Small band gap polymers synthesized via a modified nitration of 4,7-dibromo-2,1,3-benzothiadiazole.

The nitration of 4,7-dibromo-2,1,3-benzothiadiazole was modified by using CF(3)SO(3)H and HNO(3) as the nitrating agent, and the related yield was improved greatly. On the basis of this improvement, two new small band gap polymers, P1TPQ and P3TPQ, were developed. Bulk heterojunction solar cells based on P3TPQ and [6,6]-phenyl-C(71)-butyric acid methyl ester exhibit interesting results with a power conversion efficiency of 2.1% and photoresponse up to 1.1 μm.