Silafluorene-Based Polymers for Electrochromic and Polymer Solar Cell Applications Ozan Erlik, 1 Naime A. Unlu, 1 Gonul Hizalan, 1 Serife O. Hacioglu, 1 Seda Comez, 1 Esra D. Yildiz, 2 Levent Toppare, 1,3,4,5 Ali Cirpan 1,3,5,6 1 Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey 2 Department of Physics, Hitit University, 19030 C ¸orum, Turkey 3 Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey 4 Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey 5 The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University, 06800 Ankara, Turkey 6 Department of Micro and Nanotechnology, Middle East Technical University, 06800 Ankara, Turkey Correspondence to: A. Cirpan (E - mail: acirpan@metu.edu.tr) Received 10 February 2015; accepted 7 March 2015; published online 00 Month 2015 DOI: 10.1002/pola.27625 ABSTRACT: In this study, four novel silafluorene (SiF) and benzo- triazole (Btz) bearing conjugated polymers are synthesized. In the context of electrochemical and optical studies, these poly- mers are promising materials both for electrochromic device (ECD) and polymer solar cell (PSC) applications. All of the poly- mers are ambipolar (both p- and n-dopable) and multichromic. Electrochemistry experiments indicate that incorporation of sele- nophene instead of thiophene unit increases the HOMO energy level of the polymers. Power conversion efficiency of the PSCs reached 1.75% for PTBTSiF, 1.55% for PSBSSiF, 2.57% for PBTBTSiF, and 1.82% for PBSBSSiF. The hole mobilities of the polymers are estimated through space charge limited current (SCLC) model. PBTBTSiF has the highest hole mobility as 2.44 3 10 23 cm 2 Vs 21 . V C 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 00, 000–000 KEYWORDS: conducting polymers; conjugated polymers; elec- trochemistry; polymer solar cell; silafluorene INTRODUCTION The design and realization of novel donor– acceptor (D–A) alternating copolymers has been a topic of great research interest for the past decade for organic elec- tronics and electrochromic device applications. 1–7 Therefore, different D–A segments have been discovered and synthe- sized. 8,9 Combination of different donor and acceptor units allows intramolecular charge transfer (ICT) and energy level (HOMO-LUMO) modulation of donor-acceptor alternating copolymers. 10 Insertion of different bridging atoms on the polymer chain is one of the effective strategies utilized to control optical and electronic properties of conjugated poly- mers. 11 It is envisioned that different heterocyclic arene units such as thiophene, furan, pyrrole, and silole can serve as important building blocks for the development of different donor units. Among other heterocyclic arenes, donor units developed from silole units such as silafluorene and dithie- nosilole have been widely utilized for the development of new conjugated polymers. 12–19 Silole-based donor units exhibited lower band gap and low-lying LUMO energy level due to good overlapping between the r* orbital of the exocy- clic silicon-carbon bonds and p* orbital of the butadiene moiety. Besides, this interaction resulted in high fluorescence efficiency, electron affinity, and excellent electron mobil- ity. 20–27 Recent studies showed that silafluorene (SiF) could be an important building block to control photovoltaic prop- erties of D–A alternating conjugated polymers. SiF-based polymers showed high open circuit voltage due to low-lying HOMO energy level. PCEs up to 6.41% have been reported for SiF-based alternating copolymers. 27 Due to aforemen- tioned properties of silole containing alternating copolymers, we report syntheses, characterization, organic solar cell and electrochromic application of four new silafluorene-based conjugated polymers, namely, PTBTSiF, PSBSSiF, PBTBTSiF, and PBSBSSiF. In order to further tune and optimize the electronic and optical properties, selenophene and thiophene have been incorporated as the p-bridging segments. The most widely used heterocyclic conjugated molecule as a p- bridge unit is thiophene however selenophene has also been used for the past few years. Since the aromaticity of seleno- phene is less than that of the thiophene, quinoid structure is more stable for selenophene compare to the sulfur- containing counterpart. Band gap of selenophene containing Additional Supporting Information may be found in the online version of this article. V C 2015 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2015, 00, 00–00 1 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE