Magnetic Resonance Characteristics of Negative Polarons in Neat Poly(3-hexyl-thiophene)

Abstract

Magnetic resonance characteristics of carriers of both electron spin and electric charge (polarons) in conjugated polymers are required for a deeper understanding of properties of these semiconducting materials. In poly­(3-hexyl-thiophene), P3HT, which is an important material for photovoltaic applications, such information is available only for positive polarons, which are readily generated in the presence of electron acceptors via photoinduced processes. Influence of an external magnetic field on X-ray-induced delayed fluorescence from undoped P3HT and its solutions allowed us to visualize the spin interactions in the primary polaron pairs and to estimate the g-tensor components (g⊥ ≈ 2.0023, g|| ≈ 2.008) for the negative charge carriers in P3HT molecules. There is evidence that in neat regioregular P3HT, the g-tensor anisotropy for the polarons, migrating along polymer chains, is averaged partially because of irregularities in the polymer structure.

Probing an Isolated Conjugated Polymer Molecule with Radiation-Generated Spin-Correlated Polaron Pairs

“…The isotropic value of the g -factor, ( g xx + g yy + g zz )/3, for the P3HT-rra negative polarons is approximately 2.0035. This value is lower than the value of 2.0045 observed for these particles in solution but falls within the range of values of 2.0031–2.0044 obtained by EPR spectroscopy of the solid phase in steady-state experiments. − The scatter between the observed average g -values may be attributed to differences between the conformations of P3HT molecules that lead to a different partial averaging of the g -tensor anisotropy during migration of a negative polaron along the polymer chain. This result implies that in this regard, the polaron migration along a P3HT-rra molecule isolated in the PVC matrix does not differ substantially from the migration in pristine P3HT-rra.…”

“…In this case, at B = 1.8 T, the period of the Δg quantum beats in TR MFE for the PVC +• / p- TP- d 14 –• pair would be approximately 1.6 ns (see Figure 4 in ref for a comparison). The difference in the g -factors of the p- TP radical ions is much lower, of approximately 2 × 10 –4 , which explains the very weak effect of the field B = 1.8 T. Therefore, this implies that the primary holes in PVC do not contribute here to the spin dynamics. The absence of any evidence of Δg quantum beats in the presented TR MFE curve allows estimating the lifetime of PVC holes as short as several hundred picoseconds.…”

“…Let us now consider the results for the PVC matrix doped with the conjugated polymer, regiorandom poly­(3-hexylthiophene), (P3HT-rra, M w ∼ 57 kDa). This polymer has been previously studied using the TR MFE method both in solution and bulk state that allows for estimating influence of the PVC matrix on the P3HT polarons characteristics. Figure is a comparison of the TR MFE curves obtained for PVC doped with P3HT-rra at a relatively low concentration of 0.1% w/w with that for pristine P3HT-rra.…”

“…Although the experimental curves obtained for these two cases are qualitatively similar, the magnitude of the effect is significantly larger for doped PVC matrix. This observation can be explained by the fact that a valuable part of the fluorescence from the pristine polymer results from the annihilation of triplet excitons . The magnetic field effects for triplet–triplet annihilation (TTA) and the recombination fluorescence of singlet-born PPs are expected to have opposite signs .…”

“…Figure shows the modeling results for a radical pair composed of partners, for which HFCs are negligible and the principal components of the g -tensor are (2.0012; 2.0023; 20032) for a positive polaron and (2.0023; 2.0023; 2.0060) for its counterion. The same g -tensors for the polarons were used earlier to simulate the TR MFE curves for pristine regioregular P3HT, where the TTA contribution was assumed to be negligible …”