Light-induced electron paramagnetic resonance study of charge transport in fullerene and nonfullerene pbdb-t-based solar cells

We present a combined light-induced electron paramagnetic resonance (LEPR) study of photoinitiation, relaxation, and recombination of charge carriers initiated by achromatic/white (with a color temperature of 5000 K) and monochromatic (with a photon energy of 1.34-3.41 eV) light in PBDB-T-based photovoltaic systems with PC61BM, PC71BM, and ITIC-M counterions. Charge carriers, polarons on polymer chains, and respective radical anions excited in disordered composite matrixes first fill spin traps, the number, energy depth, and spatial distribution of which are determined by the structure and crystallinity of bulk heterojunctions. By deconvolution of the effective LEPR spectra, the contributions of immobilized and mobile charge carriers, as well as their main magnetic resonance parameters, were determined separately at a wide variety of experimental conditions. The interaction of spins occupying different energy levels in the bandgap of a polymer semiconductor provokes the extreme photon energy sensitivity of the spin-assisted processes carried out in the polymer composites. The density functional theory calculations of the millimeter-waveband LEPR spectrum allowed the conclusion that polarons photoinitiated in the PBDB-T backbone are delocalized over its 4-5 monomers. Side π-π-stack packaging and S-isomerization of electron acceptors were also found. Predominant nongeminate recombination of charge carriers follows multistep trapping-detrapping spin hopping between sites of polymer layers and is strongly governed by the number, energy depth, and spatial distribution of spin traps. It was shown that all spin-involving processes in composites are spin-assisted and, therefore, are determined by the main magnetic resonance properties of both the spin charge carriers. The stability of charge carriers in a polymer-based composite was demonstrated to increase by more than an order of magnitude in the series of radical anions PC61BM- → ITIC-M- → PC71BM- . A further improvement in the functionality of the composite occurs at its slight 2,5-diphenyloxazole modification. The use of low-dimensional ITIC-M instead of PCBM and/or PPO with extended π-system significantly increases the exchange interaction between the spin charge carriers situated on the adjacent layers of the composite. This blocks intrachain charge diffusion but accelerates its interlayer hopping in the polymer matrix, which increases the efficiency and functionality of the composite

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

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

New molecular complexes of fullerenes C-60 and C-70 with tetraphenylporphyrins M(tpp) , in which M = H-2, Mn, Co, Cu, Zn, and FeCl

New molecular complexes of fullerenes C-60 and C-70 with tetraphenyl-porphyrins [M(tpp)] in which M = H-2, Mn-II, Co-II, Cu-II, Zn-II and (FeCl)-Cl-III, have been synthesised. Crystal structures of two C-60 complexes with H2TPP, which differ only in the number of benzene solvated molecules, and C-60 and C-70 complexes with [Cu(tpp)] have been studied. The fullerene molecules form a honeycomb motif in H2TPP . 2C(60). 3C(6)H(6), puckcred graphite-like layers in H2TPP . 2C(60). 4C(6)H(6), zigzag chains in [Cu(tpp)].C-70.1.5C(7)H(8).0.5C(2)HCl(3) and columns in [Cu(tpp)](2).C-60. H2TPP has van der Waals contacts with C-60 through nitrogen atoms and phenyl groups. Copper atoms of the [Cu(tpp)l molecules are weakly coordinated with C- 70, but form no shortened contacts with C-60. The formation of molecular complexes with fullerenes affects the ESR spectra of [M(tpp)] (M = Mn, Co and Cu). [Mn(tpp)] in the complex with C- 70 lowers its spin state from S = 5/2 to S = 1/2, whereas [Co(tpp)] and [Cu(tpp)] change the constants of hyperfine interaction. ESR, IR, UV-visible and X-ray photoelectron spectroscopic data show no noticeable charge transfer from the porphyrinate to the fullerene molecules