Locking-In Optimal Nanoscale Structure Induced by

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Locking-In Optimal Nanoscale Structure Induced by Naphthalenediimide-Based Polymeric Additive Enables Efficient and Stable Inverted Polymer Solar Cells Kwang Hyun Park,† Yujin An,† Seungon Jung,† Hyesung Park,* and Changduk Yang* Department of Energy Engineering, School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Perovtronics Research Center, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 44919, Republic of Korea S Supporting Information *

ABSTRACT: Operational stability and high performance are the most critical issues that must be addressed in order to propel and advance the current polymer solar cell (PSC) technology to the next level, such as manufacturing and mass production. Herein, we report a high power conversion efficiency (PCE) of 11.2%, together with an excellent device stability in PTB7-Th:PC71BM-based PSCs in the inverted structure by introducing the n-type P(NDI2OD-T2) macromolecular additive (>75% PCE retention at high temperature up to 120 °C, >97% PCE retention after 6 months in inert conditions, >93% PCE retention after 2 months in air with encapsulation, and >80% PCE retention after 140 h in air without encapsulation). The PCE is the highest value ever reported in the single-junction systems based on the PTB7 family and is also comparable to the previously reported highest PCE of inverted PSCs. These promising results are attributed to the efficient optimization and stabilization of the blend film morphology in the photoactive layer, achieved using the P(NDI2OD-T2) additive. From the perspective of manufacturing, our studies demonstrate a promising pathway for fabricating low-cost PSCs with high efficiency as well as long-term stability. KEYWORDS: inverted structure, macromolecular additive, n-type conjugated polymer, polymer solar cells, thermal stability

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either cross-linking or hydrogen bonding in the active layers in order to stabilize the blend morphology.7,27−30 In line with these research directions, we have recently demonstrated a simple and effective approach for simultaneously improving and stabilizing the performance of conventional-type PSCs based on poly[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate2−6-diyl)] (PTB7-Th):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) photoactive layer via the addition of a highquality n-type macromolecular additive poly[(N,N′-bis(2octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl)-alt-5,5-(2,2-bithiophene)], P(NDI2OD-T2).31 The NDIbased n-type polymer, P(NDI2OD-T2), is one of the representative acceptor materials studied in the development of all-polymer solar cells due to its favorable properties such as high electron mobility, small optical band gap, and suitable

ulk-heterojunction polymer solar cells (PSCs) comprising semiconducting polymers as the electron donors and fullerene derivatives as the electron acceptors have been extensively studied due to their great potential for realizing light-weight, flexible, and cost-effective solar cells, advancing past the current silicon-based photovoltaic technology.1−5 Whereas in the past few years, various research efforts in the design of new semiconducting polymers and device fabrication techniques have led to PSCs with power conversion efficiencies (PCEs) exceeding 10%,6−22 these values are still not sufficiently high to enable the wide commercial use of PSCs. In addition to the improvement of the PCE, device stability is another key issue for the commercial viability of PSCs. Currently, emerging strategies for improving the stability of PSCs include (i) isolation of high-quality polymers to reduce an abrupt burn-in degradation during the device operation,23,24 (ii) use of air-stable electrodes such as Ag or Au, so-called inverted PSCs, in order to avoid the use of the corrosive and hygroscopic hole-transporting layer and low-work-function metal cathodes,25,26 and (iii) use of materials that can form © 2017 American Chemical Society

Received: May 26, 2017 Accepted: June 22, 2017 Published: June 22, 2017 7409

DOI: 10.1021/acsnano.7b03684 ACS Nano 2017, 11, 7409−7415

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Figure 1. Chemical structures and device characteristics of PTB7-Th:PC71BM-based OSCs with macromolecular additives. (a) Device structure and molecular structures of donors (PTB7-Th), acceptor (PC71BM), and macromolecular additive (P(NDI2OD-T2)). (b) Schematic flat-band energy diagram. (c) J−V characteristics. (d) EQE spectra of devices fabricated with/without P(NDI2OD-T2) additive.

Table 1. Device Performances of PTB7-Th:PC71BM-Based OSCs with/without 0.8 wt % P(NDI2OD-T2) Additive in Inverted Devices JSC (mA cm‑2)

VOC (V)

FF (%)

PCE (%) max/avg

w/o P(NDI2OD-T2), 30 °C 60 °C 80 °C 100 °C 120 °C

19.5 19.0 16.4 15.8 13.1

± ± ± ± ±

0.2 0.2 0.3 0.5 0.5

0.75 0.72 0.70 0.68 0.65

± ± ± ± ±

0.02 0.02 0.02 0.03 0.03

60 51 46 41 32

± ± ± ± ±

2 2 2 2 5

10.4/9.87 8.15/7.95 7.05/6.88 5.14/4.94 4.57/4.32

± ± ± ± ±

0.3a 0.5b 0.6b 0.6b 0.8b

0.8 wt % P(NDI2OD-T2), 30 °C 60 °C 80 °C 100 °C 120 °C

20.3 20.1 18.9 17.8 16.3

± ± ± ± ±

0.2 0.2 0.3 0.4 0.5

0.78 0.77 0.77 0.76 0.74

± ± ± ± ±

0.01 0.01 0.01 0.01 0.01

69 66 57 56 54

± ± ± ± ±

2 2 1 2 4

11.2/11.0 10.8/10.4 9.82/9.39 9.02/8.67 8.32/7.82

± ± ± ± ±

0.2a 0.2b 0.4b 0.4b 0.5b

a The average values obtained from at least 50 devices with the standard deviation. bThe average values obtained from at least 20 devices with the standard deviation.

stability (>75% PCE retention at high temperature up to 120 °C, >97% PCE retention after 6 months in inert conditions, >93% PCE retention after 2 months in air with encapsulation, and >80% PCE retention after 140 h in air without encapsulation). The demonstration of the P(NDI2OD-T2) additive for inverted PSCs with enhanced device performance successfully satisfies the important criteria for ultimately producing highly efficient and operationally stable organic photovoltaic modules.

energy level alignment with commonly used p-type polymers.32,33 Recently, application of P(NDI2OD-T2) as the additive component has also been investigated in the ternary blend organic photovoltaics system.34,35 We expect that a more thorough elucidation of the underlying mechanism and role of P(NDI2OD-T2) in the active layer should help achieve commercially viable PCE values while satisfying the stability requirements. Therefore, in this study, we focus on the effect of P(NDI2OD-T2) in the “inverted PSCs” using PTB7Th:PC71BM as the model system for the investigation of the issues described above. An exceptional PCE of 11.20% (PCEavg = 10.96%) was achieved for the P(NDI2OD-T2) additiveprocessed device, outperforming the corresponding nonadditive control device. This value is one of the best results reported for the inverted PSCs. Moreover, the P(NDI2OD-T2) additiveprocessed device demonstrated a remarkably improved device

RESULTS AND DISCUSSION Figure 1a illustrates the inverted device architecture (ITO/ ZnO/active layer/MoO3/Ag) and chemical structures of PTB7Th, PC71BM, and P(NDI2OD-T2). The high-quality P(NDI2OD-T2) was prepared using the previously reported marginal solvent-soaking technique.31 In addition, the flat-band 7410

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Figure 2. Device stability of PTB7-Th:PC71BM-based OSCs with/without 0.8 wt % P(NDI2OD-T2) additive to exposure at different temperatures (30, 60, 80, 100, and 120 °C). (a) Short-circuit current density (JSC). (b) Open-circuit voltage (VOC). (c) Fill factor (FF). (d) Power conversion efficiency (PCE).

kT/q (without), respectively. Such higher α value in the powerlaw dependence (JSC ∝ Iα, I is the illumination intensity) and smaller 1.13 kT/q (where k is the Boltzmann constant, T is the temperature, and q is the elementary charge) corroborates that less molecular recombination is involved for the devices with P(NDI2OD-T2) additive compared to the nonadditive control device. These observations partially explain the higher efficiency of the P(NDI2OD-T2) additive-processed device relative to that of the control device. In the following discussion, we explore several aspects of the PSCs’ stability issue, such as the thermal and operational stabilities. Figure 2 shows the data for the evaluation of the effect of the thermal treatments on inverted PSCs based on PTB7Th:PC71BM with and without P(NDI2OD-T2), wherein the devices were examined at various temperatures (30, 60, 80, 100, and 120 °C) for 10 min in an inert atmosphere. The corresponding photovoltaic parameters and J−V characteristics are listed in Table 1 and shown in Figure S2. Although the increase in the temperature leads to decreased values of all photovoltaic parameters in both the devices, degradation of the solar cell performance was significantly suppressed for the P(NDI2OD-T2) additive-processed device relative to the control device. For example, upon annealing at the evaluated temperature (120 °C), the PCE of P(NDI2OD-T2) additiveprocessed device retains >75% of its initial value, whereas the nonadditive control device exhibits a sharp decrease in the PCE from 9.79 to 4.57%. Furthermore, we note that the PCE value for the P(NDI2OD-T2) additive-processed device remains almost unchanged (merely decreasing from 10.44 to 10.42%) at the mild temperature annealing (60 °C) conditions, whereas the control device shows a significant decrease in the PCE (from 9.79 to 8.15%). The observed thermal stability at these temperatures is important because typical solar cells are expected to reach this temperature range in the practical operational conditions. To gain further insight into the effect of the P(NDI2OD-T2) additive on the active layer, investigations of charge recombination dynamics and film morphologies for various thermal treatment conditions were performed using lightintensity-dependence measurements of J−V characteristics,

energy diagrams of the materials employed in the device are shown in Figure 1b. The fabrication conditions for the active layer in the inverted PSCs were adopted from the previously reported optimized processes.31 Briefly, PTB7-Th:PC71BM (1:1.7 wt %, 21.3 mg mL−1) in chlorobenzene (CB):1,8diiodooctane (DIO) (97:3 vol %) was blended with 0.8 wt % P(NDI2OD-T2) (0.17 mg) based on the total weight of the active materials. A detailed description of the fabrication and testing conditions is provided in the Experimental Section. Figure 1c shows the current density−voltage (J−V) characteristics under simulated AM 1.5G illumination of 100 mW cm−2. The corresponding photovoltaic performance parameters are summarized in Table 1. The PTB7-Th:PC71BM control device without the P(NDI2OD-T2) additive exhibited a PCE of 10.36% (PCEavg = 9.87%) with a short-circuit current density (JSC) of 19.5 ± 0.2 mA cm−2, an open-circuit voltage (VOC) of 0.75 ± 0.02 V, and a fill factor (FF) of 66% ± 2%. To the best of our knowledge, this is one of the very few studies that demonstrate a power efficiency of greater than 10% for inverted single-junction PSCs based on the PTB7-Th system. Introduction of 0.8 wt % P(NDI2OD-T2) into the active layer led to simultaneous improvements in the JSC, VOC, and FF as previously observed in the conventional device architecture.31 Consequently, the best PCE that was as high as 11.20% with JSC of 20.3 ± 0.2 mA cm−2, VOC of 0.78 ± 0.01 V, and a FF of 69% ± 2 (PCEavg = 10.96%) was achieved for the P(NDI2OD-T2) additive-processed device, which is comparable to the best inverted PSCs reported previously.10−20,36−39 The integrated JSC values obtained from the external quantum efficiency spectra were 20.3 and 18.6 mA cm−2 for the PTB7-Th:PC71BM devices with and without P(NDI2OD-T2), respectively (Figure 1d), in good agreement with the current obtained from the J−V measurements. Furthermore, we also observed that the recombination process is weaker in the P(NDI2OD-T2) additive-processed device compared to that for the nonadditive control device, as evidenced by light-intensity-dependent JSC and VOC measurements (Figure S1). For example, the fitted curves from the light intensity dependence of the JSC and VOC for the devices with and without the additive yield α = 1.00 (with) and α = 0.96 (without), and 1.13 kT/q (with) and 1.77 7411

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Figure 3. AFM images of PTB7-Th:PC71BM-based OSCs with/without 0.8 wt % P(NDI2OD-T2) additive to exposure at different temperatures (30, 60, 80, 100, and 120 °C). Phase images; root-mean square roughness.

Figure 4. Two-dimensional GIWAXD characterizations of PTB7-Th:PC71BM with/without 0.8 wt % P(NDI2OD-T2) additive exposed to different temperatures (30, 60, 80, 100, and 120 °C). (a) GIWAXD images; (b) in-plane and (c) out-of-plane profiles.

Figure 3 shows the AFM height images of the PTB7Th:PC71BM blend films with and without P(NDI2OD-T2) exposed to different temperatures. The blend films without P(NDI2OD-T2) show relatively rough surface morphologies with large root-mean-square (rms) roughness values. With the addition of P(NDI2OD-T2), the surface roughness of the corresponding blend films becomes smoother, resulting in reduced rms values. Note that with increasing temperatures (30, 60, 80, 100, and 120 °C), the rms values of the nonadditive control films gradually increase (2.16 ± 0.5, 2.20 ± 0.5, 2.31 ±

atomic force microscopy (AFM), transmission electron microscopy (TEM), and grazing-incidence wide-angle X-ray diffraction (GIWAXD). As shown in Figure S3, the α values remain constant for all P(NDI2OD-T2) additive-processed devices, whereas the slope of kT/q gradually increases with increasing temperature, suggesting that geminate recombination loss rather than a bimolecular recombination mechanism is responsible for the decrease in the device performance at the evaluated temperatures. 7412

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Figure 5. Long-term stability of PTB7-Th:PC71BM with/without 0.8 wt % P(NDI2OD-T2) additive under nitrogen environment (a) and ambient air condition ((b) with and (c) without encapsulation).

blend film morphology to be “locked”, affording improved thermal stability in the P(NDI2OD-T2) additive-processed device. This conclusion is also corroborated by the AFM and TEM results discussed above. In addition to the thermal stability, the long-term stability of the PSCs was examined for 6 months at room temperature in inert conditions. The J−V measurements of the test devices were periodically recorded, as shown in Figure 5a and Figure S5. The P(NDI2OD-T2) additive-processed device retains more than 97% of the PCE (from 11.1 to 10.8%) after 6 months. This compares with the nonadditive control device for which the PCE is reduced by more than 63% (from 9.87 to 3.63%) during the same measurement period. Besides, we encapsulated the inverted devices using the well-known epoxyglass technique, as explained in Experimental Section. Then, we conducted the device aging test under an ambient atmosphere at room temperature and collected the J−V data under continuous air exposure. As shown in Figure 5b and Figure S6, the nonadditive control device exhibits a large decline of the PCE from 9.60 to 5.88% during 2 months measurement, whereas a PCE as high as 9.16% is still maintained for (NDI2OD-T2) additive-processed device. Moreover, as a part of the representative actual-use tests, the devices were also assessed in an air environment without any encapsulation. In contrast to the sharp PCE decrease observed for the nonadditive control device, the P(NDI2OD-T2) additiveprocessed device again demonstrates a remarkable air stability (>80% PCE retention after 140 h of air exposure) (Figure 5c and Figure S7). Collectively, the above stability data clearly indicate that P(NDI2OD-T2) additive plays a crucial role to enhance simultaneously thermal and long-term stabilities of bulk-heterojunction PSCs.

0.5, 3.03 ± 0.6, and 3.20 ± 0.4 nm), whereas those from the P(NDI2OD-T2) additive-processed films show only negligible changes with the temperature (1.73 ± 0.2, 1.74 ± 0.2 and 1.79 ± 0.3, 1.84 ± 0.3, and 1.87 ± 0.3 nm). This indicates that with the aid of the P(NDI2OD-T2) additive, the surface morphology of the photoactive blend layer is less affected in the elevated temperature environment, leading to the thermally robust film formation. This is an important factor for the commercial applications of organic photovoltaics. Moreover, the TEM images (Figure S4) of the films with P(NDI2OD-T2) exhibit finer phase separation with fibril-like microstructures relative to the control films, wherein the morphological features in the blend film after various thermal treatments follow trends that are similar to those observed by AFM; that is, the P(NDI2OD-T2) in the active layer plays a key role in suppressing the morphological fluctuations. Figure 4 shows the GIWAXD patterns and the corresponding line-cut profiles qz (out-of-plane) and qxy (in-plane) of the blend films at different annealing temperatures.40,41 The crystallographic parameters are summarized in Table S1 (Supporting Information). The PTB7-Th:PC71BM film without P(NDI2OD-T2) at low temperature (30 °C) exhibits a ringlike (100) peak at q = ∼ 0.28 Å−1, corresponding to lamellar dspacing of ∼22.2 Å and a broad (010) peak at qz = 1.61 ± 0.08 Å−1, which is associated with π−π stacking d-spacing of 3.9 ± 0.2 Å. This is in agreement with previous reports of face-on conformation in the PTB7-Th:PC71BM blend films.14,37 Although the addition of P(NDI2OD-T2) into the blend film at 30 °C does not cause noticeable changes in patterns and shifts of the (100) lamellar peaks, the π−π stacking peaks become fainter. Interestingly, as the temperature increases, the intensity of the (100) lamellar peak from the PTB7Th:PC71BM film without P(NDI2OD-T2) is gradually reduced, whereas only a slight change is observed for the PTB7Th:PC71BM film with P(NDI2OD-T2). This stabilization of the horizontal crystallites of PTB7-Th:PC71BM film with P(NDI2OD-T2) is directly correlated with an improved stability of PSCs as it is generally believed that the horizontal film composition is more favorable for photovoltaic devices because of its vertical charge transportation channel.15 Besides, we also observe that the crystalline correlation length (CCL100) values for P(NDI2OD-T2) additive films are somewhat smaller than those of the corresponding nonadditive films annealed up to 100 °C, but both the films at a higher temperature (120 °C) possess the same CCL values (131.6 Å) (Table S1 in Supporting Information). Taken together, we conclude that the use of the P(NDI2OD-T2) additive allows the photoactive

CONCLUSIONS We demonstrate that the simple addition of P(NDI2OD-T2) into PTB7-Th:PC71BM-based inverted PSCs offers a significant prospect for applications in highly efficient and stable PSCs. The beneficial effects originate from the crucial role played by P(NDI2OD-T2) in not only further optimizing the blend morphology but also in the locking-in of the optimal nanoscale structure. In addition to the notable PCE that is as high as 11.20% observed for the P(NDI2OD-T2) additive-processed device, herein, we emphasize the excellent stability properties of this device such as (i) >75% PCE retention at high temperature up to 120 °C, (ii) >97% PCE retention after 6 months in inert conditions, (iii) >93% PCE retention after 2 months in air with 7413

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devices with/without 0.8 wt % P(NDI2OD-T2) additive; TEM images of PTB7-Th:PC71BM blends with/without 0.8 wt % P(NDI2OD-T2) additive (PDF)

AUTHOR INFORMATION Corresponding Authors

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

EXPERIMENTAL SECTION Materials. The detailed synthesis and characterization of P(NDI2OD-T2) can be found in the Supporting Information and our previous work.31 All other commercial reagents, including solvents used in this study, were purchased from Aldrich Co., Alfa Aesar, and TCI Co. and used without further purification. Solar Cell Characterization. Inverted device was selected to evaluate the performance of PTB7-Th:PC71BM-based OSCs by P(NDI2OD-T2) additive. The device architecture was glass/ITO/ ZnO nanoparticle/photoactive materials/MoO3/Ag. First, ITO was coated on glass and cleaned using deionized water, acetone, and isopropyl alcohol, then the substrates were dried in an oven. ZnO nanoparticle was spin-coated on the substrates at 2500 rpm for 1 min and baked at 80 °C for 10 min and 120 °C for 5 min. The substrate was moved into the glovebox under nitrogen environment. PTB7-Th and PC71BM were dissolved in chlorobenzene/1,8-diiodooctane solvent (97:3 vol %) with concentrations of 12 and 40 mg mL−1, respectively. Then, the blend solution with PTB7-Th:PC71BM (2:1 vol %) with addition of the 0.8 wt % P(NDI2OD-T2) (0.17 mg) was prepared. The blend solution was spin-coated at 900 rpm for 2 min onto the ZnO nanoparticle. Subsequently, MoO3 (20 nm) and Ag (100 nm) counterelectrodes were thermally evaporated under vacuum (∼10−6 Torr), which defines the device area of 13 mm2. Device measurements were conducted in a glovebox by a xenon arc lamp solar simulator. J−V characteristics were measured under AM 1.5G illumination (100 mW cm−2) with a Keithley 2635A source measurement unit, and EQE measurement was performed under ambient conditions using a QE system (model QEX7) by PV Measurements Inc. (Boulder, Colorado). For the long-term operation lifetime tests, the devices were encapsulated within glass slides of ∼1 mm thickness using a two-part epoxy resin (Loctite Instant Mix 5 min epoxy). Light-intensity-dependent JSC and VOC were measured with neutral density filters. Characterizations. The optical properties of composite films were analyzed using a UV−vis spectrophotometer (Agilent Carry 5000). An Agilent 5500 scanning probe microscope running with a Nanoscope V controller was used to obtain AFM images of polymer thin films. AFM images were recorded in high-resolution tapping mode under ambient conditions. The GIWAXD measurements were conducted at PLS-II 9A U-SAXS beamline of the Pohang Accelerator Laboratory in Korea. X-rays coming from the in-vacuum undulator were monochromated (wavelength λ = 1.10994 Å) using a double crystal monochromator and focused both horizontally and vertically (450 (H) × 60 (V) μm2 in fwhm at sample position) using K−B-type mirrors. The GIWAXD sample stage was equipped with a 7-axis motorized stage for the fine alignment of the sample, and the incidence angle of the X-ray beam was set to be 0.13 to 0.135° for neat polymer films and blended films. The GIWAXD patterns were recorded with a 2D CCD detector (Rayonix SX165), and X-ray irradiation time was 6−9 s, dependent on the saturation level of the detector. Diffraction angles were calibrated using a sucrose standard (monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c = 7.7624 Å, β = 102.938°), and the sample-to-detector distance was ∼231 mm.

ORCID

Hyesung Park: 0000-0002-7613-8706 Changduk Yang: 0000-0001-7452-4681 Author Contributions †

K.H.P., Y.A., and S.J. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2015R1A2A1A10053397). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015R1D1A1A0105791). GIWAXD measurements at PLS-II 6D UNIST-PAL beamline and 9A beamline were supported in part by MEST, POSTECH, and UNIST UCRF. REFERENCES (1) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338. (2) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (3) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789−1791. (4) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685−688. (5) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (6) Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng, H.; Chen, Y. Solution-Processed Organic Solar Cells Based on Dialkylthiol-Substituted Benzodithiophene Unit with Efficiency near 10%. J. Am. Chem. Soc. 2014, 136, 15529−15532. (7) Liu, Y.; Zhao, J.; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-Efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (8) Yang, Y. M.; Chen, W.; Dou, L.; Chang, W.-H.; Duan, H.-S.; Bob, B.; Li, G.; Yang, Y. High-Performance Multiple-Donor Bulk Heterojunction Solar Cells. Nat. Photonics 2015, 9, 190−198. (9) Qin, T.; Zajaczkowski, W.; Pisula, W.; Baumgarten, M.; Chen, M.; Gao, M.; Wilson, G.; Easton, C. D.; Müllen, K.; Watkins, S. E. Tailored Donor−Acceptor Polymers with an A−D1−A−D2 Structure: Controlling Intermolecular Interactions to Enable Enhanced Polymer Photovoltaic Devices. J. Am. Chem. Soc. 2014, 136, 6049−6055. (10) Zhao, J.; Li, Y.; Yang, G.; Jiang, K.; Lin, H.; Ade, H.; Ma, W.; Yan, H. Efficient Organic Solar Cells Processed from Hydrocarbon Solvents. Nat. Energy 2016, 1, 15027. (11) Yan, Y.; Cai, F.; Yang, L.; Li, J.; Zhang, Y.; Qin, F.; Xiong, C.; Zhou, Y.; Lidzey, D. G.; Wang, T. Light-Soaking-Free Inverted

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03684. J−V characteristics and light intensity dependence of JSC and VOC on PTB7-Th:PC71BM-based OSCs in inverted 7414

DOI: 10.1021/acsnano.7b03684 ACS Nano 2017, 11, 7409−7415

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DOI: 10.1021/acsnano.7b03684 ACS Nano 2017, 11, 7409−7415