A compound of formula I wherein C n is a fullerene composed of n carbon atoms, optionally with one or more atoms trapped inside, Adduct is a secondary adduct or a combination of secondary adducts appended to the fullerene C n with any connectivity, and the secondary adduct is selected from the following formulae wherein R S1, R S2, R S3, R S4 and R S5 independently of each other denote H, halogen or CN, or have one of the meanings of R 5 or R 6, and Ar S1 and Ar S2 are independently of each other an aryl or heteroaryl group with 5 to 20 ring atoms, which is mono- or polycyclic, and which is substituted by one or more identical or different substituents R S, wherein R S denotes halogen or a straight-chain, branched or cyclic alkyl moiety with 1 to 30 C atoms, in which one or more CH 2 groups are optionally replaced by -O-, -S-, -C -, -C -, -C -O-, -O-C -, -S 2 -, -NR 0 -, - SiR 0 R 00 -, -CF 2 -, m is 0, an integer ≥ 1, or a non-integer > 0, o is an integer ≥ 1, R 1, R 2, R 3, R 4 independently of each other denote H, halogen, CN or are selected from the following formulae wherein R 11, R 12, R 13, R 14, R 15, R 16, R 17 and R 18 independently of each other denote H, halogen or CN, or have one of the meanings of R 6, and wherein in each of the aforementioned formulae at least one of R 11, R 12, R 13, R 14, R 15, R 16, R 17 and R 18 is different from H, R 5 denotes 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 6, R 6 is straight-chain, branched or cyclic alkyl with 4 to 30 C atoms, in which one or more CH 2 groups are optionally replaced by -O-, -S-, -C -, -C -, -C -O-, -O-C -,-NR 0 -, -SiR 0 R 00 -, -CF 2 -, and wherein one or more H atoms are replaced by fluorine atoms, Y 1 and Y 2 denote independently of each other H, F, Cl or CN, R 0, R 00 and R 000 denote independently of each other H or an alkyl group with 1 to 12 C atoms, wherein at least one of R 1, R 2, R 3 and R 4 denotes a group selected from formulae S-C-1, S-C-2, S-C-3, S-C-4, S-C-6 and S-C-8. C n is a fullerene composed of n carbon atoms, optionally with one or more atoms trapped inside, Adduct is a secondary adduct or a combination of secondary adducts appended to the fullerene C n with any connectivity, and the secondary adduct is selected from the following formulae m is 0, an integer ≥ 1, or a non-integer > 0, o is an integer ≥ 1, R 1, R 2, R 3, R 4 independently of each other denote H, halogen, CN or are selected from the following formulae R 5 denotes 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 6, R 6 is straight-chain, branched or cyclic alkyl with 4 to 30 C atoms, in which one or more CH 2 groups are optionally replaced by -O-, -S-, -C -, -C -, -C -O-, -O-C -,-NR 0 -, -SiR 0 R 00 -, -CF 2 -, and wherein one or more H atoms are replaced by fluorine atoms, Y 1 and Y 2 denote independently of each other H, F, Cl or CN, R 0, R 00 and R 000 denote independently of each other H or an alkyl group with 1 to 12 C atoms,