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Light-Induced EPR Spectroscopy of Charge Transfer in Low-Band-Gap PCDTBT:PC71BM Bulk Heterojunctions

The light-induced electron paramagnetic resonance (LEPR) study of spin charge carrier steady-state initiated by IR-Vis photons in an 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)) with a [6, 6]-phenyl-C 71-butyric acid methyl ester is reported. LEPR spectra of the composite were deconvoluted, and the main resonance parameters of polarons and anion radicals of methanofullerene were determined. The reversible formation in polymer backbone of spin traps, whose number, distribution, and depth is governed by the photon energy, has been shown. Part of the photoinduced charge carriers is pinned by such traps, resulting in the formation in the composite of polymer and fullerene domains with different band gaps and photon sensitivity. Relaxation and dynamics parameters of spin charge carriers were shown to depend on the energy of exciting photons. The mechanisms of polaron diffusion along and between polymer chains, as well as librations of methanofullerene cages in a polymer matrix, were proposed. Our results suggest that the use of C70-counterions as an electron accepter in the composite provokes its layered morphology. This hinders the formation of spin traps, increases the path length for charge carriers, and, therefore, accelerates their spin-assisted relaxation and dynamics through bulk heterojunctions within the visible and infrared regions of the sun spectrum with comparable effectiveness.