Efficient Organic Ternary Solar Cells Employing Narrow Band Gap Diketopyrrolopyrrole Polymers and Nonfullerene Acceptors

Currently, high-performance organic solar cells (OSCs) are mainly composed of narrow band gap (NBG) nonfullerene acceptors and medium band gap (MBG) polymer donors, whereas the solar cells based on NBG polymer donors and MBG nonfullerene acceptors were much less successful. Herein, we report a new diketopyrrolopyrrole (DPP) polymer (PffBT-DPP) with a band gap of 1.33 eV for use in nonfullerene OSCs. When blended with a fullerene acceptor [6, 6]-phenyl C71 butyric acid methyl ester ([70]PCBM) and an MBG nonfullerene acceptor MeIC, the binary OSCs offered a power conversion efficiency (PCE) of 6.8 and 2.0%, respectively. Interestingly, a much higher PCE of 9.0% was achieved for the PffBT-DPP/[70]PCBM/MeIC ternary OSC, which is the sum of the two binary OSCs. The phase-separated morphology in this ternary OSC was characterized and correlated to the device performance. Moreover, the PCE of the ternary OSC is the best result for OSCs employing polymer donors with an optical band gap of <1.40 eV and nonfullerene acceptors. This work demonstrates the potential of constructing high-performance OSCs by employing NBG polymer donors and MBG nonfullerene acceptors.