pi-Conjugated polymers, PTOTBT, PTEHTBT, and PTt-BTBT, composed of benzothiadiazole as an electron accepting unit and terthiophene as an electron donating unit in the backbone were prepared. PTOTBT, PTEHTBT, and PTt-BTBT contained side chain groups of n-octyl, 2-ethylhexyl, and t-butyl groups, respectively. Solubility, optical and thermal properties of the polymers showed strong dependences on their side chain groups. PTEHTBT having 2-ethylhexyl groups in the side chain exhibited absorption maximum (lambda(max)) at longer wavelength (565 nm) than PTOTBT (534 nm) and PTt-BTBT (495 nm). PTOTBT showed higher thermal stability than the others. The prepared polymers were employed to polymer solar cells (PSCs) with a configuration of ITO/PEDOT-PSS/polymer: PC(61)BH/LiF/Al. Power conversion efficiency of the PSC-based on PTEHTBT was 1.32%. (C) 2011 Wiley Periodicals, Inc. J PolymSci Part A: Polym Chem 49: 3543-3549, 2011