Effect of spin traps on charge transport in low-bandgap copolymer:fullerene composites

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

“…Because the band gap of PANI salt normally lies near or exceeds 4 eV, this effect can rather be associated with the π–π* transition of the samples with a close optical band gap. A comparative analysis of the data obtained earlier − and in the present work showed that the spin–lattice relaxation of polarons and radical anions excited in the photovoltaic composites becomes less sensitive to the number and energy of photons when combined with the PANI:TSA subsystem. This may be due to a change in the exchange interaction between these spin subsystems with such a combination.…”

“…On the other hand, the same parameters obtained for the same charge carriers photoinitiated in the F8T2:PC 61 BM and PCDTBT:PC 61 BM BHJ appeared to be more sensitive to the photon energy. The line widths of these centers change nonlinearly with h ν ph and reach maximum values around 2.6, 2.1 and 2.4, 2.1 eV, respectively. − It must be emphasized that this picture changes sufficiently when these monopolymer narrow-band-gap composites are incorporated with the PANI:TSA subsystem. The analysis of the data presented in Figure a has testified a weak dependence of the line width of LEPR components on the photon energy h ν ph .…”

“…The LEPR spectra shown in Figures and are the sum spectra of polarons stabilized in the PANI:TSA salt and the net spectra of spin-charge carriers photoinitiated and recombined in the respective copolymer:methanofullerene BHJ. It has been shown previously − that the number and composition of paramagnetic centers photoinitiated in the latter are determined by their structure, morphology, and temperature as well as by the density and energy of the initiating light photons. Similar peculiarities should be expected also for the dual-polymer composites under study.…”

“…Because polarons and methanofullerene radical anions previously bonded to the former excitons quickly spread far apart, they become independent, which allows us to use such effects for separate estimation of their spin–lattice T 1 and spin–spin T 2 relaxation times. EPR spectroscopy was used for determination of these parameters of spin-charge carriers stabilized and photoinitiated in different polymer systems , including various narrow-band-gap copolymer:methanofullerene composites. − Relaxation parameters are also important because they provide information about the interaction of paramagnetic centers with other radicals, as well as with their microenvironment. Besides, mobile spins induce an additional magnetic field at their location.…”

“…Deconvolution of effective cw LEPR spectra of a multispin system should be able to determine the main magnetic resonance parameters separately for all mobile and captured spin-charge carriers and analyze them upon variation of experimental conditions. Indeed, such a procedure allows one to estimate the composition, spatial distribution, and local concentration of spin-charge carriers excited in different organic photovoltaic composites, including narrow-band-gap mono-copolymer composites formed by F8T2, poly­[2,7-(9,9-dioctylfluorene)-alt-4,7-bis­(thiophen-2-yl)­benzo-2,1,3-thiadiazole] (PFO-DBT), and PCDTBT copolymers with PC 61 BM electron acceptors, − BHJ of various poly­(3-alkylthiophenes):C 60 embedded into the conducting poly­(2,5-dioctyloxy-phenylene vinylene) matrix (P3AT:C 60 /OOPPV) as well as dual-polymer composites formed by PC 61 BM-modified poly­(3-dodecylthiophene) and PANI:TSA (PANI:TSA/P3DDT:PC 61 BM). , Paramagnetic susceptibility of spin-charge carriers photoinitiated in the latter dual-polymer composites was determined from their cw LEPR spectra to follow antiferromagnetic or ferromagnetic behavior depending on the value of spin-coupling. It was shown that the number, composition, and distribution of paramagnetic centers excited in the mono- and bipolymer photovoltaic systems are determined by their structure, morphology, as well as by the density and energy of the initiating light photons.…”