Effects of the Number of Bromine Substitution on Photovoltaic Efficiency and Energy Loss of Benzo[1,2-b:4,5-b′]diselenophene-based Narrow-Bandgap Multibrominated Nonfullerene Acceptors

In this work, three near-infrared (NIR) absorption nonfullerene small-molecule acceptors (NF-SMAs) (BDSeIC, BDSeIC2Br, and BDSeIC4Br) based on a fused benzo[1,2-b:4,5-b ']diselenophene unit as the electron-rich central core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) without or with one or two bromine substituents as the electron-deficient group have been synthesized for polymer solar cells. Compared to BDSeIC without bromine substitution, these multibrominated materials BDSeIC2Br and BDSeIC4Br exhibit lower energy levels, stronger absorption in the range of 500-900 nm, better crystalline quality, and enhanced electron mobility. The optimal BDSeIC2Br-based devices with PM6 as the donor, achieved a high power conversion efficiency (PCE) of up to 12.5% with a relatively low energy loss (E-loss) of 0.52 eV. The PCE of 12.5% for the BDSeIC2Br-based devices are much higher than those devices based on PM6:BDSeIC (7.1%) or PM6:BDSeIC4Br (9.6%) blend films, and it is the highest reported PCE in binary PSCs with the brominated INCN end-capped NF-SMAs. Such outstanding PCE of BDSeIC2Br-based device is attributed to more balanced electron/hole mobility, higher charge dissociation and charge collection efficiency, and more proper phase separation features. These results indicate that introducing a benzo[1,2-b:4,5-b ']diselenophene core unit and bromine substiution on the end groups is an effective way to achieve high-performance NIR absorption NF-SMAs.