Rational design of (D-A) copolymers towards high efficiency organic solar cells: DFT and TD-DFT study.

In this work, we focus on designing a donor copolymer for the improvement of photovoltaic performance. Using density functional theory and time-dependent density functional theory, we investigated the electronic, optical and charge transfer properties of a series of new designed copolymers based on the reported copolymer Pa which is composed of a donor fluorene unit and an acceptor 4,7-dithien-2-yl-2,1,3-benzothiadiazole. We first obtained two copolymers Pb and Pc by replacing the benzothiadiazole (BTZ) with two different strong acceptors units to decrease the LUMO level of conjugated polymers. Then, we designed Pa, Pb and Pc copolymers by adding a substituent methyl group to the thiophene spacer unit (T). Bulk-heterojunction photovoltaic cells were designed with the copolymers as the donors and PCBM as the acceptor. Our results show that the cells based on Pb and Pc have a suitable electronic structure with energy conversion efficiency exceeding 10%. Moreover, we used Marcus theory to evaluate the intermolecular charge transfer (inter-CT) and recombination (inter-CR) rates of these cells (copolymer/PCBM). The ratio K/K of Pc/PCBM heterojunction is about 10 times higher than that of Pb/PCBM which clearly reveals that the designed donor molecule Pc will be a promising candidate for high performance organic photovoltaic devices. Our strategy to design novel donor copolymers provides a theoretical guideline for further improving in electrical, optical properties and the efficiency of the photovoltaic device.