Polymer which contains at least one structural unit of the following formulae, (IIIa) and/or (IIIb): where the symbols and indices used have the following meaning: X is C(R 1) 2, Y 1 and Y 2 are each selected, independently of one another, from C(R 1) 2, NR 1, C(R 1) 2 -C(R 1) 2, CR 1 =CR 1 and a single bond, but with the proviso that either Y 1 or Y 2 is a single bond, m is 0 or 1, R 1 is in each case selected, independently of one another, from H, F, Cl, Br, I, N(Ar 1) 2, C(=O)Ar 1, P(=O)Ar 1 2, S(=O)Ar 1, S(=O) 2 Ar 1, CR 2 =CR 2 Ar 1, CN, NO 2, Si(R 2) 3, B(OR 2) 2, OSO 2 R 2, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms, a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, each of which may be substituted by one or more radicals R 2, where one or more non-adjacent CH 2 groups may be replaced by R 2 C=CR 2, C=C, Si(R 2) 2, Ge(R 2) 2, Sn(R 2) 2, C=O, C=S, C=Se, C=NR 2, P(=O)(R 2), SO, SO 2, NR 2, O, S or CONR 2 and where one or more H atoms may be replaced by F, Cl, Br, I, CN or NO 2, an aryl, aryloxy, heteroaryl or heteroaryloxy group having 5 to 40 C atoms, which may also be substituted by one or more non-aromatic radicals R 1, where, in addition, two or more radicals, preferably two adjacent radicals, R 1 may form an aliphatic or aromatic, mono- or polycyclic ring system with one another, or an electron-transport unit R 3, but with the proviso that at least one of the radicals R 1 is an electron-transport unit R 3, Ar 1 is selected on each occurrence, in each case independently of one another, from an aryl or heteroaryl group or an aromatic or heteroaromatic ring system, R 2 is in each case, independently of one another, H, an aliphatic hydrocarbon radical having 1 to 20 C atoms or an aromatic hydrocarbon radical having 6 to 20 C atoms, where two or more radicals R 2 may also form a ring system with one another, R 3 is an electron-transport unit which is selected from the group consisting of benzimidazole, triazine, pyrimidines, pyrazines, pyridazines, phenanthrolines, ketones, phosphine oxides and derivatives thereof, where the dashed lines represent the bonds to the adjacent structural units in the polymer. X is C(R 1) 2, Y 1 and Y 2 are each selected, independently of one another, from C(R 1) 2, NR 1, C(R 1) 2 -C(R 1) 2, CR 1 =CR 1 and a single bond, but with the proviso that either Y 1 or Y 2 is a single bond, m is 0 or 1, R 1 is in each case selected, independently of one another, from H, F, Cl, Br, I, N(Ar 1) 2, C(=O)Ar 1, P(=O)Ar 1 2, S(=O)Ar 1, S(=O) 2 Ar 1, CR 2 =CR 2 Ar 1, CN, NO 2, Si(R 2) 3, B(OR 2) 2, OSO 2 R 2, a straight-chain alkyl, alkoxy or thioalkoxy group having 1 to 40 C atoms, a branched or cyclic alkyl, alkoxy or thioalkoxy group having 3 to 40 C atoms, each of which may be substituted by one or more radicals R 2, where one or more non-adjacent CH 2 groups may be replaced by R 2 C=CR 2, C=C, Si(R 2) 2, Ge(R 2) 2, Sn(R 2) 2, C=O, C=S, C=Se, C=NR 2, P(=O)(R 2), SO, SO 2, NR 2, O, S or CONR 2 and where one or more H atoms may be replaced by F, Cl, Br, I, CN or NO 2, an aryl, aryloxy, heteroaryl or heteroaryloxy group having 5 to 40 C atoms, which may also be substituted by one or more non-aromatic radicals R 1, where, in addition, two or more radicals, preferably two adjacent radicals, R 1 may form an aliphatic or aromatic, mono- or polycyclic ring system with one another, or an electron-transport unit R 3, but with the proviso that at least one of the radicals R 1 is an electron-transport unit R 3, Ar 1 is selected on each occurrence, in each case independently of one another, from an aryl or heteroaryl group or an aromatic or heteroaromatic ring system, R 2 is in each case, independently of one another, H, an aliphatic hydrocarbon radical having 1 to 20 C atoms or an aromatic hydrocarbon radical having 6 to 20 C atoms, where two or more radicals R 2 may also form a ring system with one another, R 3 is an electron-transport unit which is selected from the group consisting of benzimidazole, triazine, pyrimidines, pyrazines, pyridazines, phenanthrolines, ketones, phosphine oxides and derivatives thereof, where the dashed lines represent the bonds to the adjacent structural units in the polymer.