Two-dimensional benzo[1,2-: B:4,5- b ′]difuran-based wide bandgap conjugated polymers for efficient fullerene-free polymer solar cells

Wide bandgap conjugated polymers are important for providing complementary absorption with the state-of-the-art narrow bandgap nonfullerene electron acceptors to maximise the utilization of solar photons. In this work, two wide bandgap two-dimensional conjugated polymers, PBDFT-Bz and PBDFF-Bz, based on benzo[1,2-b:4,5-b′]difuran building block were designed and synthesized. The optical bandgaps of PBDFT-Bz and PBDFF-Bz were found to be 1.90 eV and 1.85 eV, respectively. PBDFF-Bz with 2-ethylhexylfuryl side chains possesses the lower HOMO energy level and shows denser π-π stacking compared to PBDFT-Bz with 2-ethylhexylthienyl side chains. The fullerene-free PBDFF-Bz:ITIC-based polymer solar cell (PSC) showed a PCE of 9.46% with a J_sc of 15.02 mA cm^-2, a V_oc of 0.94 V and a FF of 67%; by using m-ITIC as an electron acceptor, the PCE was further improved to 10.28% with an enhanced J_sc of 16.57 mA cm^-2, a V_oc of 0.94 V and a FF of 66%. Under the same condition, the PBDFT-Bz:m-ITIC device gave a PCE of 9.84% with a J_sc of 16.63 mA cm^-2, a V_oc of 0.85 V and a FF of 70%. In addition, it is exciting that the PBDFF-Bz based devices show a small energy loss (E_loss) of 0.63 eV, while the PBDFT-Bz based devices show an E_loss of 0.72 eV. These results are among the best for fullerene-free devices with BDF-based polymers, and demonstrate that BDF is a very promising candidate for highly efficient polymer solar cells.