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

Abstract

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–π stacked 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.

Light-Induced Electron Paramagnetic Resonance Study of Charge Transport in Fullerene and Nonfullerene PBDB-T-Based Solar Cells

“…The calculation of hyperfine coupling parameters of the PBDB-T:ITIC-M system was performed using the method/functional/basis sets (DFT/ B3LYP/6-311G) in Gaussian-2016 and (B3LYP-) in ORCA according a procedure earlier described in detail. 22 The preliminary molecular energy minimization and structure optimization have been carried out in Chem3D by MM2 force field method. The spin−lattice T 1 and spin−spin T 2 relaxation times of spin charge carriers were determined with an accuracy of 7.7% and 1.5%, respectively, using the steadystate MW saturation method, 42 adapted for the study of spin pairs photoinitiated in organic donor−acceptor nanocomposites 43 (see Supporting Information, Section III).…”

“…and ITIC-M −• ) calculated with the data summarized in Table 1 are also presented. As in the case of other copolymers, 22,39 the principal x-axis of P mob +• carriers was chosen to be parallel to the longest polymer c-axis, whereas the y-axis lies in the thiophene plane, and the z-axis is directed perpendicular to x-and y-axes. It can be seen from Figure 2a that the better spectral resolution (ca.…”

“…Similar dependences were previously recorded in the study of poly(3-hexylthiophene):PC 71 BM 51 and a series of narrow band gap composites. 22,33 They can be interpreted as follows.…”

“…Therefore, the spin state, composition, and dynamics of all charge carriers are an important task. Because both the opposite charge carriers possess spin S = 1 / 2 , direct light-induced electron paramagnetic resonance (LEPR) spectroscopy appeared to be the most effective and suitable method for detailed investigation of spin-assisted processes carried out in various organic polymer − and copolymer − systems. For example, charge separation and recombination photoinitiated in BHJ formed by poly­[2,6-(4,4-bis­(2-ethylhexyl)-4 H -cyclopenta­[2,1-b;3,4-b′]-dithiophene)- alt -4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT), its analogues, and [6,6]-phenyl-C 61 -butanoic acid methyl ester (PC 61 BM) were studied by using the time-resolved EPR (TR-EPR) method .…”

“…Such molecular ordering accelerates a delocalization of electron wave functions at donor:acceptor interfaces that significantly reduce the Coulomb attraction between interfacial electron–hole pairs. Important is the fact that lamellar structure is characteristic not only for molecular single crystals but also for BHJ of some thin films. , The exchange interaction of spin carriers was shown , to improve electronic properties of organic donor–acceptor systems, similar to how it happens in inorganic anisotropic layered 2D-systems, and therefore should indeed play a decisive role in charge transport through their BHJ. However, the effect of the structural features of nonfullerene electron acceptors on the resulting morphology of respective composites is not yet understood.…”