Synthesis and Characterization of Isoindigo-Based Polymers Using CH-Arylation Polycondensation Reactions for Organic Photovoltaics Walaa Elsawy, 1,2 Hongkyu Kang, 1 Kilho Yu, 1 Ahmed Elbarbary, 2 Kwanghee Lee, 1 Jae-Suk Lee 1 1 Department of Nanobio Materials and Electronics and School of Material Science and Engineering, Research Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology (GIST), Gwangju 500–712, Republic of Korea 2 Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt Correspondence to: K. Lee (E - mail: klee@gist.ac.kr) or J.-S. Lee (E - mail: jslee@gist.ac.kr) Received 2 June 2014; accepted 11 July 2014; published online 00 Month 2014 DOI: 10.1002/pola.27328 ABSTRACT: A series of three new low bandgap donor–acceptor– donor–acceptor / (D–A–D–A / ) polymers have been successfully synthesized based on the combination of isoindigo as the electron-deficient acceptor and 3,4-ethylenedioxythiophene as the electron-rich donor, followed by CH-arylation with different acceptors (4,7-dibromo[c][1,2,5]-(oxa, thia, and/or selena)dia- zole (4a-c)). These polymers were used as donor materials for photovoltaic applications. All of the polymers are highly stable and show good solubility in chlorinated solvents. The highest power conversion efficiency of 1.6% was achieved in the bulk heterojunction photovoltaic device that consisted of poly ((E)26-(7-(benzo-[c][1,2,5]-thiadiazol-4-yl)22,3-dihydrothieno- [3,4-b][1,4]dioxin-5-yl)26 0 -(2,3-dihydrothieno-[3,4-b][1,4]-dioxin- 5-yl)21,1 0 -bis-(2-octyldodecyl)-[3,3 0 -biindolinylidene]-2,2 0 -dione) as the donor and PC 61 BM as the acceptor, with a short-circuit current density (J sc ) of 8.10 mA/cm 2 , an open circuit voltage (V oc ) of 0.56 V and a fill factor of 35%, which indicates that these polymers are promising donors for polymer solar cell applications. VC 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 00, 000–000 KEYWORDS: conjugated polymers; polycondensation; synthesis INTRODUCTION Polymer solar cells have received much interest in recent decades, owing to the potential advantages of conjugated polymers in the fabrication as promising mate- rials for the next generation of printable flexible devices, such as solar cells, 1–9 light-emitting diodes, 10,11 field-effect transistors (FETs), 11–14 and sensors. 15 The donor–acceptor (D–A) approach 16 for the rational synthetic design of conju- gated polymers enables the precise tuning of the properties of these organic semiconductors, 6,17 ultimately leading to power conversion efficiencies (PCEs) currently exceeding 9%, with sophisticated control over the bandgaps and energy levels of the photoactive materials, morphological control of the BHJ film, and interface engineering for efficient charge collection by the electrodes. 18–21 Conjugated polymer synthesis has seen significant pro- gress. 22,23 Conjugated polymers are normally synthesized by metal catalyzed step-growth polycondensation 6 and chain- growth polymerization. 24,25 The organometallic nucleophiles can be Grignard reagents (Kumada-Corriu), 26 stannyl (Stille), 27,28 boron reagents (Suzuki-Miyaura), 29 or copper (Sonogashira). 30 However, the most efficient and widely used reactions for the preparation of polymers, Stille and Suzuki coupling, suffer from several drawbacks, such as the difficult synthesis of the metal-activated monomers, the low stability of the involved organometallic reagents, and the formation of a stoichiometric amount of toxic byproducts. In particular, the preparation of stannyl reagents for Stille coupling is syntheti- cally challenging. Moreover, these stannyl monomers are diffi- cult to purify and are unstable under the purification methods used, which affects the quality of the final polymer products. To overcome the drawbacks of these standard coupling meth- ods, the direct arylation reaction warrants further study. Direct arylation, as a new type of cross-coupling reaction, involves the coupling of an aryl halide or pseudohalide with a simple arene, 31–33 allowing the formation of carbon–carbon bonds between aromatic units having activated hydrogen atoms without the use of organometallic intermediates. These reactions have mainly been developed for the synthesis of small Walaa Elsawy and Hongkyu Kang contributed equally to this work. Additional Supporting Information may be found in the online version of this article. VC 2014 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2014, 00, 000–000 1 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE