Ligand-free direct heteroarylation approach for benzobithiophenedione-based simple small molecular acceptor toward high efficiency polymer solar cells

The development of nonfullerene acceptors has accelerated the improvement in the power conversion efficiencies (PCEs) of polymer solar cells (PSCs) in the past five years. However, synthetic accessibility without organostannanes is of great importance in the commercial application of PSCs. In this work, a ligand-free direct heteroarylation approach was introduced to realize high efficiency simple small molecular nonfullerene acceptors. More specifically, two simple A–D–A′–D–A type nonfullerene acceptors BDDEH-4F and BDDBO-4F with benzodithiophenedione (BDD) as the core moiety were synthesized in three steps without any hazardous organostannanes or any ligands. Despite the different lengths of side chains on the BDD, the simple fused-ring acceptors BDDEH-4F and BDDBO-4F exhibit very similar optical properties and energy levels. With PM6 as the donor, BDDEH-4F with 2-ethylhexyl side chains exhibits a much higher PCE of 12.59% and larger short-circuit current density (Jsc) of 22.57 mA cm−2 because the more balanced carrier mobilities and suitable phase separation result in favorable morphology compared to that of BDDBO-4F with longer alkyl side chains (PCE of 9.80%). Our results indicate that facile direct heteroarylation is an attractive approach to achieve high performance cost-effective nonfullerene solar cells.