Two conjugated main chain polymers consisting of heteroarene fused pi conjuagted donor moiety alternating with 47 bis(5 bromo 4 octylthiophen 2 yl)benzo[c][1 2 5]thiadiazole (P1) or 2 5 bis(5 bromo 4 octylthiophen 2 yl) thiazolo[5 4 d]thiazole (P2) units have been synthesized They are intrinsically amor phous in nature and do not exhibit crystalline melting tempera tures during thermal analysis The effect of the fused rings on the thermal optical electrochemical charge transport and photo voltaic properties of these polymers has been investigated The polymer (P1) containing 47 bis(5 bromo 4 octylthiophen 2 yl)benzo[c][1 2 5] thiadiazole has a broad absorption extending from 300 to 600 nm with optical bandgaps as low as 2 02 eV The HOMO levels (5 42 to 5 29 eV) are more sensitive to the choice of acceptor The polymers were employed to fabricate organic photovoltaic cells with methanofullerene [6 6] phenyl C71 butyric acid methyl ester (PC71BM) As a result the poly mer solar cell device containing P1 had the best preliminary results with an open circuit voltage of 0 61 V a short circuit current density of 6 19 mA/cm(2) and a fill factor of 0 32 offer ing an overall power conversion efficiency of 1 21% (C) 2010 Wiley Periodicals Inc J Polym Sci Part A Polym Chem 48 5921-5929 2010