Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
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Phthalimide-Based Wide Bandgap Donor Polymers for Efficient NonFullerene Solar Cells Jianwei Yu,†,‡ Jie Yang,† Xin Zhou,† Simiao Yu,‡ Yumin Tang,† Hang Wang,†,‡ Jianhua Chen,† Shiming Zhang,*,‡ and Xugang Guo*,† †
Department of Materials Science and Engineering and The Shenzhen Key Laboratory for Printed Organic Electronics, South University of Science and Technology of China, No. 1088, Xueyuan Road, Shenzhen, Guangdong 518055, China ‡ Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China S Supporting Information *
ABSTRACT: Organic solar cells (OSCs) have achieved rapid progress, recently, due to the breakthrough of using fused-ring electron acceptors (FREAs), which show broad absorption and narrow bandgap compared to fullerene derivatives. To further improve the device performance of OSCs, it is highly desired to develop suitable donor polymers which feature complementary absorption and favorable energy levels to match the non-fullerene acceptors. We report here the synthesis of two phthalimide-based wide bandgap polymers TPhI-BDT and TffPhI-DBT. The TffPhIBDT is based on a new electron acceptor unit, difluorophthalimide (ffPhI). The fluorine addition leads to TffPhI-DBT with comparable absorption but lower-lying frontier molecular orbitals versus the non-fluorinated analogue TPhI-BDT. When incorporated into non-fullerene OSCs, polymer TPhI-BDT shows a PCE of 8.31% with a Voc of 0.90 V, a Jsc of 14.07 mA cm−2, and a FF of 66.0%. The fluorine-containing analogue polymer TffPhI-BDT exhibits an improved PCE of 9.48% with a larger Voc of 0.93 V, a Jsc of 15.92 mA cm−2, and a FF of 63.9%. The performance improvement of TffPhI-BDT is mainly attributed to its lower-lying FMOs and improved charge transport characteristics. The results demonstrate that phthalimides are highly promising building blocks for enabling wide bandgap polymers, and fluorine addition leads to polymer TffPhI-DBT with further optimized electrical properties for applications in non-fullerene solar cells.
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INTRODUCTION In the past two decades, organic solar cells (OSCs) have attracted a great deal of attention due to their distinctive advantages over the conventional inorganic-based solar cells, including light weight, mechanical flexibility/strechability, and device manufacturing in a large area with a cost-effective fashion.1−3 In order to form p−n junction, OSCs are typically fabricated using a bulk heterojunction (BHJ) structure with pand n-type organic semiconductors as the electron donor and electron acceptor, respectively.3−6 Fullerene derivatives have been widely used as electron acceptor materials, and power conversion efficiencies (PCEs) greater than 11% have been achieved in polymer:fullerene solar cells due to the high electron mobility and good energy levels of frontier molecular orbitals (FMOs) of fullerene derivatives, such as [6,6]-phenylC61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71butyric acid methyl ester (PC71BM).7−9 Despite the distinctive advantages, the fullerene acceptors suffer from several intrinsic drawbacks, especially weak absorption of visible light and limited tunability of FMO energy levels.10−12 As a result, the polymer:fullerene OSCs showing the state-of-the-art performance typically feature a narrow bandgap polymer for maximizing absorption of solar irradiation,13−15 but it cannot cover © XXXX American Chemical Society
the wide range of solar spectrum due to the intrinsic requirements of energy levels of FMOs, hence limiting the PCEs in fullerene-based solar cells. In order to maximize solar absorption, various n-type nonfullerene alternatives have been synthesized and incorporated into OSCs,16−18 but showing limited success until a major breakthrough achieved recently by Zhan et al.19 Specifically, the fused-ring electron acceptors (FREAs) possess well-tailored FMOs, substantial electron mobility, and strong absorption in the wavelength range of 600−800 nm.11,19−21 The emergence of FREAs open unprecedented opportunities for organic solar cells. The narrow bandgap FREAs can effectively absorb a solar spectrum in long wavelength, which thus requires the donor polymer to absorb the solar light in short wavelength to result in complementary absorption. In addition, the interactions between donors and acceptors and film morphologies play important roles in achieving high-performance OSCs.22−26 Hence, it is important not only to design high-performance non-fullerene acceptors but also to develop matching donor Received: September 10, 2017 Revised: November 1, 2017
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DOI: 10.1021/acs.macromol.7b01958 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
can promote various noncovalent interactions, such as F···S and F···H, which should improve polymer backbone planarity, increase film crystallinity, and hence facilitate charge transport.52−56 Therefore, for non-fullerene OSCs, fluorination of polymer should also be an effective approach to enhance the solar cell performance. Among various electron donor (D) counits, benzo[1,2-b:4,5-b′]-dithiophene (BDT) is the most promising one to construct D-π−A-π type copolymers39,43,57 due to its solubilizing capability, electronic structure, and twodimensional conjugation with conjugated side chains.58 In this work, we designed and synthesized a new difluorinated phthalimide (ffPhI); then both ffPhI and non-fluorinecontaining phthalimide (PhI) were coupled with thiophene πbridges to afford two intermediates, TffPhI and TPhI (Scheme 1b), which were copolymerized with BDT to yield two wide bandgap polymers TffPhI-BDT and TPhI-BDT (Scheme 1a) for application in non-fullerene OSCs, respectively. The absorption onsets of TPhI-BDT and TffPhI-BDT are located at 618 and 612 nm with the corresponding bandgaps of 2.00 and 2.03 eV, respectively, thus resulting in complementary absorption with that of narrow bandgap non-fullerene acceptor, IDIC (Scheme 1a), showing absorption from 350 to 800 nm. After incorporating F atoms into polymers, TffPhI-BDT exhibits a deeper HOMO (−5.39 eV) than that (−5.25 eV) of TPhI-BDT. When blended with IDIC, TPhI-BDT based OSCs show a PCE of 8.31% with a Voc of 0.90 V and a Jsc of 14.07 mA/cm2. Compared to TPhI-BDT, F containing TffPhIBDT-based solar cells exhibit a larger Voc of 0.93 V and a higher Jsc of 15.92 mA/cm2 with an increased PCE up to 9.48%. The results indicate that phthalimide-based polymers are promising wide bandgap materials for efficient non-fullerene OSCs.
polymers with complementary absorption, well-tailored FMOs, favorable film morphology, and good charge transport characteristics.27 Because of their absorption in short wavelength range, wide bandgap polymers have been regarded as promising donor materials to enhance the photovoltaic performance of OSCs when blended with narrow bandgap non-fullerene acceptors.23,28−31 In addition, wide bandgap polymers typically possess low-lying highest occupied molecular orbitals (HOMOs) and high-lying lowest unoccupied molecular orbitals (LUMOs), which are beneficial to exciton dissociation and maximize open circuit voltages (Vocs).32−34 Therefore, a variety of polymer donors with wide bandgaps (Egs) and suitable FMO levels are developed in the past two years, exemplified by J71,35 PBDB-T-SF,36 PTFBDT-BZS,37 and PvBDTTAZ23 with Egs in the range of 1.8−2.0 eV,38−42 which achieve PCEs > 10% in fullerene-free OSCs. To develop high-performance wide bandgap donor polymers, the key is design and synthesis of weak electron acceptors with good solubilizing capability and optimized electrical properties. To date, 5,6-difluoro-2-alkyl2H-benzo[d][1,2,3]triazole (FTAZ),43 benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD),38 and 6,7-difluoro-2,3-bis(3alkoxyphenyl)quinoxaline (Qx-2F)44 (Figure 1) are among the most promising electron acceptor units to build wide bandgap polymers for non-fullerene solar cells.31
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RESULTS AND DISCUSSION Materials Synthesis. The synthesis of the new difluorinated phthalimide unit (ffPhI) is straightforward as depicted in Scheme 1b, and the synthetic details can be found in the Supporting Information. Before this approach, we first tried to halogenate the commercially available 5,6-difluoroisobenzofuran-1,3-dione directly in oleum, which was used for bromination of non-fluorinated phthalimide.46 However, it was found that the yield is quite low ( 1 V and High Efficiency > 10% in Fullerene-Free Polymer Solar Cells via Energy Driver. Adv. Mater. 2017, 29 (11), 1605216. (34) Yang, L.; Zhang, S.; He, C.; Zhang, J.; Yao, H.; Yang, Y.; Zhang, Y.; Zhao, W.; Hou, J. New Wide Band Gap Donor for Efficient Fullerene-Free All-Small-Molecule Organic Solar Cells. J. Am. Chem. Soc. 2017, 139 (5), 1958−1966. (35) Bin, H.; Gao, L.; Zhang, Z.-G.; Yang, Y.; Zhang, Y.; Zhang, C.; Chen, S.; Xue, L.; Yang, C.; Xiao, M.; Li, Y. 11.4% Efficiency nonfullerene polymer solar cells with trialkylsilyl substituted 2Dconjugated polymer as donor. Nat. Commun. 2016, 7, 13651. (36) 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 (21), 7148−7151.
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DOI: 10.1021/acs.macromol.7b01958 Macromolecules XXXX, XXX, XXX−XXX
