Bulk-Heterojunction Solar Cells with Benzotriazole-Based Copolymers as Electron Donors: Largely Improved Photovoltaic Parameters by Using PFN/Al Bilayer Cathode

Abstract

Three benzotriazole (BTA)-based conjugated polymers PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, with electron-donating segments of fluorene, carbazole, and dialkoxybenzene, respectively, were successfully synthesized as new polymeric donors in bulk-heterojunction (BHJ) photovoltaic cells (PVCs). All the copolymers exhibited good solubility in common organic solvents and good thermal stability. The optical band gaps for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are 2.24, 2.18, and 1.87 eV, respectively. The HOMO levels for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are −5.67, −5.54, and −5.20 eV, respectively, which were determined by the electron-donating segments. The LUMO levels for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA are −3.43, −3.36, and −3.33 eV, respectively, which were mainly dominated by the BTA unit. For a blend ratio of polymer:PCBM = 1:2, BHJ PVCs with Al cathode displayed power conversion efficiencies (PCE) of 0.9%, 1.51%, and 1.16% for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, respectively. Alcohol-soluble poly[(9,9-dioct...