Light-induced charge separation in a P3HT/PC₇₀BM composite as studied by out-of-phase electron spin echo spectroscopy

Abstract

A composite material of semiconducting polymer P3HT and fullerene derivative PCBM was studied by means of electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal was observed under laser irradiation of the composite at low temperature. We assume that during the charge separation process firstly the spin-correlated radical pairs in the singlet-polarized spin state are formed, and then the net polarization of radical pairs arises due to spin evolution. Both types of polarizations contribute to the out-of-phase ESE signal in the case of non-ideal microwave pulses. Analytical calculation of the echo shape for both types of initial polarization revealed that the contribution of the net polarization becomes zero after averaging over the whole EPR spectrum of the radical pair. This behavior was experimentally confirmed; thus the analysis of the out-of-phase ESE signal was simplified. Interspin distance distributions in the charge transfer state were obtained by modeling the out-of-phase ESE envelope modulation measured at different delays after laser flash T from 300 ns to 3.3 μs at a temperature of 65 K. Due to geminate recombination and diffusion of the radicals from the interface the distribution becomes significantly broader with larger distances prevailing at longer T values. The average distance between charges increases from 3.5 nm to 5.6 nm with an increase in T.

Charge Transfer Processes in OPV Materials as Revealed by EPR Spectroscopy

“…The latter is responsible for the distance and orientation distributions which lead to averaging out of the modulation pattern in ESEEM signals. In spite of these challenges, authors of [186][187][188][189] were able to detect after a laser flash the two-pulse out-of-phase ESE at X-band in both composites with PC 61 BM and PC 71 BM, confirming the spin-correlated nature of these CT-states. In the case of P3HT:PC 61 BM the characteristic time of the out-of-phase ESE decay with increasing delay after laser flash is 0.5 μs at 65 K, which gives a lower limit of the lifetime of these states.…”

“…Similar results were obtained for P3HT:PC 71 BM blends. [187] The authors reported the out-of-phase ESE decay time of 9 s at 65 K, which is considerably longer than for P3HT:PC 61 BM. Interspin distance distributions in the CT-state were obtained by modeling the out-of-phase ESE envelope modulation measured at different delays after Laser flash from 300 ns to 3.3 ms at a temperature of 65 K. The authors claimed that the average distance between charges increases with the increase of the delay after Laser flash from 35 Å to 56 Å.…”

“…Pulsed tr-EPR spectroscopy has also been employed to study the interfacial light-induced CT-states in P3HT:PC 61 BM and P3HT:PC 71 BM composites. [186][187][188][189] In general, time-resolved pulsed EPR spectroscopy is a powerful technique to study SCRP and has been widely used to characterize spin dynamics and geometry of SCRP in donor-acceptor systems at conventional, X-band and higher, Q-, W-, D-bands mw frequencies. [147,[174][175][190][191][192][193][194][195][196][197][198][199][200][201] Cw tr-EPR experiments are typically considerably more difficult at higher frequencies than X-band and have no yet been performed for BHJ systems.…”

Spin-chemistry concepts for spintronics scientists

“…Finally, we would like to mention that CIDEP can be due spin-selective processes involving particles with higher spin, e.g., due to the radical-triplet pair mechanism [117][118]. CIDEP effects have already been observed in materials, which are used for OPV: Behrends et al [32] and Kobori et al [119] have detected anti-phase EPR lines of photo-induced charge transfer complexes, i.e., of SCRPs, whereas Lukina et al [120] have recently reported a study of the out-of-phase electron spin echo. Until now, no CIDNP phenomenon [121][122][123][124] has been observed in spintronics, although a possibility of obtaining such effects has been mentioned [125]: "If nuclear spin resonance is found to have an impact on the spin-dependent electron transport due to the hyperfine interaction, ultimately the opposite process may become possible: storing electronic spin information in the nuclear spin."…”

“…Finally, we would like to mention that CIDEP can be due spin-selective processes involving particles with higher spin, e.g., due to the radical–triplet pair mechanism [ 117 – 118 ]. CIDEP effects have already been observed in materials, which are used for OPV: Behrends et al [ 32 ] and Kobori et al [ 119 ] have detected antiphase EPR lines of photo-induced charge transfer complexes, i.e., of SCRPs, whereas Lukina et al [ 120 ] have recently reported a study of the out-of-phase electron spin echo.…”