Regio-regular alternating diketopyrrolopyrrole-based D-A-D-A terpolymers for the enhanced performance of polymer solar cells.

We designed and synthesized regio-regular alternating diketopyrrolopyrrole (DPP)-based D-A-D-A terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) using a primary donor (D) [3,3'-difluoro-2,2'-bithiophene (F2T2)] and a secondary donor (D) [2,2'-bithiophene (T2), ()-1,2-di(thiophen-2-yl)ethene (TVT), or dithieno[3,2-:2',3'-]thiophene (DTT)]. A PDPP2DT-F2T2 D-A polymer was synthesized as well to compare optical, electronic, and photovoltaic properties. The absorption peaks of the terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) were longer ( = 801-810 nm) than the peak of the PDPP2DT-F2T2 polymer ( = 799 nm), which is associated with the high-lying HOMO levels of the terpolymers (-5.08 to -5.13 eV) compared with the level of the PDPP2DT-F2T2 polymer (-5.38 eV). The photovoltaic properties of these DPP-based polymers were investigated under simulated AM 1.5G sunlight (100 mW cm) with a conventional structure (ITO/PEDOT:PSS/polymer:PCBM/Al). The open-circuit voltages ( ) of photovoltaic devices containing the terpolymers were slightly lower (0.68-0.70 V) than the of the device containing the PDPP2DT-F2T2 polymer (0.79 V). The short-circuit current ( ) of the PDPPF2T2DPP-DTT device was significantly improved (14.14 mA cm) compared with that of the PDPP2DT-F2T2 device (8.29 mA cm). As a result, the power conversion efficiency (PCE) of the PDPPF2T2DPP-DTT device (6.35%) was increased by 33% compared with that of the simple D-A-type PDPP2DT-F2T2 device (4.78%). The highest and PCE values (the PDPPF2T2DPP-DTT device) were attributed to an optimal nanoscopically mixed morphology and strong interchain packing with a high face-on orientation in the blend film state. The study demonstrated that our strategy of using multiple donors in a regio-regular alternating fashion could fine-tune the optical, electronic, and morphological properties of D-A-type polymers, enhancing the performance of polymer solar cells.