Synthesis of regular D–A1–D–A2 copolymers via direct arylation polycondensation and application in solar cells

• Present a simple and effective method to prepare the regular D–A1–D–A2 copolymers for solar cells.
• The synthesized copolymers exhibit broad absorption, narrow band gap and order packing structure.
• The effect of dodecyl substituent on the properties of the copolymers was explored.
• The relationship between the molecular weight and photovoltaic performance in copolymer P1 was revealed.
• This work is expected to be valuable for simple synthesis of the regular D–A1–D–A2 copolymers.

Two regular D–A1–D–A2 copolymers were synthesized via direct arylation polycondensation. In these copolymers, diketopyrrolopyrrole and benzothiadiazole were used as the acceptor units, while oligothiophene was used as the donor unit. Dodecyl group were also introduced into the thiophene moiety to explore the side chain effect on the properties of these copolymers. The optimum direct arylation reaction conditions were explored in order to obtain high molecular weight regular D–A1–D–A2 copolymer. It is found that the dodecyl group on thiophene moiety can improve the coplanarity and conjugation along the main chain of D–A1–D–A2 copolymer. These copolymers exhibit a broad absorption, narrow band gap and high packing order. Such properties are helpful for improving the photovoltaic properties. The relationship between the molecular weight and the photovoltaic property in copolymer P1 was also revealed. As the result, a higher power conversion efficiency of 3.65% was achieved for the copolymer P1 with a molecular weight of 86.1 kDa.

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