Light-Induced Electron Paramagnetic Resonance Spectroscopy Of Spin-Assisted Charge Transfer In Narrow-Bandgap Copolymer:Methanofullerene Composites

Magnetic resonance, relaxation, and dynamic parameters of polarons and methanofullerene radical anions photoinduced in photovoltaic composites formed by narrow-bandgap poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(bithiophene)] (F8T2), poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PFO-DBT), and poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) copolymers as electron donors with [6,6]-phenyl-C-61-butanoic acid methyl ester (PC61BM) as electron acceptor were comparatively studied by the direct light-induced electron paramagnetic resonance (LEPR) spectroscopy in combination with spectral simulations in a wide temperature range. A number of mobile polarons are captured by deep spin traps formed in bulk heterojunctions due to their disorder. It was shown that the concentration, transport, and recombination of photoinitiated charge carriers depend on the structure of the copolymer matrix and the interaction between the other spin packets, as well as on the number, spatial distribution, and energy depth of the spin traps. The recombination of polarons and methanofullerene radical anions can be described in the framework of a second-order bimolecular process in the F8T2:PC61BM and PFO-DBT:PC61BM composites and a first-order monomolecular process in the PCDTBT:PC61BM bulk heterojunctions. The dependence of the ambipolarity of the copolymer matrix on the anisotropy of spin dynamics is shown.