Regio-regular alternating diketopyrrolopyrrole-based D(1)–A–D(2)–A terpolymers for the enhanced performance of polymer solar cells

We designed and synthesized regio-regular alternating diketopyrrolopyrrole (DPP)-based D(1)–A–D(2)–A terpolymers (PDPPF2T2DPP-T2, PDPPF2T2DPP-TVT, and PDPPF2T2DPP-DTT) using a primary donor (D(1)) [3,3′-difluoro-2,2′-bithiophene (F2T2)] and a secondary donor (D(2)) [2,2′-bithiophene (T2), (E)-1,2-di(thiophen-2-yl)ethene (TVT), or dithieno[3,2-b:2′,3′-d]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 (λ(max) = 801–810 nm) than the peak of the PDPP2DT-F2T2 polymer (λ(max) = 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(−2)) with a conventional structure (ITO/PEDOT:PSS/polymer:PC(71)BM/Al). The open-circuit voltages (V(oc)) of photovoltaic devices containing the terpolymers were slightly lower (0.68–0.70 V) than the V(oc) of the device containing the PDPP2DT-F2T2 polymer (0.79 V). The short-circuit current (J(sc)) of the PDPPF2T2DPP-DTT device was significantly improved (14.14 mA cm(−2)) compared with that of the PDPP2DT-F2T2 device (8.29 mA cm(−2)). 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 J(sc) 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.