Enhanced Charge Transfer between Fullerene and Non-Fullerene

Nov 16, 2018 - As a result, ternary OSCs based on PBDB-TF/HF-PCIC/PC71BM and PBDB-TF/HC-PCIC/PC71BM exhibit high power conversion efficiencies ...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Enhanced Charge Transfer between Fullerene and Non-Fullerene Acceptors Enables Highly Efficient Ternary Organic Solar Cells Lingling Zhan,†,§ Shuixing Li,†,§ Shuhua Zhang,† Xingzhi Chen,† Tsz-Ki Lau,‡ Xinhui Lu,*,‡ Minmin Shi,† Chang-Zhi Li,† and Hongzheng Chen*,† †

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MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China ‡ Department of Physics, Chinese University of Hong Kong, New Territories, Hong Kong 999077, P. R. China S Supporting Information *

ABSTRACT: Insufficient driving forces defined as the energetic offsets of the frontier molecular orbitals between a donor and an acceptor influence the charge separation in organic solar cells (OSCs), thus restricting the improvement of quantum efficiencies. Herein, we demonstrate that enhancing charge transfer between fullerene and non-fullerene acceptors via ternary strategy is an effective method to address this problem. By introducing an electron acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the third component to the binary blends based on the polymer donor of poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PBDB-TF) and the small-molecule acceptor of 2,2′-((2Z,2′Z)-(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1diylidene))dimalononitrile (HF-PCIC) or 2,2′-((2Z,2′Z)-(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1diylidene))dimalononitrile (HC-PCIC) with unfused cores, the quantum efficiencies can be boosted from ∼70% for binary blends to over 80% for ternary blends in the longer wavelength ranges. PC71BM shows lower energy levels and higher electron mobility, benefiting the charge transfer and transport in ternary OSCs and resulting in an enhanced quantum efficiency. As a result, ternary OSCs based on PBDB-TF/HF-PCIC/PC71BM and PBDB-TF/HC-PCIC/PC71BM exhibit high power conversion efficiencies (PCEs) of 11.55 and 12.36%, respectively. In addition, excellent thermal stabilities are realized for both ternary OSCs, which retained ∼80% initial PCEs after thermal treatment at 130 °C for 12 h, indicating that the active layer morphology containing fullerene/non-fullerene acceptors is stabilized. This work demonstrates efficient and thermally stable ternary OSCs with enhanced charge transfer between fullerene and non-fullerene acceptors via the modulation of energy levels, which helps to better understand the working mechanism of ternary OSCs. KEYWORDS: charge transfer, ternary OSCs, external quantum efficiency, fullerene acceptor, non-fullerene acceptor

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INTRODUCTION

simultaneous achievement of both high voltage and quantum efficiency remains a big challenge for improving OSC performances. However, it is difficult to address the above challenge in a single binary blend. Constructing a ternary blend by combining the advantages of two binary blends might be an effective strategy.16−24 Besides the basic charge separation and transfer in the interfaces of a donor and an acceptor, the charge transfer between the third component and the hosts could also be observed in a ternary blend.25 If the energy levels of the three components can form a cascade energy level alignment, it is

Power conversion efficiency (PCE), stability, and cost are the three key issues to realize in the commercialization of organic solar cells (OSCs).1−5 Though the PCEs of OSCs have been promoted to over 14%,6,7 it still needs to march further before the considerations of stability8,9 and cost.10,11 Obtaining a high open-circuit voltage (Voc) is beneficial for a high PCE, which requires the enlargement of difference between the lowest unoccupied molecular orbital of the acceptor (LUMOA) and the highest occupied molecular orbital of the donor (HOMOD).12−14 It inevitably results in the minimization of HOMOD−A and LUMOD−A offsets, which lowers the driving force for charge separation and transfer between the donor and the acceptor, thus influencing the quantum efficiency of photocurrent generation in OSCs.15 In other words, the © 2018 American Chemical Society

Received: September 20, 2018 Accepted: November 16, 2018 Published: November 16, 2018 42444

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structures of PBDB-TF, HF-PCIC, HC-PCIC, and PC71BM. (b) Normalized absorption spectra of PBDB-TF, HF-PCIC, HC-PCIC, and PC71BM thin films. (c) Energy level diagram of PBDB-TF, HF-PCIC, HC-PCIC, and PC71BM.

constructing a cascade energy level alignment to enhance the charge transfer by introducing a third component with deeper energy levels is worth studying.46−48 According to the requirement of deep energy levels and complementary absorption, PC71BM is one of the good candidates as the third component. Herein, we choose two binary blends of PBDB-TF/HFPCIC and PBDB-TF/HC-PCIC, in which 2,2′-((2Z,2′Z)(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4Hc yc lo pe n ta [2 , 1 -b: 3 , 4 -b′ ] d i t h i o p h e n e - 6 , 2 - d i y l ) ) b i s (methanylylidene))bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (HC-PCIC) is a new synthesized material, as the reference to construct ternary OSCs with PC71BM as the third component (see chemical structures in Figure 1a). It is found that the cascade energy level alignment indeed benefits charge separation, charge transfer, and charge transport in the ternary blends. As a result, obvious quantum efficiency enhancements from around 70% to over 80% in the longer wavelength ranges are observed, leading to high PCEs of 11.55% for PBDB-TF/HF-PCIC/ PC71BM-based ternary OSCs and 12.36% for PBDB-TF/HCPCIC/PC71BM-based ternary OSCs. In addition, both ternary OSCs show good thermal stability with ∼80% initial efficiencies remaining after thermal treatment at 130 °C for 12 h.

possible to observe the charge transfer between two donors or acceptors. Benefiting from the effective charge separation and transfer, the PCEs in ternary blends could be improved. Nowadays, non-fullerene derivatives have dominated the electron acceptor field with strong absorption for a high Jsc and tunable energy levels for a high Voc, thus promoting the PCEs of OSCs to over 14%.26−29 In addition, OSCs based on non-fullerene acceptors with small energy level offsets are also workable with low energy losses.30−32 However, fullerene derivatives, for example, [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), still possess the superiorities in electron mobility and charge-transfer abilities, compared with nonfullerene acceptors.33−39 Hopefully, combining the advantages of non-fullerene and fullerene acceptors to construct ternary OSCs might lead to an efficient charge transfer with a high Voc.25,40−45 In our previous works, we developed a new series of unfused-core electron acceptors based on cyclopentadithiophene (CPDT) units, for example, 2,2′-((2Z,2′Z)-(((2,5difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4Hc y c l o p e n t a [ 2 , 1 -b: 3, 4 -b′ ] d i t h i o p h e n e - 6 , 2 - d i y l ) ) b i s (methanylylidene))bis(3-oxo-2,3-dihydro-1H-indene-2,1diylidene))dimalononitrile (DF-PCIC) and 2,2′-((2Z,2′Z)(((2,5-difluoro-1,4-phenylene)bis(4,4-bis(2-ethylhexyl)-4Hc y c l o p e n t a [ 2 , 1 -b: 3, 4 -b′ ] d i t h i o p h e n e - 6 , 2 - d i y l ) ) b i s (methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (HF-PCIC).18,28,29 These unfused-core electron acceptors not only possess narrow band gaps but also own relatively high-lying energy levels because of the strong electron-donating ability of CPDT units. Therefore, when deep HOMO level polymer donors, such as poly[(2,6(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)-benzo[1,2b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PBDB-TF),12 are selected to pair with these unfused-core electron acceptors, both HOMOD−A and LUMOD−A offsets are minimized to be lower than 0.3 eV, thus limiting the quantum efficiency of relevant OSCs to be around 70%. Therefore,

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RESULTS AND DISCUSSION Figure 1a shows the chemical structures of PBDB-TF, HFPCIC, HC-PCIC, and PC71BM. HC-PCIC was designed and synthesized according to our previous report on HF-PCIC.29 Detailed information about the synthesis and basic properties of HC-PCIC can be found in the Supporting Information. HCPCIC owns not only good thermal stability with the decomposition temperature (5% weight loss) of 347 °C (Figure S1) but also a high absorption coefficient of 2.19 × 105 M−1 cm−1 at the highest peak of 692 nm in chloroform solution (Figure S2). Figure 1b gives the absorption spectra of PBDB-TF, HF-PCIC, HC-PCIC, and PC71BM films. The 42445

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

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ACS Applied Materials & Interfaces

Figure 2. (a) J−V curves of optimized OSCs based on two binary blends (PBDB-TF/HF-PCIC and PBDB-TF/PC71BM) and the ternary blend (PBDB-TF/HF-PCIC/PC71BM). (b) EQE curves of the corresponding OSCs. (c) J−V curves of optimized OSCs based on two binary blends (PBDB-TF/HC-PCIC and PBDB-TF/PC71BM) and the ternary blend (PBDB-TF/HC-PCIC/PC71BM). (d) EQE curves of the corresponding OSCs.

Table 1. Photovoltaic Parameters of Binary and Ternary OSCs blend

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)c

Jcalcd

PBDB-TF/HF-PCIC PBDB-TF/HF-PCIC/PC71BMa PBDB-TF/PC71BM (DIO) PBDB-TF/HC-PCIC PBDB-TF/HC-PCIC/PC71BMb PBDB-TF/PC71BM (CN)

0.89 0.90 0.95 0.88 0.89 0.95

17.24 18.10 13.24 17.54 19.29 12.23

70.98 70.81 58.63 72.69 70.18 58.46

10.90 (10.79) 11.55 (11.32) 7.29 (7.12) 11.48 (11.36) 12.36 (12.29) 6.77 (6.75)

16.14 17.14 11.74 17.13 18.72 11.81

a

Ternary blend with 30 wt % PC71BM addition. bTernary blend with 15 wt % PC71BM addition. cData in brackets are the average PCE values based on 10 individual devices. dCalculated Jsc from the corresponding EQE curves.

absorption of PBDB-TF mainly lies in the range of 500−700 nm, whereas those of both non-fullerene acceptors are mainly located at 600−800 nm. On comparison, PC71BM has the absorption in the region of 300−500 nm. Therefore, PC71BM is suitable to be the third component with the absorption complementary to both non-fullerene binary blends. Chlorine atom has larger dipole moment than the fluorine atom, thus enhancing the intermolecular interactions for HC-PCIC.49,50 As a result, HC-PCIC possesses more red-shifted absorption with a stronger 0−0 peak than HF-PCIC. The energy levels of PBDB-TF, HF-PCIC, HC-PCIC, and PC71BM are provided in Figure 1c. The values of LUMO and HOMO levels of PBDBTF,51 HF-PCIC,29 and PC71BM52 are taken from the literature. The LUMO and HOMO levels of HC-PCIC are calculated from the cyclic voltammetry measurements (Figure S3) as −3.87 and −5.54 eV, respectively. The polymer donor PBDB-TF has a low HOMO level of −5.48 eV with the fluorine atoms on the side chains of thienyl. HC-PCIC shows slightly lower energy levels than HF-PCIC because of the larger dipole moment of the Cl atom than the F atom. Because of the deep energy levels of PBDB-TF, the LUMO offsets are both lower than 0.3 eV for PBDB-TF/HF-

PCIC and PBDB-TF/HC-PCIC binary blends, and their HOMO offsets are both lower than 0.1 eV for the corresponding binary blends. Therefore, whether the driving forces are enough for OSCs based on these two binary blends are doubtful. As PC71BM owns deep LUMO and HOMO levels of −4.0 and −6.0 eV, both LUMO and HOMO offsets are larger than 0.3 eV between PBDB-TF and PC71BM.52 Therefore, cascade energy level alignments can be formed in ternary blends from PBDB-TF to non-fullerene acceptors and to PC71BM, which meet the requirement for ternary device construction. On the basis of the above two considerations in complementary absorption and cascade energy level alignments, PC71BM is fit to be the third component in our case. To check the device performances of binary and ternary OSCs, we fabricate the OSCs with an inverted device structure of indium tin oxide (ITO)/ZnO/active layer/MoO3/Ag. The optimization of PBDB-TF/HF-PCIC-based binary OSCs can be found in our reported paper,29 and here we directly use the reported conditions [donor/acceptor (D/A) = 1:1.2, 0.5% 1,8diiodooctane (DIO), 100 °C annealing for 10 min]. As for PBDB-TF/HC-PCIC-based binary OSCs, the optimization process can be found in Table S1, and the best conditions are 42446

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) PL spectra of PBDB-TF, two binary blends (PBDB-TF/HF-PCIC and PBDB-TF/PC71BM), and ternary blend (PBDB-TF/HFPCIC/PC71BM) films excited at 550 nm. (b) PL spectra of PBDB-TF, two binary blends (PBDB-TF/HC-PCIC and PBDB-TF/PC71BM), and ternary blend (PBDB-TF/HC-PCIC/PC71BM) films excited at 550 nm. (c) J−V curves of three single components (HF-PCIC, HC-PCIC, and PC71BM) and two acceptor blends (HF-PCIC/PC71BM and HC-PCIC/PC71BM). (d) Dependence of Jsc on the light intensity (Plight) of corresponding binary and ternary OSCs.

450 nm range, which is related to the enhanced chargetransport property of PBDB-TF proven below. Thus, the PCE is improved from 11.48% for HC-PCIC-based binary OSCs to 12.36% for HC-PCIC-based ternary OSCs. The above results indicate that the introduction of PC71BM to the binary blends of PBDB-TF/HF-PCIC and PBDB-TF/HC-PCIC can truly promote the charge separation or charge transfer, thus allowing both ternary OSCs to realize over 80% EQE values. To investigate the effects of adding PC71BM on the charge separation, we measure the steady photoluminescence (PL) spectra of pure PBDB-TF and relevant binary and ternary blend films excited at 550 nm, and the results are displayed in Figure 3. The PL spectrum of the pure PBDB-TF film mainly lies in the range of 625−850 nm. For the PBDB-TF/HF-PCIC blend, most fluorescence emission is quenched, but there still remains some fluorescence emission in the range of 750−850 nm. For the PBDB-TF/PC71BM blend, it is more quenched in the range of 750−850 nm than in the range of 625−750 nm. For the PBDB-TF/HF-PCIC/PC71BM blend, it is obvious that the fluorescence emission is more quenched in the range of 750−850 nm, indicating that charge separation is more efficient in the ternary blend than that in the binary blend. When HF-PCIC is changed to HC-PCIC, it can be seen that quenching is more sufficient for the PBDB-TF/HC-PCIC blend because of the lower energy levels of HC-PCIC. Therefore, the enhancement of quenching efficiency in the PBDB-TF/HC-PCIC/PC71BM blend is not so obvious as that in the PBDB-TF/HF-PCIC/PC71BM blend. These results suggest that enhancing the charge transfer can indeed promote the charge separation when the energy offsets are not sufficient. To prove whether there exists charge transfer between the non-fullerene acceptors and PC71BM, we fabricate three single component devices and two non-fullerene/fullerene acceptor

found to be a D/A weight ratio of 1:1.2 with the addition of 1.0% 1-chloronaphthalene (CN) and thermal annealing at 100 °C for 10 min. Keeping the total D/A weight ratio as 1:1.2, various weight ratios of PC71BM are added to the PBDB-TF/ HF-PCIC or PBDB-TF/HC-PCIC blends (Tables S2 and S3). The other post-treatment conditions for ternary OSCs are the same as those for relevant binary OSCs. Figure 2 presents the J−V and external quantum efficiency (EQE) curves of the optimized binary and ternary OSCs, and the corresponding photovoltaic parameters are summarized in Table 1. The binary OSCs based on PBDB-TF/HF-PCIC exhibit the best PCE of 10.90% with a Voc of 0.89 V, Jsc of 17.24 mA cm−2, and fill factor (FF) of 70.98%. The high Voc benefits from the deep HOMO level of PBDB-TF. By adding 30 wt % PC71BM to the PBDB-TF/HF-PCIC blend, the PCE of ternary OSCs increases to 11.55% with Voc and FF maintained as 0.90 V and 70.81% and Jsc enhanced to 18.10 mA cm−2. From the EQE curves in Figure 2b, we can see that the improvement of Jsc is mainly benefited from the enhancement of quantum efficiencies in the range of 600− 750 nm, indicating that the conversion efficiencies for both excitons from the polymer donor PBDB-TF and the nonfullerene acceptor HF-PCIC are boosted. Especially, the EQE value reaches 82.71% at 710 nm. We also fabricated binary OSCs based on PBDB-TF/PC71BM under the same conditions for PBDB-TF/HF-PCIC-based OSCs, and a PCE of 7.29% was achieved with a Voc of 0.95 V. Similar enhancements of quantum efficiencies and Jsc are also observed in HC-PCIC-based ternary OSCs. As HC-PCIC has deeper energy levels than HF-PCIC, the required amount of PC71BM as the third component is lowered to 15% weight ratio. Besides the increase of quantum efficiencies in the 600− 800 nm range, an enhancement is also observed in the 300− 42447

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Research Article

ACS Applied Materials & Interfaces blend devices. In Figure 3c, the HF-PCIC device shows a J−V curve with a Voc of 0.79 V and a Jsc of 0.16 mA cm−2, whereas the PC71BM device gives a J−V curve with a Voc of 0.60 V and a Jsc of 0.09 mA cm−2. After blending HF-PCIC with PC71BM, an enhanced Jsc of 0.23 mA cm−2 and an improved PCE are realized, illustrating that the electrons on the LUMO energy levels of HF-PCIC can be transferred to the LUMO energy levels of PC71BM and then collected by the cathode.53 As for the HC-PCIC device, we cannot observe a normal J−V curve. However, the device based on the HC-PCIC/PC71BM blend can exhibit a normal J−V curve with a Jsc of 0.22 mA cm−2, indicating that there also exists charge transfer from HC-PCIC to PC71BM.53 The existing of charge transfer from nonfullerene acceptors to PC71BM may depress the chargetransport barriers within the active layer and ensure the efficient exciton dissociation and charge transport to the electrode,17 thus promoting the improvement of quantum efficiencies and Jsc, as observed above. Figure 3d shows the dependence of Jsc on the light intensity (Plight), and the fitted linear lines can be described as Jsc ∝ (Plight)α in which α value represents the degree of bimolecular recombination.54 The closer to 1 the value of α is, the less bimolecular recombination there is. It is found that all α values are around 0.98, indicating that the introduction of PC71BM does not affect the bimolecular recombination. Therefore, the FFs can still be kept at high values in the ternary OSCs. As PC71BM has higher electron mobility than non-fullerene acceptors,34 the addition of PC71BM may influence the chargetransport properties of the ternary blends. Therefore, we measure the hole and electron mobilities of binary and ternary blends via the space charge limited current (SCLC) method (see the results in Figures S6 and S7 and Table S4). As expected, the hole mobilities of both HF-PCIC-based and HCPCIC-based ternary blends are improved (2.69 × 10−4 cm2 V−1 s−1 for HF-PCIC-based ternary blend and 4.34 × 10−4 cm2 V−1 s−1 for HC-PCIC-based ternary blend), but more obvious for the HC-PCIC-based ternary blend. The higher hole mobility of the HC-PCIC-based ternary blend may be responsible for the obvious quantum efficiency enhancement in the short wavelength range of 300−450 nm. As for electron mobilities, HC-PCIC-based binary blend shows higher electron mobility (2.24 × 10−4 cm2 V−1 s−1) than HF-PCICbased binary blend (0.80 × 10−4 cm2 V−1 s−1). This is why HC-PCIC-based binary OSCs show higher FF and Jsc than HF-PCIC-based binary OSCs. After the addition of PC71BM, the electron mobility is enhanced for the HF-PCIC-based ternary blend (1.11 × 10−4 cm2 V−1 s−1) but reduced a bit for the HC-PCIC-based ternary blend (1.88 × 10−4 cm2 V−1 s−1). This is why the FF of the HF-PCIC-based ternary OSC nearly has the same value as that of the corresponding binary OSC, whereas the FF of the HC-PCIC-based ternary OSC is reduced slightly relative to that of the corresponding binary OSC. The different change tendencies of electron mobilities are related with the different crystallinity changes of HF-PCIC and HCPCIC, which are proven in the following characterizations. Because the HC-PCIC-based ternary OSC shows higher electron and hole mobilities than the HF-PCIC-based ternary OSC, the former has a higher Jsc of 19.29 mA/cm2 than the latter (18.10 mA/cm2) and thus a higher PCE for the former. Anyway, the addition of PC71BM has positive effects on the charge-transport properties. The blend morphology is studied by atomic force microscopy (AFM) measurements. As shown in Figure 4,

Figure 4. AFM height images of (a) PBBD-TF/HF-PCIC, (b) PBBDTF/HF-PCIC/PC71BM, (c) PBDB-TF/PC71BM (DIO), (d) PBDBTF/HC-PCIC, (e) PBDB-TF/HC-PCIC/PC71BM, and (f) PBDBTF/PC71BM (CN) blended films.

both the non-fullerene binary blends show more obvious aggregates than the two fullerene binary blends because of the stronger crystallinities of two non-fullerene acceptors. When PC71BM is added to the non-fullerene binary blends, the two ternary blends exhibit different change tendencies. The rootmean-square (RMS) roughness of the HF-PCIC-based ternary blend is increased, whereas the RMS roughness of the HCPCIC-based ternary blend is decreased, certifying that the crystallinity of HF-PCIC is enhanced a bit and the crystallinity of HC-PCIC is reduced a bit. The enhanced crystallinity normally leads to a high mobility, which is in agreement with the SCLC results discussed above. The AFM results also conform to the phenomena observed in SCLC characterization. Grazing-incidence wide-/small-angle X-ray scattering (GIWAXS/GISAXS) was carried out to probe the molecular and nanoscale structure of the films.55,56 The results are shown in Figure 5. Comparing the overall scattering intensity, HF-PCIC systems have a higher crystallinity than the HC-PCIC systems. When mixed with PC71BM, thin films containing HF-PCIC would have increased scattering than its binary counterparts, but the trend is opposite for the HC-PCIC systems. All binary and ternary films with small molecules exhibit a face-on structure, as observed by the π−π stacking scattering peak near 1.7 Å−1 in the out-of-plane (OOP) direction. When mixed with PC71BM, the π−π stacking distance (3.61 Å, q = 1.74 Å−1) does not change for the HF-PCIC system. The π−π stacking distance of the PBDB-TF/HC-PCIC film is 3.65 Å (q = 1.72 Å−1), slightly larger than that of the PBDB-TF/HF-PCIC film. After mixing with PC71BM, the distance further increases (3.69 Å, q = 1.70 Å−1). However, the lamellar stacking distances remain the same for both systems in both OOP (19.0 Å, q = 0.330 Å−1) and in-plane (IP (20.9 Å, q = 0.300 Å−1) directions. Two-dimensional GISAXS patterns and the corresponding IP intensity profiles are shown in Figures S9 and 5, respectively. We adopt the Debye−Anderson−Brumberger model and a fractal-like network model to account for the scattering contribution from intermixing amorphous phases and acceptor domains, respectively, and estimate the corresponding domain sizes (ξ and 2Rg) by fitting. The results are listed in Table S5. For HF-PCIC systems, the addition of PC71BM helps reduce the size of the intermixing amorphous phase (ξ) from 35.8 to 26.1 nm and enhance the crystallinity of HF-PCIC. It is also 42448

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

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ACS Applied Materials & Interfaces

Figure 5. (a) GIWAXS profiles of PBDB-TF/HF-PCIC, PBDB-TF/HF-PCIC/PC71BM, PBDB-TF/PC71BM (DIO), PBDB-TF/PC71BM (CN), PBDB-TF/HC-PCIC, and PBDB-TF/HC-PCIC/PC71BM and the corresponding line integration along the (b) OOP and (c) IP directions. (d) GISAXS profile along the IP direction with fitting.

Figure 6. PCEs for the devices based on (a) PBDB-TF/HF-PCIC, PBDB-TF/HF-PCIC/PC71BM, and PBDB-TF/PC71BM (DIO) blended films and (b) PBDB-TF/HC-PCIC, PBDB-TF/HC-PCIC/PC71BM, and PBDB-TF/PC71BM (CN) blended films after thermal treatment at 130 °C for various times.

Because fullerene derivatives will normally aggregate at high temperatures and influence the device performances, we checked the thermal stability of two non-fullerene binary OSCs, two ternary OSCs, and two fullerene binary OSCs, and the results are displayed in Figure 6. The thermal stability test on photovoltaic devices was performed by heating the devices at 130 °C for various times, and the photovoltaic performances were tested. After thermal treatment at 130 °C for 12 h, both non-fullerene binary OSCs keep ∼90% initial PCEs, whereas both ternary OSCs keep ∼80% initial PCEs. On comparison, the PCE of the fullerene binary OSC with DIO as the additive

noticed that a pure PC71BM phase domain (19.2 nm) is formed in the HF-PCIC ternary film containing 30 wt % PC71BM. All these observations convinced the increased exciton dissociation and charge transportation in PBDB-TF/ HF-PCIC/PC71BM ternary solar cells, leading to the enhancement of quantum efficiency. For HC-PCIC systems, the addition of PC71BM helps optimize the HC-PCIC domains to a finer size, thus resulting in the improvement of charge separation and quantum efficiency in the ternary OSCs. The reduced domain size may also contribute to the improved PL quenching in the ternary blend displayed in Figure 3. 42449

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Research Article

ACS Applied Materials & Interfaces

51561145001, 61721005, and 51803178), the China Postdoctoral Science Foundation Funded Project (512300X91706), and Zhejiang Province Science and Technology Plan (no. 2018C01047). X.L. and T.-K.L. acknowledge the Research Grant Council of Hong Kong (General Research Fund no. 14314216) and the beam time and technical supports provided by 19U2 beamline at SSRF, Shanghai.

is largely reduced to 21% initial PCE and the PCE of the fullerene binary OSC with CN as the additive is also largely reduced to 42% initial PCE. From the above results, we can see that in fullerene binary OSCs, the active layer morphology with the CN additive will be more stable than that with the DIO additive. However, in non-fullerene OSCs, the stable active layer morphology can be maintained regardless of the change of additives. As shown from the optical microscopy images in Figure S10, both ternary blend films keep the uniform morphology without aggregation during 3 h thermal treatments at 180 °C.

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(1) Hou, J.; Inganäs, O.; Friend, R. H.; Gao, F. Organic Solar Cells Based on Non-Fullerene Acceptors. Nat. Mater. 2018, 17, 119−128. (2) Li, S.; Liu, W.; Li, C.-Z.; Shi, M.; Chen, H. Efficient Organic Solar Cells with Non-Fullerene Acceptors. Small 2017, 13, 1701120. (3) Elumalai, N. K.; Uddin, A. Open Circuit Voltage of Organic Solar Cells: An in-Depth Review. Energy Environ. Sci. 2016, 9, 391− 410. (4) Yao, H.; Cui, Y.; Yu, R.; Gao, B.; Zhang, H.; Hou, J. Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra-Narrow Band Gap. Angew. Chem., Int. Ed. 2017, 56, 3045−3049. (5) Liu, T.; Huo, L.; Chandrabose, S.; Chen, K.; Han, G.; Qi, F.; Meng, X.; Xie, D.; Ma, W.; Yi, Y.; Hodgkiss, J. M.; Liu, F.; Wang, J.; Yang, C.; Sun, Y. Optimized Fibril Network Morphology by Precise Side-Chain Engineering to Achieve High-Performance Bulk-Heterojunction Organic Solar Cells. Adv. Mater. 2018, 30, 1707353. (6) Cui, Y.; Yao, H.; Yang, C.; Zhang, S.; Hou, J. Organic Solar Cells with an Efficiency Approaching 15%. Acta Polym. Sin. 2018, 223−230. (7) Zhang, X. A Tandem Polymer Solar Cell Based on NonFullerene-Acceptors Yields an Efficiency Approaching 15%. Acta Polym. Sin. 2018, 129−131. (8) Li, N.; McCulloch, I.; Brabec, C. J. Analyzing the Efficiency, Stability and Cost Potential for Fullerene-Free Organic Photovoltaics in One Figure of Merit. Energy Environ. Sci. 2018, 11, 1355−1361. (9) Li, S.; Zhang, Z.; Shi, M.; Li, C.-Z.; Chen, H. Molecular Electron Acceptors for Efficient Fullerene-Free Organic Solar Cells. Phys. Chem. Chem. Phys. 2017, 19, 3440−3458. (10) Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K.-Y.; Marder, S. R.; Zhan, X. Non-Fullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 18003. (11) Ye, L.; Hu, H.; Ghasemi, M.; Wang, T.; Collins, B. A.; Kim, J.H.; Jiang, K.; Carpenter, J. H.; Li, H.; Li, Z.; McAfee, T.; Zhao, J.; Chen, X.; Lai, J. L. Y.; Ma, T.; Bredas, J.-L.; Yan, H.; Ade, H. Quantitative Relations between Interaction Parameter, Miscibility and Function in Organic Solar Cells. Nat. Mater. 2018, 17, 253−260. (12) Zhang, M.; Guo, X.; Ma, W.; Ade, H.; Hou, J. A Large-Bandgap Conjugated Polymer for Versatile Photovoltaic Applications with High Performance. Adv. Mater. 2015, 27, 4655−4660. (13) Sun, C.; Pan, F.; Bin, H.; Zhang, J.; Xue, L.; Qiu, B.; Wei, Z.; Zhang, Z.-G.; Li, Y. A Low Cost and High Performance Polymer Donor Material for Polymer Solar Cells. Nat. Commun. 2018, 9, 743. (14) Xue, L.; Yang, Y.; Xu, J.; Zhang, C.; Bin, H.; Zhang, Z.-G.; Qiu, B.; Li, X.; Sun, C.; Gao, L.; Yao, J.; Chen, X.; Yang, Y.; Xiao, M.; Li, Y. Side Chain Engineering on Medium Bandgap Copolymers to Suppress Triplet Formation for High-Efficiency Polymer Solar Cells. Adv. Mater. 2017, 29, 1703344. (15) Janssen, R. A. J.; Nelson, J. Factors Limiting Device Efficiency in Organic Photovoltaics. Adv. Mater. 2013, 25, 1847−1858. (16) Yu, R.; Yao, H.; Hou, J. Recent Progress in Ternary Organic Solar Cells Based on Nonfullerene Acceptors. Adv. Energy Mater. 2018, 8, 1702814. (17) An, Q.; Zhang, F.; Zhang, J.; Tang, W.; Deng, Z.; Hu, B. Versatile Ternary Organic Solar Cells: A Critical Review. Energy Environ. Sci. 2016, 9, 281−322. (18) Zhan, L.; Li, S.; Zhang, H.; Gao, F.; Lau, T.-K.; Lu, X.; Sun, D.; Wang, P.; Shi, M.; Li, C.-Z.; Chen, H. A near-Infrared Photoactive Morphology Modifier Leads to Significant Current Improvement and Energy Loss Mitigation for Ternary Organic Solar Cells. Adv. Sci. 2018, 5, 1800755.

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CONCLUSIONS In conclusion, we optimized two non-fullerene binary systems (PBDB-TF/HF-PCIC and PBDB-TF/HC-PCIC) with insufficient driving forces via the ternary strategy. PC71BM is introduced as the third component to form a cascade energy level alignment. Such an arrangement leads to an enhanced charge transfer between the non-fullerene acceptors (HF-PCIC or HC-PCIC) and PC71BM, thus benefitting the improvement of charge separation and charge transport in the ternary blends. As a result, obvious quantum efficiency improvements from ∼70% for binary blends to over 80% for ternary blends in the longer wavelength ranges are presented. Besides, high Voc values and FFs can also be maintained for both ternary OSCs. Therefore, high PCEs of 11.55 and 12.36% are realized for PBDB-TF/HF-PCIC/PC71BM and PBDB-TF/HC-PCIC/ PC71BM ternary OSCs, respectively. In addition, both the ternary OSCs show excellent thermal stabilities with ∼80% initial PCEs remaining after thermal treatment at 130 °C for 12 h. This work well-demonstrates that enhancing charge transfer and transport via energy level modulation and ternary strategy is effective in improving the quantum efficiency and device performance of those binary systems with insufficient driving forces.

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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/acsami.8b16131. Materials and methods, synthesis details, thermogravimetric analysis, cyclic voltammograms, ultraviolet− visible absorption, SCLC, GIWAXS, GISAXS, optical microscopy images, and nuclear magnetic resonance spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.L.). *E-mail: [email protected] (H.C.). ORCID

Xinhui Lu: 0000-0002-1908-3294 Hongzheng Chen: 0000-0002-5922-9550 Author Contributions §

L.Z. and S.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (nos. 21734008, 21474088, 51473142, 42450

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Research Article

ACS Applied Materials & Interfaces (19) Wang, J.; Gao, W.; An, Q.; Zhang, M.; Ma, X.; Hu, Z.; Zhang, J.; Yang, C.; Zhang, F. Ternary Non-Fullerene Polymer Solar Cells with an Efficiency of 11.6% by Simultaneously Optimizing Photon Harvesting and Phase Separation. J. Mater. Chem. A 2018, 6, 11751− 11758. (20) Zhang, M.; Gao, W.; Zhang, F.; Mi, Y.; Wang, W.; An, Q.; Wang, J.; Ma, X.; Miao, J.; Hu, Z.; Liu, X.; Zhang, J.; Yang, C. Efficient Ternary Non-Fullerene Polymer Solar Cells with Pce of 11.92% and Ff of 76.5%. Energy Environ. Sci. 2018, 11, 841−849. (21) Fu, H.; Wang, Z.; Sun, Y. Advances in Non-Fullerene Acceptor Based Ternary Organic Solar Cells. Sol. RRL 2018, 2, 1700158. (22) de Zerio, A. D.; Müller, C. Glass Forming Acceptor Alloys for Highly Efficient and Thermally Stable Ternary Organic Solar Cells. Adv. Energy Mater. 2018, 8, 1702741. (23) Zhang, S.; Zuo, L.; Chen, J.; Zhang, Z.; Mai, J.; Lau, T.-K.; Lu, X.; Shi, M.; Chen, H. Improved Photon-to-Electron Response of Ternary Blend Organic Solar Cells with a Low Band Gap Polymer Sensitizer and Interfacial Modification. J. Mater. Chem. A 2016, 4, 1702−1707. (24) Zhang, S.; Shah, M. N.; Liu, F.; Zhang, Z.; Hu, Q.; Russell, T. P.; Shi, M.; Li, C.-Z.; Chen, H. Efficient and 1,8-Diiodooctane-Free Ternary Organic Solar Cells Fabricated Via Nanoscale Morphology Tuning Using Small-Molecule Dye Additive. Nano Res. 2017, 10, 3765−3774. (25) Li, H.; Lu, K.; Wei, Z. Polymer/Small Molecule/Fullerene Based Ternary Solar Cells. Adv. Energy Mater. 2017, 7, 1602540. (26) Zhang, S.; Qin, Y.; Zhu, J.; Hou, J. Over 14% Efficiency in Polymer Solar Cells Enabled by a Chlorinated Polymer Donor. Adv. Mater. 2018, 30, 1800868. (27) Li, S.; Ye, L.; Zhao, W.; Yan, H.; Yang, B.; Liu, D.; Li, W.; Ade, H.; Hou, J. A Wide Band Gap Polymer with a Deep Highest Occupied Molecular Orbital Level Enables 14.2% Efficiency in Polymer Solar Cells. J. Am. Chem. Soc. 2018, 140, 7159−7167. (28) Li, S.; Zhan, L.; Liu, F.; Ren, J.; Shi, M.; Li, C.-Z.; Russell, T. P.; Chen, H. An Unfused-Core-Based Nonfullerene Acceptor Enables High-Efficiency Organic Solar Cells with Excellent Morphological Stability at High Temperatures. Adv. Mater. 2018, 30, 1705208. (29) Li, S.; Zhan, L.; Zhao, W.; Zhang, S.; Ali, B.; Fu, Z.; Lau, T.-K.; Lu, X.; Shi, M.; Li, C.-Z.; Hou, J.; Chen, H. Revealing the Effects of Molecular Packing on the Performances of Polymer Solar Cells Based on A-D-C-D-A Type Non-Fullerene Acceptors. J. Mater. Chem. A 2018, 6, 12132−12141. (30) Liu, J.; Chen, S.; Qian, D.; Gautam, B.; Yang, G.; Zhao, J.; Bergqvist, J.; Zhang, F.; Ma, W.; Ade, H.; Inganäs, O.; Gundogdu, K.; Gao, F.; Yan, H. Fast Charge Separation in a Non-Fullerene Organic Solar Cell with a Small Driving Force. Nat. Energy 2016, 1, 16089. (31) Zhang, H.; Li, S.; Xu, B.; Yao, H.; Yang, B.; Hou, J. FullereneFree Polymer Solar Cell Based on a Polythiophene Derivative with an Unprecedented Energy Loss of Less Than 0.5 eV. J. Mater. Chem. A 2016, 4, 18043−18049. (32) Bin, H.; Yang, Y.; Peng, Z.; Ye, L.; Yao, J.; Zhong, L.; Sun, C.; Gao, L.; Huang, H.; Li, X.; Qiu, B.; Xue, L.; Zhang, Z.-G.; Ade, H.; Li, Y. Effect of Alkylsilyl Side-Chain Structure on Photovoltaic Properties of Conjugated Polymer Donors. Adv. Energy Mater. 2018, 8, 1702324. (33) 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. (34) 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. (35) 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. (36) Cui, C.; Li, Y.; Li, Y. Fullerene Derivatives for the Applications as Acceptor and Cathode Buffer Layer Materials for Organic and Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, 1601251.

(37) Kim, M.; Lee, J.; Sin, D. H.; Lee, H.; Woo, H. Y.; Cho, K. Nonfullerene/Fullerene Acceptor Blend with a Tunable Energy State for High-Performance Ternary Organic Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 25570−25579. (38) Liu, H.; Li, J.; Xia, L.; Bai, Y.; Hu, S.; Liu, J.; Liu, L.; Hayat, T.; Alsaedi, A.; Tan, Z. Perfect Complementary in Absorption Spectra with Fullerene, Nonfullerene Acceptors and Medium Band Gap Donor for High-Performance Ternary Polymer Solar Cells. ACS Appl. Mater. Interfaces 2018, 10, 29831−29839. (39) Liu, W.; Liang, T.; Chen, Q.; Yu, Z.; Zhang, Y.; Liu, Y.; Fu, W.; Tang, F.; Chen, L.; Chen, H. Solution-Processed 8-Hydroquinolatolithium as Effective Cathode Interlayer for High-Performance Polymer Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 9254−9261. (40) Zhao, W.; Li, S.; Zhang, S.; Liu, X.; Hou, J. Ternary Polymer Solar Cells Based on Two Acceptors and One Donor for Achieving 12.2% Efficiency. Adv. Mater. 2017, 29, 1604059. (41) Gao, H.-H.; Sun, Y.; Wan, X.; Ke, X.; Feng, H.; Kan, B.; Wang, Y.; Zhang, Y.; Li, C.; Chen, Y. A New Nonfullerene Acceptor with near Infrared Absorption for High Performance Ternary-Blend Organic Solar Cells with Efficiency over 13%. Adv. Sci. 2018, 5, 1800307. (42) Zhang, T.; Zhao, X.; Yang, D.; Tian, Y.; Yang, X. Ternary Organic Solar Cells with >11% Efficiency Incorporating Thick Photoactive Layer and Nonfullerene Small Molecule Acceptor. Adv. Energy Mater. 2017, 8, 1701691. (43) Lu, H.; Zhang, J.; Chen, J.; Liu, Q.; Gong, X.; Feng, S.; Xu, X.; Ma, W.; Bo, Z. Ternary-Blend Polymer Solar Cells Combining Fullerene and Nonfullerene Acceptors to Synergistically Boost the Photovoltaic Performance. Adv. Mater. 2016, 28, 9559−9566. (44) Liu, T.; Xue, X.; Huo, L.; Sun, X.; An, Q.; Zhang, F.; Russell, T. P.; Liu, F.; Sun, Y. Highly Efficient Parallel-Like Ternary Organic Solar Cells. Chem. Mater. 2017, 29, 2914−2920. (45) Li, H.; Xiao, Z.; Ding, L.; Wang, J. Thermostable SingleJunction Organic Solar Cells with a Power Conversion Efficiency of 14.62%. Sci. Bull. 2018, 63, 340. (46) Cheng, P.; Zhang, M.; Lau, T.-K.; Wu, Y.; Jia, B.; Wang, J.; Yan, C.; Qin, M.; Lu, X.; Zhan, X. Realizing Small Energy Loss of 0.55 Ev, High Open-Circuit Voltage >1 V and High Efficiency >10% in Fullerene-Free Polymer Solar Cells Via Energy Driver. Adv. Mater. 2017, 29, 1605216. (47) Cheng, P.; Li, Y.; Zhan, X. Efficient Ternary Blend Polymer Solar Cells with Indene-C60 Bisadduct as an Electron-Cascade Acceptor. Energy Environ. Sci. 2014, 7, 2005−2011. (48) Cheng, P.; Wang, J.; Zhang, Q.; Huang, W.; Zhu, J.; Wang, R.; Chang, S.-Y.; Sun, P.; Meng, L.; Zhao, H.; Cheng, H.-W.; Huang, T.; Liu, Y.; Wang, C.; Zhu, C.; You, W.; Zhan, X.; Yang, Y. Unique Energy Alignments of a Ternary Material System toward HighPerformance Organic Photovoltaics. Adv. Mater. 2018, 30, 1801501. (49) Chen, H.; Hu, Z.; Wang, H.; Liu, L.; Chao, P.; Qu, J.; Chen, W.; Liu, A.; He, F. A Chlorinated π-Conjugated Polymer Donor for Efficient Organic Solar Cells. Joule 2018, 2, 1623−1634. (50) Zhang, H.; Yao, H.; Hou, J.; Zhu, J.; Zhang, J.; Li, W.; Yu, R.; Gao, B.; Zhang, S.; Hou, J. Over 14% Efficiency in Organic Solar Cells Enabled by Chlorinated Nonfullerene Small-Molecule Acceptors. Adv. Mater. 2018, 30, 1800613. (51) Li, S.; Ye, L.; Zhao, W.; Liu, X.; Zhu, J.; Ade, H.; Hou, J. Design of a New Small-Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor. Adv. Mater. 2017, 29, 1704051. (52) Gasparini, N.; Jiao, X.; Heumueller, T.; Baran, D.; Matt, G. J.; Fladischer, S.; Spiecker, E.; Ade, H.; Brabec, C. J.; Ameri, T. Designing Ternary Blend Bulk Heterojunction Solar Cells with Reduced Carrier Recombination and a Fill Factor of 77%. Nat. Energy 2016, 1, 16118. (53) An, Q.; Zhang, F.; Li, L.; Wang, J.; Sun, Q.; Zhang, J.; Tang, W.; Deng, Z. Simultaneous Improvement in Short Circuit Current, Open Circuit Voltage, and Fill Factor of Polymer Solar Cells through Ternary Strategy. ACS Appl. Mater. Interfaces 2015, 7, 3691−3698. 42451

DOI: 10.1021/acsami.8b16131 ACS Appl. Mater. Interfaces 2018, 10, 42444−42452

Research Article

ACS Applied Materials & Interfaces (54) Koster, L. J. A.; Kemerink, M.; Wienk, M. M.; Maturová, K.; Janssen, R. A. J. Quantifying Bimolecular Recombination Losses in Organic Bulk Heterojunction Solar Cells. Adv. Mater. 2011, 23, 1670−1674. (55) Mai, J.; Xiao, Y.; Zhou, G.; Wang, J.; Zhu, J.; Zhao, N.; Zhan, X.; Lu, X. Hidden Structure Ordering Along Backbone of Fused-Ring Electron Acceptors Enhanced by Ternary Bulk Heterojunction. Adv. Mater. 2018, 30, 1802888. (56) Mai, J.; Lau, T.-K.; Li, J.; Peng, S.-H.; Hsu, C.-S.; Jeng, U.-S.; Zeng, J.; Zhao, N.; Xiao, X.; Lu, X. Understanding Morphology Compatibility for High-Performance Ternary Organic Solar Cells. Chem. Mater. 2016, 28, 6186−6195.

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