Synthesis and Photovoltaic Properties of Thieno[3,4-b]pyrazine or Dithieno[3 0 ,2 0 :3,4;2 00 ,3 00 :5,6]benzo[1,2-d]imidazole-Containing Conjugated Polymers Mukhamed L. Keshtov, 1 Dmitri Yu. Godovsky, 1 Alexei R. Khokhlov, 1,2 Tetsunari Mizobe, 3 Hiroyuki Fujita, 3 Eisuke Goto, 4 Junya Hiyoshi, 3 Saki Nakamura, 3 Susumu Kawauchi, 3 Tomoya Higashihara, 4 Tsuyoshi Michinobu 3 1 A.N. Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences, Vavilova St. 28, 119991, GSP-1, Moscow V-334, Russia 2 Faculty of Physics, Lomonosov Moscow State University, 1-2 Leninskiye Gory, 119991, GSP-1, Moscow, Russia 3 Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8550, Japan 4 Graduate School of Science and Engineering, Yamagata University, 4-3-16, Jonan, Yonezawa, Yamagata 992-8510, Japan Correspondence to: M. L. Keshtov (E- mail: keshtov@ineos.ac.ru); T. Higashihara (E- mail: thigashihara@yz.yamagata-u.ac.jp); T. Michinobu (E - mail: michinobu.t.aa@m.titech.ac.jp) Received 3 October 2014; accepted 28 January 2015; published online 3 March 2015 DOI: 10.1002/pola.27570 ABSTRACT: Novel conjugated polymers composed of benzo[1,2- b:4,5-b 0 ]dithiophene and thieno[3,4-b]pyrazine or dithie- no[3 0 ,2 0 :3,4;2 00 ,3 00 :5,6]benzo[1,2-d]imidazole units are synthe- sized by Stille polycondensation. The resulting polymers display a longer wavelength absorption and well-defined redox activities. The effective intramolecular charge-transfer and energy levels of all polymers are elucidated by computational calculations. Bulk-heterojunction solar cells based on these polymers as p-type semiconductors and [6,6]-phenyl-C 61 - butyric acid methyl ester (PC 61 BM) as an n-type semiconductor are fabricated, and their photovoltaic performances are for the first time evaluated. VC 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 1067–1075 KEYWORDS: charge transport; computer modeling; conducting poly- mers; conjugated polymers; electrochemistry; photochemistry INTRODUCTION Over the past 10 years, polymer solar cells (PSCs) have attracted considerable attention as new alterna- tive renewable energy sources owing to their advantages such as low cost, easy fabrication, light weight, as well as the possibility of obtaining large-area flexible devices. 1,2 Among these devices, bulk-heterojunction solar cells (SCs) with composites of conjugated polymers (p-type semiconduc- tors) and fullerene derivatives (n-type semiconductors) are regarded as one of the most effective systems for the rapid dissociation of excitons and their charge transport toward the electrodes. 3–5 The recent success of this device has largely relied on the development of novel high-performance p-type semiconducting polymers. In comparison with the homopolymers, conjugated copolymers with alternating donor and acceptor units usually exhibit nar- rower band gaps, allowing for absorption of the solar spec- trum in the longer wavelength range. However, it is also important to consider the energy levels. A minimum difference of 0.3–0.4 eV in the lowest unoccupied molecular orbital (LUMO)–LUMO gap between the conjugated polymers and full- erene derivatives is required for effective charge separation at the interface. 6 Also, the difference between the highest occu- pied molecular orbital (HOMO) level of the conjugated poly- mer and the LUMO level of the fullerene derivative determines the maximum open circuit voltage (V oc ) of the SC. 7,8 As a result, the polymer structures must be carefully optimized in consideration of the band gap and energy levels. Recently, conjugated polymers containing fused thiophene units have emerged as a promising p-type semiconductor in polymer field effect transistors (FETs) and SCs. 9 For example, it is known that alternating copolymers of benzo[1,2-b:4,5-b 0 ]dithiophene (BDT) and other conjugated units have a relatively planar main chain p-system, which leads to high hole mobilities and excellent photovoltaic performance. 10,11 Thus, we became interested in the development of novel BDT polymers. We selected thieno[3,4-b]pyrazine and dithieno[3 0 ,2 0 :3,4;2 00 ,3 00 :5,6]benzo Additional Supporting Information may be found in the online version of this article. VC 2015 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 53, 1067–1075 1067 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE