light-induced charge separation dynamics in P3HT/PC70BM composite probed by pulse EPR spectroscopy at 65K

Leonid Kulik^a^,^b, Alexander Popov^a^,^b, Ekaterina Lukina^a^,^b

^aInstitute of Chemical Kinetics and Combustion SB RAS, Institutskaya 3, Novosibirsk, 630090, Russian Federation

^bNovosibirsk State University, Pirogova 2, Novosibirsk, 630090, Russian Federation

Materials for Sustainable Development Conference (MATSUS)
Proceedings of September Meeting 2016 (NFM16)

Berlin, Germany, 2016 September 5th - 13th

Organizers: Marin Alexe, Enrique Cánovas, Celso de Mello Donega, Ivan Infante, Thomas Kirchartz, Maksym Kovalenko, Federico Rosei, Lukas Schmidt-Mende, Laurens Siebbeles, Peter Strasser, Teodor K Todorov, Roel van de Krol and Ulrike Woggon

Poster, Leonid Kulik, 441
Publication date: 14th June 2016

Despite the significant progress in OPV power conversion efficiency, the mechanism of light-induced charge separation in bulk heterojunction OPV devices is still unclear. During charge separation process the intermediate state called charge transfer (CT) state is formed. Due to low dielectric constant of organic materials the estimated coulomb attraction in CT state is much larger than room temperature thermal energy. The mechanism helping CT state to separate into free charges still remains the challenge. To solve this problem the knowledge of structure and properties of charge-transfer state is required.

We studied a light-induced CT state in composite of conductive polymer P3HT and fullerene derivative PC70BM by out-of-phase electron spin echo (ESE), known to be a powerful tool for spin-correlated radical pairs studies. Recently this method was applied to another benchmark composite P3HT/PC60BM [1]. The out-of-phase ESE signal generated by two microwave pulses was observed under the laser pulse irradiation of the composite at low temperature 65K. Interspin distance distributions within the charge transfer state were obtained from modeling the dependence of out-of-phase ESE on the delay between the microwave pulses at different delays after laser flash TDAF=300 ns; 1.3 μs; 3.3 μs. The magnetic-dipolar interaction parameters of the radical pair P3HT+/PCBM- were determined from quantum chemical calculation, thus charge delocalization over P3HT chain was taken into account. The average distance between charges increases with TDAF increases from 3.5 nm at the smallest TDAF to 5.5 nm at TDAF=3.3 μs; also the distribution becomes significantly broader with larger distances prevailing at longer TDAF values. Such behavior is presumably caused by two simultaneous processes: diffusion of the radicals from the polymer/fullerene interface and geminate recombination of radical pairs with small distances between radicals.

The work was supported by the Russian Foundation for Basic Research grant â„– 15-03-07682a, by the Ministry of Science and Education of Russian Federation, and by Alexander von Humboldt Foundation research group linkage project "Light-induced processes and paramagnetic species in organic photovoltaics and photosynthesis".

[1] E.A. Lukina, A.A. Popov, M.N. Uvarov, L.V. Kulik. J. Phys. Chem. B, 2015, 119, 13543-13548.

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