Phthalimide-Based High Mobility Polymer Semiconductors for Efficient Nonfullerene Solar Cells with Power Conversion Efficiencies over 13%

Highly efficient nonfullerene polymer solar cells (PSCs) are developed based on two new phthalimide-based polymers phthalimide-difluorobenzothiadiazole (PhI-ffBT) and fluorinated phthalimide-ffBT (ffPhI-ffBT). Compared to all high-performance polymers reported, which are exclusively based on benzo[1,2-b:4,5-b′]dithiophene (BDT), both PhI-ffBT and ffPhI-ffBT are BDT-free and feature a D-A₁-D-A₂ type backbone. Incorporating a second acceptor unit difluorobenzothiadiazole leads to polymers with low-lying highest occupied molecular orbital levels (≈−5.6 eV) and a complementary absorption with the narrow bandgap nonfullerene acceptor IT-4F. Moreover, these BDT-free polymers show substantially higher hole mobilities than BDT-based polymers, which are beneficial to charge transport and extraction in solar cells. The PSCs containing difluorinated phthalimide-based polymer ffPhI-ffBT achieve a substantial PCE of 12.74% and a large V_oc of 0.94 V, and the PSCs containing phthalimide-based polymer PhI-ffBT show a further increased PCE of 13.31% with a higher J_sc of 19.41 mA cm⁻² and a larger fill factor of 0.76. The 13.31% PCE is the highest value except the widely studied BDT-based polymers and is also the highest among all benzothiadiazole-based polymers. The results demonstrate that phthalimides are excellent building blocks for enabling donor polymers with the state-of-the-art performance in nonfullerene PSCs and the BDT is not necessary for constructing such donor polymers.