Synthesis of Alkyl‐Substituted Quinoxaline‐Based Copolymers Along with Photophysical Property Modulation for Polymer Solar Cells

• Published in: Macromolecular chemistry and physics 2018-08, Vol.219 (15), p.n/a
• Main Authors: Kang, Yeongkwon, Agneeswari, Rajalingam, Lee, Jihoon, Kwon, Ji Hyeon, Tamilavan, Vellaiappillai, Park, Seong Soo, Park, Sung Heum, Jin, Youngeup
• Format: Article
• Language: English
• Subjects: BHJ solar cells; Chemical synthesis; Copolymerization; Copolymers; Donor materials; Energy conversion efficiency; Energy levels; fluorine‐based copolymers; Molecular orbitals; Photovoltaic cells; polymer solar cells; Polymers; quinoxaline‐based copolymers; random copolymers; Solar cells; Substitutes; Thermal stability
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.201800117

A series of alkyl‐substituted quinoxaline‐based copolymers are prepared and their properties are studied. Stille copolymerization of 2,5‐bis(trimethylstannyl)thiophene (M3) with different ratios of 2,7‐dibromo‐9,9‐dioctyl‐9H‐fluorene (M4) and 5,8‐dibromo‐6,7‐difluoro‐2,3‐didodecylquinoxaline (M1) affords five new random copolymers, labeled P1–P5. Suzuki copolymerization of 4,5,5‐tetramethyl‐2‐[2‐(4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolan‐2‐yl)‐9,9‐dioctyl‐9H‐fluoren‐7‐yl]‐1,3,2‐dioxaborolane (M5) and 5,8‐bis(5‐bromothiophen‐2‐yl)‐2,3‐didodecyl‐6,7‐difluoroquinoxaline (M2) yielded a new alternating copolymer, labeled P6. All copolymers show high thermal stability, and the 5% weight loss temperature is above 400 °C. The estimated optical bandgap (E g) values of random copolymers P1–P5 (E g ≈ 1.93, 1.97, 1.97, 2.02, and 2.08 eV, respectively) are found to be relatively lower than that of alternating copolymer P6 (E g ≈ 2.14 eV). All copolymers P1–P6 exhibit deep highest occupied molecular orbital (HOMO) energy levels, and the determined HOMO levels are −5.65, −5.67, −5.67, −5.60, −5.59, and −5.66 eV, respectively. The maximum power conversion efficiencies of polymer solar cells made with individual copolymers P1–P6 as a donor materials and PC70BM as an acceptor are 3.98, 2.91, 3.33, 3.55, 3.07, and 2.78%, respectively. A series of 2,3‐didodecyl‐6,7‐difluoroquinoxaline‐based copolymers incorporating different ratios of 9,9‐dioctyl‐9H‐fluorene on their main chain are prepared, and their optical, electrochemical, and photovoltaic properties are briefly studied.