Abstract: The present invention relates to polymers comprising a repeating unit of the formula—[Ar3]c—[Ar2]b—[Ar1]a—Y(R1)n1 (R2)n2—[Ar1?]a—[Ar2?]b?—[Ar3?]c?— (I), wherein ? is a bivalent heterocyclic group, or ring system, which may optionally be substituted, Ar1, Ar1?Ar2, Ar2?, Ar3 and Ar3? are independently of each other a C6-C24 arylen group, which can optionally be substituted, or a C2-C30 heteroarylen group, which can optionally be, Formula (1), substituted; at least one of R1 and R2 is a group of formula (II); and their use as organic semiconductor in organic devices, especially in organic photovoltaics and photodiodes, or in a device containing a diode and/or an organic field effect transistor. The polymers according to the invention can have excellent solubility in organic solvents and excellent film-forming properties.