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

Abstract

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-C61-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 α-polymorph with spin traps and a more structured its β-polymorph. The charge carriers, photoinitiated in the α-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 α–β 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.

Magnetic Field Effects of Charge Transfer Excitons in Organic Semiconductor Devices

“…Since the spin‐lattice relaxation time, T 1 , places an upper limit on the spin coherence time, T 2 , the latter one expected to be even shorter than 1 μs. Krinichnyi et al [45,46] . have measured for the P3HT/PCBM blend 1/ T 2 ∼10 7 s −1 , they have found it approximately independent in the temperature in the range, 77–325 K. Therefore, as CTE decay rate approaches ∼10 7 s −1 , the decay parameter , in the decay term in Eq.…”

“…However, the spin‐lattice relaxation times ( T 1 ) reported in OS have quite large values. For example, Krinichnyi et al [45,46] . have measured at room temperature T 1 ≈120 ns by electron spin resonance (ESR) experiment in photoexcited P3HT/C 60 fullerene derivative heterojunction blends.…”

“…Note that this T 1 refers to the relaxation times related to either free polarons or polarons composing PPs. At lower temperatures the spin‐lattice relaxation times are even longer [45,46] . Hence, the population in short lived CTE λ states apparently is not considerably affected by the thermal spin polarization, because the CTE spins do not have enough time to evolve towards thermal equilibrium before they dissociate.…”

“…The g‐factor values can be found by means of ESR experiments. In particular, light‐induced ESR technique is capable to reveal specifically the g‐factor values of polarons in donor‐acceptor OS compounds [46,56–58] . The polaron g‐factors also can be measured by EDMR experiments [22] …”

“…This makes the contribution of thermal spin polarization effect to MPC to be significant. The thermal spin‐lattice relaxation times for the P3HT/PCBM blend are known [46] as well as the g‐factor values [46] . Therefore, by fitting the experimental data, shown in Figure 6, Khachatryan et al [48] .…”