Poly(5,5′-bis(thiophen-2-yl)-benzo[2,1-b;3,4-b′]dithiophene) comprising as repeating units the group of the formula (I) wherein R is independently selected from a) a C1-20 alkyl group, b) a C2-20 alkenyl group, c) a C2-20 alkynyl group, d) a C1-20 alkoxy group, e) a —Y—C3-10 cycloalkyl group, f) a —Y—C6-14 aryl group, g) a —Y-3-12 membered cycloheteroalkyl group, or h) a —Y-5-14 membered heteroaryl group, wherein each of the C1-20 alkyl group, the C2-20 alkenyl group, the C2-20 alkynyl group, the C3-10 cycloalkyl group, the C6-14 aryl group, the 3-12 membered cycloheteroalkyl group, and the 5-14 membered heteroaryl group is optionally substituted with 1-4 R1 groups, R1 is independently selected from a) a S(O)m—C1-20 alkyl group, b) a S(O)m—OC1-20 alkyl group, c) a S(O)m—OC6-14 aryl group, d) a C(O)—OC1-20 alkyl group, e) a C(O)—OC6-14 aryl group, f) a C1-20 alkyl group, g) a C2-20 alkenyl group, h) a C2-20 alkynyl group, i) a C1-20 alkoxy group, j) a C3-10 cycloalkyl group, k) a C6-14 aryl group, l) a 3-12 membered cycloheteroalkyl group, or m) a 5-14 membered heteroaryl group, Y is independently selected from a divalent C1-6 alkyl group, or a covalent bond; and m is independently selected from 0, 1, or 2, and having a number average molecular weight Mn in the range of from 5,000 to 200,000 g/mol.