Heteroarene-fused πconjugated main-chain polymers containing 4,7-bis(4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazole or 2,5-bis(4- octylthiophen-2-yl)thiazolo[5,4-d]thiazole and their application to photovoltaic devices

Two conjugated main-chain polymers consisting of heteroarene-fused πconjuagted donor moiety alternating with 4,7-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 amorphous in nature and do not exhibit crystalline melting temperatures during thermal analysis. The effect of the fused rings on the thermal, optical, electrochemical, charge transport, and photovoltaic properties of these polymers has been investigated. The polymer (P1) containing 4,7-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 polymer 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/cm², and a fill factor of 0.32, offering an overall power conversion efficiency of 1.21%.