A compound of formula I wherein C n denotes a fullerene composed of n carbon atoms, optionally with one or more atoms trapped inside, Adduct is a secondary adduct which is selected from the formula S-5 R S1 and R S2 independently of each other denote H, halogen or CN, or have one of the meanings of R 5 or R 6, m is 0, 1 or 2, o is 1 or 2, n is 60 or 70, Ar S1 denotes a benzene, thiophene or naphthalene group that is optionally substituted with one or more groups R 5, R 5 and R 6 denote, independently of each other, H, halogen, CN, R 7 or R 8, R 7 denotes, on each occurrence identically or differently, a saturated or unsaturated, non-aromatic carbo- or heterocyclic group, or an aryl, heteroaryl, aryloxy or heteroaryloxy group, wherein each of the aforementioned groups has 3 to 20 ring atoms, is mono- or polycyclic, does optionally contain fused rings, and is optionally substituted by one or more halogen atoms or CN groups, or by one or more identical or different groups R 8, R 8 denotes, on each occurrence identically or differently, an alkyl group with up to 30 C atoms, which is straight-chain, branched or cyclic, and in which one or more CH 2 groups are optionally replaced by -O-, -S-, -C(=O)-, -C(=S)-,-C(=O)-O-, -O-C(=O)-, -NR 0 -, -C(=O)-NR 0 -, -NR 0 -C(=O)-,-SiR 0 R 00 -, -CF 2 -, -CHR 0 =CR 00 -, -CY 1 =CY 2 - or -C=C- in such a manner that O and/or S atoms are not linked directly to one another, and in which one or more H atoms are optionally replaced by F, CI, Br, I or CN, Y 1 and Y 2 denote, independently of each other, H, F, Cl or CN, R 0 and R 00 denote, independently of each other, H or alkyl with 1 to 12 C atoms, R 2, R 3 and R 4 are H and R 1 is selected from formulae P-RS-1 to P-RS-7 wherein Ar S2 denotes R 7 or has one of the meanings of Ar S1, i is 0 or an integer from 1 to 9, k is 0 or an integer from 1 to 9. C n denotes a fullerene composed of n carbon atoms, optionally with one or more atoms trapped inside, Adduct is a secondary adduct which is selected from the formula S-5 R S1 and R S2 independently of each other denote H, halogen or CN, or have one of the meanings of R 5 or R 6, m is 0, 1 or 2, o is 1 or 2, n is 60 or 70, Ar S1 denotes a benzene, thiophene or naphthalene group that is optionally substituted with one or more groups R 5, R 5 and R 6 denote, independently of each other, H, halogen, CN, R 7 or R 8, R 7 denotes, on each occurrence identically or differently, a saturated or unsaturated, non-aromatic carbo- or heterocyclic group, or an aryl, heteroaryl, aryloxy or heteroaryloxy group, wherein each of the aforementioned groups has 3 to 20 ring atoms, is mono- or polycyclic, does optionally contain fused rings, and is optionally substituted by one or more halogen atoms or CN groups, or by one or more identical or different groups R 8, R 8 denotes, on each occurrence identically or differently, an alkyl group with up to 30 C atoms, which is straight-chain, branched or cyclic, and in which one or more CH 2 groups are optionally replaced by -O-, -S-, -C(=O)-, -C(=S)-,-C(=O)-O-, -O-C(=O)-, -NR 0 -, -C(=O)-NR 0 -, -NR 0 -C(=O)-,-SiR 0 R 00 -, -CF 2 -, -CHR 0 =CR 00 -, -CY 1 =CY 2 - or -C=C- in such a manner that O and/or S atoms are not linked directly to one another, and in which one or more H atoms are optionally replaced by F, CI, Br, I or CN, Y 1 and Y 2 denote, independently of each other, H, F, Cl or CN, R 0 and R 00 denote, independently of each other, H or alkyl with 1 to 12 C atoms, R 2, R 3 and R 4 are H and R 1 is selected from formulae P-RS-1 to P-RS-7 Ar S2 denotes R 7 or has one of the meanings of Ar S1, i is 0 or an integer from 1 to 9, k is 0 or an integer from 1 to 9.