Synthesis and Characterization of New Selenophene-Based Conjugated Polymers for Organic Photovoltaic Cells Woo-Hyung Lee, 1 Sang Kyu Lee, 2 Seon Kyoung Son, 3 Ji-Eun Choi, 1 Won Suk Shin, 2 Kyoungkon Kim, 3 Soo-Hyoung Lee, 4 Sang-Jin Moon, 2 In-Nam Kang 1 1 Department of Chemistry, The Catholic University of Korea, Bucheon 420-743, Korea 2 Energy Materials Research Division, Korea Research Institute of Chemical Technology, Daejeon, Republic of Korea 3 Solar Cell Research Center, Korea Institute of Science and Technology, Seoul, Republic of Korea 4 School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju, Republic of Korea Correspondence to: I.-N. Kang (E-mail: inamkang@catholic.ac.kr) Received 15 September 2011; accepted 16 October 2011; published online 11 November 2011 DOI: 10.1002/pola.25064 ABSTRACT: Three new polymers poly(3,4 000 -didodecyl) hexasele- nophene) (P6S), poly(5,5 0 -bis(4,4 0 -didodecyl-2,2 0 -biselenophene- 5-yl)-2,2 0 -biselenophene) (HHP6S), and poly(5,5 0 -bis(3 0 ,4-dido- decyl-2,2 0 -biselenophene-5-yl)-2,2 0 -biselenophene) (TTP6S) that have the same selenophene-based polymer backbone but dif- ferent side chain patterns were designed and synthesized. The weight-averaged molecular weights (M w ) of P6S, HHP6S, and TTP6S were found to be 19,100, 24,100, and 19,700 with poly- dispersity indices of 2.77, 1.48, and 1.41, respectively. The UV– visible absorption maxima of P6S, HHP6S, and TTP6S are at 524, 489, and 513 nm, respectively, in solution and at 569, 517, and 606 nm, respectively, in the film state. The polymers P6S, HHP6S, and TTP6S exhibit low band gaps of 1.74, 1.95, and 1.58 eV, respectively. The field-effect mobilities of P6S, HHP6S, and TTP6S were measured to be 1.3  10 4 , 3.9  10 6 , and 3.2  10 4 cm 2 V 1 s 1 , respectively. A photovoltaic device with a TTP6S/[6,6]-phenyl C 71 -butyric acid methyl ester (1:3, w/ w) blend film active layer was found to exhibit an open circuit voltage (V OC ) of 0.71 V, a short circuit current (J SC ) of 5.72 mA cm 2 , a fill factor of 0.41, and a power conversion efficiency (PCE) of 1.67% under AM 1.5 G (100 mW cm 2 ) illumination. TTP6S has the most planar backbone of the tested polymers, which results in strong p–p interchain interactions and strong aggregation, leading to broad absorption, high mobility, a low band gap, and the highest PCE. V C 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 50: 551–561, 2012 KEYWORDS: charge transfer; conjugated polymers; functionali- zation of polymers; low band-gap polymer; organic solar cell; selenophene INTRODUCTION In recent years, conjugated polymers have been developed for applications such as polymer light-emit- ting diodes, 1–4 organic thin-film transistors (OTFTs), 5–9 and photovoltaic solar cells. 10–14 Organic solar cells based on conjugated polymers offer potential clean energy sources with the advantages of low-cost device fabrication via solu- tion-based processing and a mechanical flexibility appropri- ate for foldable or rollable substrates. 15–18 Among them, thi- ophene-based conjugated polymers have been the subject of intensive research over recent years as a result of their interesting optical and electronic properties. For example, poly(3-hexylthiophene) (P3HT) exhibits a high charge carrier mobility in OTFTs and high power conversion efficiencies (PCEs) in bulk heterojunction (BHJ) solar cells. However, regioregular head-to-tail (HT) P3HT electronic devices fabri- cated in air generally exhibit much lower performances due to oxygen and water sensitivities that arise because of the relatively high-lying highest occupied molecular orbital (HOMO) levels. To overcome this problem, some groups have developed polythiophene derivatives with low-lying HOMO levels by controlling the effective conjugation length of the polymer and controlling the alkyl side chain position and spacing to achieve better interdigitation. These approaches have effectively improved both their charge carrier mobility and their stability in ambient air. 19 In contrast to their thio- phene analogs, selenophene-based conjugated polymers have been little studied so far because of the lack of suitable syn- thetic methods and reagents. Among many other organic semiconductors, conjugated polymers based on polyseleno- phene have recently become a promising alternative mate- rial. As a class, polyselenophenes are expected to have advantages over polythiophenes. For example, interchain charge transfer should be facilitated by intermolecular Se–Se contact, which might mean that polyselenophenes have higher bulk conductivities and mobilities than polythio- phene. 20 Furthermore, polyselenophenes have a lower band gap than the corresponding polythiophenes. 21 As a result, polyselenophenes can absorb the solar spectrum more V C 2011 Wiley Periodicals, Inc. WWW.MATERIALSVIEWS.COM JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 50, 551–561 551 JOURNAL OF POLYMER SCIENCE WWW.POLYMERCHEMISTRY.ORG ARTICLE