Wide-Bandgap Conjugated Polymers Based on Alkylthiofuran-Substituted Benzo[1,2- b:4,5- b ′]difuran for Efficient Fullerene-Free Polymer Solar Cells

The alkythiofuran-based benzo[1,2-b:4,5-b′]difuran (BDFS) unit presents the planar and rigid structure with favorable inter/intramolecular interactions and is expected to be a promising building block to construct the highly efficient photovoltaic polymers. Herein, we reported two novel wide-bandgap conjugated polymers, PBDFS-Bz and PBDFS-fBz, which were based on BDFS and benzo[d][1,2,3]triazole (Bz) units, and investigated their photovoltaic performances in fullerene-free polymer solar cells (PSCs). PBDFS-Bz and PBDFS-fBz show similar optical properties with the wide bandgaps of ∼1.88 eV. The HOMO energy level of PBDFS-Bz is at -5.30 eV, whereas it is downshifted to -5.45 eV for PBDFS-fBz due to the strong electron-withdrawing fluorine substitutes. The optimized fullerene-free PSC based on PBDFS-Bz:ITIC achieved a power conversion efficiency (PCE) of 8.07% with a V_oc of 0.82 V, a J_sc of 15.14 mA cm^-2, and a FF of 65%. Under the same conditions, PBDFS-fBz:ITIC device won a promising PCE of 9.00% with an enhanced V_oc of 0.88 V, a J_sc of 15.26 mA cm^-2, and a FF of 67.0%. It is found that the improved photovoltaic performance of PBDFS-fBz:ITIC device is mainly due to the enhanced V_oc in comparison with that of PBDFS-Bz:ITIC device. To the best of our knowledge, the photovoltaic performances are among the best devices reported for fullerene-free PSCs with BDF polymers as donors. These results demonstrate that the rational design of BDF building block is highly important in obtaining the state-of-the-art photovoltaic performances.