Light-Induced Electron Paramagnetic Resonance Study of Poly(3-alkylthiophene)/Fullerene Composites

Abstract

Radical pairs, polarons, and fullerene anion radicals photoinduced by photons with an energy of 1.98−2.73 eV in bulk heterojunctions formed by poly(3-hexylthiophene) (P3HT) and poly(3-dodecylthiophene) (P3DDT) with [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and 2-(azahomo[60]fullereno)-5-nitropyrimidine (AFNP) fullerene derivatives were studied by a direct light-induced electron paramagnetic resonance (LEPR) method in a wide temperature range. LEPR spectra of the polymer/fullerene composites consist of contributions of mobile and trapped charge carriers. Concentration and magnetic resonance parameters of these charge carriers were found to depend on the energy of initiated photons. Spin−lattice and spin−spin relaxation times of polarons and fullerene anion radicals were determined by the steady-state saturation method. The interaction of most charge carriers with the lattice is characterized by monotonic temperature dependence, whereas the spin−lattice relaxation time of fullerene anion radicals trapped in the P3DDT matrix demonstrates sharper temperature dependence. Spin−spin interaction is shown to be nearly temperature independent and to be governed by structural properties of polymer/fullerene composites. Longitudinal diffusion of polarons and pseudorotation of fullerene derivatives was shown to follow the activation Elliot hopping model. The replacement of the P3HT matrix by P3DDT accelerates polaron dynamics and increases its anisotropy. The energetic barrier required for polaron interchain hopping mainly prevails upon that of its intrachain diffusion in all composites except P3DDT/AFNP one. Spin dynamics becomes easier when the PCBM fullerene derivative is replaced by the AFNP one.

Light-induced EPR study of charge transfer in P3HT/bis-PCBM bulk heterojunctions

“…The effective values g P iso = 2.0023 and g F iso = 2.0007 are close to those obtained for charge carriers photoinduced in other polymer/fullerene composites, 19,25 however, exceed those, g P iso = 2.0017 and g F iso = 1.9996, obtained for P3HT/PCBM. 21 Note, that the g P iso value of the latter measured more accurately at W-band (94 GHz) EPR is equal to 2.00187. 26 These parameters were appeared to become weakly temperature dependent than those of the P3HT/PCBM composite.…”

“…26 These parameters were appeared to become weakly temperature dependent than those of the P3HT/PCBM composite. 21 As in case of other polymer/fullerene systems, 20,21 LEPR spectra of the P3HT/bis-PCBM bulk heterojunctions consists of two Lorentzian contributions of mobile polarons and bis-methanofullerene anion radicals in quasi-pairs P…”

“…The limiting number of polarons n p and fullerene anion radicals n f simultaneously formed per each polymer unit in the P3HT/bis-PCBM bulk heterojunctions can be determined, as in the case of analogous systems, 21 by I 2 -doping of the sample. Limiting paramagnetic susceptibility χ of iodine-treated composite was determined at 310 and 77 K to be 8.7 x 10 -6 and 9.9 x 10 -5 emu/mole, respectively.…”

“…However, up today such LEPR studies are carried out mainly at helium temperatures when dynamic processes in organic photovoltaics appear to be frozen. Recently, we showed 20,21 that a LEPR spectra of charge carriers photoinduced in P3AT/PCBM bulk heterojunctions indeed contains contributions of mobile and pinned charge carriers with different spectrum shapes. Magnetic, relaxation and dynamics parameters of these radicals were demonstrated to be governed by the energy of initiated photons hν ph .…”

“…Magnetic, relaxation and dynamics parameters of these radicals were demonstrated to be governed by the energy of initiated photons hν ph . [19][20][21][22] Herein, we present results of the LEPR study of main properties of charge carriers photoinduced in the P3HT/bis-PCBM bulk heterojunctions. We demonstrate the increase of dimensionality of this composition under the replacement of PCBM by bis-PCBM.…”