Synthesis of two-dimensional π-conjugated polymers pendent with benzothiadiazole and naphtho[1,2-c：5,6-c]bis[1,2,5]thiadiazole moieties for polymer solar cells

Four new 2D donor-acceptor conjugated polymers were designed and synthesized. These new polymers comprised fluorenealt-triphenylamine or carbazole-alt-triphenylamine as the backbones, and pendants with 2,1,3-benzothiadiazole （BT） or naph- tho[1,2-c：5,6-c]bis[1,2,5]thiadiazole （NT） in a triphenylamine unit as the side groups. By changing the acceptor BT for a stronger electron-withdrawing unit of NT moiety in the side chain, the energy levels, absorption spectra, band gaps, and charge-transport abilities of the resultant polymers could be effectively tuned. Bulk heterojunction solar cells with these polymers as the electron donors and （6,6）-phenyl-C71-butyric acid methyl ester as the electron acceptor exhibited high open-circuit voltage （more than 0.8 eV）. The power conversion efficiency can be improved from 1.37% to 3.52% by replacing the BT with an NT moiety, which indicates that introducing NT as the side-chain building block can be an effective strategy to construct efficient 2D conjugated polymers for PSCs.