https://www.sciencedaily.com/releases/2019/01/190114103246.htm

Solar cells convert the energy of light into an electric direct current (DC) which is fed into an electric supply grid. Key steps are the separation of charges after light absorption and their transport to the contacts of the device. The electric currents are carried by negative (electrons) and positive charge carriers (holes) performing so called intraband motions in various electronic bands of the semiconductor. From a physics point of view, the following questions are essential: what is the smallest unit in a crystal which can provide a photo-induced direct current (DC)? Up to which maximum frequency can one generate such currents? Which mechanisms at the atomic scale are responsible for such charge transport?

The smallest unit of a crystal is the so-called unit cell, a well-defined arrangement of atoms determined by chemical bonds. The unit cell of the prototype semiconductor GaAs represents an arrangement of Ga and As atoms without a center of inversion. In the ground state of the crystal represented by the electronic valence band, the valence electrons are concentrated on the bonds between the Ga and the As atoms. Upon absorption of near-infrared or visible light, an electron is promoted from the valence band to the next higher band, the conduction band. In the new state, the electron charge is shifted towards the Ga atoms. This charge transfer corresponds to a local electric current, the interband or shift current, which is fundamentally different from the electron motions in intraband currents. Until recently, there has been a controversial debate among theoreticians whether the experimentally observed photo-induced currents are due to intraband or interband motions.