The present invention relates to polymers, comprising a repeating unit of the formula (I), and compounds of formula (II), wherein Y, Y15, Y16 and Y17 are independently of each other a group of formula (a) characterized in that the polymers and compounds comprise silicon-containing solubilizing side chains 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 and compounds according to the invention can have excellent solubility in organic solvents and excellent film-forming properties. In addition, high efficiency of energy conversion, excellent field-effect mobility, good on/off current ratios and/or excellent stability can be observed, when the polymers and compounds according to the invention are used in organic field effect transistors, organic photovoltaics and photodiodes.