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

Most wide-bandgap (WBG) conjugated polymers with E-g > 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-(trirnethylstannyl)thiophen-2-yl)ethane (TVT) results in two WBG polymers PDBND-2T (E-g 2.32 eV) and PDBND-TVT (E-g 2.23 eV), respectively. Both polymers act as excellent donors in high-performance organic solar cells (OSCs). When blended with phenyl-C-71-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.