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Indolo[3,2‑b]indole-Containing Donor–Acceptor Copolymers for High-Efficiency Organic Solar Cells

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journal contribution
posted on 03.01.2017, 00:00 by Jaeyoung Hwang, Jeonghun Park, Yu Jin Kim, Yeon Hee Ha, Chan Eon Park, Dae Sung Chung, Soon-Ki Kwon, Yun-Hi Kim
The organic solar cell (OSC) performance of a series of new donor–acceptor copolymers containing indolo­[3,2-b]­indole as a key donor block and benzothiadiazole (BT) units with various degrees of fluorination as acceptors is reported. Compared with the simple carbazole unit, the strategically developed indolo­[3,2-b]­indole unit is found to significantly extend π-conjugation and thus increase the intermolecular interactions of the resulting copolymer, as probed by density functional theory calculations, photophysical studies, and structural/morphological analyses. In addition, fluorination of BT can facilitate nanostructuring of the copolymers, mainly due to further planarization of the backbone, which leads to apparently higher hole/electron charge carrier mobilities. The OSC properties of this series of new copolymers blended with fullerene show a strong dependence on the fine and continuous fibrous nanostructure of the blend film. The indolo­[3,2-b]­indole-based copolymer with singly fluorinated BT units possesses optimal intermolecular interactions and achieves the highest power conversion efficiency of 8.84% under AM 1.5G illumination. This result shows the potential of π-extended carbazole moieties for achieving high-performance OSCs with many of the favorable properties induced by large heteroacene blocks.