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

Abstract

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-C61-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.

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

“…Figure S1 shows the NIR−vis−UV spectra of the PANI:TS A/F8T2:PC 6 1 BM, P ANI:TSA/PFO-DBT:PC 61 BM, and PANI:TSA/PCDTBT:PC 61 BM dual-polymer composites. For the comparison, optical spectra obtained 42 for respective copolymer photovoltaic composites are also shown in Figure S1 42 differing by a slight growth in optical density at the high-energy region due to the overlapping signals of both polymers. The system PANI:TSA/PFO-DBT:PC 61 BM demonstrates a characteristic set of absorption bands at 608 and 571 nm (2.04 and 2.17 eV) inherent to the benzatriazole moiety and lines at 415 and 397 nm (2.99 and 3.12 eV) attributed to the fluorine subunits.…”

“…Structurally, the latter two absorption spectra also correspond to those earlier obtained for the PFO-DBT:PC 61 BM and PCDTBT:PC 61 BM composites. 42 The only difference is that the presence of the PANI:TSA subsystem increases the intensity of the UV−vis absorption band at near 290 nm, typical of a π−π* transition. 54,55 Thus, polyaniline salt, while dispersed in these composites, leads to a slight increase in the density of optical spectra while maintaining their structures.…”

“…As a result of such a procedure, each sample contained in sum 0.1 mg of a S1 in comparison with the spectra obtained for the respective copolymer photovoltaic composites. 42 The compositions and positions of the spectral components were determined accurately using the differentiation of experimental absorption spectra.…”

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

“…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.…”