The Multiple Exciton Generation (MEG) Effect
It has been experimentally demonstrated that it is possible to utilize the lost photon energy in order to produce multiple excitons and consequently an amplified electrical current signal from a single photon. The multiple-exciton energy can never exceed the amount of energy carried by the incident photon, so that the whole process is in accordance with the conservation of energy principle.
A similar process is even possible when bulk materials are used and it is called impact ionization. However, the enhancement of solar-energy conversion is only possible in quantum-confined systems, such as quantum dots, where the Multiple Exciton Generation Effect – MEG is observed.
The observation of the MEG can only be explained through the complicated quantum dynamics of nanostructured semiconductors. More specifically, these quantum confined systems have a small Exciton Bohr Radius which is a measure of the maximum distance between an electron and the hole. In a bulk semiconductor the Exciton Bohr Radius is very small compared to the dimensions of the crystal, so the exciton can wander throughout the crystal. In the case of quantum dots, the exciton can not move freely throughout the crystal due to the small size of the dot.
The use of quantum dots for the increase in conversion efficiency, still has a certain weakness. The exciton's lifetime is only a few hundred femtoseconds (order of 10-15sec). This means that an electron can only stay in the conductive band for a limited period of time before it reunites with a hole and the exciton disappears. The exciton's brief lifetime is due to the small Bohr radius of the quantum dot.