The use of at least one cobalt complex of formula (I-a) or (I-b) below: wherein: - n is an integer that varies from 0 to 5; - R 1 is chosen from I, C 1 -C 12 alkyl, trimethylsilyl, HgCl, -C (C 1 -C 4)alkyl, and an oxazole group optionally substituted by a C 1 -C 4 alkyl radical, it being understood that when n>1, all the R 1 radicals of a given compound of formula (I-a) or (I-b) are identical, - the groups A 1, A' 1, A 2 and A' 2 are identical in pairs and are chosen from the groups of formulae (II-1) to (II-9) below: and wherein: - R 2, R 3, and R 4, which are identical or different, represent a hydrogen, iodine or bromine atom, a nitro, linear C 1 -C 12 alkyl, trifluoromethyl, di(C 1 -C 4)alkylamino, -C (C 1 -C 4)alkyl or linear C 1 -C 4 alkoxy radical; - R 6, R 8, R 9, R 10, R 12, R 13, R 14, R 15 and R 16, which are identical or different, represent a hydrogen or bromine atom, a linear C 1 -C 12 alkyl or linear C 1 -C 4 alkoxy radical, R 9 and R 10 together and/or R 13 and R 14 together and/or R 15 and R 16 together may also form an ethylenedioxy group (-O-(CH 2) 2 -O-); - R 7, R 11 and R 17 represent a hydrogen, bromine or iodine atom, a nitro, linear C 1 -C 12 alkyl, linear C 1 -C 4 alkoxy, -CHO, -C (C 1 -C 4)alkyl or -C (C 1 -C 4)alkoxy radical or a thiophene ring optionally bearing one or more substituents chosen from Br, I, nitro, linear C 1 -C 12 alkyl, linear C 1 -C 4 alkoxy, -C (C 1 -C 4)alkyl and - C (C 1 -C 4)alkoxy; - R 18 to R 36, which are identical or different, represent a hydrogen atom, a linear C 1 -C 4 alkoxy radical, a nitro radical or a -C (C 1 -C 4)alkoxy radical, as a electron donor and in combination with an electron acceptor, for the preparation of an active layer in a photovoltaic conversion cell. - n is an integer that varies from 0 to 5; - R 1 is chosen from I, C 1 -C 12 alkyl, trimethylsilyl, HgCl, -C (C 1 -C 4)alkyl, and an oxazole group optionally substituted by a C 1 -C 4 alkyl radical, it being understood that when n>1, all the R 1 radicals of a given compound of formula (I-a) or (I-b) are identical, - the groups A 1, A' 1, A 2 and A' 2 are identical in pairs and are chosen from the groups of formulae (II-1) to (II-9) below: and wherein: - R 2, R 3, and R 4, which are identical or different, represent a hydrogen, iodine or bromine atom, a nitro, linear C 1 -C 12 alkyl, trifluoromethyl, di(C 1 -C 4)alkylamino, -C (C 1 -C 4)alkyl or linear C 1 -C 4 alkoxy radical; - R 6, R 8, R 9, R 10, R 12, R 13, R 14, R 15 and R 16, which are identical or different, represent a hydrogen or bromine atom, a linear C 1 -C 12 alkyl or linear C 1 -C 4 alkoxy radical, R 9 and R 10 together and/or R 13 and R 14 together and/or R 15 and R 16 together may also form an ethylenedioxy group (-O-(CH 2) 2 -O-); - R 7, R 11 and R 17 represent a hydrogen, bromine or iodine atom, a nitro, linear C 1 -C 12 alkyl, linear C 1 -C 4 alkoxy, -CHO, -C (C 1 -C 4)alkyl or -C (C 1 -C 4)alkoxy radical or a thiophene ring optionally bearing one or more substituents chosen from Br, I, nitro, linear C 1 -C 12 alkyl, linear C 1 -C 4 alkoxy, -C (C 1 -C 4)alkyl and - C (C 1 -C 4)alkoxy; - R 18 to R 36, which are identical or different, represent a hydrogen atom, a linear C 1 -C 4 alkoxy radical, a nitro radical or a -C (C 1 -C 4)alkoxy radical, as a electron donor and in combination with an electron acceptor, for the preparation of an active layer in a photovoltaic conversion cell.