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 electro absorption spectroscopy (TREAS) in order to follow the fate of charge pairs photo generated 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 ultra short 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 femto-second 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 of 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.