The procedure depends 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 result is referred to as “electro-absorption” or the “Stark effect.”
Natalie Banerji in Jaques Moser’s laboratory over at EPFL used ultrafast time-resolved electroabsorption spectroscopy (TREAS) in order 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 throughout the last three years. This allows real-time measurements of the separation distance of charges generated by light in the active layer of a photovoltaic or PV solar cell.
An ultrashort laser pulse then generates charges. These begin to separate, inducing a counter electrical field that opposes the externally administered one. As a result, a decrease of the amplitude of the electro-absorption signal can possibly be detected in real-time with pico- to femtosecond resolution.
The data from the study develop a better understanding of the mechanisms of light-induced charge separation in this type of photovoltaic, in addition to the effect of the morphology of the polymer: fullerene blend, which in turn is really necessary for developing a lot more effective solar energy converters.