Spin and charge transport in poly(3-octylthiophene)

In this chapter, the light-induced electron paramagnetic resonance study of magnetic, relaxation, and dynamic parameters of spin charge carriers stabilized and photoinduced in polymer:fullerene and polymer/polymer:fullerene nanocomposites is described. The interaction of positively charged polarons and negatively charged fullerene anion radicals becomes smaller in spin pairs preinitiated, due to interaction with their microenvironment, and affects a charge separation in such systems. A nanoscope probe can supply information about the structure and dynamics of a microenvironment. Illumination initiates the appearance in a polymer matrix of spin traps, whose number and energy depth are governed by photon energy. A part of charge carriers can be captured by such traps that decrease the conversion efficiency of a composite. The decay of long-living spins captured depends on the spatial distance between the charge carriers photoinduced, on the structure of a composite, and also on the energy of exciting photons. Magnetic, relaxation, and dynamics parameters of spin charge carriers are governed by the photon energy due to inhomogeneous distribution of polymer and fullerene domains in the composite. An interaction of different domestic and/or photoinduced spin packets deepens the overlapping of their wave functions and leads to the increase in the energy barrier which overcomes the polaron crossing through bulk heterojunctions of polymer nanocomposite. This can open new horizons for the creation of flexible and scalable organic molecular devices with handled spin-assisted electronicproperties.

Light-induced electron paramagnetic resonance (LEPR) study the steady-state of spin charge carriers initiated by Vis-NIR irradiation with the energy (wavelength) of 1.32–2.73 eV (940–455 nm) in organic composite of a low-band-gap poly(N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (PCDTBT) with a [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) within a wide temperature range is reported. LEPR spectra of the PCDTBT:PC₇₁BM composite were deconvoluted and the main resonance parameters of polarons and anion radicals of methanofullerene were determined. The reversible formation of spin traps in polymer backbone, whose number, distribution and depth governed by the photon energy, has being shown. A part of photoinduced charge carriers is pinned by such traps resulting in formation of domains with different band gaps and photon sensitivity. Relaxation and dynamics parameters of all the charge carriers determined separately by the steady-state saturation method were shown to depend extremely on the energy of exciting photons. Polaron diffusion along polymer chains was analyzed in terms of spin interaction with the lattice phonons of crystalline domains embedded into amorphous polymer matrix. Activation of polaron hopping over the energetic barrier is characteristic for the charge transfer between polymer chains. Small-angle librations of methanofullerene cages in the polymer matrix were shown to follow the Markus model. Our results suggest that C₇₀-counter-ions embedded into the PCDTBT backbone strengthen an overlapping of molecular orbitals and provoke a layered morphology of appropriate composite. This regularizes the polymer matrix, hinders the formation of spin traps and accelerates spin dynamics that in turn facilitates charge transport through bulk heterojunctions. It was shown that the spin-assisted photon-electron conversion is realized in the composite within the visible and near-infrared range of the sun spectrum with comparable efficiency.

The results of an investigation at different (10–140 GHz) wavebands EPR of magnetic, relaxation and dynamics parameters of mobile paramagnetic charge carriers (polarons) in low-dimensional solid-state poly(3-alkylthiophenes) semiconductors are discussed. At high registration frequency all components of the g-tensor of polarons are registered. Relaxation and diffusion rates of such paramagnetic impurities are determined by the method of steady-state saturation of spin packets.