Bulk-Heterojunction Solar Cells with Benzotriazole-Based Copolymers as Electron Donors: Largely Improved Photovoltaic Parameters by Using PFN/A1 Bilayer Cathode

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-diocty1-2,7-fluorene)-alt-(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) was selected for cathode modification. For BHJ PVCs with PF-DTBTA and PCz-DTBTA as the donors, open-circuit voltages (V-oc,) of the PVCs could all be elevated with the PFN/Al bilayer cathode, whereas no such improvements were found for PVCs with PPh-DTBTA as the donor. The behaviors could be attributed to the different V-oc losses when using Al cathode. Using PFN/Al bilayer cathode could also improve short-curcuit current and fill factor for the three BTA-based copolymers. The N-N interactions between the BTA-based polymers and the PFN may modify interfacial contact, resulting in enhancement of the electron extraction from the acceptor phase to the cathode and decreasing hole-electron recombination in the active layer. Consequently, PFN/Al bilayer cathode elevated PCE values to 1.3%, 2.75%, and 1.39% for PF-DTBTA, PCz-DTBTA, and PPh-DTBTA, respectively. The most significant increasing of PCE with a calculated value of 80% was found for PCz-DTBTA as the donor, and this might be related to the additional N-N interactions between the carbazole segments and the PFN. The results would supply useful information to understand the contribution of an interfacial layer on the photovoltaic performance. Our results also suggest that the bilayer cathode would have great potential to elevate PCE of BHJ PVCs.