Impact of Spin-Exchange Interaction on Charge Transfer in Dual-Polymer Photovoltaic Composites

Abstract

Copyright © 2020 American Chemical Society. Magnetic resonance, relaxation, and dynamic parameters of spin-charge carriers photoinitiated in dual-polymer composites formed by narrow-band-gap 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 modified with [6,6]-phenyl-C61-butanoic acid methyl ester (PC61BM) as a photovoltaic spin subsystem and polyaniline salt doped with para-toluenesulfonic acid (PANI:TSA) as a guest spin subsystem were comparatively studied by the direct light-induced electron paramagnetic resonance (LEPR) spectroscopy in a wide photon energy and temperature range. Irradiation of dual-polymer composites by the photons leads to the formation in its photovoltaic subsystem of polarons and methanofullerene radical anions whose concentration and dynamics are determined by the density and energy of the initiating light photons. A part of such polarons first filled high-energetic spin traps formed in the matrix due to its disordering. A crucial role of exchange interaction between different spin ensembles in the charge excitation, relaxation, and transport in multispin narrow-band-gap composites was demonstrated. These processes were interpreted within the framework of hopping of polarons along copolymer chains of photovoltaic subsystems and their exchange interaction with neighboring spin ensembles. Such an interaction was shown to facilitate the transfer of charges and inhibit their recombination in multispin dual-polymer composites. The distribution of spin density over polymer chains in the dual-polymer composites with the π-πstacked architecture was analyzed in the framework of the density functional theory (DFT). It confirmed the transfer of electron spin density between neighboring polymer chains that made formation more likely of radical pairs in triplet state than in singlet one and inhibited their fast geminate recombination. Spin interactions eliminate the selectivity of these systems to the photon energy, extend the range of optical photons they absorb, and, therefore, increase their efficiency to converse the light energy. Handling electronic properties via intra-and intersubsystem spin interactions in such multispin composites allows one to create on their base more efficient and functional electronic and spintronic elements.