Rhodanine side-chained thiophene and indacenodithiophene copolymer for solar cell applications Meijie Fan a, b , Linrui Duan b , Yuanhang Zhou b , Shuguang Wen b , Feng Li c , Deyu Liu b , Mingliang Sun a, * , Renqiang Yang b a Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China b CAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China c Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, China article info Article history: Received 14 April 2017 Received in revised form 10 June 2017 Accepted 9 July 2017 Available online 19 July 2017 Keywords: Polymer solar cells Side chain Indacenodithiophene Rhodanine abstract In this work, 2-(3-ethyl-4-oxothiazolidin-2-ylidene)malononitrile is connected on the b position of thiophene, and this monomer is copolymerized with indacenodithiophene (IDT) to build one novel photovoltaic polymer (PIDTPCR). The highest occupied molecular orbital (HOMO) and the lowest un- occupied molecular orbital (LUMO) level of the polymer is 5.26 eV and 3.45 eV, respectively. Compared with reference polymer without rhodanine side chain, PIDTPCR polymer shows low band gap and low energy level which contribute to the J sc and V oc of the PSCs devices. The best performance parameters of the solar cells devices are V oc (0.91 V), J sc (10.1 mA/cm 2 ), FF (52.4%) and PCE (5.18%). © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Polymer solar cells (PSCs) have drawn a great deal of attention due to their unique features for low cost, easy processing, low weight, flexibility, and large-area fabrication [1e5]. It is an efficient strategy using donoreacceptor (DeA) copolymer to optimize the photovoltaic polymers properties [6,7], because DeA copolymer can form an intramolecular charge transfer (ICT) state which widen the polymer light absorption. The strength of ICT state varies with different donor and accepter units, so the changes of donor and accepter units can optimize polymers photovoltaic performances [8]. Through donor and accepter design, the polymer optical band gap match well with solar spectrum and the energy levels match with that of PCBM, and then the PSCs devices performance can be improved [9]. Therefore, appropriate donor and acceptor units play a very essential role in DeA polymer design. On this basis, many novel donoreacceptor (DeA) conjugated copolymers have been widely developed and applied to PSCs with PCE over 12% [10e14]. Indacenodithiophene (IDT) with highly planar structure has been demonstrated as a promising build block for polymer donor materials because of its extraordinary electron-donating and hole transporting performance [15,16]. IDT based conjugated polymers were widely applied in PSCs and showed broad absorption and high molar extinction coefficient, which is beneficial to obtaining high short circuit current (J sc ) [17,18]. The solubility of IDT polymer can be easily increased by attaching functional side chains due to the effect of sp3-hybridized carbon atoms on the IDT [16,19,20]. Rho- danine, as an electron withdrawing unit, can effectively induce intramolecular charge transfer and improve optical absorption properties. Rhodanine is widely used as end capping group in photovoltaic small molecule, while there is very few report about rhodanine used in photovoltaic polymer [21e23]. As an alternative method to design DeA polymer with donor and acceptor unit both in polymer backbone, electron deficient side chains introduced to polymer backbone can also effectively broaden absorption spec- trum and deepen energy levels [24]. This method has been proved to be successful in many polymer systems [25e28]. * Corresponding author. E-mail addresses: mlsun@ouc.edu.cn (M. Sun), yangrq@qibebt.ac.cn (R. Yang). Contents lists available at ScienceDirect Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/ http://dx.doi.org/10.1016/j.mtener.2017.07.007 2468-6069/© 2017 Elsevier Ltd. All rights reserved. Materials Today Energy 5 (2017) 287e292