Bandgap Engineering of Dual Acceptor-Containing Naphthalene

Nov 13, 2018 - State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics , Chinese Academy of Sciences, Dalian National Laboratory for C...
13 downloads 0 Views 2MB Size
Download PDF
Letter pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Bandgap Engineering of Dual Acceptor-Containing Naphthalene Diimide Polymers for All-Polymer Solar Cells Dandan Tu,†,‡ Xuan Liu,†,‡ Jing Zhang,†,‡ Qing Yang,†,‡ Shuwen Yu,†,∥ Xin Guo,*,†,∥ and Can Li*,†,∥ †

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, Zhongshan Road 457, Dalian 116023, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China ∥ The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian 116023, China

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV STRASBOURG on 11/16/18. For personal use only.

S Supporting Information *

ABSTRACT: Naphthalene diimide (NDI)-based polymers, as nonfullerene acceptors for all-polymer solar cells (allPSCs), are usually investigated by manipulating electrondonating units, which have a narrow bandgap showing absorption spectra mostly overlapping with high-performance donor polymers like PTB7-Th. In order to gain complementary absorptions between NDI-based polymers and PTB7-Th, we report three NDI-based polymers (P1−P3) designed by a dual-acceptor strategy and systematically study their effect on the bandgap, molecular configuration, and charge transport property. The absorption bands of these polymers range from 300 to 800 nm; in particular, P1 with a maximum absorption at 530 nm exhibits a good complementary absorption with the PTB7-Th. The device based on the PTB7-Th:P1 blend provides higher Jsc and efficiency than those based on other two polymers. The result suggests that the dual-acceptor strategy is effective to design acceptor polymers with adjustable absorption and molecular configuration for the all-PSCs. KEYWORDS: Naphthalene diimide polymers, All-polymer solar cells, Complementary absorption, Dual-acceptor strategy

■

INTRODUCTION

donor polymers, such as PTB7-Th. This limits the light harvest in the visible region, resulting in low photocurrent in devices. In addition to varying the electron-donating moiety for tuning the bandgap and absorption behavior of the donor− acceptor (D-A) type polymers, introducing another electronwithdrawing unit into the backbone of these polymers can play the same role. Such a dual-acceptor strategy has been proposed to construct D-A1-D-A2 copolymers for organic field-effect transistors.24−27 It is also employed to design NDI-based acceptor polymers for all-PSCs.28−31 For example, Yang et al. have incorporated a strong electron-deficient unit diketopyrrolopyrrole into the NDI-thiophene polymer backbone.31 However, very few NDI-based polymers containing an additional acceptor unit have been systematically studied; in particular, large-bandgap NDI-based polymers showing complementary absorption with high-performance donor polymers like PTB7-Th are barely reported. It is thus of importance to choose an additional acceptor unit with adjustable electronwithdrawing ability in order to obtain large-bandgap NDIbased polymers absorbing light in the region where the highperformance donor polymer exhibits extremely weak absorption.

In recent years, increasing efforts have been devoted to developing nonfullerene acceptors1−5 for polymer solar cells due to their readily tunable photoelectronic properties and low synthesis costs. Compared with the small-molecule acceptors such as “ITIC”, spiro-type compounds, and their derivatives,6−9 polymer acceptors offer additional merits of high morphological stability, superior mechanical properties and suitability for large-area production, making them capable of being used in all-polymer solar cells (all-PSCs).10−17 However, there are only a few polymer acceptors that can achieve high efficiencies for all-PSCs. Most of the widely used polymer acceptors with good device performances are derived from naphthalene diimide (NDI) and perylene diimide (PDI) building blocks,18 exhibiting excellent ambient and thermal stability, strong electron affinities, and high electron mobilities. Among them, an NDI-bithiophene copolymer19 with a commercial name of N2200 is well-known for its impressive electron mobility and has been intensively studied as acceptor for application in the all-PSCs. Many following works are reported to fine-tune the absorption bands, energy levels, and charge transport properties of the NDI-based polymers, yet mostly focusing on the manipulation of electron-donating units.20−23 No matter what the comonomers are, resultant polymers have a narrow or medium bandgap, leading to their overlapped absorption scope with excellent narrow-bandgap © XXXX American Chemical Society

Received: September 30, 2018 Revised: October 25, 2018 Published: November 13, 2018 A

DOI: 10.1021/acssuschemeng.8b05021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

The backbone conformations of P1−P3 were calculated by density function theory (DFT) using the Gaussian 09 program at the B3LYP/6-31G (d,p) level. To avoid excessive computation demands, two repeat units were chosen and alkyl chains attached to the backbones were replaced by methyl groups in the calculation. The energy-minimized geometry and corresponding electron distributions are shown in Figure 2. It is found that the highest occupied molecular orbitals (HOMOs) of all polymers are delocalized over the whole backbone, while the lowest unoccupied molecular orbitals (LUMOs) are primarily resident on the NDI unit owing to its strong electron affinity. It can be observed that dihedral angles between fluorinated additional comonomers and adjacent thiophenes in P1 and P3 are smaller than those in P2, which could be ascribed to the C−H···F−C hydrogen bonding interaction.32 In addition, the symmetrical F atoms in TF unit induce a relatively high backbone coplanarity of P1, which is beneficial for the improved charge mobility as compared to other polymers. The DFT results reveal that the backbone conformation of the NDI-based polymers could be well tuned by the additional acceptor unit. The UV−visible absorption spectra of P1−P3 in toluene and as films are shown in Figure 3 and the data are summarized in Table 1. All the polymers exhibit two distinct bands at 300− 500 and 500−800 nm that are respectively ascribed to π−π* transition and intramolecular charge transfer (ICT) transition. In toluene solution, the λmax is 692 nm (P3), 634 nm (P2), and 530 nm (P1), respectively, which should result from the influence of the second electron acceptor unit on the electronic structure. Notably, the polymer P1 shows a broad absorption band covering the visible region of 450−650 nm, which represents a most obviously hypsochromic shift for the NDIbased polymers. The absorption bands of these polymers are also in good agreement with their color change from green of P3 to purple of P1 (Figure 3a). The extinction coefficients (ε) of the polymers at the λmax range from 3.90 × 104 cm−1 to 7.46 × 104 cm−1. It is clear that the P1 shows a higher ε than others, suggesting its stronger light-harvesting property. In film state, while the P3 presents similar absorption spectra as that in solution, probably because of the suppression of the intermolecular interactions caused by the steric hindrance of bulky substituent on the FTAZ unit, the P1 and P2 exhibit broadened and red-shifted absorption bands. Particularity, the λmax of P1 remarkably shift from 530 to 582 nm, indicative of stronger aggregation of P1 in the film. The optical bandgaps (Eopt g ) of P1−P3 were estimated to be between 1.56 and 1.81 eV from the onset of the film absorption spectra, indicating that the three polymers present variable bandgaps by tuning the electron affinity of the additional comonomer. Noted that the absorption band of narrow-bandgap polymer donor PTB7Th locates at 600−750 nm; obviously, the P1 represents a complementary absorption with the PTB7-Th in a broad wavelength region from 300 to 750 nm. Thus, the blend film of

In this work, we utilize the dual-acceptor strategy to construct NDI-based polymers by incorporating variable electron-affinity units, namely, 2,2′-(perfluoro-1,4-phenylene) dithiophene (TF), benzothiadiazole (BT) and fluorinated benzotriazole (FTAZ), respectively, into the N2200 backbone (Figure 1). The bandgaps of resultant polymers (P1−P3,

Figure 1. Illustration of structural evolution from N2200 to designed dual-acceptor NDI-based polymers.

Scheme 1) are successively adjusted, with the absorption region ranging from 300 to 800 nm. Especially, the polymer P1 displays an absorption maximum (λmax) at 530 nm that is well complementary with that of the PTB7-Th. Meanwhile, P1 shows a better backbone coplanarity than other two polymers, in favor of increasing the charge transport. The all-PSC based on P1 blended with the PTB7-Th yields the highest PCE in this series of polymers without using any processing additives and post-treatment. The result can be attributed to its high absorption coefficient, complementary absorption spectrum with the donor component, and balanced charge mobility.

■

RESULTS AND DISCUSSION The three polymers were prepared by the Stille coupling polymerization between dibromo-substituted NDI and distannyl comonomers, as shown in Scheme 1. The synthetic details and complete characterizations are described in Supporting Information. The P3 shows good solubility in common solvents such as chloroform, toluene and chlorobenzene, while the P1 and P2 are less soluble in these solvents at room temperature but could be dissolved upon moderate heating above 50 °C. The molecular weight and polydispersity index (PDI) values of P1 and P2 were determined by gel permeation chromatography (GPC) at 150 °C with 1,2,4-trichlorobenzene (TCB) as the eluent, while that of P3 was determined at 35 °C with tetrahydrofuran (THF) as the eluent. The relatively low molecular weight of P2 could be ascribed to its poor solubility during the polymerization process. The thermal property of P1−P3 was measured by thermogravimetric analysis (TGA) in a nitrogen atmosphere at a heating rate of 10 °C min−1. The results (Figure S1) reveal that the P1−P3 exhibit excellent thermal stability with decomposition temperatures (Td) above 450 °C at 5% weight loss under nitrogen. Scheme 1. Synthetic Procedure of NDI-Based Polymers P1−P3

B

DOI: 10.1021/acssuschemeng.8b05021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 2. Frontier molecular orbitals (HOMO and LUMO) and optimized geometries of P1−P3 obtained from DFT calculations.

Figure 3. UV−vis absorption spectra of polymers P1−P3 (a) in toluene solution (10−5 M) and (b) in thin film (spin-coated on glass substrate in a concentration of 3 mg/mL).

Figure 4. (a) J−V characteristics of PTB7-Th:P1−P3-based solar cells and (b) EQE spectra measured under 100 mW cm−2 AM 1.5G solar illumination.

PTB7-Th:P1 should be highly potential in photovoltaic performances in this series of polymers. The electrochemical properties of P1−P3 films were investigated by cyclic voltammetry in acetonitrile solution containing 0.1 M Bu4NPF6 at a scan rate of 20 mV s−1. The LUMO levels of P1−P3 were approximated from the onset of the reduction peak in each voltammogram, using the Ag/ AgNO3 electrode as the reference (see Figure S2). The ELUMO of P1 is higher than those of P2 and P3 due to the strong electron-withdrawing TF group (Table 1). The HOMO levels were calculated using the optical bandgaps and the LUMOs. The photovoltaic behaviors of P1−P3 were investigated in all-PSCs using narrow-bandgap polymer PTB7-Th as the donor, and the all-PSCs were fabricated with inverted device architecture of ITO/ZnO/PTB7-Th:P1−P3/MoO3/Ag. The J−V curves of the devices measured under AM 1.5G illumination at 100 mW cm−2 are shown in Figure 4 and the photovoltaic parameters are summarized in Table 2. It is found that along with the blue-shifted absorption spectra of P1−P3, the short-circuit current density (Jsc) of the all-PSCs gradually increases, in line with the complementary absorption of the acceptor polymer with the PTB7-Th (Figure S3). Especially,

the P1-based device affords a highest Jsc as well as PCE, which is achieved at the optimal D/A ratio of 1:3 without thermal annealing and additives, contributed by the most matching absorption with the PTB7-Th. Although the P1 exhibits a higher LUMO level than the P2 and P3, the open-circuit voltage (Voc) of its device is lower than those based on the latters. It could be ascribed to the interfacial energetic disorder caused by different molecular orientation of PTB7-Th and P1 at their mixed region and the large charge transfer (CT) state binding energy.33−37 In contrast, the FTAZ-containing polymer P3 shows extremely poor photovoltaic performance, which is related to its unbalanced electron/hole mobility and inferior film morphology (see details below). External quantum efficiencies (EQE) were measured to evaluate the spectral responses of the all-PSCs. As shown in Figure 4, the photoresponse of the EQE profiles over 300−850 nm is consistent with the absorption spectra of the polymer blend films. The PTB7-Th:P1 device shows a more efficient photoresponse between 350 and 750 nm than other cases. The Jsc calculated from integration of the EQE spectra is in good agreement with those obtained from the J−V measurements (Table 2).

Table 1. Molecular Weights, Thermal Stability, Optical and Electrochemical Properties of P1−P3 Polymer

Mna (kDa)

PDI

Tdb (°C)

c λsol max (nm)

ε ×104 (cm−1)

d λfilm max (nm)

Eopt g (eV)

LUMOe (eV)

HOMOf (eV)

P1 P2 P3

12.6 7.0 16.5

1.34 1.23 1.07

460 459 455

352, 530 335, 479, 634 413, 629, 692

7.46 5.13 3.90

373, 582 365, 501, 644 427, 631, 693

1.81 1.56 1.61

−3.87 −4.05 −4.05

−5.68 −5.61 −5.69

a Mn of P1 and P2 was measured with the 1,2,4-trichlorobenzene (TCB) as the eluent, and that of P3 was measured with THF as eluent. bTd of P1−P3 was measured by TGA at a heating rate of 10 °C min−1 under nitrogen atmosphere. cSolution absorptions were measured in toluene with a concentration of 10−5 M. dFilms for the absorption measurement were spin-coated from a 3 mg/mL solution on the glass substrate. eEnergy levels were measured using the cyclic voltammetry method (see details in Supporting Information). fHOMO energy levels were calculated according to the LUMO energy levels and Eopt g .

C

DOI: 10.1021/acssuschemeng.8b05021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering Table 2. Device Performances of All-PSCs Based on Blend Films of PTB7-Th:P1−P3 Donor:acceptor

Jsc (mA cm−2)

PTB7-Th:P1 PTB7-Th:P2 PTB7-Th:P3

a

8.03 (7.99 ) 7.11 (6.60a) 4.10 (3.78a)

Voc (V) 0.79 0.80 0.86

FF

PCE (%)

0.51 0.51 0.43

μh (cm2 v−1 s−1) −5

6.6 × 10 4.0 × 10−5 4.9 × 10−5

3.24 2.93 1.52

μe (cm2 v−1 s−1)

μh/μe

4.9 × 10−5 6.3 × 10−6 1.1 × 10−6

1.35 6.37 44.5

a

Jsc calculated from EQE.

Figure 5. AFM height images of PTB7-Th:P1−P3 blend films spin-coated on ITO/ZnO substrates.

strength of the additional comonomers. In particular, the P1 possesses a large bandgap, affording a most complementary light absorption with the PTB7-Th. The device based on the PTB7-Th:P1 yields a higher PCE than other two polymersbased devices. The relatively high Jsc was attributed to the most complementary light absorption of the P1 with the donor, and the high Jsc and FF were also associated with its coplanar backbone structure facilitating the electron transport and the balanced charge transport. This work demonstrates that the dual-acceptor strategy is promising for fine-tuning bandgap and absorption, backbone conformation, and charge transport behavior of nonfullerene polymers, which are crucial for improving all-PSCs performance.

Space-charge-limited current (SCLC) measurement was performed to investigate the charge carrier transport characteristics to better understand the photovoltaic properties of the P1−P3:PTB7-Th blends. Hole-only devices with the configuration of ITO/PEDOT:PSS/blend/MoO3/Au and electrononly devices with the configuration of ITO/ZnO/blend/Ca/Al were fabricated and characterized. The charge carrier mobilities obtained from the SCLC fittings of the J1/2 − (V−Vbi) characteristics of the hole-only and electron-only devices are shown in Figure S3 and Table 2. The blend film based on PTB7-Th:P1 presents hole/electron charge mobilities of 6.6 × 10−5/4.9 × 10−5 cm2 V−1 s−1 and balanced charge transport (μh/μe = 1.35), accounting for the high FF.38 The relatively higher electron mobility of the P1-containing blend film compared to others benefits from its coplanar backbone, in accordance with its stronger aggregation in film as stated above. On the contrary, the steric hindrance of bulky substituents on FTAZ in P3 suppresses the intermolecular interaction, resulting in their low electron mobility and thus low FF. Tapping-mode atomic force microscopy (AFM) was used to investigate the surface morphology of the blend films in the allPSCs, as shown in Figure 5. It is found that the surface morphology is obviously affected by the additional comonomers. The PTB7-Th:P1 blend film shows well mixed feature, and the blend film has smooth surface with a root-mean-square (RMS) roughness of 1.48 nm (10 μm × 10 μm). The smooth surface morphology would result in a good interfacial adhesion and accordingly facilitating the charge carrier transport. The PTB7-Th:P2 blend film displays a distinct phase separation and a relatively smooth surface with RMS roughness of 2.69 nm (10 μm × 10 μm). However, the PTB7-Th:P3 blend film presents a very rough surface with a large phase-separation size. This is unfavorable for the exciton separation and will lead to poor interfacial contact between the blend film and the hole transport layer, thus accounting for the low FF and PCE.23,37,39

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05021. Experimental section; synthetic details; all-PSCs fabrication and characterization; TGA curves of P1−P3; cyclic voltammograms of P1−P3 in films; mobility measurement; 1H and 13C NMR spectra (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Prof. Xin Guo. Email: [email protected]. *Prof. Can Li. Email: [email protected]. ORCID

Xin Guo: 0000-0003-4995-8378 Can Li: 0000-0002-9301-7850

■

Notes

The authors declare no competing financial interest.

CONCLUSION We have synthesized three NDI-based polymers (P1−P3) using a dual-acceptor strategy and systematically investigated the influence of the additional acceptor unit on the photophysical properties and the photovoltaic performances of the polymers. The absorption spectra of P1−P3 are gradually blue-shifted with the increased electron-withdrawing

■

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21644008 and 51773204) and the “Thousand Talents Program for Young Scholars” of China. D

DOI: 10.1021/acssuschemeng.8b05021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

■

Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%. Adv. Mater. 2016, 28, 1884−1890. (18) Guo, X.; Tu, D.; Liu, X. Recent advances in rylene diimide polymer acceptors for all-polymer solar cells. J. Energy Chem. 2015, 24, 675−685. (19) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. A high-mobility electron-transporting polymer for printed transistors. Nature 2009, 457, 679−686. (20) Geng, Y.; Huang, J.; Tajima, K.; Zeng, Q.; Zhou, E. A low band gap n-type polymer based on dithienosilole and naphthalene diimide for all-polymer solar cells application. Polymer 2015, 63, 164−169. (21) Xue, L.; Yang, Y.; Bin, H.; Zhang, Z.-G.; Zhang, J.; Yang, Y.; Li, Y. Synthesis and Characterization of Arylenevinylenearylene-Naphthalene Diimide Copolymers as Acceptor in All-Polymer Solar Cells. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1757−1764. (22) Li, X.; Sun, P.; Wang, Y.; Shan, H.; Xu, J.; Song, X.; Xu, Z.-x.; Chen, Z.-K. A random copolymer approach to develop nonfullerene acceptors for all-polymer solar cells. J. Mater. Chem. C 2016, 4, 2106− 2110. (23) Li, X.; Sun, P.; Wang, Y.; Shan, H.; Xu, J.; You, C.; Xu, Z.-x.; Chen, Z.-K. Design of three-component randomly incorporated copolymers as non-fullerene acceptors for all-polymer solar cells. Polym. Chem. 2016, 7, 2230−2238. (24) An, C.; Marszalek, T.; Guo, X.; Puniredd, S. R.; Wagner, M.; Pisula, W.; Baumgarten, M. Tuning the optoelectronic properties of dual-acceptor based low-bandgap ambipolar polymers by changing the thiophene-bridge length. Polym. Chem. 2015, 6, 6238−6245. (25) Kim, M. J.; Choi, S.; Lee, M.; Heo, H.; Lee, Y.; Cho, J. H.; Kim, B. Photoresponsive Transistors Based on a Dual Acceptor-Containing Low-Bandgap Polymer. ACS Appl. Mater. Interfaces 2017, 9, 19011− 19020. (26) Zhao, Z.; Yin, Z.; Chen, H.; Zheng, L.; Zhu, C.; Zhang, L.; Tan, S.; Wang, H.; Guo, Y.; Tang, Q.; Liu, Y. High-Performance, Air-Stable Field-Effect Transistors Based on Heteroatom-Substituted Naphthalenediimide-Benzothiadiazole Copolymers Exhibiting Ultrahigh Electron Mobility up to 8.5 cm V-1 s(−1). Adv. Mater. 2017, 29, 1602410. (27) Luzio, A.; Fazzi, D.; Nuebling, F.; Matsidik, R.; Straub, A.; Komber, H.; Giussani, E.; Watkins, S. E.; Barbatti, M.; Thiel, W.; Gann, E.; Thomsen, L.; McNeill, C. R.; Caironi, M.; Sommer, M. Structure-Function Relationships of High-Electron Mobility Naphthalene Diimide Copolymers Prepared Via Direct Arylation. Chem. Mater. 2014, 26, 6233−6240. (28) Kim, H. J.; Park, G. E.; Lee, D. H.; Cho, M. J.; Choi, D. H. Importance of varying electron-accepting moieties in regular conjugated terpolymers for use in polymer solar cells. Org. Electron. 2016, 38, 256−263. (29) Keshtov, M. L.; Kuklin, S. A.; Khokhlov, A. R.; Osipov, S. N.; Radychev, N. A.; Godovskiy, D. Y.; Konstantinov, I. O.; Chen, F. C.; Koukaras, E. N.; Sharma, G. D. Polymer solar cells based low bandgap A1-D-A2-D terpolymer based on fluorinated thiadiazoloquinoxaline and benzothiadiazole acceptors with energy loss less than 0.5 eV. Org. Electron. 2017, 46, 192−202. (30) Weng, C.; Liu, Z.; Guo, H.; Tan, S. The Photoelectric Properties of Polymer Acceptors Containing Oxadiazole and Thiadiazole. Macromol. Chem. Phys. 2017, 218, 1700094. (31) Li, Y.; Yang, Y.; Bao, X.; Qiu, M.; Liu, Z.; Wang, N.; Zhang, G.; Yang, R.; Zhang, D. New pi-conjugated polymers as acceptors designed for all polymer solar cells based on imide/amide-derivatives. J. Mater. Chem. C 2016, 4, 185−192. (32) Schroeder, B. C.; Huang, Z.; Ashraf, R. S.; Smith, J.; D’Angelo, P.; Watkins, S. E.; Anthopoulos, T. D.; Durrant, J. R.; McCulloch, I. Silaindacenodithiophene-Based Low Band Gap Polymers - The Effect of Fluorine Substitution on Device Performances and Film Morphologies. Adv. Funct. Mater. 2012, 22, 1663−1670. (33) Burke, T. M.; Sweetnam, S.; Vandewal, K.; McGehee, M. D. Beyond Langevin Recombination: How Equilibrium Between Free Carriers and Charge Transfer States Determines the Open-Circuit Voltage of Organic Solar Cells. Adv. Energy Mater. 2015, 5, 1500123.

DEDICATION Dedicated to the 70th anniversary of Dalian Institute of Chemical Physics, CAS.

■

REFERENCES

(1) Mori, D.; Benten, H.; Okada, I.; Ohkita, H.; Ito, S. Highly efficient charge-carrier generation and collection in polymer/polymer blend solar cells with a power conversion efficiency of 5.7%. Energy Environ. Sci. 2014, 7, 2939−2943. (2) Earmme, T.; Hwang, Y.-J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor. J. Am. Chem. Soc. 2013, 135, 14960−14963. (3) Hwang, Y.-J.; Courtright, B.A.E.; Ferreira, A. S.; Tolbert, S. H.; Jenekhe, S. A. 7.7% Efficient All-Polymer Solar Cells. Adv. Mater. 2015, 27, 4578−4584. (4) Lee, J.; Singh, R.; Sin, D. H.; Kim, H. G.; Song, K. C.; Cho, K. A Nonfullerene Small Molecule Acceptor with 3D Interlocking Geometry Enabling Efficient Organic Solar Cells. Adv. Mater. 2016, 28, 69−76. (5) Zhong, Y.; Trinh, M. T.; Chen, R.; Purdum, G. E.; Khlyabich, P. P.; Sezen, M.; Oh, S.; Zhu, H.; Fowler, B.; Zhang, B.; Wang, W.; Nam, C.-Y.; Sfeir, M. Y.; Black, C. T.; Steigerwald, M. L.; Loo, Y.-L.; Ng, F.; Zhu, X. Y.; Nuckolls, C. Molecular helices as electron acceptors in high-performance bulk heterojunction solar cells. Nat. Commun. 2015, 6, 8242. (6) Zhao, W.; Qian, D.; Zhang, S.; Li, S.; Inganas, O.; Gao, F.; Hou, J. Fullerene-Free Polymer Solar Cells with over 11% Efficiency and Excellent Thermal Stability. Adv. Mater. 2016, 28, 4734−4739. (7) Bin, H.; Zhang, Z.-G.; Gao, L.; Chen, S.; Zhong, L.; Xue, L.; Yang, C.; Li, Y. Non-Fullerene Polymer Solar Cells Based on Alkylthio and Fluorine Substituted 2D-Conjugated Polymers Reach 9.5% Efficiency. J. Am. Chem. Soc. 2016, 138, 4657−4664. (8) Lin, Y.; Wang, J.; Zhang, Z.-G.; Bai, H.; Li, Y.; Zhu, D.; Zhan, X. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174. (9) Zhao, W.; Li, S.; Yao, H.; Zhang, S.; Zhang, Y.; Yang, B.; Hou, J. Molecular Optimization Enables over 13% Efficiency in Organic Solar Cells. J. Am. Chem. Soc. 2017, 139, 7148−7151. (10) Diao, Y.; Zhou, Y.; Kurosawa, T.; Shaw, L.; Wang, C.; Park, S.; Guo, Y.; Reinspach, J. A.; Gu, K.; Gu, X.; Tee, B. C. K.; Pang, C.; Yan, H.; Zhao, D.; Toney, M. F.; Mannsfeld, S. C. B.; Bao, Z. Flowenhanced solution printing of all-polymer solar cells. Nat. Commun. 2015, 6, 7955. (11) Kim, T.; Kim, J.-H.; Kang, T. E.; Lee, C.; Kang, H.; Shin, M.; Wang, C.; Ma, B.; Jeong, U.; Kim, T.-S.; Kim, B. J. Flexible, highly efficient all-polymer solar cells. Nat. Commun. 2015, 6, 8547. (12) Facchetti, A. Polymer donor-polymer acceptor (all-polymer) solar cells. Mater. Today 2013, 16, 123−132. (13) Guo, Y.; Li, Y.; Awartani, O.; Han, H.; Zhao, J.; Ade, H.; Yan, H.; Zhao, D. Improved Performance of All-Polymer Solar Cells Enabled by Naphthodiperylenetetraimide-Based Polymer Acceptor. Adv. Mater. 2017, 29, 1700309. (14) Fan, B.; Ying, L.; Zhu, P.; Pan, F.; Liu, F.; Chen, J.; Huang, F.; Cao, Y. All-Polymer Solar Cells Based on a Conjugated Polymer Containing Siloxane-Functionalized Side Chains with Efficiency over 10%. Adv. Mater. 2017, 29, 1703906. (15) Fan, B.; Ying, L.; Wang, Z.; He, B.; Jiang, X.-F.; Huang, F.; Cao, Y. Optimisation of processing solvent and molecular weight for the production of green-solvent-processed all-polymer solar cells with a power conversion efficiency over 9%. Energy Environ. Sci. 2017, 10, 1243−1251. (16) Fan, B.; Zhu, P.; Xin, J.; Li, N.; Ying, L.; Zhong, W.; Li, Z.; Ma, W.; Huang, F.; Cao, Y. High-Performance Thick-Film All-Polymer Solar Cells Created Via Ternary Blending of a Novel Wide-Bandgap Electron-Donating Copolymer. Adv. Energy Mater. 2018, 8, 1703085. (17) Gao, L.; Zhang, Z.-G.; Xue, L.; Min, J.; Zhang, J.; Wei, Z.; Li, Y. All-Polymer Solar Cells Based on Absorption-Complementary E

DOI: 10.1021/acssuschemeng.8b05021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering (34) Verlaak, S.; Beljonne, D.; Cheyns, D.; Rolin, C.; Linares, M.; Castet, F.; Cornil, J.; Heremans, P. Electronic Structure and Geminate Pair Energetics at Organic-Organic Interfaces: The Case of Pentacene/C-60 Heterojunctions. Adv. Funct. Mater. 2009, 19, 3809−3814. (35) Song, C.-p.; Qu, Y.; Liu, J.-g.; Han, Y.-c. Phase-Separation Mechanism and Morphological Control in All-polymer Solar Cells. Acta Polym. Sin. 2018, 2, 145−163. (36) Ye, L.; Jiao, X.; Zhou, M.; Zhang, S.; Yao, H.; Zhao, W.; Xia, A.; Ade, H.; Hou, J. Manipulating Aggregation and Molecular Orientation in All-Polymer Photovoltaic Cells. Adv. Mater. 2015, 27, 6046−6054. (37) Li, M.; Liu, J.; Cao, X.; Zhou, K.; Zhao, Q.; Yu, X.; Xing, R.; Han, Y. Achieving balanced intermixed and pure crystalline phases in PDI-based non-fullerene organic solar cells via selective solvent additives. Phys. Chem. Chem. Phys. 2014, 16, 26917−26928. (38) Bartesaghi, D.; Pérez, I. d. C.; Kniepert, J.; Roland, S.; Turbiez, M.; Neher, D.; Koster, L. J. A. Competition between recombination and extraction of free charges determines the fill factor of organic solar cells. Nat. Commun. 2015, 6, 7083. (39) Wang, Y.; Hasegawa, T.; Matsumoto, H.; Mori, T.; Michinobu, T. D-A(1)-D-A(2) Backbone Strategy for Benzobisthiadiazole Based n-Channel Organic Transistors: Clarifying the Selenium-Substitution Effect on the Molecular Packing and Charge Transport Properties in Electron-Deficient Polymers. Adv. Funct. Mater. 2017, 27, 1701486.

F

DOI: 10.1021/acssuschemeng.8b05021 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX