Shows silicon a photoelectric effect

Surprising dynamic in the photo effect

In solar cells, electrons are released into semiconductor materials such as silicon with the help of sunlight, which enables electrical current to flow. Physicists are now reporting in the journal "Science" that the underlying photo effect takes place with more amazing dynamics than previously thought. They analyzed the movement of released electrons with extremely short laser flashes and thus lay the foundation for a more detailed understanding of the photo effect.

Photo effect in semiconductors

For their experiments, Fabian Siek from Bielefeld University and his colleagues used a semiconductor made from tungsten selenide. Using ultraviolet light pulses, they knocked four electrons out of this material from different trajectories around the atomic nuclei of the crystalline compound - the orbitals. The kinetic energy of the photoelectrons varied significantly between 32.2 and 87 electron volts, depending on their origin. Each of these stimuli was followed by a second, extremely short infrared laser pulse within a few attoseconds - an attosecond is a billionth of a billionth of a second. The movement of the electrons could be determined through the interaction of the light with the electrons.

This method of short-term spectroscopy with attosecond pulses showed that the fastest electrons moved around the atoms of the crystal for an extremely short time before they flew away with a slight delay. In contrast, slower electrons separated from the atoms much faster. The physicists see the reason for this in an interaction between the atoms of the crystal. The angular momentum that acted locally on the respective electrons before they were knocked out played an important role. The greater this angular momentum, the greater the kinetic energy of the electrons and the longer they moved around the atoms in the crystal lattice.

Not only the speed, but also the movements of photoelectrons depend heavily on the respective starting position. "The dynamics of the photo effect are not completely and correctly captured with the usual models," the scientists say. Further experiments with extremely short attosecond pulses and above all a detailed theoretical consideration of this phenomenon could lead to a more precise and therefore more correct description of the photo effect in the future.