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EPFL Scientists Make Breakthrough in Cutting-Edge Solar Cells

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EPFL scientists have used an innovative spectroscopic technique to make a much-needed breakthrough in advanced photovoltaics.

Normally, light produces opposite charges in the active layer of a photovoltaic cell. Then the produced charges have to be separated immediately in order to keep them from recombining.

An integrated electric field drives positive charge carriers to one metallic contact, making negative charge carriers move in the opposite direction to become another electrode. EPFL researchers have been able to track the outcome of charge pairs in an innovative solar cell that is currently under research. The study is published in Nature Communications.

At Jacques Moser’s lab at EPFL, Martina Causa’ and Jelissa De Jonghe-Risse used a novel technique called ultrafast time-resolved electroabsorption spectroscopy (TREAS) to track the outcome of electron-hole pairs photogenerated in polymer: fullerene combination used in plastic solar cells.

The TREAS technique has been developed at Moser’s lab over the past three years. It enables real-time analysis of the separation distance of opposite charges produced by light in the active layer of a solar cell.

The TREAS technique relies on the optical probing of the efficient electric field encountered by a material. An external field is then applied to the device and causes “electroabsorption” or the “Stark effect”, where the external field influences the absorption spectrum of materials that create its photoactive layer.

Then, an ultrashort laser pulse produces charge carriers, which start to separate and make a counter electric field that opposes the external field. Consequently, the amplitude of the electroabsorption signal declines in real-time with pico- to femtosecond resolution.

The research findings could lead to an improved understanding of the light-induced charge separation mechanisms in this type of photovoltaic solar cell and the outcome of the morphology of the polymer:fullerene combination required to design more effective solar energy converters.

EPFL, Imperial College London and the group of Natalie Banerji at the University of Fribourg collaborated on this research. Swiss National Science Foundation (SNSF), NCCR- MUST, the University of Fribourg, King Abdullah University of Science and Technology and the European Research Council (ERC) Starting Independent Researcher Fellowship provided funding support for the research work.

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