starting polymer, we tried to increase the rotation barrier and hence stabilize it...
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A Fluorinated Polythiophene Derivative with Stabilized Backbone Conformation for Highly Efficient Fullerene and Non-Fullerene Polymer Solar Cells Shaoqing Zhang,†,‡ Yunpeng Qin,†,‡ Mohammad Afsar Uddin,§ Bomee Jang,§ Wenchao Zhao,† Delong Liu,† Han Young Woo,*,§ and Jianhui Hou*,†,‡ †

School of Chemistry and Biology Engineering, University of Science and Technology Beijing, Beijing 100083, China State Key Laboratory of Polymer Physics and Chemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China § Department of Chemistry, College of Science, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Here, taking a polythiophene derivative (PBDD4T) as a starting polymer, we tried to increase the rotation barrier and hence stabilize its backbone conformation by introducing fluorine into the β- and β′-position of the αlinked bithiophene segments and then synthesized a new polymer named as PBDD4T-2F. Our results demonstrate that the rotation barrier between the α-linked bithiophene significantly increases after the fluorination, so PBDD4T-2F has a more stable backbone conformation than PBDD4T. Compared to PBDD4T, PBDD4T-2F shows stronger aggregation effect in solution state and more compact π−π stacking in solid thin film and also possesses deeper HOMO level. These properties make PBDD4T-2F being an ideal donor material in PSCs. When blended with PC71BM, a fullerene acceptor, the PBDD4T-2F-based device showed a power conversion efficiency (PCE) of 9.04%, which is 38% higher than that of the PBDD4T-based device; when blended with ITIC, a non-fullerene acceptor, the PBDD4T-2F-based device showed a PCE of 8.69%, which is almost 20 times higher than that of the PBDD4T-based device. What is more, the tandem cell, in which the blend of PBDD4T-2F:PC61BM was used for making the front subcell, exhibited a high PCE of 10.12%. The photovoltaic results indicate that the fluorination is an effective method to enhance interchain π−π interaction for the polythiophene and hence to tune its photovoltaic properties in PSCs, especially for the fullerene-free device based on ITIC. In recent years, our group carried out a series of studies8−10 to reveal the correlations among backbone conformations, aggregation structures, and photovoltaic properties of conjugated polymers. For example, we found that the polymers with straight backbones exhibited stronger interchain π−π stacking effect and more ordered lamellar packing structures compared to their counterparts with zigzagged backbones, and the PSCs based on the straight-conformational polymers often presented higher PCEs. Very recently, a few research groups reported encouraging results by employing conjugated polymers with strong interchain π−π interaction.11−15 For instance, as reported by Yan et al., the polymer named as PffBT4T-2OD showed strong aggregation effect in the solution state and could form a highly crystalline structure in the solid state, and the corresponding devices exhibited excellent PCEs over 10%;11 Hwang et al. synthesized PBT-ttTPD with enhanced interchain π−π interaction in solid film and realized a PCE of 9.21%.16 Therefore, from the point of view of

1. INTRODUCTION Conjugated polymers have been broadly used in polymer solar cells (PSCs) with the bulk heterojunction (BHJ) structure.1 The BHJ active layer in a PSC device commonly consisted of a blend of a conjugated polymer as electron donor and a polymeric or small molecular material as electron acceptor, and its photovoltaic property is not only determined by intrinsic characteristics of the donor and the acceptor but also greatly affected by the morphological properties of the BHJ active layer. In order to improve device performance of PSCs, various of strategies have been investigated and successfully applied to design molecular structures of conjugated polymers, by which the optical absorption, molecular energy levels, carrier mobilities, and/or morphological properties of the materials can be effectively tuned.2−7 In fact, to reveal the correlation between the chemical structures and the aforementioned properties is a key to develop molecular design strategies, by means of which high-performance photovoltaic materials could be produced. As known, for conjugated polymers, the interchain interaction is much stronger than van der Waals force and thus plays an important role in affecting their aggregation states. © XXXX American Chemical Society

Received: February 1, 2016 Revised: April 2, 2016

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DOI: 10.1021/acs.macromol.6b00248 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Chemical Structures and Synthesis Routes of PBDD4T and PBDD4T-2Fa

a Reagents and conditions: (i) toluene, Pd(PPh3)4, inert atmosphere, 110 °C, 16 h; (ii) CHCl3, NBS, 0 °C; (iii) toluene, Pd(PPh3)4, inert atmosphere, 110 °C, 17 h.

Figure 1. (a) DFT-calculated HOMO and LUMO surfaces of the optimal geometries at the B3LYP/6-31G(d,p) level. (b) Torsional energy profiles for the bithiophene segments in the two polymers.

2. RESULTS AND DISCUSSION Consideration for Chemical Structures of the Polymers. As shown in Scheme 1, the newly designed polythiophene derivative (named as PBDD4T-2F) is an alternative copolymer based on 1,3-bis(4-(2-ethylhexyl)thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) and 3,3′-difluoro-2,2′-bithiophene (T-T-2F), which is a difluorinated derivative of PBDD4T.17 The synthesis of the new polymer is provided in the Experimental Section; meanwhile, PBDD4T was also prepared and used as the control polymer. The numberaverage molecular weights (Mn) of PBDD4T-2F and PBDD4T in this work are 22.2 and 12.2 kDa with polydisperisity index (PDI) of 1.98 and 2.41, respectively. These two polymers show similar thermal stability in thermogravimetric analysis (TGA), i.e., thermal decomposition temperature (Td) at approximately 400 °C (see Figure S1 in Supporting Information). As known, fluorine can form weak nonbonding interaction with sulfur atom,12,18−21 and consequently, the torsional barrier between the two thiophenes in T-T-2F unit will be different than that in 2,2′-bithiophene (T-T). Here, we initially calculated the optimal geometries of the dimers of PBDD4T2F and PBDD4T by density functional theory (DFT) at the B3LYP/6-31G(d,p) level. As shown in Figure 1a, the optimal geometries of these two polymers present very similar conformation. In addition, we carried out the theoretical calculations to analyze the thermodynamically stabilities of the rotational conformers of T-T-2F and T-T. As demonstrated in Figure 1b, the optimal torsional angle of the stable state of T-T2F is 0° and that in T-T is 25°; from 0 to 180°, these two units show metastable states at 140° and 145° for T-T-2F and T-T,

molecular design, the strategies that may affect the interchain interaction can be used to manipulate the aggregation structure of conjugated polymers and hence to tune morphological properties of the BHJ active layers. Although the conjugated segments in conjugated polymers are all rigid, these adjacent segments are often linked with rotatable single bonds. Therefore, a conjugated polymer with noncentrosymmetric building blocks like thiophene and its derivatives will have many backbone rotational conformers (also known as rotamers). We speculate that for such a conjugated polymer that if we could increase the rotation barriers between the adjacent rotatable segments and/or enlarge the total energy difference between the optimal and the metastable geometries, the conformer with the optimal geometry will be more thermodynamically stable, which may be favorable for achieving more ordered interchain packing and hence affect its photovoltaic properties. However, it has to be said that although this speculation seems reasonable, to the best of our knowledge, it has not been verified in molecular design of conjugated photovoltaic materials. Thus, we carried out the study in this work to prove the above-mentioned presumption and developed a new polythiophene derivative with high photovoltaic performance in device. By utilizing this newly designed polymer as donor, we get PCEs of 9.04% and 8.69% in fullerene-based and fullerene-free PSCs, respectively. In addition, this polymer could also be used as the wide band gap absorber in highly efficient double-junction tandem PSCs, in which a PCE of 10.12% was obtained. B

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Figure 2. Temperature-dependent UV−vis spectra of the two polymers solutions in chlorobenzene: (a) PBDD4T and (b) PBDD4T-2F.

Figure 3. 2D GIWAXS patterns of pristine films of (a) PBDD4T and (b) PBDD4T-2F and (c) the corresponding in-plane and out-of-plane line-cut profiles.

significantly red-shifted from 490 nm (110 °C) to 590 nm (−10 °C). However, for the PBDD4T solution, it only shows a very weak absorption hump under −30 °C, and also the redshift of the main absorption peak is much smaller than that observed in PBDD4T-2F. For a conjugated polymer solution, interchain interaction can be reduced by increasing temperature, so its main absorption peaks originated from the intramolecular π−π* transition; under low temperature in solution, the interchain interaction will increase and result in an absorption shoulder of intermolecular π−π* transition at long wavelength range, which have been observed in many conjugated polymers.11,22−25 That is to say, higher absorption shoulder means stronger interchain interaction. Therefore, the TD-Abs results indicate that PBDD4T-2F shows the stronger interchain π−π interaction in solution state than PBDD4T. Moreover, the solid thin film of PBDD4T-2F also shows a higher absorption extinction coefficient and stronger shoulder peak than PBDD4T (see Figure S3). Electrochemical cyclic voltammetry (CV) measurement was performed to determine the highest occupied molecular orbital (HOMO) levels of the polymers. As shown in Figure S4 (see Supporting Information), the p-doping onset potentials (Eox) of PBDD4T-2F and PBDD4T are 0.59 and 0.50 V, corresponding to the HOMO levels of −5.39 and −5.30 eV, respectively. These results indicate that the fluorination of 2,2′-dithiophene can reduce the molecular energy levels of the polymer, which is in consequence of the strong electronegativity of fluorine

respectively. For T-T-2F, there is a 16.9 kJ/mol energy barrier from the stable state to the metastable state, and the barrier is 6.5 kJ/mol from the metastable state to the stable state; for T-T unit, these two barriers are 11.4 and 8.5 kJ/mol, respectively. On the basis of the theoretical calculation results, we can get the following conclusions. First, T-T-2F tends to form a much more planar structure than T-T; second, the optimal geometry of T-T-2F should be more stable than that of PBDD4T; third, in metastable state, T-T-2F can more easily rotate to its stable state than T-T. Therefore, we can reasonably infer that although the optimal backbone conformations of these two polymers are very similar, the optimal geometry of PBDD4T2F should be more stable than that of PBDD4T, and hence PBDD4T-2F should have stronger interchain interaction and/ or possess higher crystallinity than PBDD4T in the aggregation state. Temperature-Dependent UV−Vis Absorption Spectra and Molecular Energy Levers. Then, we employed temperature-dependent UV−vis absorption spectroscopy (TD-Abs) to investigate the aggregation effects of the two polymers in solution state. As shown in Figures 2a and 2b, under high temperature, i.e. 90 °C, these two polymers show very similar absorption spectra with onsets at ca. 600 nm. For the PBDD4T-2F solution, an absorption shoulder appears at longer wavelength when the temperature reduces to 30 °C, and it increases greatly and forms a peak under low temperature (see the spectra at −10 °C). In addition, it could be observed that the main absorption peak of the PBDD4T-2F solution is C

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Figure 4. (a) J−V curves and (b) the EQE curves of the single-junction PSC devices with a structure of ITO/PFN/BHJ/MoO3/Al under the illumination of AM 1.5G 100 mW/cm2.

Table 1. Photovoltaic Characteristics of PSC Devices Based on the Two Polymer Blends with PC71BM and ITIC BHJ layer

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

PBDD4T:PC71BM PBDD4T:ITIC PBDD4T-2F:PC71BM PBDD4T-2F:ITIC

0.86 0.88 0.88 0.94

11.16 2.04 15.13 15.04

0.68 0.25 0.68 0.61

6.53 0.45 9.04 8.69

PCEav (%)

thickness (nm)

± ± ± ±

78 85 91 90

6.24 0.36 8.68 8.13

0.24 0.08 0.29 0.33

dooctane (CB:DIO = 100:1, v/v) as processing solvent, a donor/acceptor (D/A) weight ratio of 1.5:1, and an active layer thickness of approximately 90 nm. The PBDD4T:PC71BMbased PSCs were also prepared by following the procedure in our previous work17 and used as the control devices. The current-density/voltage (J−V) curves of the best performing PSCs based on the PBDD4T-2F:PC71BM and PBDD4T:PC 71 BM are shown in Figure 4a, and the corresponding photovoltaic parameters are collected in Table 1. Compared to the control device, the PBDD4T-2F:PC71BMbased device showed a higher open circuit voltage (Voc) of 0.88 V, an improved short circuit current density (Jsc) of 15.13 mA/ cm2, and a similar fill factor (FF) of 0.68, resulting in a higher PCE of 9.04%. Therefore, when blended with PC71BM, PBDD4T-2F showed improved photovoltaic properties than PBDD4T. Furthermore, we also fabricated two types of fullerene-free PSCs by employing PBDD4T-2F and PBDD4T as donor, respectively, and a promising non-fullerene-base small molecular acceptor named as ITIC31 (the chemical structure is shown in Scheme S1). The device structure is the same as that used in the PC71BM-based PSCs, and the best performing PSCs can be fabricated by using CB:diphenyl ester (CB:DPE = 100:1, v/v) as processing solvent and a D/A weight ratio of 1:1 (see Experimental Section). J−V curves of the PSCs based on PBDD4T:ITIC and PBDD4T-2F:ITIC are shown in Figure 4a, and the photovoltaic parameters are collected in Table 1. The PBDD4T:ITIC PSC show a good Voc (0.88 V) but very poor Jsc (1.15 mA/cm2) and a miserable FF (0.25), resulting in a low PCE of 0.45%, which is much lower than the PBDD4T:PC71BM-based device and also much lower than those of the devices using ITIC and the likes as acceptor and other polymers as donor in reported works.31−34 Surprisingly, the PBDD4T-2F:ITIC device exhibits a Voc of 0.94 V, a Jsc of 15.04 mA/cm2, and a FF of 0.61, resulting in an impressive PCE of 8.69%, which is among the top values in fullerene-free PSCs. It should be noted that the molecular weights of the polymers have not been fully optimized yet; further studies on the correlation between molecular weight and photovoltaic performance are still needed.

substituents and should be helpful for improving open circuit voltage in PSCs.2,5,6,21,26−29 2-D Grazing Incident Wide-Angle X-ray Scattering (GIWAXS) Analysis. 2-D Grazing incident wide-angle X-ray scattering (GI-WAXS) was used to characterize the crystalline structural features of the polymers neat films. Figures 3a and 3b show the 2D scattering images of spin-coated thin films of the two polymers casted from chlorobenzene, and also the in-plane and out-of-plane diffraction profiles are provided in Figure 3c. As shown, PBDD4T and PBDD4T-2F samples present broad peaks (100) at ∼0.35 and ∼0.32 Å−1, corresponding to a lamellar d-spacing of 18.0 and 19.6 Å, respectively. A clear reflection (010) peak could be observed at 1.60 Å−1 for PBDD4T-2F, which corresponds to the d-spacing for π−π stacking of 3.93 Å. However, PBDD4T sample shows quite weak broad (010) peak at 1.50 Å−1, corresponding to a dspacing of 4.19 Å. In addition, we measured the hole mobilities (μh) of the two pristine polymer films by the space-chargelimited current (SCLC) method, and the μh of PBDD4T and PBDD4T-2F show similar results of 2.9 × 10−3 and 3.5 × 10−3 cm2 V s−1, respectively. The results indicated that (1) the lamellar packing distances of the two polymers are mainly determined by the identical alkyls attached to the BDD units so the smaller lamellar d-spacing of PBDD4T implies the building blocks in its backbones may be more twisted and (2) by introducing fluorine into the bithiophene unit, PBDD4T-2F has stronger π−π stacking effect in solid film. Therefore, the findings in GIWAXs analysis are coincident with that obtained in TD-Abs. Photovoltaic Properties and Morphological Characteristics. Photovoltaic properties of the blends of PBDD4T2F:PC71BM were investigated by fabricating the PSCs with device architecture of ITO/PFN/BHJ-active-layer/MoO3/Al, where an ultrathin film PFN, poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene)], was used as cathode interlayer30 and 10 nm thick MoO3 was used as anode interlayer. The optimal device fabrication conditions of the devices were obtained by the typically used procedures (see Experimental Section), and we found the best performing device can be fabricated by employing chlorobenzene:diioD

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transport,4,35−38 and since such a phase separation morphology cannot be formed in the PBDD4T:ITIC blend film, the corresponding device should have poor photovoltaic performance. In addition to phase separation, photovoltaic performance of a BHJ PSC also closely correlates with the crystalline structures of the active layer materials; i.e., stronger π−π stacking effect with more ordered lamellar packing is helpful for facilitating intermolecular charge transport.4,11 As shown in Figure S5, the blend film of PBDD4T-2F:ITIC showed much stronger π−π interaction than the PBDD4T:ITIC blend, which may be another reason for the significantly improved PCE in the PBDD4T-2F:ITIC device. Therefore, according to the observations in AFM, TEM, and GIWAXS measurements, we can say that although both PBDD4T-2F and PBDD4T can form nanoscle phase separation when they are blended with PC71BM, these two polymers show different miscibilities with ITIC so the PBDD4T-2F:ITIC and PBDD4T:ITIC films exhibited distinctly different phase separation morphologies, and the favorable morphology of the PBDD4T-2F:ITIC film leads to good photovoltaic performance in device. Considering PBDD4T-2F has a band gap of 1.78 eV and the single-junction device based on PBDD4T-2F:PC71BM exhibits high EQE ranging from 500 to 700 nm, we think it should be an ideal candidate for making front cell in double-junction tandem PSCs. Here, we fabricated the double-junction tandem PSCs by using a similar device structure reported in our recent work39 in which PBDD4T-2F and PDPP4T40−42 were used as the donor materials in the front and rear cell, respectively (see Figure S8). As is known, getting symmetric current densities in the front and rear cells is the key to realize highly efficient double-junction PSCs, so we deliberately selected PC61BM as the acceptor in the front cell to reserve more sunlight for the rear cell because it has weaker optical absorption than PC71BM in the region from 400 to 650 nm, so the symmetric current densities can be obtained from the two subcells. The device structures of the two subcells and the tandem cells are shown in Figure S8, and the J−V curves and the corresponding photovoltaic parameters are provided in Figure 6a and Table 2. It shows that the Voc of the tandem cell is 1.53 V, which is equal to the summation of those of the front and rear cells. Jsc of the tandem cell is 11.91 mA/cm2, which is an outstanding result for double-junction tandem PSCs.43−46 We also measured the EQE of the front and rear cells by using 750 and 600 nm light bias, respectively (see Figure 6b). Both the front and rear cell show EQE close to 70%, and the integrated Jsc are 11.92 and 11.61 mA/cm2, respectively, which agree well with that in the J−V measurement. The overall efficiency of the tandem cell is 10.12%, and this outstanding result indicates PBDD4T-2F is a promising material for making the front cells in tandem PSCs. In conclusion, on the basis of PBDD4T as a reference polymer, we investigated the influence of fluorination on improving the backbone conformation stability and synthesized a new polythiophene derivative, PBDD4T-2F, with stronger and more compact π−π stacking effect. The results of theoretical calculation, TD-Abs, CV, and GIWAXs measurements clearly demonstrate that the fluorinated polymer, PBDD4T-2F, exhibited stronger aggregation effect in solution and stronger π−π stacking effect in solid film than PBDD4T, and also the former has a deeper HOMO level than the latter. When blended with PC71BM, the PBDD4T-2F-based device showed a PCE of 9.04%, which is 38% higher than that of the PBDD4T-based device; when blended with ITIC, the

External quantum efficiency (EQE) curves of the aforementioned four devices are shown in Figure 4b. For the two devices with PC71BM, the EQE curves are similar in shape, but the PBDD4T-2F:PC71BM device showed higher EQE than the PBDD4T:PC71BM device; the integral current densities obtained from the EQE curves are 10.15 and 14.53 mA/cm2, respectively, which are coincident with those obtained from the J−V measurements. The two devices with ITIC show broader EQE spectra than the two devices with PC71BM, and according to the absorption spectra of the films (see Figure S6 in Supporting Information), the additional photoresponse from 700 to 800 nm are contributed by ITIC. For the PBDD4T:ITIC device, EQE in the whole response range is below 10%, while the PBDD4T-2F:ITIC device exhibits much higher EQE in the whole response range (over 60% in the range from 550 to 740 nm), implying both the donor and the acceptor in the PBDD4T-2F:ITIC device make good contribution to photogenerated current generation. In order to understand the reasons for why these two polymers have such different photovoltaic properties, we investigated the morphological properties of the four types of donor:acceptor blends films by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The two blends films with PC71BM show very similar surface (see Figure S7 in the Supporting Information) and bulk (see Figure 5a,b) morphologies; i.e., root-mean-square surface roughness

Figure 5. TEM images of thin films of (a) PBDD4T:PC71BM (1.5:1, w/w) and (b) PBDD4T-2F:PC71BM (1.5:1, w/w) casted from CB:DIO = 100:1 (v/v) and (c) PBDD4T:ITIC (1:1, w/w) and (d) PBDD4T-2F:ITIC (1:1, w/w) casted from CB:DPE = 100:1 (v/v).

(Rq) of PBDD4T:PC71BM and PBDD4T-2F:PC71BM blends films are 1.69 and 1.82 nm, respectively. However, the two blends films with ITIC are very different in AFM and TEM images. As shown in Figures 5c and 5d, the PBDD4T:ITIC blends shows homogeneous morphology, and no distinct phase separation can be observed, while phase separation in the PBDD4T-2F:ITIC blend film is clear and nanoscale fibrils can be distinguished. As known, for BHJ active layers in PSCs, to form nanoscale bicontinuous phase separation is the prerequisite for achieving efficient exciton dissociation and charge E

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Figure 6. (a) J−V curves of the single-junction and tandem PSCs. (b) EQE curves of the front and rear subcells in the tandem device. stirred at room temperature for 2 h. 50 mL of water was added into the flask, and the product was extracted by chloroform. After removing the solvent by rotary evaporation, the product was purified by silica gel chromatography by using hexane/dichloromethane (20:1) as eluent. Compound 4 was obtained as light yellow solid (yield 80.5) 1H NMR (CDCl3, 400 MHz): 7.38 (s, 2H), 3.36−3.26 (m, 4H), 2.54−2.52 (d, 4H), 1.78−1.75 (t, 2H), 1.68−1.61 (m, 2H), 1.43−1.33 (m, 32H), 0.95−0.90 (m, 24H). PBDD4T-2F. M1 (0.2459 g, 0.248 mmol) and M2 (0.1311 g, 0.248 mmol) were dissolved into 8 mL of toluene in a two-neck roundbottom flask. The solution was flushed with argon for 5 min, and 20 mg of Pd(PPh3)4 was added into the flask subsequently. The mixture was flushed with argon for another 15 min and then allowed to stir at 110 °C for 18 h under an argon atmosphere. Then, the reactant was cooled down to room temperature, and the polymer was precipitated into 100 mL of methanol. The polymer was collected by filtration and further purified by silica gel chromatography by using chlorobenzene as eluent. The polymer was precipitated again in 100 mL of methanol and obtained as dark purple solid with a yield of ∼60%. C58H74O2S6 (PBDD4T): Calculated C: 69.97 H: 7.49. Found C: 69.86 H: 7.52. PBDD4T. This polymer was polymerized by the same condition of PBDD4T-2F. C58H72O2S6F2 (PBDD4T-2F): Calculated C: 67.53 H: 7.04. Found C: 67.41 H: 7.04. Device Fabrication. General Procedure. The ITO-coated glass substrates (10−15 ohm/square) were cleaned by deionized water, acetone, and isopropanol, successively. Then the dry substrates were treated with UV-ozone for 15 min. PFN was mixed into methanol with a concentration of 2.0 mg/mL, and a small volume (0.25 vol %) of acetic acid was added into the mixture to help to dissolve PFN. The PFN solution is stable for use in a few months. Fabrication of Inverted Single-Junction Devices Based on Polymer:PC71BM and Polymer:ITIC System. In a nitrogen filled glovebox, 50 μL of PFN solution was spin-coated onto the ITO substrate with a spin speed of 3000 rpm for 30 s, and the substrates were annealed at 150 °C for 10 min. The polymer:PC71BM blend (1.5:1, w/w) or polymer:ITIC blend (1:1, w/w) was dissolved in CB:DIO (100:1, v/v) and CB:DPE (100:1, v/v) with a concentration of 15 mg/mL (calculated for polymer), respectively. The solution was stirred for ∼5 h at room temperature and then was spin-coated onto the surface of PFN. The thickness of the active layer was controlled by employing different spin speed during the process. Then, 10 nm thick of MoO3 and 80 nm thick of Al were thermal deposited onto the active layer under high vacuum (∼3 × 10−4 Pa). The cell area was 4.0 mm2. Fabrication of Tandem Devices. The PFN solution was spincoated on the surface ITO substrates at 3000 rpm in a nitrogen glovebox, and the substrates were annealed at 150 °C for 10 min. Then, PBDD4T-2F:PC61BM solution was spin-coated, and the thickness could be tuned by varying different spin speeds. Subsequently, 7 nm MoO3 and 0.5 nm ultrathin Ag layer were evaporated on the active layer under high vacuum, successively. Then the PFN solution was casted onto the top of the ultrathin Ag layer. The solution of PDPP4T:PC71BM was spin-coated to get active layer with a thickness of 90−100 nm. Finally, MoO3 (7 nm) and Al (100

Table 2. Photovoltaic Characteristics of the Single- and Double-Junction PSCs

PBDD4T2F:PC61BM PDPP4T:PC71BM tandem cell

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

thickness (nm)

0.89

13.99

0.63

7.80

95

0.66 1.53

16.06 11.91

0.62 0.56

6.57 10.12

110

PBDD4T-2F-based device showed a PCE of 8.69%, which is 20 times higher than that of the PBDD4T-based device. The PSC device results indicate that the fluorination is an effective method to increase the torsional barrier with enhanced chain planarity and interchain π−π interaction and hence to tune its morphological and photovoltaic properties of PSCs, especially for the fullerene-free device based on ITIC. What is more, the tandem cell in which the blend of PBDD4T-2F:PC61BM was used for making the front subcell exhibited a high PCE of 10.12%. Overall, in this study, we not only correlate theoretical calculation with the crystallinity of a polythiophene derivative and hence suggest an effective method to rationally tune morphological properties of conjugated polymers but also report a promising polythiophene derivative for multiple photovoltaic applications, i.e., for single-junction PSCs based on fullerene or non-fullerene acceptor and for tandem PSCs.

3. EXPERIMENTAL SECTION Materials. PFN was synthesized according to the reported literatures.30 PC61BM, PC71BM, ITIC, and M2-3 were purchased from Solarmer Materials Inc. Pd(PPh3)4 was purchased from Frontier Scientific Inc. All of the commercial available compounds and reagents were used as received. The compounds 3, M1, and the polymers were synthesized as the following procedures. 1,3-Bis(2-ethylhexyl)-5,7-bis(4-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (3). In a 50 mL two-neck roundbottom flask, compound 1 (2 g, 3.32 mmol), 2 (6.5 g, 13.28 mmol), and Pd(PPh3)4 (200 mg) were mixed in 15 mL of toluene under argon protection. The solution was stirred at 115 °C for 16 h. Subsequently, the reaction was cooled to room temperature, and the solvent was removed by rotary evaporation. Then, the raw NBS product was purified by silica gel chromatography using hexane/dichloromethane (10:1), and compound 3 was obtained as light yellow oil (yield 83.2%). 1H NMR (CDCl3, 400 MHz): 7.57 (s, 2H), 7.07 (s, 2H), 7.37−7.26 (m, 4H), 2.59−2.57 (d, 4H), 1.78−1.75 (t, 2H), 1.61−1.59 (t, 2H), 1.43−1.31 (m, 32H), 0.94−0.90 (m, 24H). 1,3-Bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-5,7-bis(2ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (M1). In a 100 mL round-bottom flask, compound 3 (2.3 g, 2.76 mmol) was dissolved into 30 mL of chloroform, and then the solution was cooled down to 0 °C by ice bath. Then, N-bromosuccinimide (NBS) (0.98 g, 5.53 mmol) was added into the flask in one portion, and the reactant was F

DOI: 10.1021/acs.macromol.6b00248 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules nm) were deposited on the top of the rear active layer successively. The cell area was 4.0 mm2. Instruments and Measurements. The UV−vis absorption spectroscopy measurements including temperature-dependent UV measurements were conducted on a Hitachi UH4150 spectrophotometer. TGA measurements were carried out on TGA-2050 from TA Instruments, Inc. Gel permeation chromatography (GPC) was carried out to provide the molecular weight and the polydispersity (PDI) by using trichlorobenzene as eluent at 160 °C. The tapping mode atom force microscopy (AFM) measurements were performed on a Nanoscope V (Vecco) AFM. Cyclic voltammograms (CV) measurements were recorded on a Zahner IM6e electrochemical workstation using glassy carbon discs as the working electrode, Pt wire as the counter electrode, and Ag/AgCl electrode as the reference electrode with a scanning rate of 20 mV/s in a 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) solution, and the potential of Ag/Ag+ reference electrode was internally calibrated by using ferrocene/ ferroncenium (Fc/Fc+) as the redox couple. The J−V characteristics of all the devices were measured with a Keithley 2400 Precision Source/ Measure unit. The PCEs of the resulting polymer solar cells were measured under AM1.5G spectrum (100 mW cm−2) using a XES-70S1 solar simulator (SAN-EI Electric Co., Ltd., AAA grade, 70 mm × 70 mm photobeam size). A KG-3 sigle-crystal Si reference cell (20 mm × 20 mm) was used to calibrate the irradiation power of the simulator. The external quantum efficiency (EQE) was measured by a Solar Cell Spectral Response Measurement System QE-R3011 (Enlitech, Taiwan).

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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.6b00248. TGA of the polymers, 1H NMR spectra for all compounds and monomers, energy scan of the other adjacent rotamers in the two polymers, UV−vis absorption spectra of the polymers films, 2D GI-WAXS data and AFM images of the blend films, device structures of the inverted single junction and tandem cells (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (J.H.). *E-mail [email protected] (H.Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Science Foundation of China (Nos. 91333204, 21325419), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB12030200), and the CASCroucher Funding Scheme for Joint Laboratories (CAS14601). This work was also supported by the National Research Foundation (NRF) of Korea (2015R1A2A1A15055605, H.Y.W.).

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DOI: 10.1021/acs.macromol.6b00248 Macromolecules XXXX, XXX, XXX−XXX