The present invention relates to polymers comprising a repeating unit of the formula I, or III and their use as organic semiconductor in organic devices, especially an organic field effect transistor (OFET), or a device containing a diode and/or an organic field effect transistor. The polymers according to the invention 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 according to the invention are used in organic field effect transistors.