Effect of thiophene, 3-hexylthiophene, selenophene, and Thieno[3,2-b] thiophene spacers on OPV device performance of novel 2,1,3- benzothiadiazole based alternating copolymers Cansu Zeytun Karaman a , Seza Göker b , Ümmügülsüm S ¸ ahin a , Serife O. Hacioglu c , Sultan Tas ¸ kaya Aslan a , Tuğba Hacıefendioğlu a , Gonul Hizalan d,e , Erol Yıldırım a,f,g , Ali Çırpan a,e,f,g , Levent Toppare a,e,g,⇑ a Chemistry Department, Middle East Technical University, Ankara 06800, Turkey b Solid Propellant Department, Roketsan Missiles Inc., Ankara 06500, Turkey c Department of Engineering Basic Sciences of Engineering, Faculty of Engineering and Natural Sciences, Iskenderun Technical University, Hatay 31200, Turkey d ODTÜ-GÜNAM, Middle East Technical University, 06800 Ankara, Turkey e The Center for Solar Energy Research and Application (GUNAM), Middle East Technical University, 06800 Ankara, Turkey f Department of Micro and Nanotechnology, Middle East Technical University, 06800 Ankara, Turkey g Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey ARTICLE INFO Keywords: Benzothiadiazole Fluorene Organic bulk heterojunction solar cell Suzuki cross coupling reaction ABSTRACT Four novel alternating copolymers bearing 5-fluoro-6-((2-octyldodecyl)oxy)benzo[c][1,2,5]thiadiazole as a strong acceptor unit and 9,9-dioctylfluorene as a strong donor unit with bridging units namely, thienothio- phene, selenophene, 3-hexylthiophene, and thiophene were designed and synthesized. The polymers were characterized via 1 H NMR spectroscopy, and weight average molecular weights were reported via gel perme- ation chromatography (GPC). For synthesized novel polymers, the bulk heterojunction solar cells were con- structed. Besides, the effects of bridging units on electronic, optical, photovoltaic, and morphological properties were investigated. Among the polymers, the thienothiophene containing polymer P1 exhibited the highest PCE as 4.25% under the illumination of AM 1.5 G with 100 mW/cm 2 . 1. Introduction A couple decades ago, polymers were known as a good insulator until conductive polyacetylene was discovered [1]. With this discov- ery, both academia and industry show tremendous interest in conduct- ing polymers. Conjugated polymers (CPs) are macromolecules that alter single and double bonds along their skeleton, and they show properties like low-cost, lightweight, solution processability, and flex- ibility [2]. Therefore, CPs have found many application areas such as biosensors [3], electrochromic devices (EDCs) [4], organic light-emit- ting diodes (OLED) [5], organic solar cells (OPV) [6], and organic field-effect transistors (OFETs) [7]. In order to control the band gap, which plays a pivotal role in the optoelectronic properties of the con- jugated polymers, different approaches are put into practice. The donor–acceptor (D-A) approach is the most effective one among the effects of interchain, planarity, bond-length alternation, and resonance stabilization. For this purpose, different acceptors are introduced into literature like benzo[c][1,2,5]triazole (BTz) [8], benzo[c][1,2,5]thia- diazole (BT) [9], benzo[c][1,2,5]selenadiazole (BS) [10], quinoxaline [11], and isoindigo [12]. BT is one of the most preferred acceptor moi- eties due to favoring narrow band gaps. It has a strong electron-with- drawing character, which reveals its rigid, planar skeleton containing two imines (-C@N-) bonds. Besides, the dominant quinoid form makes contributions to lower the band gap [13]. Incorporation of substituents like fluorine atoms increases the BT moiety's electron-withdrawing character due to being the most electronegative atom in the periodic table. In addition, the fluorine atom can make noncovalent interac- tions such as C-F···H and F···S, and its small size reduces the steric hin- drance. For the OPV devices, fluorinated BT moiety shows a higher short circuit current and power conversion efficiency, and by lowering the HOMO and LUMO levels exhibits good thermal and oxidative sta- bilities [14]. The main drawback of the BT moiety is the low solubility. The incorporation of the alkoxy unit containing bulk branched alkyl chains increases the solubility of polymers bearing this unit [15]. Therefore, BT is one of the strongest acceptors to obtain low band gap CP. In the past decade, fluorene derivatives are the most https://doi.org/10.1016/j.jelechem.2021.115483 Received 11 May 2021; Received in revised form 18 June 2021; Accepted 18 June 2021 Available online 23 June 2021 1572-6657/© 2021 Elsevier B.V. All rights reserved. ⇑ Corresponding author at: Chemistry Department, Middle East Technical University, Ankara 06800, Turkey. E-mail address: toppare@metu.edu.tr (L. Toppare). Journal of Electroanalytical Chemistry 895 (2021) 115483 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem