Enhancing the photovoltaic properties of low bandgap terpolymers based on benzodithiophene and phenanthrophenazine by introducing different second acceptor units

Two novel donor-acceptor (D-A) type random conjugated terpolymers PBDTT-PPzIID and PBDTT-PPzDPP were synthesized by copolymerizing electron-rich alkylthienyl benzodithiophene (BDTT) and two different electron-deficient units, in which phenanthrophenazine (PPz) was the first acceptor unit, and isoindigo (IID) or diketopyrrolopyrrole (DPP) with a strong electron-withdrawing ability was the second acceptor unit, respectively. Compared to their parent polymer PTTPPz-BDTT, a broadening of the band in the long wavelength region from 500 to 850 nm and a significant decrease in HOMO levels were observed for the random terpolymers. Ca and PFN-Br were used as interfacial layers to optimize the devices, respectively. Pleasingly, after replacing Ca/Al with PFN-Br/Al as the cathode, the performance of both polymer devices improved greatly. A maximum power conversion efficiency (PCE) of 5.91% was obtained with a V_oc of 0.73 V, a J_sc of 11.75 mA cm^-2 and a FF of 68.8% in the PBDTT-PPzDPP/PC₇₁BM-based device, while the PBDTT-PPzIID/PC₇₁BM-based device achieved a PCE of 4.21%. To the best of our knowledge, this is one of the highest recorded FF values among PSCs based on previously reported phenazine copolymers.