Influence of Photogeneration Frequency on the Transport of Spin Charge Carriers in the Copolymer–Methanofullerene Composite: EPR Study

Light-Induced Electron Paramagnetic Resonance Spectroscopy of Spin-Assisted Charge Transfer in Narrow-Bandgap Copolymer:Methanofullerene Composites

“…Such a recombination process becomes bimolecular and should follow a second order. This parameter obtained for the F8T2:PC 61 BM, PFO–DBT:PC 61 BM, and PCDTBT:PC 61 BM composites appeared to be 2.23, 2.25, and 1.23, respectively. − The key role in recombination of spin charge carriers should play their dynamics and exchange interaction, as well as the formation of spin traps in copolymer backbone. So, a different mechanism of charge recombination may indicate a difference not only in their structural ordering but also in spatial distribution and energetic depth of the spin traps in these systems.…”

“…Figure depicts the temperature dependences of the number of localized polarons P loc +• , mobile methanofullerene radical anions m F mob –• (inset), and also the ratio of their concentrations, n mob m F / n loc P , photoinitiated in the composites under study by photons of white light with CCT of 5500 K. From the figure it can be seen that the number of polarons captured by spin traps in the F8T2:PC 61 BM and PCDTBT:PC 61 BM BHJ varies little with temperature. On the other hand, these centers photoinduced in the PFO–DBT:PC 61 BM BHJ demonstrate an extreme temperature dependence with a characteristic point around T ≈ 120 K. As in the case of other polymer systems, this dependence can be caused by a collective change in the diffusion rate of spin charge carriers, by their coupling with other spin packets, and also by the dissociation of some diamagnetic bipolarons into polaron pairs. ,, The possible capture of a part of polarons by spin traps and the release of their parts under the light illumination − should also be taken into account. The data obtained show that as the composites are heated up to the above characteristic temperature, the number of mobile charge carriers, as well as the corresponding parameter n mob m F / n loc P decreases significantly, possible due to the acceleration of recombination of mobile charge carriers.…”

“…Previously, it was shown − that the shape and intensity of the LEPR spectra of organic photovoltaic systems depend also on the energy of the initiating photons and on the properties of the radical microenvironment. Some charge carriers may be captured by deep spin traps formed in organic composites due to the disordering of their polymer matrices.…”

“…The difference in microwave saturation and in g -factors of the polymer and fullerene moiety allows separately determining their effective relaxation parameters. Earlier, it was shown − that these parameters of both charge carriers depend not only on the structure of a copolymer composite but also on the energy of the initiated photons h ν ph . They should also reflect spin interaction with other spin ensembles and environmental lattice phonons.…”

“…Because both the polaron and the fullerene radical anion possess the spin S = 1 / 2 and weakly anisotropic g -factors, they demonstrate at the often-used X-band EPR partly overlapping doublet of their contributions. This method was used for the study of the main magnetic resonance parameters of spin charge carriers photoinitiated in some organic polymer and copolymer composites. ,,− A preliminary cw EPR study of some copolymer composites − allowed to determine the main values of magnetic resonance parameters of charge carriers and to identify some correlations of their spin state and dynamic properties with the structure and morphology of such systems. It was demonstrated the capture of a part of mobile polarons by high-energetic spin traps formed due to structural disordering of their matrices.…”