The paper presents the results of the light-induced EPR and optical absorption study of spin charge carriers photoexcited by light photons with an energy in the range of 1.34 - 4.52 eV in composites of regioregular poly(3-dodecylthiophene) (P3DDT) with methyl ether [6,6]-phenyl-C₆₁-butyric acid (PC₆₁BM) modified by various molecular nanoadditives. The morphology of the polymer matrix suggests the presence of two structural polymorphs - a chaotically structured α-polymorph and a more crystalline β-polymorph. Light irradiation leads to the formation in these polymorphs of spin charge carriers, polarons and methanofullerene radical anions. Magnetic resonance, relaxation and dynamic parameters of such carriers depend on the energy of exciting light photons, as well as on the balance between the α-polymorph of the polymer with spin traps and its more structured β-polymorph [1,2]. These parameters can be changed by adding to the system small quantity (3-6% by weight) of 2D molecules with an extended π-structure, for example, acenes. During the preparation of the composite this initiates its structuring process which efficiency is proportional to the acene band gap. It was found that small flat acene molecules introduced into the initial composite solution indirectly bring the nearest macromolecules of the polymer matrix together. It was shown that the concentration and other properties of charge carriers photoinitiated in modified composites correlate with the band structure of the introduced acene. This effect was interpreted in favor of a trigger-like polymorphic α-β-transition caused in the system by planar acene molecules. The calculations of the zone structure of acenes allowed us to indicate a close relationship between their band gap energy and the stimulated properties of spin charge carriers in so modified composites. The amplification of the π-π-interaction of small molecules was supposed to cause a greater overlap of the wave functions of polymer chains and nearby additives, which accelerates the electronic relaxation of all spin ensembles formed in the systems under study, reduces the number of spin traps and widens their high-energy optical absorption band. Charge carriers photoinitiated in the α-polymorph are characterized by an extreme dependence of their main parameters on the energy of exciting photons. The concentration and stability of both mobile charge carriers were observed to be increased in both polymorphs of acene-modified polymer:methanofullerene solar cells. An analogous effect was registered for a composite weakly doped with naphthalene and other molecules. Ultimately, the addition of small 2D molecules increases the stability of spin charge carriers photoinitiated in a composite doped with anthracene or naphthalene molecules by more than 2 and 5 times, respectively. This eliminates the selectivity of spin charge carriers to photon energy and changes the mechanism of their transfer to the electrodes. If in the amorphous α-phase of the composite charge transfer is carried out mainly by polarons along polymer chains, whereas the charge hopping between 2D layered stacks prevails in its more crystalline β-polymorphs. Polymorphism and modification of the organic polymer:fullerene composite allow shifting the balance of their metastable amorphous α-phase towards a more stable and crystalline β-polymorph. This can be realized either by irreversible trigger modification of the composite by small flat molecules with an extended π-π-structure, or/and its episodic processing by certain light photons. The spin-assisting nature of relaxation and dynamics processes occurring in the initial and slightly modified composites is shown. This makes it possible to significantly accelerate the transfer of charge carriers, increase their stability and thereby increase the functionality and efficiency of energy conversion in the system. The results obtained confirm the universal proposal for the use of polymers optimally doped with small 2D molecules and their composites modified with fullerenes to create electronic and spintronic devices with spin-assisted parameters. The proposed approach can also be used for similar studies of other organic donor-acceptor systems with different spin charge dynamics, increasing their stability and thereby increasing the functionality and energy efficiency of the system. The results obtained confirm the universal proposal on the use of polymers optimally doped with acenes and their composites modified with fullerenes to create electronic and spintronic devices with spin-regulated parameters.

Acknowledgement: This work was done by the State Assignment No. AAAA-A19-119032690060-9.

References:

1. Krinichnyi, V. I.; Yudanova E. I.; Denisov, N. N. ; Bogatyrenko V. R., Effects of small-molecule-doping on spin-assisted processes in P3DDT:PC₆₁BM photovoltaics . Synth. Met., 2020, N 267, P. 116462; DOI: 10.1016/j.synthmet.2020.116462.
2. Krinichnyi, V.I.; Yudanova, E.I.; Denisov, N.N. Light-Induced EPR Study of Polymorphic Acene-Stipulated Transition in P3DDT:PC₆₁BM Composit. J. Phys. Chem. C, 2022, V. 126, P. 4495-4507; DOI: http://doi.org/10.1021/acs.jpcc.1c10407.