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2015: Ultrafast charge dynamics in organic solar elements

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MSU physicists (group of Prof. Dmitry Paraschuk) together with their Russian and foreign colleagues studied thecharge generationprocesses in organic solar cells based on novel materials.

Organic solar cells (OSCs) are a promising alternative to conventional solar cells because of a number of unique advantages, such as flexibility, semitransparency, ease of manufacturing etc. Due to relatively low dielectric constant of the organic materials, strongly bound Frenkel excitons are formed there upon photoexcitation. To split the photogenerated excitons, the so-called bulk heterojunction is used, which is a nanoscaled mixture of organic donor and acceptor materials. Therefore, charge photogeneration in the OSC active layer is a complex and multistep process, which occurs at sub-ns timescales. Any losses of photons, excitons and/or charges even at such ultrafast timescales lead to reduced efficiency of the OSC and, therefore, the early-time processes of the charge photogeneration have to be thoroughly studied.

Physicists from the Faculty of Physics, Lomonosov Moscow State University in collaboration with researches from the Institute of Synthetic Polymer Materials RAS, University of Mons (Belgium) and The Zernike Institute for Advanced Materials (University of Groningen, the Netherlands) demonstrated that the ultrafast spectroscopy is a unique tool for studying charge generation and recombination processes in the OSC active layer. A series of photovoltaic blends based on novel star-shaped small molecular donor and [70]PCBM fullerene acceptor were used as a model system. The processes of charge generation and recombination was studied by time-resolved photoinduced absorption (PIA) spectroscopy with the time resolution of ~100 fs. It was shown that both electron transfer after donor excitation and hole transfer after fullerene excitation are equally significant for generation of long-lived charges. The efficiency of charge photogeneration strongly depends on both donor chemical structure and donor:acceptor ratio. In a broader context, the results presented herein demonstrate how ultrafast spectroscopy can be used to reveal pathways of charge generation and losses, thereby providing an efficient means for optimization of the OSCs materials.

The results of this work have been published in the paper: O. Kozlov et al., “Ultrafast Charge Generation Pathways in Photovoltaic Blends Based on Novel Star-Shaped Conjugated Molecules,” Adv. Energy Mater. 5(7), 1401657 (2015), and is featured at the cover page of the journal.