Polymer of formula I which is formed by identical or different repeating units of formula III wherein A is on each occurrence identically or differently a group selected from the following formulae B is on each occurrence identically or differently a group selected from the following formulae X 1 and X 2 are on each occurrence identically or differently, and independently of each other, O, S or Se, R 1 and R 2 are on each occurrence identically or differently, and independently of each other, straight-chain or branched alkyl, alkoxy, thioalkyl with 1 to 30 C atoms, or ketone (-CO-R y) or carboxylic acid ester (-CO-O-R y), wherein R y is straight-chain, branched or cyclic alkyl with 1 to 30 C atoms, and wherein in all aforementioned groups one or more H atoms are optionally replaced by F, R 3 and R 4 on each occurrence identically or differently, and independently of each other, denote H or have one of the meanings given for R 1, Ar is on each occurrence identically or differently selected from the group consisting of benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkyl-benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkoxybenzo[2,1,3]thiadiazole-4,7-diyl, benzo[2,1,3]selenadiazole-4,7-diyl, 5,6-dialkoxy-benzo[2,1,3]selenadiazole-4,7-diyl, benzo[1,2,5]thiadiazole-4,7,diyl, benzo[1,2,5]selenadiazole-4,7,diyl, benzo[2,1,3]oxadiazole-4,7-diyl, 5,6-dialkoxybenzo[2,1,3]oxadiazole-4,7-diyl, 2H-benzotriazole-4,7-diyl, 2,3-dicyano-1,4-phenylene, 2,5-dicyano-1,4-phenylene, 2,3-difluro-1,4-phenylene, 2,5-difluoro-1,4-phenylene, 2,3,5,6-tetrafluoro,1,4-phenylene, 3,4-difluorothiophene-2,5-diyl, thieno[3,4-b]pyrazine-2,5-diyl, quinoxaline-5,8-diyl, thieno[3,4-b]thiophene-4,6-diyl, thieno[3,4-b]thiophene-6,4-diyl, 3,6- pyrrolo[3,4-c]pyrrole-1,4-dione, all of which are unsubstituted, mono- or polysubstituted with R 1, and wherein "alkyl" means a straight-chain or branched alkyl group with 1 to 30 C atoms and "alkoxy" means a straight-chain or branched alkoxy group with 1 to 30 C atoms, m is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit m is 1, o is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit o is 1, i and k are on each occurrence identically or differently, and independently of each other, 0 or 1, wherein in at least one repeating unit at least one of i and k is 1, n is an integer > 1, and wherein the polymer contains repeating units of formula III wherein i=m=1 and k=o=0 and repeating units of formula III wherein i=m=0 and k=o=1. A is on each occurrence identically or differently a group selected from the following formulae B is on each occurrence identically or differently a group selected from the following formulae X 1 and X 2 are on each occurrence identically or differently, and independently of each other, O, S or Se, R 1 and R 2 are on each occurrence identically or differently, and independently of each other, straight-chain or branched alkyl, alkoxy, thioalkyl with 1 to 30 C atoms, or ketone (-CO-R y) or carboxylic acid ester (-CO-O-R y), wherein R y is straight-chain, branched or cyclic alkyl with 1 to 30 C atoms, and wherein in all aforementioned groups one or more H atoms are optionally replaced by F, R 3 and R 4 on each occurrence identically or differently, and independently of each other, denote H or have one of the meanings given for R 1, Ar is on each occurrence identically or differently selected from the group consisting of benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkyl-benzo[2,1,3]thiadiazole-4,7-diyl, 5,6-dialkoxybenzo[2,1,3]thiadiazole-4,7-diyl, benzo[2,1,3]selenadiazole-4,7-diyl, 5,6-dialkoxy-benzo[2,1,3]selenadiazole-4,7-diyl, benzo[1,2,5]thiadiazole-4,7,diyl, benzo[1,2,5]selenadiazole-4,7,diyl, benzo[2,1,3]oxadiazole-4,7-diyl, 5,6-dialkoxybenzo[2,1,3]oxadiazole-4,7-diyl, 2H-benzotriazole-4,7-diyl, 2,3-dicyano-1,4-phenylene, 2,5-dicyano-1,4-phenylene, 2,3-difluro-1,4-phenylene, 2,5-difluoro-1,4-phenylene, 2,3,5,6-tetrafluoro,1,4-phenylene, 3,4-difluorothiophene-2,5-diyl, thieno[3,4-b]pyrazine-2,5-diyl, quinoxaline-5,8-diyl, thieno[3,4-b]thiophene-4,6-diyl, thieno[3,4-b]thiophene-6,4-diyl, 3,6- pyrrolo[3,4-c]pyrrole-1,4-dione, all of which are unsubstituted, mono- or polysubstituted with R 1, and wherein "alkyl" means a straight-chain or branched alkyl group with 1 to 30 C atoms and "alkoxy" means a straight-chain or branched alkoxy group with 1 to 30 C atoms, m is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit m is 1, o is on each occurrence identically or differently 0 or 1, wherein in at least one repeating unit o is 1, i and k are on each occurrence identically or differently, and independently of each other, 0 or 1, wherein in at least one repeating unit at least one of i and k is 1, n is an integer > 1,