A weak electron acceptor, 4,9-bis(5-bromothiophen-2-yl)-2-(2-ethylhexyl)-benzo[f]isoindole-1,3-dione (BIDO) was designed and synthesized. Polymer P1 derived from BIDO and 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester by Suzuki cross-coupling reaction has a band gap of 2.41 eV. To expand the absorption range, a different amount of BIDO was copolymerized with a diketopyrrolopyrrole monomer to produce a series of copolymers P_a–c. The optical properties, electrochemical behavior, and energy levels of these four copolymers were investigated. The photovoltaic properties copolymers P_a–c were studied. A photovoltaic device containing P_c and [70]PCBM with a ratio of 1:2 had a power conversion efficiency of 1.17%.

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