Using Cyclopenta[2,1‐b:3,4‐c′]dithiophene‐4‐one as a Building Block for Low‐Bandgap Conjugated Copolymers Applied in Solar Cells

A novel electron‐accepting unit cyclopenta2,1‐b:3,4‐c′]dithiophene‐4‐one (CPDTO‐c′), which is an isomer of CPDTO‐b′ was developed. CPDTO‐c′ can be incorporated into the D–A backbone through 5, 7 positions. The 2 position of CPDTO‐c′ can be easily functionalized with an electron‐withdrawing chain. By copolymerizing CPDTO‐c′ with four different donor units: benzo1,2‐b:4,5‐b′]dithiophene (BDT), dithieno3,2‐b:2′,3′‐d]silole (DTS), carbazole, and fluorene, four new conjugated copolymers P1 – P4 were obtained. All these polymers have good solubility and low‐lying HOMO energy levels (?5.41 ～ ?5.92 eV). Among them, P1 and P2 exhibit broad absorption and narrow optical bandgaps of 1.91 and 1.72 eV, respectively. Solar cells based on P1 /PC₇₁BM afforded a PCE up to 2.72% and a high V_oc up to ～0.9 V.