Phthalimide-Based Wide Bandgap Donor Polymers for Efficient Non-Fullerene Solar Cells

Organic solar cells (OSCs) have achieved rapid progress, recently, due to the breakthrough of using fused-ring electron acceptors (FREAs), which show broad absorption and narrow bandgap compared to fullerene derivatives. To further improve the device performance of OSCs, it is highly desired to develop suitable donor polymers which feature complementary absorption and favorable energy levels to match the non-fullerene acceptors. We report here the synthesis of two phthalimide-based wide bandgap polymers TPhI-BDT and TffPhI-DBT. The TffPhI-BDT is based on a new electron acceptor unit, difluorophthalimide (ffPhI). The fluorine addition leads to TffPhI-DBT with comparable absorption but lower-lying frontier molecular orbitals versus the non-fluorinated analogue TPhI-BDT. When incorporated into non-fullerene OSCs, polymer TPhI-BDT shows a PCE of 8.31% with a V_oc of 0.90 V, a J_sc of 14.07 mA cm^-2, and a FF of 66.0%. The fluorine-containing analogue polymer TffPhI-BDT exhibits an improved PCE of 9.48% with a larger V_oc of 0.93 V, a J_sc of 15.92 mA cm^-2, and a FF of 63.9%. The performance improvement of TffPhI-BDT is mainly attributed to its lower-lying FMOs and improved charge transport characteristics. The results demonstrate that phthalimides are highly promising building blocks for enabling wide bandgap polymers, and fluorine addition leads to polymer TffPhI-DBT with further optimized electrical properties for applications in non-fullerene solar cells.