Wide-bandgap donor polymers based on a dicyanodivinyl indacenodithiophene unit for non-fullerene polymer solar cells

A wide-bandgap polymer donor with improved efficiency plays an important role in improving the photovoltaic performance of polymer solar cells (PSCs). In this study, two novel wide-bandgap polymer donors, PBDT and PBDT-S, were designed and synthesized based on a dicyanodivinyl indacenodithiophene (IDT-CN) moiety, in which benzo[1,2-b:4,5-b']dithiophene (BDT) building blocks and IDT-CN are used as electron-sufficient and -deficient units, respectively. In our study, the PBDT and PBDT-S polymer donors exhibited similar frontier-molecular-orbital energy levels and optical properties, and both copolymers showed good miscibility with the widely used narrow-bandgap small molecular acceptor Y6. Non-fullerene polymer solar cells (NF-PSCs) based on PBDT:Y6 exhibited an impressive power conversion efficiency of 10.04% with an open circuit voltage of 0.88 V, a short-circuit current density of 22.16 mA cm^-2 and a fill factor of 51.31%, where the NF-PSCs based on PBDT-S:Y6 exhibited a moderate power conversion efficiency of 6.90%. The enhanced photovoltaic performance, realized by virtue of the improved short-circuit current density, can be attributed to the slightly enhanced electron mobility, higher exciton dissociation rates, more efficient charge collection and better nanoscale phase separation of the PBDT-based device. The results of this work indicate that the IDT-CN unit is a promising building block for constructing donor polymers for high-performance organic photovoltaic cells.