A New Technique Opens up Advanced Solar Cells

Using a novel spectroscopic technique, EPFL scientists have made a much-needed breakthrough in cutting-edge photovoltaics.

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In a photovoltaic cell, light generates opposite charges in the active layer. The charges must then be separated as quickly as possible to keep them from recombining. Positive charge carriers are driven by a built-in electric field to one metallic contact, while negative charge carriers migrate in the opposite direction to another electrode. Using a unique ultrafast spectroscopic technique, EPFL scientists have now been able to track the fate of charge pairs in an advanced type of solar cell currently under intense research. The work is published in Nature Communications.

Jelissa De Jonghe-Risse and Martina Causa’ at Jacques Moser’s lab at EPFL used ultrafast time-resolved electroabsorption spectroscopy (TREAS) to follow the fate of electron-hole pairs photogenerated in polymer:fullerene blends used in plastic solar cells. TREAS has been developed in Moser’s lab during the last three years. It allows real-time measurements of the separation distance of opposites charges generated by light in the active layer of a photovoltaic solar cell.

The technique relies on the optical probing of the effective electric field experienced by a material. An external field is applied to the device and affects the absorption spectrum of materials that make up its photoactive layer. The effect is known as “electroabsorption” or the “Stark effect”.

An ultrashort laser pulse then generates charge carriers. These begin to separate, inducing a counter electric field that opposes the externally applied one. As a result, a decrease of the amplitude of the electroabsorption signal can be detected in real time with pico- to femtosecond resolution.

The data from the study create a better understanding of the mechanisms of light-induced charge separation in this type of photovoltaics, as well as of the effect of the morphology of the polymer:fullerene blend, which is necessary for designing more efficient solar energy converters.