In this study, light-induced electron paramagnetic resonance was employed to determine the magnetic, relaxation, and dynamic parameters of background spin charge carriers photoinduced in the bulk heterojunctions of composites formed by poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) and poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) with methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester. Some polarons were captured by deep spin traps, where their number and energy depth were governed by the structure and morphology of the copolymer matrix as well as by the photon energy. Both composites exhibited photo-responses within the photon energy/wavelength regions of 1.32–3.14 eV/940–395 nm, which were wider than those of other polymer composites. The magnetic, relaxation, and dynamic parameters of the spin charge carriers were governed by their exchange interactions and photon energy. The specific morphology of the composites was responsible for the selectivity of these parameters in terms of the photon energy. The anisotropy of the spin mobility through bulk heterojunctions reflected the system dimensionality and it was governed by the photon properties. The replacement of the PFO-DBT backbone by the PCDTBT matrix increased the ordering of the copolymer, decreased the number of spin traps, and changed the mechanism of charge recombination. The decay of free charge carriers was interpreted in terms of the trapping–detrapping spin diffusion in bulk heterojunctions.In this study, light-induced electron paramagnetic resonance was employed to determine the magnetic, relaxation, and dynamic parameters of background spin charge carriers photoinduced in the bulk heterojunctions of composites formed by poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT) and poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) with methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester. Some polarons were captured by deep spin traps, where their number and energy depth were governed by the structure and morphology of the copolymer matrix as well as by the photon energy. Both composites exhibited photo-responses within the photon energy/wavelength regions of 1.32–3.14 eV/940–395 nm, which were wider than those of other polymer composites. The magnetic, relaxation, and dynamic parameters of the spin charge carriers were governed by their exchange interactions and photon energy. The specific morphology of the composites was responsible for the selectivity of these parameters in terms of the photon energy. The anisotropy of the spin mobility through bulk heterojunctions reflected the system dimensionality and it was governed by the photon properties. The replacement of the PFO-DBT backbone by the PCDTBT matrix increased the ordering of the copolymer, decreased the number of spin traps, and changed the mechanism of charge recombination. The decay of free charge carriers was interpreted in terms of the trapping–detrapping spin diffusion in bulk heterojunctions.