New Spectroscopy to Follow PV-Charges

Schematic three-dimensional representation of the interface in a PV cell (Source: EPFL / NPG)

Schematic three-dimensional representation of the interface in a PV cell (Source: EPFL / NPG)

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 charges are driven by a built-in electric field to one metallic contact, while negative charges migrate in the opposite direction to another electrode. Using a unique ultra-fast spectro­scopic technique, EPFL scien­tists have now been able to track the fate of charged pairs in an advanced type of solar cells currently under intense research.

Natalie Banerji at Jaques Moser’s lab at EPFL used ultrafast time-resolved electro­absorption spectro­scopy (TREAS) to follow the fate of charge 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 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. An ultra­short laser pulse then generates charges. These begin to separate, inducing a counter electrical 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 femto-second 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. (Source: EPFL)

Reference: M. Causa et al.: The fate of electron–hole pairs in polymer:fullerene blends for organic photovoltaics, Nat. Comm. 7, 12556 (2016); DOI: 10.1038/ncomms12556

Link: Lausanne Centre for Ultrafast Science (LACUS), Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

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