Two-step excitation in three-level systems

If interaction with phonons cannot be prevented, thermalization losses can be reduced by dividing the incident spectrum over more than one transition as we have seen with tandem cells. In a three-level system, where the levels can be bands as well, three different transitions may occur in a single material: directly from the lower level to the upper level and in addition by a two-step process from the lower level to the intermediate level and from there to the upper level. In both ways electrons are generated at the upper level and holes at the lower level.