Light-Induced EPR Study of Polymorphic Acene-Stipulated Transition in P3DDT:PC61BM Composite

We report the light-induced electron paramagnetic resonance (LEPR) and optics study of spin charge carriers initiated by light photons with the energy of 1.34-4.52 eV in the initial and slightly acene-treated composites of a regioregular poly(3-dodecylthiophene) (P3DDT) with a [6,6]-phenyl-C-61-butyric acid methyl ester (PC61BM). Light irradiation leads to the formation in both 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 a balance of a chaotically structured polymer alpha-polymorph with spin traps and a more structured its beta-polymorph. The charge carriers, photoinitiated in the alpha-polymorph, are characterized by an extreme dependence of their main parameters from the energy of exciting photons. The increase in the concentration and stability of both mobile charge carriers were reached upon modification of the initial composite by the simplest conjugated acenes. The greatest effect was registered for the composite, weakly doped by naphthalene molecules. The concentration and other properties of charge carriers photoinitiated in a modified composite were found to correlate with the band structure of the acene introduced. Such an effect was interpreted in favor of a triggerlike polymorphic alpha-beta transition stipulated in the system by planar acene molecules. It leads to an acceleration of spin-lattice relaxation of mobile charge carriers, a decrease in the anisotropic diffusion of polarons between polymer chains, and a slow down of the recombination of charge carriers. This opens horizons to use optimally acene-doped polymers and their composites for the creation of electronic and spintronic devices with spin-photon-assisted parameters.