Influence of morphology of low-band-gap PCDTBT:PC71BM composite on photoinduced charge transfer: LEPR spectroscopy study

• We report spin-assisted photon-electron conversion by a PCDTBT:PC71BM composite within sun visible and infrared regions.
• Relaxation and dynamics of spin charge carriers were studied upon background IR-Vis illumination.
• PC71BM anions order polymer matrix that accelerates spin dynamics in the composite.
• Spin-assisted charge transfer in the composite depends on the photon energy.

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.

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