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From Isoindigo to Dibenzonaphthyridinedione: A Building Block for Wide-Bandgap Conjugated Polymers with High Power Conversion Efficiency

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journal contribution
posted on 30.08.2016 by Mian Cai, Xichang Bao, Xiao Wang, Huanrui Zhang, Meng Qiu, Renqiang Yang, Chunming Yang, Xiaobo Wan
Most wide-bandgap (WBG) conjugated polymers with Eg > 2.2 eV exhibit low power conversion efficiency (PCE) due to their limited absorption window. Here we report the synthesis of a novel tetracyclic fused building block dibenzonaphthyridinedione (DBND) from isoindigo and its application as an acceptor building block for wide-bandgap copolymers with improved PCE. The Stille copolymerization of this building block with 5,5′-bis­(trimethylstannyl)-2,2′-bithiophene (2T) and (E)-1,2-bis­(5-(trimethylstannyl)­thiophen-2-yl)­ethane (TVT) results in two WBG polymers PDBND-2T (Eg 2.32 eV) and PDBND-TVT (Eg 2.23 eV), respectively. Both polymers act as excellent donors in high-performance organic solar cells (OSCs). When blended with phenyl-C71-butyric acid methyl ester (PC71BM), PDBND-2T based OSCs exhibit a PCE of 5.75%, which makes it the broadest bandgap OSCs with PCE over 5%. PDBDN-TVT based OSCs featured a high PCE up to 6.32%. Such efficiency is the highest reported to date for a conjugated polymer at such a broad bandgap. Moreover, without additives or annealing process, PDBND-TVT based OSCs exhibit an efficiency around 6.0% with a thick active layer (240 nm) and the performance shows little sensitivity to polymer:PC71BM weight ratios (range from 1:1.5 to 1:3), which makes PDBND-TVT a potential material for processable large-area tandem or ternary OSCs.